Spectrum Analyzer Mode Measurement Guide

Spectrum Analyzer Mode Measurement Guide
Agilent X-Series
Signal Analyzer
This manual provides documentation for
the following X-Series Instruments:
PXA Signal Analyzer N9030A
MXA Signal Analyzer N9020A
EXA Signal Analyzer N9010A
CXA Signal Analyzer N9000A
MXE EMI Receiver N9038A
Spectrum Analyzer Mode
Measurement Guide
Notices
© Agilent Technologies, Inc. 2008-2012
Manual Part Number
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agreement and written consent from Agilent Technologies, Inc. as governed by
United States and international copyright
laws.
N9060-90034
Supersedes: N9060-90033
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Print Date
February 2012
Printed in USA
Agilent Technologies, Inc.
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Safety Notices
CAUTION
A CAUTION notice denotes a
hazard. It calls attention to an
operating procedure, practice, or
the like that, if not correctly performed or adhered to, could result
in damage to the product or loss of
important data. Do not proceed
beyond a CAUTION notice until
the indicated conditions are fully
understood and met.
WARNING
A WARNING notice denotes a
hazard. It calls attention to an
operating procedure, practice,
or the like that, if not correctly
performed or adhered to, could
result in personal injury or
death. Do not proceed beyond a
WARNING notice until the indicated conditions are fully
understood and met.
If software is for use in the performance
of a U.S. Government prime contract or
subcontract, Software is delivered and
2
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This Agilent technologies instrument product is warranted against defects in material and workmanship for
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Where to Find the Latest Information
Documentation is updated periodically. For the latest information about this analyzer, including firmware
upgrades, application information, and product information, see the following URLs:
http://www.agilent.com/find/pxa
http://www.agilent.com/find/mxa
http://www.agilent.com/find/exa
http://www.agilent.com/find/cxa
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Information on preventing analyzer damage can be found at:
http://www.agilent.com/find/tips
3
4
Contents
1 Getting Started with the Spectrum Analyzer Measurement Application
Making a Basic Measurement
Using the front panel
12
12
Presetting the signal analyzer
Viewing a signal
13
14
Recommended Test Equipment
Accessories Available
50 ohm load
17
18
18
50 ohm/75 ohm minimum loss pad
75 ohm matching transformer
AC probe
18
18
18
AC probe (low frequency)
19
Broadband preamplifiers and power amplifiers
GPIB cable
19
USB/GPIB cable
19
RF and Transient Limiters
Power splitters
19
19
20
Static safety accessories
20
2 Measuring Multiple Signals
Comparing Signals on the Same Screen Using Marker Delta
22
Comparing Signals not on the Same Screen Using Marker Delta
Resolving Signals of Equal Amplitude
25
28
Resolving Small Signals Hidden by Large Signals
33
Decreasing the Frequency Span Around the Signal
37
Easily Measure Varying Levels of Modulated Power Compared to a Reference
3 Measuring a Low−Level Signal
Reducing Input Attenuation
44
Decreasing the Resolution Bandwidth
47
Using the Average Detector and Increased Sweep Time
Trace Averaging
52
5
50
39
Contents
4 Improving Frequency Resolution and Accuracy
Using a Frequency Counter to Improve Frequency Resolution and Accuracy
56
5 Tracking Drifting Signals
Measuring a Source Frequency Drift
Tracking a Signal
58
61
6 Making Distortion Measurements
Identifying Analyzer Generated Distortion
64
Third-Order Intermodulation Distortion
67
7 Measuring Noise
Measuring Signal-to-Noise
72
Measuring Noise Using the Noise Marker
74
Measuring Noise-Like Signals Using Band/Interval Density Markers
78
Measuring Noise-Like Signals Using the Channel Power Measurement
Measuring Signal-to-Noise of a Modulated Carrier
83
Improving Phase Noise Measurements by Subtracting Signal Analyzer Noise
8 Making Time-Gated Measurements
Generating a Pulsed-RF FM Signal
Signal source setup
Analyzer setup
94
94
95
Digitizing oscilloscope setup
97
Connecting the Instruments to Make Time-Gated Measurements
Gated LO Measurement
100
Gated Video Measurement
104
Gated FFT Measurement
108
9 Measuring Digital Communications Signals
Channel Power Measurements
112
Occupied Bandwidth Measurements
Troubleshooting hints
114
115
6
81
99
88
Contents
Making Adjacent Channel Power (ACP) Measurements
Making Statistical Power Measurements (CCDF)
Making Burst Power Measurements
121
126
Spurious Emissions Measurements
Troubleshooting hints
116
132
134
Spectrum Emission Mask Measurements
Troubleshooting hints
135
136
10 Demodulating AM Signals
Measuring the Modulation Rate of an AM Signal
140
Measuring the Modulation Index of an AM Signal
144
11 IQ Analyzer Measurement
Capturing Wideband Signals for Further Analysis
Complex Spectrum Measurement
148
149
IQ Waveform (Time Domain) Measurement
151
12 Using Option BBA Baseband I/Q Inputs
Baseband I/Q measurements available for X-Series Signal Analyzers
Baseband I/Q measurement overview
156
157
13 Option EXM External Mixing
Using Option EXM with the Agilent 11970 Series Mixers.
Amplitude calibration
161
Loading conversion loss data for the PXA Signal Analyzer
Signal ID
160
161
165
Image shift
165
Using the M1970 Series Mixers with X-Series Signal Analyzers (Option EXM)
Operating precautions
Equipment set up
Operation
167
168
169
Amplitude calibration
LO adjustment
170
170
7
167
Contents
Viewing the external mixer setup screen
171
14 Concepts
Resolving Closely Spaced Signals
174
Resolving signals of equal amplitude
174
Resolving small signals hidden by large signals
Trigger Concepts
174
176
Selecting a trigger
Time Gating Concepts
176
179
Introduction: Using Time Gating on a Simplified Digital Radio Signal
How time gating works
181
Measuring a complex/unknown signal
187
“Quick Rules” for making time-gated measurements
192
Using the Edge Mode or Level Mode for triggering
Noise measurements using Time Gating
195
196
AM and FM Demodulation Concepts
197
Demodulating an AM signal using the analyzer as a fixed tuned receiver
(Time-Domain)
197
Demodulating an FM signal using the analyzer as a fixed tuned receiver
(Time-Domain)
197
IQ Analysis Concepts
Purpose
198
198
Complex Spectrum measurement
IQ Waveform Measurement
198
199
Spurious Emissions Measurement Concepts
Purpose
200
200
Measurement method
200
Spectrum Emission Mask Measurement Concepts
Purpose
201
Measurement method
201
Occupied Bandwidth Measurement Concepts
Purpose
201
202
Measurement method
202
8
202
179
Contents
Baseband I/Q Inputs (Option BBA) Measurement Concepts
What are Baseband I/Q inputs?
What are Baseband I/Q signals?
203
203
Why make measurements at baseband?
204
Selecting input probes for baseband measurements
Baseband I/Q measurement views
206
9
205
203
Contents
10
Getting Started with the Spectrum Analyzer Measurement
Application
1
Getting Started with the
Spectrum Analyzer
Measurement Application
This chapter provides some basic information about using the Spectrum Analyzer
and IQ Analyzer Measurement Application Modes. It includes topics on:
•
“Making a Basic Measurement” on page 12
•
“Recommended Test Equipment” on page 17
•
“Accessories Available” on page 18
Technical
Documentation
Summary:
Your Signal Analysis measurement platform:
Getting Started
Turn on process, Windows XP use/configuration, Front and rear panel
Specifications
Specifications for all available Measurement Applications and optional
hardware (for example, Spectrum Analyzer and W-CDMA)
Functional Testing
Quick checks to verify overall instrument operation.
Instrument Messages
Descriptions of displayed messages of Information, Warnings and Errors
Measurement Application specific documentation:
(for example, Spectrum Analyzer Measurement Application or W-CDMA Measurement Application)
Measurement Guide
Examples of measurements made using the front panel keys or over a
remote interface.
User’s/Programmer’s
Reference
Descriptions of front panel key functionality and the corresponding SCPI
commands. May also include some concept information.
11
Getting Started with the Spectrum Analyzer Measurement Application
Making a Basic Measurement
Making a Basic Measurement
Refer to the description of the instrument front and rear panels to improve your
understanding of the Agilent Signal Analyzer measurement platform. This
knowledge will help you with the following measurement example.
This section includes:
CAUTION
•
“Using the front panel” on page 12
•
“Presetting the signal analyzer” on page 13
•
“Viewing a signal” on page 14
Make sure that the total power of all signals at the analyzer input does not exceed
+30 dBm (1 watt).
Using the front panel
Entering data
When setting measurement parameters, there are several ways to enter or modify the
value of the active function:
Knob
Increments or decrements the current value.
Arrow Keys
Increments or decrements the current value.
Numeric
Keypad
Enters a specific value. Then press the desired terminator (either a
unit softkey, or the Enter key).
Unit Softkeys
Terminate a value that requires a unit-of-measurement.
Enter Key
Terminates an entry when either no unit of measure is needed, or
you want to use the default unit.
Using Menu Keys
Menu Keys (which appear along the right side of the display) provide access to many
analyzer functions. Here are examples of menu key types:
Toggle
Allows you to activate/deactivate states.
Example:
Submenu
Toggles the selection (underlined choice) each time you
press the key.
Displays a new menu of softkeys.
12
Getting Started with the Spectrum Analyzer Measurement Application
Making a Basic Measurement
Example:
Choice
A submenu key allows you to view a new menu of softkeys
related to the submenu key category.
Allows you to make a selection from a list of values.
Example:
Adjust
A choice key displays the currently selected submenu
choice, in this example, dBm. When the choice is made, the
submenu automatically returns.
Highlights the softkey and sets the active function.
Examples:
Press this type of key and enter a value.
The default for softkeys with an automatic (Auto) or
manual (Man) choice is automatic. After you enter a
value, the selection changes to manual. You can also press
the softkey twice to change to manual.
Presetting the signal analyzer
The preset function provides a known starting point for making measurements. The
analyzer has two main types of preset:
User Preset
Restores the analyzer to a user-defined state.
Mode Preset
This type of preset restores the currently selected mode to a
default state.
For details, see the help or User’s and Programmer’s Reference.
Creating a user preset
If you constantly use settings which are not the normal defaults, use the following
steps to create a user-defined preset:
Step
Action
1 Set analyzer parameters as
desired
2 Set the current parameters
as the user preset state
•
Press User Preset, Save User Preset
3 To select a preset state
•
Press User Preset, User Preset
13
Getting Started with the Spectrum Analyzer Measurement Application
Making a Basic Measurement
Viewing a signal
Step
Action
Notes
1 Return the current
mode settings to
factory defaults.
•
Press Mode Preset.
2 Route the internal
50 MHz signal to the
analyzer input.
•
Press Input/Output,
RF Calibrator, 50,
MHz.
3 Set the reference
level to 10 dBm.
•
Press AMPTD Y
Scale, 10, dBm.
4 Set the center
frequency to 40
MHz.
•
Press FREQ
Channel, Center
Freq, 40, MHz.
5 Set the frequency
span to 50 MHz.
•
Press SPAN, 50, MHz.
The 50 MHz reference
signal appears on the
display
Reading frequency and amplitude
Step
Action
Notes
1 Activate a marker
and place it on the
highest amplitude
signal.
•
Press Peak
Search.
The frequency and amplitude
of the marker appear in the
active function block in the
upper-right of the display.
You can use the knob, the
arrow keys, or the softkeys in
the Peak Search menu to
move the marker around on
the signal.
2 To return the marker
to the peak of the
signal.
•
Press Peak
Search.
Changing reference level
Step
Action
Notes
1 Change the reference
level.
a. Press AMPLTD
Y Scale.
The reference level is now the
active function.
b. Press Marker −>,
Mkr −> Ref Lvl.
14
Getting Started with the Spectrum Analyzer Measurement Application
Making a Basic Measurement
Improving frequency accuracy
NOTE
When you use the frequency count function, if the ratio of the resolution bandwidth
to the span is less than 0.002, you will get a display message that you need to reduce
the Span/RBW ratio. This is because the resolution bandwidth is too narrow.
Step
Action
1 Activate the Marker
Count menu.
•
Press Marker,
More 1 of 2,
Marker Count.
2 To increase the
accuracy of the
frequency reading in
the marker
annotation.
•
Press Counter .
3 Move the signal peak
to the center of the
display.
•
Notes
The marker active function
annotation changes from
Mkr1 to Cntr1.
The displayed resolution in
the marker annotation
improves.
Press Marker −>,
Mkr −> CF.
Valid marker count range
NOTE
Marker count functions properly only on CW signals or discrete peaks. For a valid
reading, the marker must be ≥26 dB above the noise.
Step
Action
1 Move the marker
down the skirt of the
50 MHz peak.
Notes
Although the readout in the
active function changes, as
long as the marker is at least
26 dB above the noise, the
counted value (upper-right
corner of display) does not
change.
For an accurate count, the
marker does not have to be
exactly at the displayed peak
15
Getting Started with the Spectrum Analyzer Measurement Application
Making a Basic Measurement
Step
Action
Notes
2 Enter a new value.
•
Press BW, Res
BW and enter a
value.
This action makes the
resolution bandwidth (RBW)
the active function and allows
you to experiment with
different resolution
bandwidth values.
3 Turn off the marker.
•
Press Marker,
Off.
16
Getting Started with the Spectrum Analyzer Measurement Application
Recommended Test Equipment
Recommended Test Equipment
The following table list the test equipment you will need to perform the example
measurements describe in this manual.
NOTE
To find descriptions of specific analyzer functions, for the N9060A Spectrum
Analyzer Measurement Application, refer to the Agilent Technologies X-Series
User’s and Programmer’s Reference.
Test Equipment
Specifications
Recommended Model
0.25 MHz to 4.0 GHz
Ext Ref Input
E443XB series or
E4438C
Signal Sources
Signal Generator (2)
Adapters
Type-N (m) to BNC (f) (6)
1250-0780
Cables
BNC, 122 cm (48 in) (3)
10503A
Miscellaneous
Directional Bridge
86205A
17
Getting Started with the Spectrum Analyzer Measurement Application
Accessories Available
Accessories Available
A number of accessories are available from Agilent Technologies to help you
configure your analyzer for your specific applications. They can be ordered through
your local Agilent Sales and Service Office and are listed below.
NOTE
There are also some instrument options available that can improve your
measurements. Some options can only be ordered when you make your original
equipment purchase. But some are also available as kits that you can order and install
later. Order kits through your local Agilent Sales and Service Office.
For the latest information on Agilent signal analyzer options and upgrade kits, visit
the following Internet URL: http://www.agilent.com/find/sa_upgrades
50 ohm load
The Agilent 909 series of loads come in several models and options providing a
variety of frequency ranges and VSWRs. Also, they are available in either 50 ohm or
75 Ohm. Some examples include the:
909A: DC to 18 GHz
909C: DC to 2 GHz
909D: DC to 26.5 GHz
50 ohm/75 ohm minimum loss pad
The HP/Agilent 11852B is a low VSWR minimum loss pad that allows you to make
measurements on 75 Ohm devices using an analyzer with a 50 Ohm input. It is
effective over a frequency range of dc to 2 GHz.
75 ohm matching transformer
The HP/Agilent 11694A allows you to make measurements in 75 Ohm systems
using an analyzer with a 50 Ohm input. It is effective over a frequency range of 3 to
500 MHz.
AC probe
The Agilent 85024A high frequency probe performs in-circuit measurements
without adversely loading the circuit under test. The probe has an input capacitance
of 0.7 pF shunted by 1 megohm of resistance and operates over a frequency range of
300 kHz to 3 GHz. High probe sensitivity and low distortion levels allow
measurements to be made while taking advantage of the full dynamic range of the
signal analyzer.
18
Getting Started with the Spectrum Analyzer Measurement Application
Accessories Available
AC probe (low frequency)
The Agilent 41800A low frequency probe has a low input capacitance and a
frequency range of 5 Hz to 500 MHz.
Broadband preamplifiers and power amplifiers
Preamplifiers and power amplifiers can be used with your signal analyzer to enhance
measurements of very low-level signals.
•
The Agilent 8447D preamplifier provides a minimum of 25 dB gain from 100
kHz to 1.3 GHz.
•
The Agilent 87405A preamplifier provides a minimum of 22 dB gain from 10
MHz to 3 GHz. (Power is supplied by the probe power output of the analyzer.)
•
The Agilent 83006A preamplifier provides a minimum of 26 dB gain from 10
MHz to 26.5 GHz.
•
The Agilent 85905A CATV 75 ohm preamplifier provides a minimum of 18 dB
gain from 45 MHz to 1 GHz. (Power is supplied by the probe power output of the
analyzer.)
•
The 11909A low noise preamplifier provides a minimum of 32 dB gain from 9
kHz to 1 GHz and a typical noise figure of 1.8 dB.
GPIB cable
The Agilent 10833 Series GPIB cables interconnect GPIB devices and are available
in four different lengths (0.5 to 4 meters). GPIB cables are used to connect
controllers to a signal analyzer.
USB/GPIB cable
The Agilent 82357A USB/GPIB interface provides a direct connection from the
USB port on your laptop or desktop PC to GPIB instruments. It comes with the SICL
and VISA software for Windows® 2000/XP. Using VISA software, your existing
GPIB programs work immediately, without modification. The 82357A is a standard
Plug and Play device and you can interface with up to 14 GPIB instruments.
RF and Transient Limiters
The Agilent 11867A and N9355B RF and Microwave Limiters protect the analyzer
input circuits from damage due to high power levels. The N9355B operates over a
frequency range of dc to 1800 MHz and begins reflecting signal levels over 1 mW up
to 10 W average power and 100 watts peak power. The 11693A microwave limiter
(0.1 to 18 GHz) guards against input signals over 10 milliwatt up to 1 watt average
power.
19
Getting Started with the Spectrum Analyzer Measurement Application
Accessories Available
The Agilent 11947A Transient Limiter protects the analyzer input circuits from
damage due to signal transients. It specifically is needed for use with a line
impedance stabilization network (LISN). It operates over a frequency range of 9 kHz
to 200 MHz, with 10 dB of insertion loss.
Power splitters
The Agilent 11667A/B/C power splitters are two-resistor type splitters that provide
excellent output SWR, at 50 Ω impedance. The tracking between the two output
arms, over a broad frequency range, allows wideband measurements to be made with
a minimum of uncertainty.
11667A: DC to 18 GHz
11667B: DC to 26.5 GHz
11667C: DC to 50 GHz
Static safety accessories
9300-1367
Wrist-strap, color black, stainless steel. Four adjustable links and a
7 mm post-type connection.
9300-0980
Wrist-strap cord 1.5 m (5 ft.)
20
Measuring Multiple Signals
2
Measuring Multiple Signals
21
Measuring Multiple Signals
Comparing Signals on the Same Screen Using Marker Delta
Comparing Signals on the Same Screen Using Marker Delta
Using the analyzer, you can easily compare frequency and amplitude differences
between signals, such as radio or television signal spectra. The analyzer delta marker
function lets you compare two signals when both appear on the screen at one time.
In this procedure, the analyzer 10 MHz signal is used to measure frequency and
amplitude differences between two signals on the same screen. Delta marker is used
to demonstrate this comparison.
Figure 2-1
An Example of Comparing Signals on the Same Screen
Step
Action
Notes
1 Connect the 10 MHz OUT
from the rear panel to the
front panel RF input.
2 Select the mode.
•
Press Mode, Spectrum Analyzer.
3 Preset the mode.
•
Press Mode Preset.
4 Configure the analyzer
settings.
a. Press FREQ Channel, Center
Freq, 30, MHz.
b. Press SPAN X Scale, Span, 50,
MHz.
c. Press AMPTD Y Scale, Ref Level,
10, dBm.
22
This sets the analyzer center
frequency, span and reference
level to view the 10 MHz
signal and its harmonics up to
50 MHz.
Measuring Multiple Signals
Comparing Signals on the Same Screen Using Marker Delta
Step
Action
Notes
5 Place a marker at the
highest peak on the display
(10 MHz).
•
Press Peak Search.
The Next Pk Right and Next
Pk Left softkeys are
available to move the marker
from peak to peak. The
marker should be on the
10 MHz reference signal.
6 Anchor the first marker
and activate a second delta
marker.
•
Press Marker, Delta.
The symbol for the first
marker is changed from a
diamond to a cross with a
label that now reads 2,
indicating that it is the fixed
marker (reference point). The
second marker symbol is a
diamond labeled 1Δ2,
indicating it is the delta
marker. When you first press
the Delta key, both markers
are at the same frequency
with the symbols
superimposed over each
other. It is not until you move
the delta marker to a new
frequency that the different
marker symbols are easy to
discern.
7 Move the delta marker to
another signal peak.
•
Press Peak Search, Next Peak.
The amplitude and frequency
difference between the
markers is displayed in the
marker results block of the
screen. Refer to the upper
right portion of the screen.
See Figure 2-2.
23
Measuring Multiple Signals
Comparing Signals on the Same Screen Using Marker Delta
Step
Figure 2-2
NOTE
Action
Notes
Using the Delta Marker Function
The frequency resolution of the marker readings can be increased by turning on the
marker count function.
24
Measuring Multiple Signals
Comparing Signals not on the Same Screen Using Marker Delta
Comparing Signals not on the Same Screen Using Marker Delta
Measure the frequency and amplitude difference between two signals that do not
appear on the screen at one time. (This technique is useful for harmonic distortion
tests when narrow span and narrow bandwidth are necessary to measure the low
level harmonics.)
In this procedure, the analyzer 10 MHz signal is used to measure frequency and
amplitude differences between one signal on screen and one signal off screen. Delta
marker is used to demonstrate this comparison.
Figure 2-3
Comparing One Signal on Screen with One Signal Off Screen
Step
Action
Notes
1 Connect the 10 MHz OUT
from the rear panel to the
front panel RF input.
2 Select the mode.
•
Press Mode, Spectrum
Analyzer.
3 Preset the mode.
•
Press Mode Preset.
4 Configure the analyzer
settings.
a. Press FREQ Channel,
Center Freq, 10, MHz.
b. Press SPAN X Scale, Span,
5, MHz.
c. Press AMPTD Y Scale,
Ref Level, 10, dBm.
25
This sets the analyzer center
frequency, span and reference level
to view the 10 MHz signal and its
harmonics up to 50 MHz.
Measuring Multiple Signals
Comparing Signals not on the Same Screen Using Marker Delta
Step
Action
5 Place a marker at the
highest peak on the
display (10 MHz).
•
Press Peak Search.
6 Set the center frequency
step size equal to the
marker frequency.
•
Press Marker →, Mkr →
CF Step.
7 Activate the marker delta
function.
.
8 Increase the center
frequency by 10 MHz.
•
Press FREQ Channel,
Center Freq, ↑.
The first marker and delta markers
move to the left edge of the screen, at
the amplitude of the first signal peak.
9 Move the delta marker to
the new center frequency.
•
Press Peak Search.
Figure 2-4 shows the reference
annotation for the first marker (52) at
the left side of the display, indicating
that the 10 MHz reference signal is at
a lower frequency than the frequency
range currently displayed. The delta
marker (1Δ2) appears on the peak of
the 20 MHz component. The delta
marker results block displays the
amplitude and frequency difference
between the 10 and 20 MHz signal
peaks.
Figure 2-4
Notes
Delta Marker with Reference Signal Off-Screen
26
Measuring Multiple Signals
Comparing Signals not on the Same Screen Using Marker Delta
Step
Action
10 Turn the markers off.
•
Notes
Press Marker, Off.
27
Measuring Multiple Signals
Resolving Signals of Equal Amplitude
Resolving Signals of Equal Amplitude
In this procedure a decrease in resolution bandwidth is used in combination with a
decrease in video bandwidth to resolve two signals of equal amplitude with a
frequency separation of 100 kHz. Notice that the final RBW selection to resolve the
signals is the same width as the signal separation while the VBW is slightly narrower
than the RBW.
Step
Action
Notes
a. Set the frequency of signal
generator #1 to 300 MHz.
The amplitude of both signals
should be approximately
−20 dBm at the output of the
bridge/directional coupler.
(The −4 dBm setting plus
−16 dB coupling factor of the
86205A results in a −20 dBm
signal.)
1 Connect two sources to the
analyzer RF INPUT as
shown.
2 Set up the signal sources.
b. Set the frequency of signal
generator #2 to 300.1 MHz.
c. Set signal generator #1 amplitude
to −20 dBm.
d. Set signal generator #2 amplitude
to −4 dBm.
3 Select the mode.
•
Press Mode, Spectrum
Analyzer.
4 Preset the analyzer.
•
Press Mode Preset.
5 Set up the analyzer to view
the signals.
a. Press FREQ Channel, Center
Freq, 300, MHz.
b. Press BW, Res BW, 300, kHz.
c. Press SPAN X Scale, Span, 2,
MHz.
28
A single signal peak is
visible. See Figure 2-5.
Measuring Multiple Signals
Resolving Signals of Equal Amplitude
Step
Figure 2-5
Action
Notes
Unresolved Signals of Equal Amplitude
6 Change the RBW.
•
Press BW, Res BW, 100, kHz.
The RBW setting is less than
or equal to the frequency
separation of the two signals
7 Decrease the video BW.
•
Press Video BW, 10, kHz.
Notice that the peak of the
signal has become two peaks
separated by a 2.5 dB dip
indicating that two signals
may be present. See Figure
2-6.
29
Measuring Multiple Signals
Resolving Signals of Equal Amplitude
Step
Figure 2-6
Action
Notes
Unresolved Signals of Equal Amplitude
8 Decrease the RBW.
•
Press BW, Res BW, 10, kHz.
30
Two signals are now visible,
see Figure 2-7. You can use
the front-panel knob or step
keys to further reduce the
resolution bandwidth and
better resolve the signals.
Measuring Multiple Signals
Resolving Signals of Equal Amplitude
Step
Figure 2-7
Action
Notes
Resolving signals of equal amplitude
As the resolution bandwidth is decreased, resolution of the individual signals is
improved and the sweep time is increased. For fastest measurement times, use the
widest possible resolution bandwidth. Under mode preset conditions, the resolution
bandwidth is “coupled” (or linked) to the span.
Since the resolution bandwidth has been changed from the coupled value, a # mark
appears next to Res BW in the lower-left corner of the screen, indicating that the
resolution bandwidth is uncoupled. (For more information on coupling, refer to the
Auto Couple key description in the Agilent Technologies X-Series User’s and
Programmer’s Reference.)
NOTE
An alternative method for resolving two equal amplitude signals is to use the Auto
Tune Function as follows:
Press Mode Preset.
Press Freq Channel, Auto Tune.
The two signals are fully resolved with a marker placed on the highest peak. Refer to
Figure 2-8.
31
Measuring Multiple Signals
Resolving Signals of Equal Amplitude
Figure 2-8
Resolving Signals of Equal Amplitude
32
Measuring Multiple Signals
Resolving Small Signals Hidden by Large Signals
Resolving Small Signals Hidden by Large Signals
This procedure uses narrow resolution bandwidths to resolve two input signals with
a frequency separation of 50 kHz and an amplitude difference of 60 dB.
Step
Action
Notes
1 Connect two sources to the
analyzer RF INPUT as
shown.
2 Set up the signal sources.
a. Set the frequency of signal
generator #1 to 300 MHz.
b. Set the frequency of the signal
generator #2 to 300.05 MHz.
c. Set signal generator #1 amplitude
to −10 dBm.
d. Set signal generator #2 amplitude
to −54 dBm.
3 Select the mode
•
Press Mode, Spectrum Analyzer.
4 Preset the analyzer:
•
Press Mode Preset.
5 Set up the analyzer to view
the signals.
a. Press FREQ Channel, Center
Freq, 300, MHz.
b. Press BW, Res BW, 30, kHz.
c. Press SPAN X Scale, Span, 500,
kHz.
33
This signal is 50 kHz higher
in frequency than the first
signal.
The −54 dBm setting plus
−16 dB coupling factor of
the 86205A results in a
signal 60 dB below the first
signal.
Measuring Multiple Signals
Resolving Small Signals Hidden by Large Signals
Step
Action
6 Set the 300 MHz signal
peak to the reference level.
•
Figure 2-9
Notes
Press Peak Search, Mkr → Ref
Lvl.
The Signal Analyzer 30 kHz
filter shape factor of 4.1:1
has a bandwidth of 123 kHz
at the 60 dB point. The
half-bandwidth, or 61.5 kHz,
is NOT narrower than the
frequency separation of
50 kHz, so the input signals
can not be resolved. See
Figure 2-9.
Signal resolution with a 30 kHz RBW
7 Change the RBW.
•
Press BW, Res BW, 10, kHz.
The reduced resolution
bandwidth filter allows you
to view the smaller hidden
signal.
8 Place a delta marker on the
smaller signal.
•
Press Peak Search, Marker
Delta, 50, kHz.
Notice that the peak of the
signal has become two peaks
separated by a 2.5 dB dip
indicating that two signals
may be present. See.Figure
2-10.
34
Measuring Multiple Signals
Resolving Small Signals Hidden by Large Signals
Step
Figure 2-10
Action
Notes
Unresolved Signals of Equal Amplitude
9 Decrease the RBW.
•
Press BW, 10, kHz.
35
The Signal Analyzer 10 kHz
filter shape factor of 4.1:1
has a bandwidth of 4.1 kHz
at the 60 dB point. The
half-bandwidth, or 20.5 kHz,
is narrower than 50 kHz, so
the input signals can be
resolved. See Figure 2-11.
Measuring Multiple Signals
Resolving Small Signals Hidden by Large Signals
Step
Figure 2-11
NOTE
Action
Notes
Signal resolution with a 10 kHz RBW
To make the separate signals more clear in the display, you may need to use
averaging. To set the averaging to 10 averages:
Press Meas Setup.
Press Average/Hold Number, 10, Enter.
36
Measuring Multiple Signals
Decreasing the Frequency Span Around the Signal
Decreasing the Frequency Span Around the Signal
Using the analyzer signal track function, you can quickly decrease the span while
keeping the signal at center frequency. This is a fast way to take a closer look at the
area around the signal to identify signals that would otherwise not be resolved.
This procedure uses signal tracking with span zoom to view the analyzer 50 MHz
reference signal in a 200 kHz span.
Step
Action
Notes
1 Select the mode.
•
Press Mode, Spectrum Analyzer.
2 Preset the analyzer.
•
Press Mode Preset.
3 Enable the internal
50 MHz amplitude
reference signal.
•
Press Input/Output, RF
Calibrator, 50 MHz.
4 Set the start and stop
frequencies.
a. Press FREQ Channel, Start
Freq, 20, MHz.
b. Press FREQ Channel, Stop Freq,
1, GHz.
5 Turn on the signal
tracking function.
•
Press Span X Scale, Signal Track
(Span Zoom) (On).
This places a marker at the
peak,. moves the signal to the
center of the screen. and
initiates Signal Track.
See Figure 2-12.
Figure 2-12
Signal Tracking on Before Span Decrease
37
Measuring Multiple Signals
Decreasing the Frequency Span Around the Signal
Step
Action
Notes
6 Set the calibration signal
to the reference level.
•
Press Mkr →, Mkr →Ref Lvl.
Because the signal track
function automatically
maintains the signal at the
center of the screen, you can
reduce the span quickly for a
closer look. If the signal drifts
off of the screen as you
decrease the span, use a wider
frequency span.
7 Reduce the span and
resolution bandwidth.
•
Press SPAN X Scale, Span, 200,
kHz.
If the span change is large
enough, the span decreases in
steps as automatic zoom is
completed. You can also use
the front-panel knob or step
keys to decrease the span and
resolution bandwidth values.
See Figure 2-13.
Figure 2-13
Signal Tracking After Zooming in on the Signal
8 Turn Signal tracking off.
•
Press SPAN X Scale, Signal
Track (Off).
38
Measuring Multiple Signals
Easily Measure Varying Levels of Modulated Power Compared to a Reference
Easily Measure Varying Levels of Modulated Power Compared to a
Reference
This section demonstrates a method to measure the complex modulated power of a
reference device or setup and then compare the result of adjustments and changes to
that or other devices.
The Delta Band/Interval Power Marker function will be used to capture the
simulated signal power of a reference device or setup and then compare the resulting
power level due to adjustments or DUT changes.
An important key to making accurate Band Power Marker measurements is to insure
that the Average Type under the Meas Setup key is set to “Auto”.
Step
Action
Notes
1 Connect the source RF
OUTPUT to the analyzer
RF INPUT as shown.
2 Set up the signal sources.
a. Set up a 4-carrier W-CDMA
signal.
b. Set the source frequency to 1.96
GHz.
c. Set the source amplitude to
−10 dBm.
3 Select the mode.
•
Press Mode, Spectrum Analyzer.
4 Preset the analyzer.
•
Press Mode Preset.
5 Tune to W-CDMA signal.
•
Press FREQ Channel, Auto
Tune, 300, MHz.
6 Set the analyzer reference
level.
•
Press AMPTD Y Scale, Ref
Level, 0, dBm.
39
Measuring Multiple Signals
Easily Measure Varying Levels of Modulated Power Compared to a Reference
Step
Action
Notes
7 Enable trace averaging.
•
Press Trace/Detector, Select
Trace, Trace 1, Trace Average.
8 Enable the Band/Interval
Power Marker function.
•
Press Marker Function,
Band/Interval Power.
This measures the total power
of the reference 4-carrier
W-CDMA signal
9 Center the frequency of
the Band/Interval Power
marker.
•
Press Select Marker, Marker 1,
1.96, GHz.
This centers the marker on the
4-carrier reference signal
envelope.
10 Adjust the width (or span)
of the Band/Interval
Power marker.
•
Press Marker Function, Band
Adjust, Band/Interval Span, 20,
MHz.
This encompasses the entire
4-carrier W-CDMA reference
signal. See Figure 2-14.
Note the green vertical lines
of Marker 1 representing the
span of signals included in the
Band/Interval Power
measurement and the carrier
power indicated in Markers
Result Block.
Figure 2-14
Measured Power of Reference 4-carrier W-CDMA Signal Using Band/Interval
Power Marker
40
Measuring Multiple Signals
Easily Measure Varying Levels of Modulated Power Compared to a Reference
Step
Action
Notes
11 Enable the Delta Band
Power Marker
functionality.
•
Press Marker, Select Marker,
Marker 1, Delta.
This will change the reference
Band Power Marker into a
fixed power value (labeled
X2) and initiate a second
Band Power Marker (labeled
1 Δ 2) to measure any changes
in power levels relative to the
reference Band Power Marker
X2.
12 Simulate a varying power
level resulting from either
adjustments, changes to
the reference DUT, or a
different DUT by
lowering the signal source
power.
•
Set the source amplitude to
–20 dBm.
Note the Delta Band Power
Marker value displayed in the
Marker Result Block showing
the 10 dB difference between
the modulated power of the
reference and the changed
power level.
See Figure 2-15
Figure 2-15
Delta Band Power Markers Displaying Lower Modulated Power Level
Compared to a Reference
41
Measuring Multiple Signals
Easily Measure Varying Levels of Modulated Power Compared to a Reference
42
Measuring a Low−Level Signal
3
Measuring a Low−Level
Signal
43
Measuring a Low−Level Signal
Reducing Input Attenuation
Reducing Input Attenuation
The ability to measure a low-level signal is limited by internally generated noise in
the signal analyzer. The measurement setup can be changed in several ways to
improve the analyzer sensitivity.
The input attenuator affects the level of a signal passing through the instrument. If a
signal is very close to the noise floor, reducing input attenuation can bring the signal
out of the noise.
CAUTION
Ensure that the total power of all input signals at the analyzer RF input does not
exceed +30 dBm (1 watt).
Step
Action
Notes
1 Set up the signal generator.
a. Set the frequency to 300 MHz.
b. Set the amplitude to −80 dBm.
2 Connect the source RF
OUTPUT to the analyzer
RF INPUT as shown.
3 Select the mode.
•
Press Mode, Spectrum Analyzer.
4 Preset the mode.
•
Press Mode Preset.
5 Set the center frequency,
span and reference level.
a. Press FREQ Channel, Center
Freq, 300, MHz.
b. Press SPAN X Scale, Span, 5,
MHz.
c. Press AMPTD Y Scale, Ref Level,
40, −dBm.
6 Move the peak to the center
of the display.
•
Press Peak Search, Marker −>,
Mkr −> CF.
44
Measuring a Low−Level Signal
Reducing Input Attenuation
Step
Action
Notes
7 Reduce the span.
•
Press Span, 1, MHz.
If necessary re-center the
peak.
8 Set the attenuation.
•
Press AMPTD Y Scale,
Attenuation, Mech Atten or Atten
(Man), 20, dB.
Increasing the attenuation
moves the noise floor closer
to the signal level.
A “#” mark appears next to
the Atten annotation at the
top of the display, indicating
that the attenuation is no
longer coupled to other
analyzer settings.
See Figure 3-1.
Figure 3-1
NOTE
Measuring a Low-Level Signal Using Mechanical Attenuation
The CXA does not have a mechanical attenuator. Therefore, the Attenuation menu will be
different than the one shown.
9 Change the attenuation to
see the signal more clearly.
•
Press AMPTD Y Scale,
Attenuation, Mech Atten or Atten
(Man), 0, dB.
45
Measuring a Low−Level Signal
Reducing Input Attenuation
Step
Figure 3-2
CAUTION
Action
Notes
Measuring a Low-Level Signal Using 0 dB Attenuation
When you finish this example, increase the attenuation to protect the analyzer RF
input:
Press AMPTD Y Scale, Attenuation, Mech Atten or Atten (Auto), or press
Auto Couple.
46
Measuring a Low−Level Signal
Decreasing the Resolution Bandwidth
Decreasing the Resolution Bandwidth
Resolution bandwidth settings affect the level of internal noise without affecting the
level of continuous wave (CW) signals. Decreasing the RBW by a decade reduces
the noise floor by 10 dB.
Step
Action
Notes
1 Set up the signal generator.
a. Set the frequency to 300
MHz.
b. Set the amplitude to
−80 dBm.
2 Connect the source RF
OUTPUT to the analyzer
RF INPUT as shown.
3 Select the mode.
•
Press Mode, Spectrum
Analyzer.
4 Preset the mode.
•
Press Mode Preset.
5 Set the center frequency,
span and reference level.
a. Press FREQ Channel,
Center Freq, 300, MHz.
b. Press SPAN X Scale,
Span, 50, MHz.
c. Press AMPTD Y Scale,
Ref Level, 40, −dBm.
47
See Figure 3-3.
Measuring a Low−Level Signal
Decreasing the Resolution Bandwidth
Step
Figure 3-3
Action
Default resolution bandwidth
6 Decrease the RBW.
Figure 3-4
Notes
•
Press BW, 47, kHz.
The low-level signal appears more
clearly because the noise level is
reduced. See Figure 3-4.
Decreasing Resolution Bandwidth
A “#” mark appears next to the Res BW annotation in the lower left corner of the screen, indicating that
the resolution bandwidth is uncoupled.
48
Measuring a Low−Level Signal
Decreasing the Resolution Bandwidth
RBW Selections
You can use the step keys to change the RBW in a 1−3−10 sequence.
All the Signal Analyzer RBWs are digital and have a selectivity ratio of 4.1:1.
Choosing the next lower RBW (in a 1−3−10 sequence) for better sensitivity
increases the sweep time by about 10:1 for swept measurements, and about 3:1 for
FFT measurements (within the limits of RBW). Using the knob or keypad, you can
select RBWs from 1 Hz to 3 MHz in approximately 10% increments, plus 4, 5, 6 and
8 MHz. This enables you to make the trade off between sweep time and sensitivity
with finer resolution.
49
Measuring a Low−Level Signal
Using the Average Detector and Increased Sweep Time
Using the Average Detector and Increased Sweep Time
When the analyzer noise masks low-level signals, changing to the average detector
and increasing the sweep time smooths the noise and improves the signal visibility.
Slower sweeps are required to average more noise variations.
Step
Action
Notes
1 Set up the signal
generator.
a. Set the frequency to 300
MHz.
b. Set the amplitude to
−80 dBm.
2 Connect the source RF
OUTPUT to the analyzer
RF INPUT as shown.
3 Select the mode.
•
Press Mode, Spectrum
Analyzer.
4 Preset the mode.
•
Press Mode Preset.
5 Set the center frequency,
span and reference level.
a. Press FREQ Channel,
Center Freq, 300, MHz.
b. Press SPAN X Scale,
Span, 5, MHz.
c. Press AMPTD Y Scale,
Ref Level, 40, −dBm.
50
Measuring a Low−Level Signal
Using the Average Detector and Increased Sweep Time
Step
Action
Notes
6 Select the average
detector.
•
Press Trace/Detector,
More 1 of 2, Detector,
Average (Log/RMS/V).
The number 1 (Trace 1 indicator) in
the Trace/Detector panel (in the upper
right-hand corner of the display)
changes from green to white,
indicating that the detector has been
chosen manually. In addition, the letter
in the Det row has ben set to “A”
indicating that the Average detector
has been selected. See Figure 3-5.
7 Increase the sweep time.
•
Press Sweep/Control,
Sweep Time (Man), 100,
ms.
The noise smooths out, as there is
more time to average the values for
each of the displayed data points.
8 Change the average type
to log averaging.
•
Press Meas Setup,
Average Type, Log-Pwr
Avg (Video).
Note how the noise level drops.
Figure 3-5
Varying the sweep time with the average detector
51
Measuring a Low−Level Signal
Trace Averaging
Trace Averaging
Averaging is a digital process in which each trace point is averaged with the previous
average for the same trace point. Selecting averaging, when the analyzer is
autocoupled, changes the detection mode from normal to sample. Sample mode may
not measure a signal amplitude as accurately as normal mode, because it may not
find the true peak.
NOTE
This is a trace processing function and is not the same as using the average detector
(as described on page 50).
Step
Action
Notes
1 Set up the signal generator.
a. Set the frequency to 300 MHz.
b. Set the amplitude to −80 dBm.
2 Connect the source RF
OUTPUT to the analyzer
RF INPUT as shown.
3 Select the mode.
•
Press Mode, Spectrum Analyzer.
4 Preset the mode.
•
Press Mode Preset.
5 Set the center frequency,
span and reference level.
a. Press FREQ Channel, Center
Freq, 300, MHz.
b. Press SPAN X Scale, Span, 5,
MHz.
c. Press AMPTD Y Scale, Ref Level,
40, −dBm.
52
Measuring a Low−Level Signal
Trace Averaging
Step
Action
Notes
6 Turn on Averaging.
•
Press Trace/Detector, Trace
Average.
As the averaging routine
smooths the trace, low level
signals become more
visible. Avg/Hold >100
appears in the measurement
bar near the top of the
screen. SeeFigure 3-6.
7 Set number of averages.
•
Press Meas Setup, Average/Hold
Number, 25, Enter.
Annotation above the
graticule in the
measurement bar to the right
of center shows the type of
averaging, Log-Power. Also,
the number of traces
averaged is shown on the
Average/Hold Number key.
Figure 3-6
Trace Averaging
Changing most active functions restarts the averaging, as does pressing the Restart
key. Once the set number of sweeps completes, the analyzer continues to provide a
running average based on this set number.
NOTE
If you want the measurement to stop after the set number of sweeps, use single
sweep: Press Single and then press the Restart key.
53
Measuring a Low−Level Signal
Trace Averaging
54
Improving Frequency Resolution and Accuracy
4
Improving Frequency
Resolution and Accuracy
55
Improving Frequency Resolution and Accuracy
Using a Frequency Counter to Improve Frequency Resolution and Accuracy
Using a Frequency Counter to Improve Frequency Resolution and
Accuracy
This procedure uses the signal analyzer internal frequency counter to increase the
resolution and accuracy of the frequency readout.
Step
Action
Notes
1 Select the mode.
•
Press Mode, Spectrum Analyzer.
2 Preset the mode.
•
Press Mode Preset.
3 Enable the internal
reference signal.
•
Press Input/Output, RF
Calibrator, 50 MHz.
4 Set the center frequency
and span.
a. Press FREQ Channel, Center
Freq, 50, MHz
b. Press SPAN X Scale, Span, 80,
MHz.
5 Turn the frequency
counter on.
Figure 4-1
•
Press Marker, More, Marker
Count, Counter (On).
Using Marker Counter
6 Turn off the marker
counter.
•
Press Marker, More, Marker
Count, Count (Off).
Or
Press Marker, Off.
56
The marker counter remains
on until turned off.
Tracking Drifting Signals
5
Tracking Drifting Signals
57
Tracking Drifting Signals
Measuring a Source Frequency Drift
Measuring a Source Frequency Drift
The analyzer can measure the short- and long-term stability of a source. The
maximum amplitude level and the frequency drift of an input signal trace can be
displayed and held by using the maximum-hold function. You can also use the
maximum hold function if you want to determine how much of the frequency
spectrum a signal occupies.
This procedure using signal tracking to keep the drifting signal in the center of the
display. The drifting is captured by the analyzer using maximum hold.
Step
Action
Notes
1 Set up the signal sources.
a. Set the frequency of the
signal source to 300 MHz.
b. Set the source amplitude to
−20 dBm
2 Instrument setup.
•
Connect the source RF
OUTPUT to the analyzer
RF INPUT as shown.
3 Set the analyzer to the
Spectrum Analyzer mode.
•
Press Mode, Spectrum
Analyzer.
4 Preset the analyzer
•
Press Mode Preset.
5 Set the analyzer center
frequency, span and
reference level.
a. Press FREQ Channel,
Center Freq, 300, MHz.
b. .Press SPAN X Scale,
Span, 10, MHz.
c. Press AMPTD Y Scale,
Ref Level, 10, −dBm.
6 Place a marker on the peak
of the signal.
•
Press Peak Search.
58
This enables the spectrum analyzer
measurements.
Tracking Drifting Signals
Measuring a Source Frequency Drift
Step
Action
Notes
7 Turn on the signal tracking
function.
•
Press SPAN X Scale,
Signal Track (On).
8 Reduce the span to
500 kHz.
•
Press SPAN, 500, kHz.
9 Turn off the signal track
function.
•
Press SPAN X Scale,
Signal Track (Off).
10 Measure the excursion of
the signal.
•
Press Trace/Detector, Max
Hold.
Notice that the signal is held in the
center of the display.
As the signal varies, maximum hold
maintains the maximum responses of
the input signal.
Annotation in the Trace/Detector
panel, upper right corner of the
screen, indicates the trace mode. In
this example, the M in the Type row
under TRACE 1 indicates trace 1 is
in maximum-hold mode.
11 Activate trace 2.
•
Press Trace/Detector,
Select Trace, Trace (2).
12 Change the mode to
continuous sweeping.
•
Press Clear Write.
13 Slowly change the
frequency of the signal
generator ± 50 kHz in
1 kHz increments.
Trace 1 remains in maximum hold
mode to show any drift in the signal.
Your analyzer display should look
similar to Figure 5-1.
59
Tracking Drifting Signals
Measuring a Source Frequency Drift
Step
Figure 5-1
Action
Notes
Viewing a Drifting Signal With Max Hold and Clear Write
60
Tracking Drifting Signals
Tracking a Signal
Tracking a Signal
The signal track function is useful for tracking drifting signals that drift relatively
slowly by keeping the signal centered on the display as the signal drifts. This
procedure tracks a drifting signal.
Note that the primary function of the signal track function is to track unstable
signals, not to track a signal as the center frequency of the analyzer is changed. If
you choose to use the signal track function when changing center frequency, check to
ensure that the signal found by the tracking function is the correct signal.
Step
Action
Notes
1 Set up the signal sources.
a. Set the frequency of the signal
source to 300 MHz.
b. Set the source amplitude to
−20 dBm.
2 Instrument setup.
•
Connect the source RF OUTPUT to
the analyzer RF INPUT as shown.
3 Set the analyzer to the
Spectrum Analyzer mode.
•
Press Mode, Spectrum Analyzer.
4 Preset the analyzer.
•
Press Mode Preset.
5 Set the analyzer center
frequency, span and
reference level.
•
Press FREQ Channel, Center
Freq, 301, MHz.
•
.Press SPAN X Scale, Span, 10,
MHz.
•
Press Peak Search.
6 Place a marker on the peak
of the signal.
61
This enables the spectrum
analyzer measurements.
Tracking Drifting Signals
Tracking a Signal
Step
Action
Notes
7 Turn on the signal tracking
function.
•
Press SPAN X Scale, Signal Track
(On).
Notice that signal tracking
places a marker on the
highest amplitude peak and
then brings the selected
peak to the center of the
display. After each sweep
the center frequency of the
analyzer is adjusted to keep
the selected peak in the
center.
8 Turn on the delta marker.
•
Press Marker, Delta.
9 Tune the frequency of the
signal generator in 100
KHz increments.
Figure 5-2
Notice that the center
frequency of the analyzer
also changes in 100 kHz
increments, centering the
signal with each increment.
Tracking a Drifting Signal
62
Making Distortion Measurements
6
Making Distortion
Measurements
63
Making Distortion Measurements
Identifying Analyzer Generated Distortion
Identifying Analyzer Generated Distortion
High level input signals may cause internal analyzer distortion products that could
mask the real distortion measured on the input signal. Using trace 2 and the RF
attenuator, you can determine which signals, if any, are internally generated
distortion products.
Using a signal from a signal generator, determine whether the harmonic distortion
products are generated by the analyzer.
Step
Action
Notes
1 Set up the signal
generator.
a. Set the frequency to 200 MHz.
b. Set the amplitude to 0 dBm.
2 Connect the source RF
OUTPUT to the
analyzer RF INPUT as
shown.
3 Select the mode.
•
Press Mode, Spectrum Analyzer.
4 Set the analyzer center
frequency, span, and
video bandwidth.
a. Press FREQ Channel, Center
Freq, 400, MHz.
b. Press SPAN X Scale, Span, 500,
MHz.
c. Press BW, Video BW, 30, kHz.
64
The signal produces harmonic
distortion products (spaced 200
MHz from the original 200
MHz signal) in the analyzer
input mixer as shown in the
following graphic. See Figure
6-1
Making Distortion Measurements
Identifying Analyzer Generated Distortion
Step
Figure 6-1
Action
Notes
Harmonic Distortion
5 Change the center
frequency to the value
of the second harmonic.
•
Press Peak Search, Next Peak,
Mkr→CF.
6 Change the span to
50 MHz and re-center
the signal.
a. Press SPAN X Scale, Span, 50,
MHz.
b. Press Peak Search, Mkr→CF.
7 Set the attenuation to
0 dB.
•
Press AMPTD Y Scale,
Attenuation, 0, dB.
8 Save the trace data in
trace 2.
•
Press Trace/Detector, Select
Trace, Trace 2, Clear Write.
9 Allow trace 2 to update.
•
Press Trace/Detector,
View/Blank, View, Trace On.
Minimum of two sweeps.
10 Place a delta marker on
the harmonic of trace 2.
•
Press Peak Search, Marker Delta.
The analyzer display shows the
stored data in trace 2 and the
measured data in trace 1. The
ΔMkr1 amplitude reading is the
difference in amplitude
between the reference and
active markers.
65
Making Distortion Measurements
Identifying Analyzer Generated Distortion
Step
Action
Notes
11 Increase the RF
attenuation to 10 dB.
•
Notice the ΔMkr1 amplitude
reading. This is the difference
in the distortion product
amplitude readings between
0 dB and 10 dB input
attenuation settings. If the
ΔMkr1 amplitude absolute
value is approximately ≥1 dB
for an input attenuator change
of 10 dB, the distortion is being
generated, at least in part, by
the analyzer. In this case more
input attenuation is necessary.
Increase the input attenuation
until ΔMkr1 amplitude stops
increasing or decreasing in
value. Return to the previous
attenuator step and the input
signal distortion measured will
be minimally impacted by the
analyzer internally generated
distortion. See Figure 6-2.
Figure 6-2
Press AMPTD Y Scale,
Attenuation, 10, dB.
RF Attenuation of 10 dB
66
Making Distortion Measurements
Third-Order Intermodulation Distortion
Third-Order Intermodulation Distortion
Two-tone, third-order intermodulation distortion is a common test in communication
systems. When two signals are present in a non-linear system, they can interact and
create third-order intermodulation distortion products that are located close to the
original signals. These distortion products are generated by system components such
as amplifiers and mixers.
This procedure tests a device for third-order intermodulation using markers. Two
sources are used, one set to 300 MHz and the other to 301 MHz. This combination of
signal generators and a directional coupler (used as a combiner) results in a two-tone
source with very low intermodulation distortion. Although the distortion from this
setup may be better than the specified performance of the analyzer, it is useful for
determining the TOI performance of the source/analyzer combination. After the
performance of the source/analyzer combination has been verified, the
device-under-test (DUT) (for example, an amplifier) would be inserted between the
directional coupler output and the analyzer input.
Step
Action
Notes
1 Connect two sources to the
analyzer RF INPUT as
shown.
The coupler should have a
high degree of isolation
between the two input ports so
the sources do not
intermodulate.
67
Making Distortion Measurements
Third-Order Intermodulation Distortion
Step
Action
Notes
2 Set up the signal sources.
a. Set the frequency of signal
generator #1 to 300 MHz.
b. Set the frequency of signal
generator #2 to 300.1 MHz.
This produces a frequency
separation of 1 MHz.
c. Set signal generator #1 amplitude
to −5 dBm.
Sets the sources equal in
amplitude as measured by the
analyzer.
d. Set signal generator #2 amplitude
to −5 dBm.
3 Select the mode.
•
Press Mode, Spectrum
Analyzer.
4 Preset the analyzer.
•
Press Mode Preset.
5 Set the analyzer center
frequency and span.
a. Press FREQ Channel, Center
Freq, 300.5, MHz.
b. Press SPAN X Scale, Span, 5,
MHz
6 Set the analyzer detector to
Peak.
•
Press Trace/Detector, Detector,
Peak.
7 Set the mixer level to
improve dynamic range.
•
Press AMPTD Y Scale,
Attenuation, Max Mixer Lvl,
–10, dBm.
8 Move the signal to the
reference level.
•
Press Peak Search, Mkr →, Mkr
→ Ref Lvl.
9 Reduce the RBW until the
distortion products are
visible.
•
Press BW, Res BW, ↓.
10 Activate the second
marker and place it on the
peak of the distortion
product closest to the
marker test signal.
•
Press Peak Search, Marker
Delta, Next Left or Next Right
(as appropriate).
11 Measure the other
distortion product
•
Press Marker, Normal,
Peak Search, Next Peak.
68
The analyzer automatically
sets the attenuation so that a
signal at the reference level
has a maximum value of
−10 dBm at the input mixer.
Use the Next Right key (if the
first marker is on the right test
signal) or Next Left key (if
the first marker is on the left
test signal):
Making Distortion Measurements
Third-Order Intermodulation Distortion
Step
Action
Notes
12 Activate the second
marker and place it on the
peak of the distortion
product closest to the
marked test signal.
•
See Figure 6-3.
Figure 6-3
Press Marker, Normal, Marker
Delta, Next Left or Next Right
(as appropriate).
Measuring the Distortion Product
69
Making Distortion Measurements
Third-Order Intermodulation Distortion
70
Measuring Noise
7
Measuring Noise
71
Measuring Noise
Measuring Signal-to-Noise
Measuring Signal-to-Noise
Signal-to-noise is a ratio used in many communication systems as an indication of
noise in a system. Typically the more signals added to a system adds to the noise
level, reducing the signal-to-noise ratio making it more difficult for modulated
signals to be demodulated. This measurement is also referred to as carrier-to-noise in
some communication systems.
The signal-to-noise measurement procedure below may be adapted to measure any
signal in a system if the signal (carrier) is a discrete tone. If the signal in your system
is modulated, it is necessary to modify the procedure to correctly measure the
modulated signal level.
In this example the 50 MHz amplitude reference signal is used as the fundamental
signal. The amplitude reference signal is assumed to be the signal of interest and the
internal noise of the analyzer is measured as the system noise. To do this, you need to
set the input attenuator such that both the signal and the noise are well within the
calibrated region of the display.
Step
Action
Notes
1 Set the analyzer to the
Spectrum Analyzer
mode.
•
Press Mode, Spectrum Analyzer.
This enables the spectrum
analyzer measurements.
2 Preset the analyzer.
•
Press Mode Preset.
3 Enable the internal
reference signal.
•
Press Input/Output, RF
Calibrator, 50, MHz.
4 Set the center frequency,
span, reference level and
attenuation.
a. Press FREQ Channel, Center
Freq, 50, MHz.
b. Press SPAN X Scale, Span, 1,
MHz.
c. Press AMPTD Y Scale, Ref Level,
−10, dBm.
d. Press AMPTD Y Scale,
Attenuation, 40, dB.
5 Place a marker on the
peak of the signal and
place a delta marker in
the noise.
•
Press Peak Search, Marker Delta,
200, kHz.
6 Turn on the marker noise
function.
•
Press Marker Function, Marker
Noise.
72
This enables you to view the
signal-to-noise measurement
results.
Measuring Noise
Measuring Signal-to-Noise
Step
Figure 7-1
Action
Notes
Measuring signal-to-noise
Read the signal-to-noise in dB/Hz, that is with the noise value determined for a 1 Hz
noise bandwidth. If you wish the noise value for a different bandwidth, decrease the
ratio by 10 × log ( BW ) . For example, if the analyzer reading is −70 dB/Hz but you
have a channel bandwidth of 30 kHz:
S/N = – 70 dB/Hz + 10 × log ( 30 kHz ) = – 25.23 dB ⁄ ( 30 kHz )
NOTE
When Noise Marker is activated, the display detection mode is set to Average.
NOTE
When using the Noise Marker, if the delta marker is closer than one quarter of a
division from the edge of a discrete signal response, the amplitude reference signal in
this case, there is a potential for error in the noise measurement. See “Measuring
Noise Using the Noise Marker” on page 74.
73
Measuring Noise
Measuring Noise Using the Noise Marker
Measuring Noise Using the Noise Marker
This procedure uses the marker function, Marker Noise, to measure noise in a 1 Hz
bandwidth. In this example the noise marker measurement is made near the 50 MHz
reference signal to illustrate the use of Marker Noise.
Step
Action
Notes
1 Set the analyzer to the
Spectrum Analyzer
mode.
•
Press Mode, Spectrum
Analyzer.
This enables the spectrum analyzer
measurements
2 Preset the analyzer.
•
Press Mode Preset.
3 Enable the internal
reference signal.
•
Press Input/Output, RF
Calibrator, 50, MHz.
4 Set the center frequency,
span, reference level and
attenuation.
a. Press FREQ Channel,
Center Freq, 49.98, MHz.
b. Press SPAN X Scale,
Span, 100, kHz.
c. Press AMPTD Y Scale,
Ref Level, −10, dBm.
d. Press AMPTD Y Scale,
Attenuation, Mech Atten
Man, 40, dB.
5 Turn on the noise marker.
•
Press Marker Function,
Marker Noise.
Note that display detection
automatically changes to “Avg”;
average detection calculates the noise
marker from an average value of the
displayed noise. Notice that the noise
marker floats between the maximum
and the minimum displayed noise
points. The marker readout is in dBm
(1 Hz) or dBm per unit bandwidth.
For noise power in a different
bandwidth, add 10 × log ( BW ) . For
example, for noise power in a 1 kHz
bandwidth, dBm (1 kHz), add
10 × log ( 1000 ) or 30 dB to the noise
marker value.
6 Reduce the variations of
the sweep-to-sweep
marker value by
increasing the sweep
time.
•
Press Sweep/Control,
Sweep Time, 3, s.
74
Increasing the sweep time when the
average detector is enabled allows the
trace to average over a longer time
interval, thus reducing the variations in
the results (increases measurement
repeatability).
Measuring Noise
Measuring Noise Using the Noise Marker
Step
Action
Notes
7 Move the marker.
•
Press Marker, 50, MHz.
The noise marker value is based on the
mean of 5% of the total number of
sweep points centered at the marker in
the initially selected span. The points
that are averaged span one-half of a
division. Changing spans after
enabling the noise marker will result in
the marker averaging a progressively
wider or narrower portion of the newly
selected span and corresponding sweep
points. This occurs because the marker
is locked to 5% of the initially selected
span.
8 Adjust the width of the
noise marker relative to
the span.
•
Press Marker Function,
Band Adjust,
Band/Interval Span, and
adjust the value to the
desired marker width.
Notice that the marker does not go to
the peak of the signal unless the
Band/Interval Span is set to 0 Hz
because otherwise there are not enough
points at the peak of the signal. With a
Band Interval Span greater than 0 Hz,
the noise marker is also averaging
points below the peak due to the
narrow RBW.
9 Widen the resolution
bandwidth.
•
Press BW, Res BW, 10,
kHz.
This allows the marker to make a more
accurate peak power measurement
using the noise marker as shown in
Figure 7-2.
75
Measuring Noise
Measuring Noise Using the Noise Marker
Step
Figure 7-2
Action
Notes
Noise marker
10 Set the analyzer to zero
span at the marker
frequency.
a. Press Mkr→, Mkr→CF.
b. Press SPAN X Scale, Zero
Span.
c. Press Marker.
Note that the marker amplitude value
is now correct since all points averaged
are at the same frequency and not
influenced by the shape of the
bandwidth filters. See Figure 7-3.
Remember that the noise marker
calculates a value based on an average
of the points around the frequency of
interest. Generally when making
power measurements using the noise
marker on discrete signals, first tune to
the frequency of interest and then
make your measurement in zero span
(time domain).
76
Measuring Noise
Measuring Noise Using the Noise Marker
Step
Figure 7-3
Action
Notes
Noise Marker with Zero Span
77
Measuring Noise
Measuring Noise-Like Signals Using Band/Interval Density Markers
Measuring Noise-Like Signals Using Band/Interval Density Markers
Band/Interval Density Markers let you measure power over a frequency span. The
markers allow you to easily and conveniently select any arbitrary portion of the
displayed signal. However, while the analyzer, when autocoupled, makes sure the
analysis is power-responding (rms voltage-responding), you must set all of the other
parameters.
Step
Action
Notes
1 Set the analyzer to the
Spectrum Analyzer mode.
•
Press Mode, Spectrum
Analyzer.
This enables the spectrum
analyzer measurements.
2 Preset the analyzer
•
Press Mode Preset.
3 Set the center frequency,
span, reference level and
attenuation.
a. Press FREQ Channel, Center
Freq, 50, MHz.
b. Press SPAN X Scale, Span, 100,
kHz.
c. Press AMPTD Y Scale, Ref
Level, −20, dBm.
d. Press AMPTD Y Scale,
Attenuation, 40, dB.
4 Measure the total noise
power between the
markers.
•
Press Marker Function,
Band/Interval Density.
5 Set the band span.
•
Press Band Adjust,
Band/Interval Span, 40, kHz.
6 Set the resolution and
video bandwidths.
a. Press BW, Res BW, 1, kHz.
7 Enable the internal
50 MHz amplitude
reference signal of the
analyzer.
•
b. Press BW, Video BW, 10, kHz.
Press Input/Output, RF
Calibrator, 50 MHz.
78
Common practice is to set the
resolution bandwidth from 1%
to 3% of the measurement
(marker) span, 40 kHz in this
example.
Adds a discrete tone to see the
effects on the reading. See
Figure 7-4.
Measuring Noise
Measuring Noise-Like Signals Using Band/Interval Density Markers
Step
Figure 7-4
Action
Notes
Band/Interval Density Measurement
8 Set the Band/Interval
Density Markers.
•
Press Marker Function,
Band/Interval Density.
79
This allows you to move the
markers (set at 40 kHz span)
around without changing the
Band/Interval span. Use the
front-panel knob to move the
band power markers and note
the change in the power
reading
Measuring Noise
Measuring Noise-Like Signals Using Band/Interval Density Markers
Step
Figure 7-5
NOTE
Action
Notes
Band/Interval Density Measurement
Band/Interval Density Markers can be changed to read the total absolute power by pressing
Marker Function, Band/Interval Power.
80
Measuring Noise
Measuring Noise-Like Signals Using the Channel Power Measurement
Measuring Noise-Like Signals Using the Channel Power Measurement
You may want to measure the total power of a noise-like signal that occupies some
bandwidth. Typically, channel power measurements are used to measure the total
(channel) power in a selected bandwidth for a modulated (noise-like) signal.
Alternatively, to manually calculate the channel power for a modulated signal, use
the noise marker value and add 10 × log ( channel BW ) . However, if you are not certain
of the characteristics of the signal, or if there are discrete spectral components in the
band of interest, you can use the channel power measurement. This example uses the
noise of the analyzer, adds a discrete tone, and assumes a channel bandwidth
(integration bandwidth) of 2 MHz. If desired, a specific signal may be substituted.
Step
Action
Notes
1 Set the analyzer to the
Spectrum Analyzer mode
•
Press Mode, Spectrum Analyzer.
This enables the spectrum
analyzer measurements.
2 Preset the analyzer.
•
Press Mode Preset.
3 Set the center frequency,
•
Press FREQ Channel, Center
Freq, 50, MHz.
4 Start the channel power
measurement.
•
Press Meas, Channel Power,
5 Enable the bar graph.
•
Press View/Display, Bar Graph,
On.
6 Enable the internal
50 MHz amplitude
reference signal.
•
Press Input/Output, RF
Calibrator, 50 MHz.
This adds a discrete tone to
see the effects on the reading.
7 Optimize the analyzer
attenuation level setting.
•
Press AMPTD, Attenuation,
Adjust Atten for Min Clip.
Your display should be
similar to Figure 7-6.
81
Measuring Noise
Measuring Noise-Like Signals Using the Channel Power Measurement
Step
Figure 7-6
Action
Notes
Measuring Channel Power
The power reading is essentially that of the tone; that is, the total noise power is far
enough below that of the tone that the noise power contributes very little to the total.
The algorithm that computes the total power works equally well for signals of any
statistical variant, whether tone-like, noise-like, or combination.
82
Measuring Noise
Measuring Signal-to-Noise of a Modulated Carrier
Measuring Signal-to-Noise of a Modulated Carrier
Signal-to-noise (or carrier-to-noise) is a ratio used in many communication systems
as indication of the noise performance in the system. Typically, the more signals
added to the system or an increase in the complexity of the modulation scheme can
add to the noise level. This can reduce the signal-to-noise ratio and impact the
quality of the demodulated signal. For example, a reduced signal-to-noise in digital
systems may cause an increase in EVM (error vector magnitude).
With modern complex digital modulation schemes, measuring the modulated carrier
requires capturing all of its power accurately. This procedure uses the Band Power
Marker with a RMS average detector to correctly measure the carrier's power within
a user adjustable region. A Noise Marker (normalized to a 1 Hz noise power
bandwidth) with an adjustable noise region is also employed to allow the user to
select and accurately measure just the system noise of interest. An important key to
making accurate Band Power Marker and Noise Power measurements is to insure
that the Average Type under the Meas Setup key is set to “Auto”.
In this example a 4 carrier W-CDMA digitally modulated carrier is used as the
fundamental signal and the internal noise of the analyzer is measured as the system
noise.
Step
Action
Notes
1 Set up the signal sources.
a. Setup a 4 carrier W-CDMA signal.
b. Set the source frequency to 1.96
GHz.
c. Set the source amplitude to –10
dBm.
2 Instrument setup.
•
Connect the source RF OUTPUT to
the analyzer RF INPUT as shown.
3 Set the analyzer to the
Spectrum Analyzer mode.
•
Press Mode, Spectrum Analyzer
4 Preset the analyzer.
•
Press Mode Preset.
83
This enables the spectrum
analyzer measurements
Measuring Noise
Measuring Signal-to-Noise of a Modulated Carrier
Step
Action
5 Tune to the W-CDMA
signal.
•
Press FREQ Channel, Auto Tune.
6 Enable the Band Power
Marker function.
•
Press Marker Function,
Band/Interval Power.
7 Center the frequency of the
Band Power marker on the
signal.
•
Press Select Marker 1, 1.96, GHz
8 Adjust the width (or span)
of the Band Power marker.
•
Press Marker Function, Band
Adjust, Band/Interval Span, 20,
MHz.
Figure 7-7
Notes
This measures the total
power of the 4 carrier
W-CDMA signal.
This encompasses the entire
4 carrier W-CDMA signal.
4 Carrier W-CDMA Signal Power using Band Power Marker
Note the green vertical lines of Marker 1 representing the span of signals included in the Band Power
measurement and the carrier power indicated in Markers Result Block.
9 Enable the Noise Marker
using marker 2.
•
Press Marker Function, Select
Marker, Marker 2, Marker Noise.
This measures the system
noise power.
10 Move the Noise Marker 2
to the system noise
frequency of interest.
•
Press Select Marker 2, 1.979, GHz.
This encompasses the
desired noise power.
11 Adjust the width of the
noise marker region.
•
Press Marker Function, Select
Marker, Marker 2, Band Adjust,
Band/Interval, 5, MHz.
See Figure 7-8.
84
Measuring Noise
Measuring Signal-to-Noise of a Modulated Carrier
Step
Figure 7-8
Action
Notes
Noise Marker Measuring System Noise
Note the green “wings” of Marker 2 outlining the noise region to be included in the measurement and
the resulting noise power expressed in dBm/Hz as shown in the Marker Results Block.
12 Measure carrier-to-noise by
making the Noise Marker
relative to the carrier's
Band Power Marker.
•
Press Marker, Properties, Select
Marker, Marker 2, Relative to,
Marker 1.
85
See Figure 7-9.
Measuring Noise
Measuring Signal-to-Noise of a Modulated Carrier
Step
Figure 7-9
Action
Notes
Signal-to-noise measurement
13 Simultaneously measure
carrier-to-noise on a
second region of the
system by enabling another
Noise Marker.
a. Press Marker Function, Select
Marker, Marker 3, Marker Noise.
Up to 11 are available.
b. Press Select Marker 3, 1.941, GHz.
c. Press Return, Band Adjust,
Band/Interval, 5, MHz.
d. Press Marker, Properties, Select
Marker, Marker 3, Relative to,
Marker 1.
14 Enable the Marker Table.
•
Press Marker, More 1 of 2,
Marker Table, On.
86
This enables you to view
results of both
carrier-to-noise
measurements and all other
markers. See Figure 7-10.
Measuring Noise
Measuring Signal-to-Noise of a Modulated Carrier
Step
Figure 7-10
Action
Notes
Multiple Signal-to Noise Measurements with Marker Table
87
Measuring Noise
Improving Phase Noise Measurements by Subtracting Signal Analyzer Noise
Improving Phase Noise Measurements by Subtracting Signal Analyzer
Noise
Making noise power measurements (such as phase noise) near the noise floor of the
signal analyzer can be challenging where every dB improvement is important.
Utilizing the analyzer trace math function Power Diff and 3 separate traces allows
measurement of the DUT phase noise in one trace, the analyzer noise floor in a
second trace and then the resulting subtraction of those two traces displayed in a
third trace with the analyzer noise contribution removed.
Step
Action
Notes
1 Set up the signal sources.
a. Setup an unmodulated signal.
b. Set the source frequency to 1.96
GHz.
c. Set the source amplitude to –30
dBm.
2 Instrument setup.
•
Connect the source RF OUTPUT to
the analyzer RF INPUT as shown.
3 Set the analyzer to the
Spectrum Analyzer mode.
•
Press Mode, Spectrum Analyzer.
4 Preset the analyzer.
•
Press Mode Preset.
5 Tune to the unmodulated
carrier, adjust the span and
RBW.
a. Press FREQ Channel, Auto Tune.
This enables the spectrum
analyzer measurements.
b. Press Span, Span, 200, kHz.
c. Press BW, Res BW, 910, Hz.
6 Measure and store the DUT
phase noise plus the
analyzer noise.
a. Press Trace/Detector, Select Trace,
Trace 1, Trace Average
After sufficient averaging:
b. Press View/Blank, View
88
Allow time for sufficient
averaging before initiating
action b.
See Figure 7-11.
Measuring Noise
Improving Phase Noise Measurements by Subtracting Signal Analyzer Noise
Step
Figure 7-11
Action
Notes
Measurement of DUT and Analyzer Noise
7 Measure only the analyzer
noise using trace 2 (blue
trace) with trace averaging.
a. Turn off or remove the DUT signal
to the RF input of the analyzer.
b. Press Trace/Detector, Select Trace,
Trace 2, Clear Write, Trace
Average.
c. Press View/Blank, View.
89
Allow time for sufficient
averaging.
See Figure 7-12.
Measuring Noise
Improving Phase Noise Measurements by Subtracting Signal Analyzer Noise
Step
Figure 7-12
Action
Notes
Measurement of Analyzer Noise
8 Subtract the noise from the
DUT phase noise
measurement using the
Power Diff math function.
a. Press Trace/Detector, Select Trace,
Trace 3, Clear Write.
b. Press More, More, Math, Power
Diff, Trace Operands, Operand 1,
Trace 1, Operand 2, Trace 2.
Notice the phase noise
improvement at 100 kHz
offset between trace 1
(yellow trace) and trace 3
(magenta trace).
See Figure 7-13.
90
Measuring Noise
Improving Phase Noise Measurements by Subtracting Signal Analyzer Noise
Step
Figure 7-13
Action
Notes
Improved Phase Noise Measurement
9 Measure the noise
measurement improvement
with delta Noise markers
between traces.
a. Press Marker, Select Marker,
Marker 1, Normal.
b. Press Properties, Select Marker,
Marker 1, Marker Trace, Trace 1.
c. Using the knob, adjust Marker 1 to
approximately 90 kHz offset from
the carrier on trace 1.
d. Press Return, Select Marker,
Marker 2, Normal.
e. Press Properties, Select Marker,
Marker 2, Marker Trace, Trace 3.
f. Press Relative To, Marker 1.
g. Using the knob, adjust Marker 2 to
approximately 90 kHz offset from
the carrier on trace 3.
h. Press Marker Function, Select
Marker, Marker 1, Marker Noise.
i. Press Select Marker, Marker 2,
Marker Noise.
91
Note the up to 6 dB
improvement in the Marker
Results Block. See Figure
7-14.
Measuring Noise
Improving Phase Noise Measurements by Subtracting Signal Analyzer Noise
Step
Figure 7-14
Action
Notes
Improved Phase Noise Measurement with Delta Noise Markers
92
Making Time-Gated Measurements
8
Making Time-Gated
Measurements
Traditional frequency-domain spectrum analysis provides only limited information
for certain signals. Examples of these difficult-to-analyze signal include the
following:
•
•
•
•
•
Pulsed-RF
Time multiplexed
Interleaved or intermittent
Time domain multiple access (TDMA) radio formats
Modulated burst
The time gating measurement examples use a simple frequency-modulated,
pulsed-RF signal. The goal is to eliminate the pulse spectrum and then view the
spectrum of the FM carrier as if it were continually on, rather than pulsed. This
reveals low-level modulation components that are hidden by the pulse spectrum.
93
Making Time-Gated Measurements
Generating a Pulsed-RF FM Signal
Generating a Pulsed-RF FM Signal
When performing these measurements you can use a digitizing oscillascope or your
Agilent X-Series Signal Analyzer (using Gate View) to set up the gated signal. Refer
back to these first three steps to set up the pulse signal, the pulsed-RF FM signal and
the oscilloscope settings when performing the gated LO procedure (page 100), the
gated video procedure (page 104) and gated FFT procedure (page 108).
For an instrument block diagram and instrument connections see “Connecting the
Instruments to Make Time-Gated Measurements” on page 99.
Signal source setup
Step 1. Set up the pulse signal with a period of 5 ms and a width of 4 ms:
There are many ways to create a pulse signal. This example demonstrates how to
create a pulse signal using a pulse generator or by using the internal function
generator in the ESG.See Table 8-1 if you are using a pulse generator or Table 8-2 if
you are using a second ESG. Select either the pulse generator or a second ESG to
create the pulse signal.
Table 8-1
Table 8-2
81100 Family Pulse Generator Settings
Period
5 ms (or pulse frequency equal to 200 Hz)
Pulse width
4 ms
High output level
2.5 V
Waveform
pulse
Low output level
-2.5 V
Delay
0 or minimum
ESG #2 Internal Function Generator (LF OUT) Settings
LF Out Source
FuncGen
LF Out Waveform
Pulse
LF Out Period
5 ms
LF Out Width (pulse
width)
4 ms
LF Out Amplitude
2.5 Vp
LF Out
On
RF On/Off
Off
Mod On/Off
On
94
Making Time-Gated Measurements
Generating a Pulsed-RF FM Signal
Step 2. Set up ESG #1 to transmit a pulsed-RF signal with frequency modulation. Set the
FM deviation to 1 kHz and the FM rate to 50 kHz:
ESG #1 generates the pulsed FM signal by frequency modulating the carrier signal
and then pulse modulating the FM signal. The pulse signal created in step 1 is
connected to the EXT 2 INPUT (on the front of ESG #1). The ESG RF OUTPUT is
the pulsed-RF FM signal to be analyzed by the spectrum analyzer.
Table 8-3
ESG #1 Instrument Connections
Frequency
40 MHz
Amplitude
0 dBm
Pulse
On
Pulse Source
Ext2 DC
FM
On
FM Path
1
FM Dev
1 kHz
FM Source
Internal
FM Rate
50 kHz
RF On/Off
On
Mod On/Off
On
Analyzer setup
If you are using an Agilent X-Series Signal Analyzer (using Gate View), set up the
analyzer to view the gated RF signal (see Figure 8-1 and Figure 8-2 for examples of
the display).
Step
Action
Notes
1 Select Spectrum
Analyzer mode and
Preset.
•
2 Set the analyzer
center frequency,
span and reference
level.
a. Press FREQ Channel, Center
Freq, 40, MHz.
Press Mode, Spectrum Analyzer,
Mode Preset.
b. Press SPAN X Scale, Span, 500,
kHz.
c. Press AMPTD Y Scale, Ref Level,
10, dBm.
3 Set the analyzer
bandwidth.
•
Press BW, Res BW (Man), 100,
kHz.
95
Making Time-Gated Measurements
Generating a Pulsed-RF FM Signal
Step
Action
Notes
4 Set the gate source to
the rear external
trigger input.
•
5 Enable Gate View
and Gate.
a. Press Sweep/Control, Gate, Gate
View (On).
Press Sweep/Control, Gate, More,
Gate Source, External 1.
b. Press Gate View Sweep Time, 10,
ms.
6 Set the gate delay and
gate length so that the
gate will open during
the middle third of
the pulse.
a. Press Sweep/Control, Gate, Gate
Delay, 1.33, ms.
b. Press Gate Length, 1.33, ms.
c. Press More, Control (Edge).
See Figure 8-1below.
For this example, this would
result in a Gate Delay of
approximately 1.33 ms and a Gate
Length of approximately 1.33 ms.
Also, check that the gate trigger is
set to edge.
Figure 8-1
Gated RF Signal
96
Making Time-Gated Measurements
Generating a Pulsed-RF FM Signal
Step
Action
Notes
7 Set the RBW to auto,
gate view to off, gate
method to LO, and
gate to on.
a. Press Sweep/Control, Gate, Gate
View (Off).
b. Press BW, Res BW (Auto).
c. Press Sweep/Control, Gate, Gate
Method, LO.
d. Press Gate (On).
Figure 8-2
Gated RF Signal with Auto RBW
Digitizing oscilloscope setup
If you are using a digitizing oscillascope, set up the oscilloscope to view the trigger,
gate and RF signals (see Figure 8-3 for an example of the oscilloscope display):
Table 8-4
Agilent Infiniium Oscilloscope with 3 or more input channels: Instrument
Connections
Timebase
1 ms/div
Channel 1
ON, 2 V/div, OFFSET = 2 V, DC coupled, 1 M Ω input,
connect to the pulse signal (ESG LF OUTPUT or pulse
generator OUTPUT). Adjust channel 1 settings as
necessary.
97
Making Time-Gated Measurements
Generating a Pulsed-RF FM Signal
Table 8-4
Figure 8-3
Agilent Infiniium Oscilloscope with 3 or more input channels: Instrument
Connections
Channel 2
ON, 500 mV/div, OFFSET = 2 V, DC coupled, 1 M Ω
input, connect to the signal analyzer TRIGGER 2 OUT
connector. Adjust channel 2 settings as needed when
gate is active.
Channel 3
ON, 500 mV/div, OFFSET = 0 V, Timebase = 20 ns/div,
DC coupled, 50 Ω input, connect to the ESG RF
OUTPUT pulsed-RF signal. Adjust channel 3 settings as
necessary.
Channel 4
OFF
Trigger
Edge, channel 1, level = 1.5 V, or as needed
Viewing the Gate Timing with an Oscilloscope
Figure 8-3 oscilloscope channels:
1. Channel 1 (yellow trace) - the trigger signal.
2. Channel 2 (green trace) - the gate signal (gate signal is not active until the gate is
on in the spectrum analyzer).
3. Channel 3 (purple) - the RF output of the signal generator.
98
Making Time-Gated Measurements
Connecting the Instruments to Make Time-Gated Measurements
Connecting the Instruments to Make Time-Gated Measurements
Figure 8-4 shows a diagram of the test setup. ESG #1 produces a pulsed FM signal
by using an external pulse signal. The external pulse signal is connected to the front
of the ESG #1 to the EXT 2 INPUT to control the pulsing. The pulse signal is also
used as the trigger signal. The oscilloscope is useful for illustrating timing
interactions between the trigger signal and the gate. The Gate View feature of the
X-Series signal analyzer could be used in place of the oscilloscope.
Using this measurement setup allows you to view all signal spectra on the spectrum
analyzer and all timing signals on the oscilloscope. This setup is helpful when you
perform gated measurements on unknown signals. If an oscilloscope is not available,
begin by using the Gate View feature to set up the gate parameters and then turn Gate
View Off to view the signal spectra, refer to Figure 8-5
Figure 8-4
Instrument Connection Diagram with Oscilloscope
Figure 8-5
Instrument Connection Diagram without Oscilloscope
99
Making Time-Gated Measurements
Gated LO Measurement
Gated LO Measurement
This procedure utilizes gated LO to gate the FM signal. For concept and theory
information about gated LO see “How time gating works” on page 181.
Step
Action
Notes
1 Set the analyzer to the
Spectrum Analyzer
mode.
•
Press Mode, Spectrum
Analyzer.
This enables the spectrum analyzer
measurements.
2 Preset the analyzer.
•
Press Mode Preset.
3 Set the center frequency,
span, reference level and
attenuation.
a. Press FREQ Channel,
Center Freq, 40, MHz.
b. Press SPAN X Scale,
Span, 500, kHz.
c. Press AMPTD Y Scale,
Ref Level, 10, dBm.
4 Set the gate source to the
rear external trigger
input.
•
.Press Sweep/Control,
Gate, More, Gate
Source, External 1.
5 Set the gate delay, gate
length, gate sweep time,
and gate trigger.
a. Press Sweep/Control,
Gate, Gate Delay, 2,
ms.
b. Press Gate Length, 1,
ms.
c. Press Gate View Sweep
Time, 5, ms.
d. Press More1 of 2,
Control (Edge).
6 Access the analyzer gate
view display.
•
Press Sweep/Control,
Gate, Gate View (On).
100
Use this function to confirm the gate
“on” time during the RF burst interval
(alternatively you could also use the
oscilloscope to view the gate settings).
Making Time-Gated Measurements
Gated LO Measurement
Step
Figure 8-6
Action
Notes
Viewing the Gate Settings with Gated LO
The blue vertical line (the far left line outside of the RF envelope) represents the location equivalent to
a zero gate delay.
The vertical green parallel bars represent the gate settings. The first (left) bar (GATE START) is set at
the delay time while the second (right) bar (GATE STOP) is set at the gate length, measured from the
first bar. The trace of the signal in this time-domain view is the RF envelope. The gate signal is
triggered off of the positive edge of the trigger signal.
When positioning the gate, a good starting point is to have it extend from 20% to 80% of the way
through the pulse.
While gate view mode is on, move the gate delay, length and polarity around. Notice the changes in the
vertical gate bars while making your changes. Set the gate delay, length and polarity back to the step 3
settings.
NOTE
The analyzer time gate triggering mode uses positive edge, negative edge, and level
triggering.
7 Turn the gate view off.
•
Press Sweep/Control,
Gate, Gate View (Off).
101
See Figure 8-7.
Making Time-Gated Measurements
Gated LO Measurement
Step
Figure 8-7
Action
Notes
Pulsed RF FM Signal
The moving signals are a result of the pulsed signal. Using delta markers with a time readout, notice that
the period of the spikes is at 5 ms (the same period as the pulse signal). Using time gating, these signals
well be blocked out, leaving the original FM signal.
8 Enable the gate settings.
•
Press Gate (On).
102
See Figure 8-8.
Making Time-Gated Measurements
Gated LO Measurement
Step
Figure 8-8
Action
Notes
Pulsed and Gated Signal
9 Turn off the pulse
modulation on ESG #1.
•
Press Pulse, Pulse so
that Off is selected.
103
Notice that the gated spectrum is much
cleaner than the ungated spectrum (as
seen in the Pulsed-RF FM Signal
above). The spectrum you see with the
gate on is the same as a frequency
modulated signal without being pulsed.
The displayed spectrum does not change
and in both cases, you can see the two
low-level modulation sidebands caused
by the narrow-band FM.
Making Time-Gated Measurements
Gated Video Measurement
Gated Video Measurement
This procedure utilizes gated video to gate the FM signal. For concept and theory
information about gated video see “How time gating works” on page 181.
Step
Action
Notes
1 Set the analyzer to
the Spectrum
Analyzer mode.
•
Press Mode,
Spectrum Analyzer.
This enables the spectrum analyzer
measurements.
2 Preset the analyzer
•
Press Mode Preset.
3 Set the center
frequency, span,
reference level and
attenuation.
a. Press FREQ Channel,
Center Freq, 40,
MHz.
b. Press SPAN X Scale,
Span, 500, kHz.
c. Press AMPTD Y
Scale, Ref Level, 10,
dBm.
4 Set analyzer points to
401 and sweep time
to 2000 ms.
a. Press Sweep/Control,
Points, 401, Enter.
For gated video, the calculated sweep time
should be set to at least
( # sweep points – 1 ) × PRI (pulse repetition interval )
to
ensure that the gate is on at least once during
each of the 401 sweep points. In this example,
the PRI is 5 ms, so you should set the sweep
time to 401 minus 1 times 5 ms, or 2 s. If the
sweep time is set too fast, some trace points
may show values of zero power or other
incorrect low readings. If the trace seems
incomplete or erratic, try a longer sweep time.
b. Press Sweep Time,
2000, ms.
Good practices for determining the minimum sweep time for gated video:
In the event that the signal is not noisy, the sweep time can be set to less than
(as calculated above). Instead of using PRI in the
previous sweep time calculation, we can use the “gate off time” where sweep time equals
( # sweep points – 1 ) × gate off time . (Gate off time is defined as PRI – GL, where GL = Gate Length.)
In our example we could use a sweep time of 400 points times 1 ms or 400 ms –
( 401 – 1 ) × ( 5ms – 4ms ) = 400ms . Increase the video bandwidth to improve the probability of
capturing the pulse using “gate off time”. If trace points are still showing values of zero
power, increase the sweep time by small increments until there are no more dropouts.
( # sweep points – 1 ) × PRI (pulse repetition interval )
5 Set the Gate source to
the external trigger
input on the rear
panel:
•
Press Sweep/Control,
Gate, More, Gate
Source, External 1.
104
Making Time-Gated Measurements
Gated Video Measurement
Step
Figure 8-9
Action
Notes
Viewing a Pulsed RF FM Signal (without gating)
6 Set the gate delay and
gate length.
a. Press Sweep/Control,
Gate, More, Control
(Edge).
b. Press More, Gate
Delay, 2, ms.
c. Press Gate Length, 1,
ms.
7 Turn the gate on.
a. Press Sweep/Control,
Gate, Gate Method,
Video.
b. Press Gate (On).
105
Ensure that the gate control is set to Edge.
Making Time-Gated Measurements
Gated Video Measurement
Step
Figure 8-10
Action
Notes
Viewing the FR Signal of a Pulsed RF Signal using Gated Video
Notice that the gated spectrum is much cleaner than the ungated spectrum (as seen in Figure 8-9). The
spectrum you see is the same as a frequency modulated signal without being pulsed. To prove this, turn
off the pulse modulation on ESG #1 by pressing Pulse, Pulse so that Off is selected. The displayed
spectrum does not change.
If you have used an oscilloscope, check the oscilloscope display and ensure that the gate is positioned
under the pulse. The gate should be set so that it is on somewhere between 20% to 80% of the pulse. If
necessary, adjust gate length and gate delay. Figure 8-11 shows the oscilloscope display when the gate is
positioned correctly (the bottom trace).
106
Making Time-Gated Measurements
Gated Video Measurement
Step
Figure 8-11
Action
Notes
The Oscilloscope Display
107
Making Time-Gated Measurements
Gated FFT Measurement
Gated FFT Measurement
This procedure utilizes gated FFT to gate the FM signal. For concept and theory
information about gated FFT see “How time gating works” on page 181.
Step
Action
Notes
1 Set the analyzer to the
Spectrum Analyzer mode.
•
Press Mode, Spectrum
Analyzer.
This enables the spectrum analyzer
measurements.
2 Preset the analyzer
•
Press Mode Preset.
3 Set the center frequency,
span, reference level and
attenuation.
a. Press FREQ Channel,
Center Freq, 40, MHz.
b. Press SPAN X Scale,
Span, 500, kHz.
c. Press AMPTD Y Scale,
Ref Level, 10, dBm.
4 Set the Gate source to the
external trigger input on the
rear panel.
a. Press Sweep/Control,
Gate, More, Gate
Source, External 1.
5 Set the gate method and turn
gate on.
a. Press Sweep/Control,
Gate, Gate Method,
FFT.
b. Press Gate (On).
6 Select the minimum
resolution bandwidth
required.
•
Press BW, Res BW
(Auto).
108
See Figure 8-12.
Making Time-Gated Measurements
Gated FFT Measurement
Step
Action
Figure 8-12
Notes
Viewing the Gated FFT Measurement results
The duration of the analysis required is determined by the RBW. Divide 1.83 by 4 ms to calculate the
minimum RBW. The pulse width in our case is 4 ms so we need a minimum RBW of 458 Hz. In this case
because the RBW is so narrow let the analyzer choose the RBW for the current analyzer settings (span).
Check that the RBW is greater than 458 Hz. Vary the RBW settings and note the signal changes shape as
the RBW transitions from 1 kHz to 300 Hz.
NOTE
If the trigger event needs to be delayed use the Trig Delay function under the Trigger menu.
Apply some small amount of trigger delay to allow time for the device under test to settle.
109
Making Time-Gated Measurements
Gated FFT Measurement
110
Measuring Digital Communications Signals
9
Measuring Digital
Communications Signals
The Signal Analyzer makes power measurements on digital communication signals
fast and repeatable by providing a comprehensive suite of power-based one-button
automated measurements with pre-set standards-based format setups. The automated
measurements also include pass/fail functionality that allow the user to quickly
check if the signal passed the measurement.
111
Measuring Digital Communications Signals
Channel Power Measurements
Channel Power Measurements
This section explains how to make a channel power measurement on a W-CDMA
(3GPP) mobile station. (A signal generator is used to simulate a base station.) This
test measures the total RF power present in the channel. The results are displayed
graphically as well as in total power (dB) and power spectral density (dBm/Hz).
Step
Action
Notes
1 Set up the signal
generator.
a. Set the mode to W-CDMA.
b. Set the frequency to 1.920
GHz.
c. Set the amplitude to
−20 dBm.
2 Connect the source RF
OUTPUT to the
analyzer RF INPUT as
shown.
3 Select the mode.
•
Press Mode, Spectrum
Analyzer.
4 Preset the analyzer.
•
Press Mode Preset.
5 Set the radio standard
and toggle the device to
mobile station.
•
Press Mode Setup, Radio
Std, 3GPP W-CDMA,
3GPP W-CDMA, Device
(MS).
6 Set the center frequency.
•
Press FREQ Channel,
1.920, GHz.
7 Initiate the channel
power measurement.
•
Press Meas, Channel
Power.
112
The Channel Power measurement
result should look like “Channel
Power Measurement Result.” on
page 113.
Measuring Digital Communications Signals
Channel Power Measurements
Step
Figure 9-1
Action
Notes
Channel Power Measurement Result.
The graph window and the text window showing the absolute power and its mean power spectral
density values over 5 MHz are displayed.
To change the measurement parameters from their default condition:
Press Meas Setup.
113
Measuring Digital Communications Signals
Occupied Bandwidth Measurements
Occupied Bandwidth Measurements
This section explains how to make the occupied bandwidth measurement on a
W-CDMA (3GPP) mobile station. (A signal generator is used to simulate a base
station.) The instrument measures power across the band, and then calculates its
99.0% power bandwidth.
Step
Action
Notes
1 Set up the signal generator.
a. Set the mode to
W-CDMA.
b. Set the frequency to 1.920
GHz.
c. Set the amplitude to
−20 dBm.
2 Connect the source RF
OUTPUT to the analyzer
RF INPUT as shown.
3 Select the mode.
•
Press Mode, Spectrum
Analyzer.
4 Preset the analyzer.
•
Press Mode Preset.
5 Set the radio standard and
toggle the device to mobile
station.
•
Press Mode Setup, Radio
Std, 3GPP W-CDMA,
3GPP W-CDMA, Device
(MS).
6 Set the center frequency.
•
Press FREQ Channel,
1.920, GHz.
7 Initiate the occupied
bandwidth measurement.
•
Press Meas, Occupied
BW.
114
The Occupied BW measurement
result should look like “Occupied BW
Measurement Result” on page 115.
Measuring Digital Communications Signals
Occupied Bandwidth Measurements
Step
Action
Figure 9-2
Notes
Occupied BW Measurement Result
Troubleshooting hints
Any distortion such as harmonics or intermodulation, for example, produces
undesirable power outside the specified bandwidth.
Shoulders on either side of the spectrum shape indicate spectral regrowth and
intermodulation. Rounding or sloping of the top shape can indicate filter shape
problems.
115
Measuring Digital Communications Signals
Making Adjacent Channel Power (ACP) Measurements
Making Adjacent Channel Power (ACP) Measurements
The adjacent channel power (ACP) measurement is also referred to as the adjacent
channel power ratio (ACPR) and adjacent channel leakage ratio (ACLR). We use the
term ACP to refer to this measurement.
ACP measures the total power (rms voltage) in the specified channel and up to six
pairs of offset frequencies. The measurement result reports the ratios of the offset
powers to the main channel power.
The following example shows how to make an ACP measurement on a W-CDMA
base station signal broadcasting at 1.96 GHz. (A signal generator is used to simulate
a base station.)
Step
Action
Notes
1 Set up the signal
generator.
a. Set the mode to W-CDMA.
b. Set the frequency to 1.920
GHz.
c. Set the amplitude to
−10 dBm.
2 Connect the source RF
OUTPUT to the analyzer
RF INPUT as shown.
3 Select the mode.
•
Press Mode, Spectrum
Analyzer.
4 Preset the analyzer.
•
Press Mode Preset.
5 Set the analyzer radio
mode to W-CDMA as a
base station device.
a. Press Mode Setup, Radio
Std, 3GPP W-CDMA,
3GPP W-CDMA,
b. Press Mode Setup, Radio
Std Setup, Device (BTS).
116
Measuring Digital Communications Signals
Making Adjacent Channel Power (ACP) Measurements
Step
Action
Notes
6 Set the center frequency.
•
Press FREQ Channel,
1.920, GHz.
7 Initiate the adjacent
channel power
measurement.
•
Press Meas, ACP.
The Occupied BW measurement
result should look like the following
graphic.
8 Optimize the attenuation
setting.
•
Press AMPTD,
Attenuation, Adjust Atten
for Min Clip.
Adjust Atten for Min Clip protects
against input signal overloads, but
does not necessarily set the input
attenuation and reference level for
optimum measurement dynamic
range.
To improve the measurement
repeatability, increase the sweep
time to smooth out the trace
(average detector must be selected).
Measurement repeatability can be
traded off with sweep time.
9 To increase dynamic
range, Noise Correction
can be used to factor out
the added power of the
noise floor effects.
•
Press Meas Setup, More,
More, Noise Correction
(On).
117
Measuring Digital Communications Signals
Making Adjacent Channel Power (ACP) Measurements
Step
Figure 9-3
Action
Notes
ACP Measurement on a Base Station W-CDMA Signal
Two vertical white lines, in the center of the screen, indicate the bandwidth limits of the central channel
being measured.
The frequency offsets, channel integration bandwidths, and span settings can all be modified from the
default settings.
Offsets A and B are designated by the adjacent pairs of white lines, in this case: 5 MHz and 10 MHz
from the center frequency respectively.
10 View the results using the
full screen.
•
Press Full Screen.
Press the Full Screen key again to
exit the full screen display without
changing any parameter values.
11 Define a new third pair of
offset frequencies.
•
Press Meas Setup,
Offset/Limits, Offset, C,
Offset Freq (On), 15,
MHz.
This third pair of offset frequencies
is offset by 15.0 MHz from the
center frequency (the outside offset
pair) as shown in Figure 9-4 Three
further pairs of offset frequencies
(D, E and F) are also available.
118
Measuring Digital Communications Signals
Making Adjacent Channel Power (ACP) Measurements
Step
Figure 9-4
Action
Notes
Measuring a Third Adjacent Channel
12 Set pass/fail limits for
each offset.
•
Press Meas Setup,
Offset/Limits, Offset, A,
More, Rel Limit (Car),
−55, dB, Offset, B, Rel
Limit (Car), −75, dB,
Offset, C, Rel Limit (Car),
−60, dB.
13 Turn the limit test on.
•
Press Meas Setup, More,
Limit Test (On).
119
In Figure 9-5 notice that offsets A
and C have passed, however offset B
has failed. Power levels that fall
above our specified –75 dB for
offset B, fail. The offset bar graph
and the associated power level value
are shaded red to identify a failure.
The offset limits are shown as
dashed lines.
Measuring Digital Communications Signals
Making Adjacent Channel Power (ACP) Measurements
Step
Figure 9-5
NOTE
Action
Notes
Setting Offset Limits
You may increase the repeatability by increasing the sweep time.
120
Measuring Digital Communications Signals
Making Statistical Power Measurements (CCDF)
Making Statistical Power Measurements (CCDF)
Complementary cumulative distribution function (CCDF) curves characterize a
signal by providing information about how much time the signal spends at or above a
given power level. The CCDF measurement shows the percentage of time a signal
spends at a particular power level. Percentage is on the vertical axis and power (in
dB) is on the horizontal axis.
All CDMA signals, and W-CDMA signals in particular, are characterized by high
power peaks that occur infrequently. It is important that these peaks are preserved
otherwise separate data channels can not be received properly. Too many peak
signals can also cause spectral regrowth. If a CDMA system works well most of the
time and only fails occasionally, this can often be caused by compression of the
higher peak signals.
The following example shows how to make a CCDF measurement on a W-CDMA
signal broadcasting at 1.96 GHz. (A signal generator is used to simulate a base
station.)
Step
Action
Notes
1 Set up the signal
generator.
a. Setup a W-CDMA down
link signal.
b. Set the frequency to 1.96
GHz.
c. Set the amplitude to
−10 dBm.
2 Connect the source RF
OUTPUT to the analyzer
RF INPUT as shown.
3 Select the mode.
•
Press Mode, Spectrum
Analyzer.
4 Preset the analyzer.
a. Press Mode Preset.
121
Measuring Digital Communications Signals
Making Statistical Power Measurements (CCDF)
Step
Action
5 Set the analyzer radio
mode to W-CDMA as a
base station device.
•
Press Mode Setup, Radio
Std, 3GPP W-CDMA,
3GPP W-CDMA, Device
(BTS).
6 Set the center frequency.
•
Press FREQ Channel,
1.98, GHz
7 Select the CCDF
measurement and
optimize the attenuation
level and attenuation
settings suitable for the
CCDF measurement.
a. Press Meas, Power Stat
CCDF.
Figure 9-6
Notes
b. Press AMPTD,
Attenuation, Adjust Atten
for Min Clip.
Power Statistics CCDF Measurement on a W-CDMA Signal
8 Store your current
measurement trace for
future reference.
•
Press Trace/Detector, Store
Ref Trace.
122
When the power stat CCDF
measurement is first made, the
graphical display should show a
signal typical of pure noise. This is
labelled Gaussian, and is shown in
aqua. Your CCDF measurement is
displayed as a yellow plot. You have
stored this measurement plot to
make for easy comparison with
subsequent measurements. Refer to
Figure 9-6.
Measuring Digital Communications Signals
Making Statistical Power Measurements (CCDF)
Step
Action
Notes
9 Display the stored trace.
•
Press Trace/Detector, Ref
Trace (On).
Press the Full Screen key again to
exit the full screen display without
changing any parameter values.
10 Change the measurement
bandwidth to 1 MHz.
•
Press BW, Info BW, 1,
MHz.
The stored trace from your last
measurement is displayed as a
magenta plot (as shown in Figure
9-7), and allows direct comparison
with your current measurement
(yellow trace).
Figure 9-7
NOTE
Storing and Displaying a Power Stat CCDF Measurement
If you choose a measurement bandwidth setting that the analyzer cannot display, it
automatically sets itself to the closest available bandwidth setting.
11 Change the number of
measured points from
10,000,000 (10.0Mpt) to
1,000 (1kpt).
•
Press Meas Setup, Counts,
1, kpt.
123
Reducing the number of points
decreases the measurement time,
however the number of points is a
factor in determining measurement
uncertainty and repeatability. Notice
how the displayed plot loses a lot of
its smoothness. You are gaining
speed but reducing repeatability and
increasing measurement uncertainty.
refer to Figure 9-8.
Measuring Digital Communications Signals
Making Statistical Power Measurements (CCDF)
Step
Figure 9-8
NOTE
Action
Notes
Reducing the Measurement Points to 1 kpt
The number of points collected per sweep is dependent on the sampling rate and the
measurement interval. The number of samples that have been processed are indicated at the
top of the screen. The graphical plot is continuously updated so you can see it getting
smoother as measurement uncertainty is reduced and repeatability improves.
12 Change the scaling of the
X-axis to 1 dB per
division to optimize your
particular measurement.
•
Press SPAN X Scale,
Scale/Div, 1, dB.
124
Refer to Figure 9-9.
Measuring Digital Communications Signals
Making Statistical Power Measurements (CCDF)
Step
Figure 9-9
Action
Notes
Reducing the X Scale to 1 dB
125
Measuring Digital Communications Signals
Making Burst Power Measurements
Making Burst Power Measurements
The following example demonstrates how to make a burst power measurement on a
Bluetooth™ signal broadcasting at 2.402 GHz. (A signal generator is used to
simulate a Bluetooth™ signal.)
Step
Action
Notes
1 Set up the signal source.
a. Setup a Bluetooth™ signal
transmitting DH1 packets.
b. Set the source frequency to
2.402 GHz.
c. Set the source amplitudes to
−10 dBm.
d. Set the source amplitudes to
−10 dBm.
2 Connect the source RF
OUTPUT to the analyzer
RF INPUT as shown.
3 Set the analyzer to the
Spectrum Analyzer
mode.
•
Press Mode, Spectrum
Analyzer.
4 Preset the analyzer.
•
Press Mode Preset.
5 Set the analyzer center
frequency.
•
Press FREQ Channel,
Center Freq, 2.402, GHz.
6 Set the analyzer radio
mode to Bluetooth™.
•
Press Mode Setup, Radio
Std, More, Bluetooth,
Bluetooth, DH1.
126
Check to make sure packet type DH1
is selected.
Measuring Digital Communications Signals
Making Burst Power Measurements
Step
Action
7 Select the burst power
measurement and
optimize the attenuation
level.
a. Press Meas, Burst Power.
8 View the results of the
burst power measurement
using the full screen).
•
Figure 9-10
NOTE
Notes
b. Press AMPTD,
Attenuation, Adjust Atten
for Min Clip.
Press Full Screen.
See Figure 9-10.
Full Screen Display of Burst Power Measurement Results
Press the Full Screen key again to exit the full screen display without changing any
parameter values. Refer to Figure 9-11
127
Measuring Digital Communications Signals
Making Burst Power Measurements
Step
Figure 9-11
Action
Notes
Normal Screen Display of Burst Power Measurement Results
9 Select one of the
following three trigger
methods to capture the
bursted signal:
•
Periodic Timer
Triggering
•
Video
•
RF Burst Wideband
Triggering (RF burst is
recommended, if
available.
10 Set the relative threshold
level above which the
burst power measurement
is calculated.
•
Press Trigger, RF Burst.
For more information on trigger
selections see “Trigger Concepts” on
page 176.
Although the trigger level allows the
analyzer to detect the presence of a
burst, the time samples contributing
to the burst power measurement are
determined by the threshold level, as
described next.
•
Press Meas Setup,
Threshold Lvl (Rel), −10,
dB.
128
The burst power measurement
includes all points above the
threshold and no points below. The
threshold level is indicated on the
display by the green horizontal line.
In this example, the threshold level
has been set to be 10 dB below the
relative level of the burst. The mean
power of the burst is measured from
all data above the threshold level.
Refer to Figure 9-12.
Measuring Digital Communications Signals
Making Burst Power Measurements
Step
Figure 9-12
Action
Notes
Burst Power Measurement Results with Threshold Level Set
11 Set the burst width to
measure the central
200 μs of the burst and
enable bar graph.
a. Press View/Display, Bar
Graph (On).
b. Press Meas Setup, Meas
Method, Measured Burst
Width, Burst
Width (Man), 200, μs.
129
The burst width is indicated on the
screen by two vertical white lines and
a blue power bar. Manually setting
the burst width allows you to make it
a long time interval (to include the
rising and falling edges of the burst)
or to make it a short time interval,
measuring a small central section of
the burst. Refer to Figure 9-13.
Measuring Digital Communications Signals
Making Burst Power Measurements
Step
Action
Figure 9-13
NOTE
Notes
Bar Graph Results with Measured Burst Width Set
If you set the burst width manually to be wider than the screen's display, the vertical white
lines move off the edges of the screen. This could give misleading results as only the data on
the screen can be measured.
The Bluetooth™ standard states that power measurements should be taken over at least 20%
to 80% of the duration of the burst.
12 Increase the sweep time
to display more than one
burst at a time.
•
Press Sweep/Control,
Sweep Time, 6200, μs (or
6.2, ms).
130
The screen display shows several
bursts in a single sweep as in Figure
9-14. The burst power measurement
measures the mean power of the first
burst, indicated by the vertical white
lines and blue power bar.
Measuring Digital Communications Signals
Making Burst Power Measurements
Step
Action
Figure 9-14
NOTE
Notes
Displaying Multiple Bursts
Although the burst power measurement still runs correctly when several bursts are displayed
simultaneously, the timing accuracy of the measurement is degraded. For the best results
(including the best trade-off between measurement variations and averaging time), it is
recommended that the measurement be performed on a single burst.
131
Measuring Digital Communications Signals
Spurious Emissions Measurements
Spurious Emissions Measurements
The following example demonstrates how to make a spurious emissions
measurement on a multitone signal used to simulate a spurious emission in a
measured spectrum.
Step
Action
Notes
1 Setup the signal source.
a. Setup a multitone signal
with 8 tones with a 2.0 MHz
frequency spacing.
b. Set the source frequency to
1.950 GHz.
c. Set the source amplitudes to
−50 dBm.
2 Connect the source RF
OUTPUT to the analyzer
RF INPUT as shown.
3 Set the analyzer to the
Spectrum Analyzer mode.
•
Press Mode, Spectrum
Analyzer.
4 Preset the analyzer.
•
Press Mode Preset.
5 Set the analyzer center
frequency.
•
Press FREQ Channel,
Center Freq, 1.950, GHz.
6 Select the spurious
emissions measurement.
•
Press Meas, More,
Spurious Emissions.
132
Measuring Digital Communications Signals
Spurious Emissions Measurements
Step
Action
Notes
7 You may Focus the display
on a specific spurious
emissions signal.
a. Press Meas Setup, Spur, 1,
Enter (or enter the number
of the spur of interest)
The Spurious Emission result
should look like Figure 9-15. The
graph window and a text window
are displayed. The text window
shows the list of detected spurs.
Each line item includes the spur
number, the range in which the spur
was detected, the power of the spur,
and the limit value against which
the spur amplitude is tested.
b. Press Meas Type to
highlight Examine
Figure 9-15
Spurious Emission Measurement Result
8 You may customize the
tested ranges for spurious
emissions (initially 5
default ranges and
parameters are loaded into
the range table).
•
Press Meas Setup, Range
Table, then select and edit
the available parameters.
133
Measuring Digital Communications Signals
Spurious Emissions Measurements
Troubleshooting hints
Spurious emissions measurements can reveal the presence of degraded or defective
parts in the transmitter section of the UUT. The following are examples of problems
which, once indicated by testing, may require further attention:
•
•
•
•
•
Faulty DC power supply control of the transmitter power amplifier
RF power controller of the pre-power amplifier stage
I/Q control of the baseband stage
Reduction in the gain and output power level of the amplifier due to a degraded
gain control and/or increased distortion
Degradation of amplifier linearity and other performance characteristics
Power amplifiers are one of the final stage elements of a base transmitter and play a
critical part in meeting the important power and spectral efficiency specifications.
Measuring the spectral response of these amplifiers to complex wideband signals is
crucial to linking amplifier linearity and other performance characteristics to the
stringent system specifications.
134
Measuring Digital Communications Signals
Spectrum Emission Mask Measurements
Spectrum Emission Mask Measurements
This section explains how to make the spectrum emission mask measurement on a
W-CDMA (3GPP) mobile station. (A signal generator is used to simulate a mobile
station.) SEM compares the total power level within the defined carrier bandwidth
and the given offset channels on both sides of the carrier frequency, to levels allowed
by the standard. Results of the measurement of each offset segment can be viewed
separately.
Step
Action
Notes
1 Set up the signal source.
a. Setup a W-CDMA uplink
signal.
b. Set the source frequency to
1,920 MHz (Channel.
Number: 5 × 1,920 =
9,600).
c. Set the source amplitudes
to 0 dBm.
2 Connect the source RF
OUTPUT to the analyzer
RF INPUT as shown.
3 Select the mode.
•
Press Mode, Spectrum
Analyzer.
4 Preset the analyzer.
•
Press Mode Preset.
5 Set the radio standard
and toggle the device to
mobile station.
•
Press Mode Setup, Radio
Std, 3GPP W-CDMA,
3GPP W-CDMA, Device
(MS).
6 Set the center frequency.
•
Press FREQ Channel,
1.920, GHz.
135
Measuring Digital Communications Signals
Spectrum Emission Mask Measurements
Step
Action
Notes
7 Initiate the spectrum
emission mask
measurement.
•
The Spectrum Emission Mask
measurement result should look like
Figure 9-16. The text window shows
the reference total power and the
absolute peak power levels which
correspond to the frequency bands on
both sides of the reference channel.
Figure 9-16
Press Meas, More,
Spectrum Emission
Mask.
Spectrum Emission Mask Measurement Result - (Default) View
Troubleshooting hints
This spectrum emission mask measurement can reveal degraded or defective parts in
the transmitter section of the UUT. The following examples are those areas to be
checked further.
•
Faulty DC power supply control of the transmitter power amplifier.
•
RF power controller of the pre-power amplifier stage.
•
I/Q control of the baseband stage.
•
Some degradation in the gain and output power level of the amplifier due to the
degraded gain control and/or increased distortion.
•
Some degradation of the amplifier linearity or other performance characteristics.
136
Measuring Digital Communications Signals
Spectrum Emission Mask Measurements
Power amplifiers are one of the final stage elements of a base or mobile transmitter
and are a critical part of meeting the important power and spectral efficiency
specifications. Since spectrum emission mask measures the spectral response of the
amplifier to a complex wideband signal, it is a key measurement linking amplifier
linearity and other performance characteristics to the stringent system specifications.
137
Measuring Digital Communications Signals
Spectrum Emission Mask Measurements
138
Demodulating AM Signals
10 Demodulating AM Signals
139
Demodulating AM Signals
Measuring the Modulation Rate of an AM Signal
Measuring the Modulation Rate of an AM Signal
This section demonstrates how to determine parameters of an AM signal, such as
modulation rate and modulation index (depth) by using frequency and time domain
measurements (see the concepts chapter “AM and FM Demodulation Concepts” on
page 197 for more information).
To obtain an AM signal, you can either connect a source transmitting an AM signal,
or connect an antenna to the analyzer input and tune to a commercial AM broadcast
station. For this demonstration an RF source is used to generate an AM signal.
Step
Action
Notes
1 Set up the signal
source.
a. Set the source frequency to
300 MHz.
b. Set the source amplitudes
to −10 dBm.
c. Set the AM depth to 80%.
d. Set the AM rate to 1 kHz.
e. Turn AM on.
2 Connect an Agilent
ESG RF signal source
to the analyzer RF
INPUT as shown.
3 Select the mode.
•
Press Mode, Spectrum
Analyzer.
4 Preset the analyzer.
•
Press Mode Preset.
140
Demodulating AM Signals
Measuring the Modulation Rate of an AM Signal
Step
Action
Notes
5 Set the center
frequency, span, RBW
and the sweep time.
a. Press FREQ Channel,
Center Freq, 300, MHz.
b. Press SPAN X Scale,
Span, 500, kHz.
c. Press BW, Res BW, 30,
kHz.
d. Press Sweep/Control,
Sweep Time, 20, ms.
6 Change the y-scale
type to linear.
•
Press AMPTD Y Scale,
Scale Type (Lin).
7 Position the signal peak
near the first graticule
below the reference
level.
•
Press AMPTD Y Scale,
Ref Level, (rotate
front-panel knob).
8 Set the analyzer in zero
span to make
time-domain
measurements
a. Press SPAN X Scale, Zero
Span
9 Use the video trigger to
stabilize the trace.
•
The y-axis units will automatically set to
volts.
b. Press Sweep/Control,
Sweep Time, 5, ms
Press Trigger, Video.
Since the modulation is a steady tone,
you can use video trigger to trigger the
analyzer sweep on the waveform and
stabilize the trace, much like an
oscilloscope. See Figure 10-1
If the trigger level is set too high or too
low when video trigger mode is
activated, the sweep stops. You need to
adjust the trigger level up or down with
the front-panel knob until the sweep
begins again.
10 Measure the AM rate
using delta markers.
•
Press Peak Search,
Marker Delta, Next
Right or Next Left.
141
Use markers and delta markers to
measure the AM rate. Place the marker
on a peak and then use a delta marker to
measure the time difference between the
peaks (this is the AM rate of the signal)
Demodulating AM Signals
Measuring the Modulation Rate of an AM Signal
Step
Action
Figure 10-1
NOTE
Notes
Measuring Time Parameters
Make sure the delta markers above are placed on adjacent peaks. See Figure 10-1 The
frequency or the AM rate is 1 divided by the time between adjacent peaks:
AM Rate = 1/1.0 ms = 1 kHz
The signal analyzer can also make this rate calculation by changing the marker readout to
inverse time.
Press Marker, Properties, X Axis Scale, Inverse Time.
142
Demodulating AM Signals
Measuring the Modulation Rate of an AM Signal
Step
Figure 10-2
Action
Notes
Measuring Time Parameters with Inverse Time Readout
Another way to calculate the modulation rate would be to view the signal in the frequency domain and
measure the delta frequency between the peak of the carrier and the first sideband.
143
Demodulating AM Signals
Measuring the Modulation Index of an AM Signal
Measuring the Modulation Index of an AM Signal
This procedure demonstrates how to use the signal analyzer as a fixed-tuned
(time-domain) receiver to measure the modulation index as a percent AM value of an
AM signal.
Step
Action
Notes
1 Set up the signal source.
a. Set the source frequency to
300 MHz.
b. Set the source amplitudes to
−10 dBm.
c. Set the AM depth to 80%.
d. Set the AM rate to 1 kHz.
e. Turn AM on.
2 Connect an Agilent ESG
RF signal source to the
analyzer RF INPUT as
shown.
3 Select the mode.
•
Press Mode, Spectrum
Analyzer.
4 Preset the analyzer.
•
Press Mode Preset.
5 Set the center frequency,
span, RBW and the
sweep time.
a. Press FREQ Channel,
Center Freq, 300, MHz.
b. Press SPAN X Scale, Span,
500, kHz.
c. Press BW, Res BW, 30,
kHz.
d. Press Sweep/Control,
Sweep Time, 20, ms.
144
Demodulating AM Signals
Measuring the Modulation Index of an AM Signal
Step
Action
Notes
6 Set the y-axis units to
volts.
•
Press AMPTD Y Scale,
More, Y-Axis Units, V
(Volts).
7 Position the signal peak
near the reference level.
•
Press AMPTD Y Scale, Ref
Level, (rotate front-panel
knob).
8 Change the y-scale type
to linear.
•
Press AMPTD Y Scale,
Scale Type (Lin).
9 Set the analyzer in zero
span to make
time-domain
measurements.
a. Press SPAN X Scale, Zero
Span.
10 Place the analyzer in
free run trigger mode.
•
11 Increase the sweep time
and decrease the VBW.
a. Press Sweep/Control,
Sweep Time, 5, s.
b. Press Sweep/Control,
Sweep Time, 5, ms
Press Trigger, Free Run.
The waveform is displayed as a flat
horizontal signal.
b. Press BW, Video BW, 30,
Hz.
12 Center the flat
waveform at the
mid-point of the y-axis
and then widen the
VBW and decrease the
sweep time to display
the waveform as a sine
wave.
13 Measure the modulation
index of the AM signal.
a. Press AMPTD Y Scale, Ref
Level, (rotate front-panel
knob).
b. Press BW, Video BW, 100,
kHz.
c. Press Sweep/Control,
Sweep Time, 5, ms.
To measure the modulation index as % AM, read the trace as follows (see
Figure 10-3 for display examples): 100% AM extends from the top
graticule down to the bottom graticule. 80% AM (as in this example) is
when the top of the signal is at 1 division below the top graticule and 1
division above the bottom graticule. To determine % AM of your signal
count each y-axis division as 10%.
145
Demodulating AM Signals
Measuring the Modulation Index of an AM Signal
Step
Figure 10-3
Action
Notes
AM Signal Measured in the Time Domain
LEFT: 100% AM Signal (Modulation Index = 1)
RIGHT: 80% AM Signal (Modulation Index = 0.8)
146
IQ Analyzer Measurement
11 IQ Analyzer Measurement
147
IQ Analyzer Measurement
Capturing Wideband Signals for Further Analysis
Capturing Wideband Signals for Further Analysis
This section demonstrates how to capture complex time domain data from wide
bandwidth RF signals. This mode preserves the instantaneous vector relationships of
time, frequency, phase and amplitude contained within the selected digitizer span or
analysis BW, at the analyzer's center frequency, for output as IQ data. This IQ data
can then be utilized internally or output over LAN, USB or GPIB for use with
external analysis tools. Each measurement description specifies the types of data
available remotely for that measurement.
The standard 10 MHz Analysis BW and optional 25 MHz Analysis BW digitizers
used to capture the wide bandwidth RF signals can be accessed from the front panel
in IQ Analyzer (Basic) mode. This IQ Analyzer mode provides basic setup, RF (FFT
based) and IQ analysis tools
Within the IQ Analyzer mode, basic frequency domain, time domain and IQ
measurements are available as initial signal and data verification tools in preparation
for deriving the IQ data output.
The Complex Spectrum measurement provides a display in the upper window of
power versus frequency with current (yellow trace) and average (blue trace) data. In
addition, an IQ waveform of voltage versus time is provided in the lower window.
The IQ Waveform measurement provides a time domain view of the RF signal
envelope with power versus time or an IQ waveform of voltage versus time.
148
IQ Analyzer Measurement
Complex Spectrum Measurement
Complex Spectrum Measurement
This section explains how to make a waveform (time domain) measurement on a
W-CDMA signal. (A signal generator is used to simulate a base station.) The
measurement of I and Q modulated waveforms in the time domain disclose the
voltages which comprise the complex modulated waveform of a digital signal.
Step
Action
Notes
1 Set up the signal source.
a. Set the mode to W-CDMA
3GPP with 4 carriers.
b. Set the frequency of the
signal source to 1.0 GHz.
c. Set the source amplitude to
-10 dBm.
2 Connect the source RF
OUTPUT to the analyzer
RF INPUT as shown.
3 Select the mode.
•
Press Mode, IQ Analyzer
(Basic).
4 Preset the analyzer.
•
Press Mode Preset
5 Set the measurement center
frequency.
•
Press Freq Channel, 1,
GHz.
6 Set the measurement
span/analysis bandwidth.
•
Press Span X Scale, 10,
MHz (25 MHz if option
B25 installed).
7 Enable the Complex
Spectrum measurement.
•
Press Meas, Complex
Spectrum.
149
Refer to the default view in Figure
11-1 or Figure 11-2.
IQ Analyzer Measurement
Complex Spectrum Measurement
Step
Action
Notes
Figure 11-1
Spectrum and I/Q Waveform (Span 10 MHz)
Figure 11-2
Spectrum and I/Q Waveform (Span 25 MHz)
NOTE
A display with both an FFT derived spectrum in the upper window and an IQ Waveform in
the lower window will appear when you activate a Complex Spectrum measurement. The
active window is outlined in green. Changes to Frequency, Span or Amplitude will affect
only the active window. Use the Next Window key to select a different window, and Zoom
key to enlarge the window.
150
IQ Analyzer Measurement
IQ Waveform (Time Domain) Measurement
IQ Waveform (Time Domain) Measurement
This section explains how to make a waveform (time domain) measurement on a
W-CDMA signal. (A signal generator is used to simulate a base station.) The
measurement of I and Q modulated waveforms in the time domain disclose the
voltages which comprise the complex modulated waveform of a digital signal.
Step
Action
Notes
1 Set up the signal source.
a. Set the mode to
W-CDMA 3GPP with 4
carriers.
b. Set the frequency of the
signal source to 1.0 GHz.
c. Set the source amplitude
to -10 dBm.
2 Connect the source RF
OUTPUT to the analyzer
RF INPUT as shown.
3 Select the mode.
•
Press Mode, IQ
Analyzer (Basic).
4 Preset the analyzer.
•
Press Mode Preset.
5 Set the measurement
center frequency.
•
Press Freq Channel, 1,
GHz.
6 Set the measurement
span/analysis bandwidth.
•
Press Span X Scale, 10,
MHz (25 MHz if option
B25 installed).
7 Enable the IQ Waveform
measurement.
•
Press Meas, IQ
Waveform.
8 View the RF envelope.
•
Press View/Display, RF
Envelope.
151
IQ Analyzer Measurement
IQ Waveform (Time Domain) Measurement
Step
Action
Notes
9 Set the analysis
bandwidth.
•
This view provides a waveform display
of power versus time of the RF signal in
the upper window with metrics for mean
and peak-to-mean in the lower window.
Refer to Figure 11-3 or Figure 11-4.
Press BW, Info BW, 10,
MHz (25 MHz if option
B25 installed).
Figure 11-3
IQ Waveform Measurement - Time domain View (10 MHz BW)
Figure 11-4
IQ Waveform Measurement - Time domain View (25 MHz BW)
152
IQ Analyzer Measurement
IQ Waveform (Time Domain) Measurement
Step
Action
10 View the IQ Waveform.
•
Press View/Display, IQ
Waveform.
11 Set the time scale.
•
Press Span X Scale,
Scale/Div, 100, ns.
12 Enable markers.
•
Press Marker,
Properties, Marker
Trace, IQ Waveform,
500, ns.
Figure 11-5
Notes
This view provides a display of voltage
versus time for the I and Q waveforms.
Markers enable measurement of the
individual values of I and Q. Refer to
Figure 11-5.
IQ Waveform Measurement - with Markers
153
IQ Analyzer Measurement
IQ Waveform (Time Domain) Measurement
154
Using Option BBA Baseband I/Q Inputs
12 Using Option BBA
Baseband I/Q Inputs
155
Using Option BBA Baseband I/Q Inputs
Baseband I/Q measurements available for X-Series Signal Analyzers
Baseband I/Q measurements available for X-Series Signal Analyzers
The following table shows the measurements that can be made using Baseband I/Q
inputs:
Table 12-1
BBIQ Supported Measurements vs. Mode
Mode
Measurements
GSM
IQ Waveform
GMSK Phase & Freq
EDGE EVM
802.16 OFDMA
IQ Waveform
Power Stat CCDF
Modulation Analysis
TD-SCDMA
IQ Waveform
Power Stat CCDF
Code Domain
Mod Accuracy
Cdma2000
IQ Waveform
Power Stat CCDF
Code Domain
Mod Accuracy
QPSK EVM
IQ Analyzer
(Basic)
IQ Waveform
Complex Spectrum
156
Using Option BBA Baseband I/Q Inputs
Baseband I/Q measurement overview
Baseband I/Q measurement overview
The Baseband I/Q functionality is a hardware option, Option BBA. If the option is
not installed in the instrument, the I/Q functionality cannot be enabled.
The Baseband I/Q option provides four input ports and one Calibration Output port.
The input ports are I, I-bar, Q, and Q-bar. The I and I-bar together compose the I
channel and the Q and Q-bar together compose the Q channel. Each channel has two
modes of operation:
Mode
Description
Single Ended
In this mode, only the main port (I or Q) is used and the
complementary ports (I-bar or Q-bar) is ignored. The I and Q
ports are in single-ended mode when Differential “Off” is
selected.
(unbalanced)
Differential
(balanced)
In this mode, both main and complementary ports are used. To
activate this mode, select Differential “On” from the I and Q
Setup softkey menus.
The system supports a variety of input passive probes as well as the Agilent 1153A
active differential probe using the InfiniMax probe interface.
NOTE
To avoid duplication, this section describes only the details unique to using the
baseband I/Q inputs. For generic measurement details, refer to the previous “Making
Measurements” sections.
To make measurements using baseband I/Q Inputs, make the following selections:
Step
Notes
1. Select measurement.
Select a measurement that supports baseband I/Q
inputs.
2. Select the I/Q Path
Press Input/Output, I/Q, I/Q Path.
Select from the choices present on the screen. The path
selected is shown at the top of the measurement screen.
3. Select the
appropriate circuit
location and
probe(s) for
measurements.
For details see ““Selecting Input Probes for Baseband
Measurements” on page 153” in the Concepts chapter.
4. Select baseband I/Q
input connectors.
157
Using Option BBA Baseband I/Q Inputs
Baseband I/Q measurement overview
Step
Notes
5. Set up the I Path (if
required).
If you have set the I/Q Path to I+jQ or to I Only, press
I Setup.
a. Select whether Differential (Balanced) inputs is
On or Off.
b. Select the input impedance, Input Z.
c. Input a Skew value in seconds.
d. Set up the I Probe by pressing I Probe
i. Select probe Attenuation
ii. Calibrate the probe. Press Calibrate... to start
the calibration procedure. Follow the calibration
procedure, clicking Next at the end of each step.
5. Set up the Q Path (if
required).
If you have set the I/Q Path to I+jQ or to Q Only,
press Q Setup.
a. Select whether Differential (Balanced) inputs is
On or Off.
b. Select the input impedance, Input Z.
c. Input a Skew value in seconds.
d. Set up the Q Probe by pressing Q Probe
i. Select probe Attenuation
ii. Calibrate the probe. Press Calibrate... to start
the calibration procedure. Follow the calibration
procedure, clicking Next at the end of each step.
5. Select the reference
impedance.
Press Reference Z, then input a value from one ohm to
one megohm.
The impedance selected is shown at the top of the
measurement screen.
6. Calibrate the cable
(if required).
If you using cables that were not calibrated in the
probe calibration step, press I/Q Cable Calibrate...
Follow the calibration procedure, clicking Next at the
end of each step.
7. Make the desired
measurement.
158
Option EXM External Mixing
13 Option EXM External
Mixing
159
Option EXM External Mixing
Using Option EXM with the Agilent 11970 Series Mixers.
Using Option EXM with the Agilent 11970 Series Mixers.
The following examples explain how to, connect the external mixers to the signal
analyzer using a diplexer, choose the band of interest, activate conversion loss
correction data, and how to use the signal-identification functions.
Step
Action
Notes
1 Set up the equipment
•
Connect the signal source,
diplexer, and harmonic mixer to
the signal analyzer as shown:
2 Set the analyzer to the
Spectrum Analyzer mode
•
Press Mode, Spectrum Analyzer
3 Preset the analyzer
•
Press Mode Preset.
4 Select external mixing.
•
Press Input/Output, External
Mixer, External Mixer,
External Mixer Setup
5 To select Q, U, V or W
bands
a. Press Mixer Presets, Agilent
11970.
b. Select the appropriate key
corresponding to 11970 frequency
band.
6 View the spectrum display
•
Press Return twice.
160
The default is 11970A
Option EXM External Mixing
Using Option EXM with the Agilent 11970 Series Mixers.
Step
Action
7 Tune the analyzer to the
input signal frequency
a. Press FREQ Channel.
8 Turn on the Signal ID
function
Notes
b. Enter a center frequency or a start
and stop frequency
•
Press Input Output, External
Mixing, Signal ID Mode, Image
Suppress, Signal ID On.
This enables you to identify
true signals from images and
harmonics.
See “Signal ID” on page 165
for more information.
Amplitude calibration
See “Loading conversion loss data for the PXA Signal Analyzer” on page 161. This
will guide you through entering the conversion loss data provided with the mixer,
into a corrections file that can be activated to provide amplitude corrected
measurements.
Action
Notes
1 To access corrections, press
Input/Output, More, Corrections,
Select Correction.
NOTE
2 Choose a correction from the list.
Note that you used one of the
Corrections numbered 1-6 when you
entered the conversion loss data into
the instrument.
3 Press Correction On.
This applies the corrections to the
measurement.
Assure only ONE correction file is turned on because it is possible to turn on
multiple correction files, and if some of the files share the same frequency points, the
correction that results for those shared frequency points will cause measurement
errors. Therefore assure that only the correction file required for the measurement is
turned on, and turn off all other corrections.
Loading conversion loss data for the PXA Signal Analyzer
The conversion loss data supplied with your mixer can be loaded into your signal
analyzer from one of two sources:
161
Option EXM External Mixing
Using Option EXM with the Agilent 11970 Series Mixers.
•
By downloading the 70xxxxxx_X.csv file located on the CD ROM disk that is
provided with your mixer. The 70xxxxxx_X.csv file is transferred from the CD to
the USB memory stick (also provided), and then the USB memory stick inserted
into the PXA, and the file transferred to one of the analyzer corrections array
locations.
•
The mixer ships with a printed copy of the conversion loss data. Find the printed
copy conversion loss data that has the text "For Use with Agilent X-Series
analyzers only". The conversion loss data will need to be manually entered as
frequency and amplitude pairs into the analyzer corrections file. An example of
calibration data is shown in Figure 13-1 on page 166.
The CD also contains 70xxxxxx_X.pdf files of the conversion loss data that can
be printed, and then manually entered into a Correction array location. You will
need to enter the frequency and amplitude pairs into the analyzer corrections file.
An example of calibration data is shown in Figure 13-1 on page 166.
Down loading the conversion loss .csv files to the analyzer corrections array
Action
Notes
1 Install the CD ROM provided
with the mixer, into a PC.
You can view the contents of the CD.
2 Locate the 70xxxxxx_X.csv file.
3 Copy the .csv file to the USB
memory stick provided with the
mixer.
4 Insert the USB memory stick into
one of the USB ports on the signal
analyzer.
It is recommended that you connect a
mouse and keyboard to the signal
analyzer.
5 Press Input/Output, More,
Corrections, Select Correction.
6 Choose a correction array from
the list of Correction 1 through
Correction 6.
Correction 1 has a provision to store
antenna corrections, so if antenna
corrections are required, reserve this
array for that use.
7 To see if anything is already
stored in a particular correction,
press Correction, Edit.
Selecting Edit turns the correction ON.
Be sure to turn the correction OFF after
determining the contents of the
correction array.
8 To delete a correction table, press
Return, assure the Select
Correction key corresponds to
the correction you want to delete,
and press Delete Correction.
162
Option EXM External Mixing
Using Option EXM with the Agilent 11970 Series Mixers.
Action
Notes
9 Once a correction array number is
selected, press Recall, Data
(Import), Amplitude
Correction, and choose one of the
correction array numbers.
10 Press Open, use the pull down
arrow in the "look in" box to
navigate to the USB memory
stick, and locate the
70xxxxxx_X.csv file.
11 Select the 70xxxxxx_X.csv file
and click Open.
The conversion loss data will now load
into the specified corrections array.
12 To view the contents of the
corrections array in the conversion
loss table, press Input/Output,
More, Corrections, select the
corrections array number, and
press Edit.
13 Press Return to go back to the
measurement screen.
NOTE
Loading the .csv file automatically populates the Description and Comment fields
found under the Corrections, Properties key. To edit these fields, press Input/
Output, More, Corrections, Select Correction, select the correction number, press
Properties, Description or Comment.
163
Option EXM External Mixing
Using Option EXM with the Agilent 11970 Series Mixers.
Manually entering conversion loss data
Action
Notes
1 Locate the printed copy of the
conversion loss data that has the
text "For use with Agilent
X-Series analyzers only".
Or
Insert the CD provided with the
mixer into a PC and navigate to
the 70xxxxxx_X.pdf file.
The file contains tabular and graphic
conversion loss data. Be careful to
select the correct file since there are
three files provided for the 11970A, Q
and V band mixers. Print the
70xxxxxx_X.pdf file to create a printed
copy.
2 Press Input/Output, More,
Corrections, Select Correction.
3 Choose a correction array from
the list of Correction 1 through
Correction 6.
Correction 1 has a provision to store
antenna corrections, so if antenna
corrections are required, reserve this
array for that use.
4 To see if anything is already
stored in a particular correction,
press Correction, Edit.
Selecting Edit turns the correction ON.
Be sure to turn the correction OFF after
determining the contents of the
correction array.
5 To delete a correction table, press
Return, assure the Select
Correction key corresponds to
the correction you want to delete,
and press Delete Correction.
6 Select the correction number and
press Edit.
When finished press Return.
NOTE
Use the keys provided to enter the
frequency and amplitude (conversion
loss) points from the calibration data
table. Conversion loss values are
entered as positive numbers.
It is possible to add a description and a comment of what the selected correction is,
and have this description appear on the Description or Comment key. Press
Properties, and connect a keyboard to the instrument. For example, press Comment
and type 11970V Serial XXxxxxxxxx. Press Done.
164
Option EXM External Mixing
Using Option EXM with the Agilent 11970 Series Mixers.
Signal ID
Image Suppress
The Image Suppress mode of Signal ID mathematically removes all image and
multiple responses of signals present at the mixer input. Two hidden sweeps are
taken in succession. The second sweep is offset in LO frequency by 2*IF/N. For each
point in each trace, the smaller amplitude from the two traces is taken and placed in
that point in Trace 1. Responses of each trace that lie on top of one another will
remain and are valid signals, others are images and are suppressed.
NOTE
This function takes control of and uses Trace 1. Any data in this trace prior to
activating Image Suppress will be lost.
In Image Suppress Mode, synchronization is ensured by first turning off Signal ID,
initiating a single sweep, then turning on Signal ID followed by two single sweeps.
Image shift
Like the Image Suppress mode, Image Shift is a two sweep sequence. The data from
the first sweep is placed in Trace 1 and the data from the second (LO frequency
shifted by 2*IF/N) sweep is placed in Trace 2. Signal responses of Trace 1 and Trace
2 that have the same horizontal position are considered to be in the current band and
therefore can be analyzed with the amplitude and frequency measurement systems of
the SA. All other responses are invalid and should be ignored.
NOTE
This function takes control of and uses Trace 1 and Trace 2. Any data in these traces
prior to activating Image Shift will be lost.
To synchronize in Image Shift Mode, turn off Signal ID and then initiate a single
sweep. Then turn on Signal ID and initiate two single sweeps. The results of the first
sweep after Signal ID is turned on are available in Trace 1. The next sweep is shifted
and the data from that sweep is available in Trace 2. The unshifted and shifted data
can then be compared.
165
Option EXM External Mixing
Using Option EXM with the Agilent 11970 Series Mixers.
Figure 13-1
166
Option EXM External Mixing
Using the M1970 Series Mixers with X-Series Signal Analyzers (Option EXM)
Using the M1970 Series Mixers with X-Series Signal Analyzers (Option
EXM)
Operating precautions
WARNING
Do not exceed the maximum ratings listed below or permanent damage to the
mixer will result.
RF input power
Use the following parameters:
CW
No greater than 20 dBm
Peak Pulse
No greater than 24 dBm at <1 μsec
See the gain compression values for each model mixer. Use an appropriate
waveguide attenuator if the output power of the unit under test exceeds the gain
compression value.
LO input power
LO input power is set by the internal signal analyzer LO alignment. However, the
LO input must not exceed 20 dBm.
Always use a high quality low loss SMA cable since this will allow the longest cable
length, and prevent damage to the mixer SMA female connector.
Electrostatic discharge
When installing the mixer, always connect the LO/IF SMA cable to the signal
analyzer BEFORE connecting to the mixer. This will minimize the danger of an
electrostatic discharge damaging the mixer diodes.
Connect only one mixer at a time
The automatic LO adjustment and mixer ID process assumes only one mixer is
connected to the analyzer's USB and SMA EXT MIXER connections.
Waveguide protection foam
Do not remove, displace or damage the nonconductive foam installed in the open end
of the waveguide. This foam keeps small objects from entering the waveguide.
Provide mechanical support for the mixer
Assure the mixer body is properly supported so it does not present any stress on the
waveguide connection. Instrument option 301 provides a jackstand that allows a
stable support and height adjustment.
167
Option EXM External Mixing
Using the M1970 Series Mixers with X-Series Signal Analyzers (Option EXM)
Avoiding waveguide flange damage
Install the waveguide flange cap whenever the mixer is not connected to a device
under test. This will protect the waveguide flange mating surface.
Mixer waveguide connections
Assure the shoulder of the mixer waveguide flange is properly aligned with the
flange of the device under test. To ensure proper mating, it is important to tighten all
screws equally. To do this, tighten opposed screws in pairs by a small amount until
all are snug. Final torque must not exceed 7 in pounds.
For additional information regarding use with a particular X-Series analyzer, refer to
that analyzer's User's and Programmer's Reference Guide.
Equipment set up
The following examples explain how to connect the external mixers to the signal
analyzer, and how to use the signal-identification functions.
Step
Action
Notes
1. SMA
connection
a. Connect an SMA
cable from the
mixer to the
spectrum analyzer
EXT MIXER front
panel SMA.
The torque for the SMA cables and adapters should not
exceed 8 in-lbs.
a. Connect a USB
cable from the
mixer to the
spectrum analyzer.
When a connection is made, the green LED on the
mixer lights up indicating that the mixer has power and
the processor inside the mixer is running. The
spectrum analyzer is automatically switched to
external mixing mode, and the mixer model number
and mixer option are used to set the start and stop
frequencies for the mixing band.
2. USB
connection
The LO/IF connection between the mixer and the
signal analyzer must be in place before the USB cable
from the mixer is connected. Connecting the USB
cable automatically triggers the LO adjustment, and if
the LO/IF cable is not connected the adjustment will
not complete and an error will occur.
168
Option EXM External Mixing
Using the M1970 Series Mixers with X-Series Signal Analyzers (Option EXM)
The spectrum analyzer performs an LO alignment using a detector in the mixer to set
the LO Power. During the alignment the yellow Busy LED turns on. Once the
alignment completes, the mixer is ready to make calibrated measurements since the
conversion loss values stored in the mixer are automatically loaded to the signal
analyzer.
Operation
Step
Action
Notes
1. Tune the analyzer
a. Apply a signal to the
mixer input.
.
b. Press FREQ Channel
and enter a center
frequency or a start and
stop frequency
Multiple responses may appear on
screen. Most of these responses are
images or multiples of the mixing
process and not true signals.
a. Press Input/Output,
External Mixer, Signal
ID to On, Signal ID
Mode, Image Suppress.
This enables you to identify true
signals from images and
harmonics. Refer to the following
graphic.
2. Turn on the signal
identification function.
169
Option EXM External Mixing
Using the M1970 Series Mixers with X-Series Signal Analyzers (Option EXM)
Step
Action
Notes
3. With wide spans, increasing
the number of sweep points may
improve the effectiveness of
image suppressing.
a. Press Sweep/Control,
Points, enter the number,
and press Enter.
Amplitude calibration
The M1970 Series mixers automatically download a conversion loss file to the signal
analyzer, and this conversion loss table is used to correct the measured amplitude. It
is not possible to view or edit the conversion loss file. The Certificate of Calibration
that is provided with each mixer lists the conversion loss measured at the factory.
LO adjustment
The LO adjustment will run each time the USB connection is made, and if the signal
analyzer Alignments are set to Normal, the LO adjustment will be performed based
on time and temperature rules similar to the other signal analyzer alignments. Both
the mixer and the spectrum analyzer temperature sensors are monitored by the
alignment algorithm. If the Alignment is set to OFF, the LO Alignment will not be
performed based on time and temperature, and it will be necessary for the user to
determine when to perform alignments.
To manually trigger an LO alignment from the front panel, press System,
Alignments, Align Now, External Mixer.
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Option EXM External Mixing
Using the M1970 Series Mixers with X-Series Signal Analyzers (Option EXM)
Viewing the external mixer setup screen
The mixer setup screen contains information such as mixer serial and model number,
harmonic mixing used, and the status of the mixer connection. When the USB mixer
is connected, the Mixer Preset, Mixer Bias and Edit Harmonic Table keys are grayed
out since these keys do not apply. However, if you unplug the USB cable, the Mixer
Presets key will become functional and the connection status will show USB Mixer
not connected.
To view the setup screen, press Input /Output, External Mixer, Ext Mix Setup.
171
Option EXM External Mixing
Using the M1970 Series Mixers with X-Series Signal Analyzers (Option EXM)
172
Concepts
14 Concepts
173
Concepts
Resolving Closely Spaced Signals
Resolving Closely Spaced Signals
Resolving signals of equal amplitude
Two equal-amplitude input signals that are close in frequency can appear as a single
signal trace on the analyzer display. Responding to a single-frequency signal, a
swept-tuned analyzer traces out the shape of the selected internal IF (intermediate
frequency) filter (typically referred to as the resolution bandwidth or RBW filter). As
you change the filter bandwidth, you change the width of the displayed response. If a
wide filter is used and two equal-amplitude input signals are close enough in
frequency, then the two signals will appear as one signal. If a narrow enough filter is
used, the two input signals can be discriminated and appear as separate peaks. Thus,
signal resolution is determined by the IF filters inside the analyzer.
The bandwidth of the IF filter tells us how close together equal amplitude signals can
be and still be distinguished from each other. The resolution bandwidth function
selects an IF filter setting for a measurement. Typically, resolution bandwidth is
defined as the 3 dB bandwidth of the filter. However, resolution bandwidth may also
be defined as the 6 dB or impulse bandwidth of the filter.
Generally, to resolve two signals of equal amplitude, the resolution bandwidth must
be less than or equal to the frequency separation of the two signals. If the bandwidth
is equal to the separation and the video bandwidth is less than the resolution
bandwidth, a dip of approximately 3 dB is seen between the peaks of the two equal
signals, and it is clear that more than one signal is present.
For Signal Analyzers in swept mode, sweep time is automatically set to a value that
is inversely proportional to the square of the resolution bandwidth (1/BW2),to keep
the analyzer measurement calibrated. So, if the resolution bandwidth is reduced by a
factor of 10, the sweep time is increased by a factor of 100 when sweep time and
bandwidth settings are coupled. For the shortest measurement times, use the widest
resolution bandwidth that still permits discrimination of all desired signals. Sweep
time is also a function of which detector is in use, peak and normal detectors sweep
as fast or more quickly than sample or average detectors. The analyzer allows RBW
selections up to 8 MHz in 1, 3, 10 steps and it has the flexibility to fine tune RBWs in
increments of 10% for a total of 160 RBW settings.
For best sweep times and keeping the analyzer calibrated set the sweep time
(Sweep/Control, Sweep Time) to Auto, and the sweep type (Sweep/Control,
Sweep Setup, Sweep Type) to Auto. Use the widest resolution bandwidth and the
narrowest span that still permits resolution of all desired signals.
Resolving small signals hidden by large signals
When dealing with the resolution of signals that are close together and not equal in
amplitude, you must consider the shape of the IF filter of the analyzer, as well as its
3 dB bandwidth. (See “Resolving signals of equal amplitude” on page 174 for more
information.) The shape of a filter is defined by the selectivity, which is the ratio of
the 60 dB bandwidth to the 3 dB bandwidth. If a small signal is too close to a larger
signal, the smaller signal can be hidden by the skirt of the larger signal.
174
Concepts
Resolving Closely Spaced Signals
To view the smaller signal, select a resolution bandwidth such that k is less than a
(see Figure 14-1). The separation between the two signals (a) must be greater than
half the filter width of the larger signal (k), measured at the amplitude level of the
smaller signal.
The digital filters in the analyzer have filter widths about one-third as wide as typical
analog RBW filters. This enables you to resolve close signals with a wider RBW (for
a faster sweep time).
Figure 14-1
RBW Requirements for Resolving Small Signals
175
Concepts
Trigger Concepts
Trigger Concepts
NOTE
The trigger functions let you select the trigger settings for a sweep or measurement.
When using a trigger source other than Free Run, the analyzer will begin a sweep
only with the selected trigger conditions are met. A trigger event is defined as the
point at which your trigger source signal meets the specified trigger level and
polarity requirements (if any). In FFT measurements, the trigger controls when the
data is acquired for FFT conversion.
Selecting a trigger
1. Free Run Triggering
Pressing this key, when it is not selected, selects free-run triggering. Free run
triggering occurs immediately after the sweep/measurement is initiated.
Press Trigger, Free Run
2. Video Triggering
The Video trigger condition is met when the video signal (the filtered and
detected version of the input signal, including both RBW and VBW filtering)
crosses the video trigger level. Video triggering triggers the measurement at the
point at which the rising signal crosses the video trigger horizontal green line on
the display:
Press Trigger, Video, Video, Trigger Level, −30, dBm. (If Video trigger has not
already been selected, you need to press Video a second time to get to the Trigger
Level function.)
3. External Triggering
In the event that you have an external trigger available that can be used to
synchronize with the burst of interest, connect the trigger signal to the rear of the
analyzer using either the Trigger 1 In or Trigger 2 In input connector. It
might be necessary to adjust the trigger level by rotating the front panel knob or
by entering the numeric value on the keypad.
Press Trigger, External 1 or External 2, Trigger Level, and adjust as necessary.
4. RF Burst Wideband Triggering
RF burst triggering occurs in the IF circuitry chain, as opposed to after the video
detection circuitry with video triggering. In the event video triggering is used, the
detection filters are limited to the maximum width of the resolution bandwidth
filters. Set the analyzer in RF burst trigger mode.
Press Trigger, RF Burst.
5. Line Triggering
176
Concepts
Trigger Concepts
Line triggering selects the line signal as the trigger. A new sweep/measurement
will start synchronized with the next cycle of the line voltage. Pressing this key,
when it is already selected, access the line trigger setup menu.
Press Trigger, Line.
6. Periodic Timer Triggering
This feature selects the internal periodic timer signal as the trigger. Trigger
occurrences are set by the Periodic Timer parameter, which is modified by the
Sync Source and Offset. Figure 14-2 shows the action of the periodic timer
trigger. Before reviewing the figure, we'll explain some uses for the periodic
trigger.
A common application is measuring periodic burst RF signals for which a trigger
signal is not easily available. For example, we might be measuring a TDMA
radio which bursts every 20 ms. Let's assume that the 20 ms period is very
consistent. Let's also assume that we do not have an external trigger source
available that is synchronized with the period, and that the signal-to-noise ratio of
the signal is not high enough to provide a clean RF burst trigger at all of the
analysis frequencies. For example, we might want to measure spurious
transmissions at an offset from the carrier that is larger than the bandwidth of the
RF burst trigger. In this application, we can set the Periodic Timer to a 20.00 ms
period and adjust the offset from that timer to position our trigger just where we
want it. If we find that the 20.00 ms is not exactly right, we can adjust the period
slightly to minimize the drift between the period timer and the signal to be
measured.
A second way to use this feature would be to use Sync Source temporarily,
instead of Offset. In this case, we might tune to the signal in a narrow span and
use the RF Burst trigger to synchronize the periodic timer. Then we would turn
the sync source off so that it would not mistrigger. Mistriggering can occur when
we are tuned so far away from the RF burst trigger that it is no longer reliable.
A third example would be to synchronize to a signal that has a reference time
element of much longer period than the period of interest. In some CDMA
applications, it is useful to look at signals with a short periodicity, by
synchronizing that periodicity to the “even-second clock” edge that happens
every two seconds. Thus, we could connect the even-second clock trigger to Ext1
and use then Ext1 as the sync source for the periodic timer.
The figure below illustrates this third example. The top trace represents the
even-second clock. It causes the periodic timer to synchronize with the leading
edge shown. The analyzer trigger occurs at a time delayed by the accumulated
offset from the period trigger event. The periodic timer continues to run, and
triggers continue to occur, with a periodicity determined by the analyzer time
base. The timer output (labeled “late event”) will drift away from its ideal time
due to imperfect matching between the time base of the signal being measured
and the time base of the analyzer, and also because of imperfect setting of the
period parameter. But the synchronization is restored on the next even-second
clock event. (“Accumulated offset” is described in the in the Offset function
section.)
177
Concepts
Trigger Concepts
Figure 14-2
Frame Triggering
a. Period
Sets the period of the internal periodic timer clock. For digital
communications signals, this is usually set to the frame period of your current
input signal. In the case that sync source is not set to OFF, and the external
sync source rate is changed for some reason, the periodic timer is
synchronized at the every external synchronization pulse by resetting the
internal state of the timer circuit.
Press Trigger, Periodic Timer.
b. Offset
Adjusts the accumulated offset between the periodic timer events and the
trigger event. Adjusting the accumulated offset is different than setting an
offset, and requires explanation.
The periodic timer is usually unsynchronized with any external events, so the
timing of its output events has no absolute meaning. Since the timing relative
to external events (RF signals) is important, you need to be able to adjust
(offset) it. However, you have no direct way to see when the periodic timer
events occur. All that you can see is the trigger timing. When you want to
adjust the trigger timing, you will be changing the internal offset between the
periodic timer events and the trigger event. Because the absolute value of that
internal offset is unknown, we will just call that the accumulated offset.
Whenever the Offset parameter is changed, you are changing that
accumulated offset. You can reset the displayed offset using Reset Offset
Display. Changing the display does not change the value of the accumulated
offset, and you can still make additional changes to accumulated offset.
Press Trigger, Periodic Timer.
c. Reset Offset Display
Resets the value of the periodic trigger offset display setting to 0.0 seconds.
The current displayed trigger location may include an offset value defined
with the Offset key. Pressing this key redefines the currently displayed trigger
location as the new trigger point that is 0.0 s offset. The Offset key can then
be used to add offset relative to this new timing
Press Trigger, Periodic Timer.
178
Concepts
Time Gating Concepts
Time Gating Concepts
Introduction: Using Time Gating on a Simplified Digital Radio Signal
This section shows you the concepts of using time gating on a simplified digital
radio signal. The section on Making Time-Gated Measurements demonstrates time
gating examples.
Figure 14-3 shows a signal with two radios, radio 1 and radio 2, that are time-sharing
a single frequency channel. Radio 1 transmits for 1 ms then radio 2 transmits for
1 ms.
Figure 14-3
Simplified Digital Mobile-Radio Signal in Time Domain
We want to measure the unique frequency spectrum of each transmitter.
A signal analyzer without time gating cannot do this. By the time the signal analyzer
has completed its measurement sweep, which lasts about 50 ms, the radio
transmissions switch back and forth 25 times. Because the radios are both
transmitting at the same frequency, their frequency spectra overlap, as shown in
Figure 14-4 The signal analyzer shows the combined spectrum; you cannot tell
which part of the spectrum results from which signal.
Figure 14-4
Frequency Spectra of the Combined Radio Signals
179
Concepts
Time Gating Concepts
Time gating allows you to see the separate spectrum of radio 1 or radio 2 to
determine the source of the spurious signal, as shown in Figure 14-5
Figure 14-5
Time-Gated Spectrum of Radio 1
Figure 14-6
Time-Gated Spectrum of Radio 2
Time gating lets you define a time window (or time gate) of when a measurement is
performed. This lets you specify the part of a signal that you want to measure, and
exclude or mask other signals that might interfere.
180
Concepts
Time Gating Concepts
How time gating works
Time gating is achieved by the signal analyzer selectively interrupting the path of the
detected signal, with a gate, as shown in Figure 14-9 and Figure 14-8 The gate
determines the times at which it captures measurement data (when the gate is turned
“on,” under the Gate menu, the signal is being passed, otherwise when the gate is
“off,” the signal is being blocked). Under the right conditions, the only signals that
the analyzer measures are those present at the input to the analyzer when the gate is
on. With the correct signal analyzer settings, all other signals are masked out.
There are typically two main types of gating conditions, edge and level:
•
With edge gating, the gate timing is controlled by user parameters (gate delay and
gate length) following the selected (rising or falling) edge of the trigger signal.
The gate passes a signal on the edge of the trigger signal (after the gate delay time
has been met) and blocks the signal at the end of the gate length.
With edge gating, the gate control signal is usually an external periodic TTL
signal that rises and falls in synchronization with the rise and fall of the pulsed
radio signal. The gate delay is the time the analyzer waits after the trigger event
to enable the gate (see Figure 14-7).
•
Figure 14-7
With level gating, the gate will pass a signal when the gate signal meets the
specified level (high or low). The gate blocks the signal when the level conditions
are no longer satisfied (level gating does not use gate length or gate delay
parameters).
Edge Trigger Timing Relationships
With Agilent signal analyzers, there are three different implementations for time
gating: gated LO, gated video and gated FFT.
181
Concepts
Time Gating Concepts
Gated video concepts
Gated video may be thought of as a simple gate switch, which connects the signal to
the input of the signal analyzer. When the gate is “on” (under the Gate menu) the
gate is passing a signal. When the gate is “off,” the gate is blocking the signal.
Whenever the gate is passing a signal, the analyzer sees the signal. In Figure 14-8
notice that the gate is placed after the envelope detector and before the video
bandwidth filter in the IF path (hence “gated video”).
The RF section of the signal analyzer responds to the signal. The selective gating
occurs before the video processing. This means that there are some limitations on the
gate settings because of signal response times in the RF signal path.
With video gating the analyzer is continually sweeping, independent of the position
and length of the gate. The analyzer must be swept at a minimum sweep time (see the
sweep time calculations later in this chapter) to capture the signal when the gate is
passing a signal. Because of this, video gating is typically slower than gated LO and
gated FFT.
Figure 14-8
Gated Video Signal Analyzer Block Diagram
Gated LO Concepts
Gated LO is a very sophisticated type of time gating that sweeps the LO only while
the gate is “on” and the gate is passing a signal. See Figure 14-9 for a simplified
block diagram of gated LO operation. Notice that the gate control signal controls
when the scan generator is sweeping and when the gate passes or blocks a signal.
This allows the analyzer to sweep only during the periods when the gate passes a
signal. Gated LO is faster than Gated Video because Gated Video must constrain
sweep time so that each point is long enough to include a burst event. On the other
hand, when in Gated LO, multiple points may be swept during each gate.
182
Concepts
Time Gating Concepts
Figure 14-9
Gated LO Signal Analyzer Block Diagram
Gated FFT Concepts
Gated FFT (Fast-Fourier Transform) is an FFT measurement which begins when the
trigger conditions are satisfied.
The process of making a spectrum measurement with FFTs is inherently a “gated”
process, in that the spectrum is computed from a time record of short duration, much
like a gate signal in swept-gated analysis.
Using the analyzer in FFT mode, the duration of the time record to be gated is:
1.83
FFT Time Record (to be gated) = -----------RBW
The duration of the time record is within a tolerance of approximately 3% for
resolution bandwidths up through 1 MHz. Unlike swept gated analysis, the duration
of the analysis in gated FFT is fixed by the RBW, not by the gate signal. Because
FFT analysis is faster than swept analysis (up to 7.99 MHz), the gated FFT
measurements can have better frequency resolution (a narrower RBW) than swept
analysis for a given duration of the signal to be analyzed.
183
Concepts
Time Gating Concepts
Figure 14-10
Gated FFT Timing Diagram
Time gating basics (Gated LO and Gated Video)
The gate passes or blocks a signal with the following conditions:
•
Trigger condition - Usually an external transistor-transistor logic (TTL) periodic
signal for edge triggering and a high/low TTL signal for level triggering.
•
Gate delay - The time after the trigger condition is met when the gate begins to
pass a signal.
•
Gate length - The gate length setting determines the length of time a gate begins
to pass a signal.
To understand time gating better, consider a spectrum measurement performed on
two pulsed-RF signals sharing the same frequency spectrum. You will need to
consider the timing interaction of three signals with this example:
•
The composite of the two pulsed-RF signals.
•
The gate trigger signal (a periodic TTL level signal).
184
Concepts
Time Gating Concepts
•
The gate signal. This TTL signal is low when the gate is “off” (blocking) and
high when the gate is “on” (passing).
The timing interactions between the three signals are best understood if you observe
them in the time domain (see Figure 14-11).
The main goal is to measure the spectrum of signal 1 and determine if it has any
low-level modulation or spurious signals.
Because the pulse trains of signal 1 and signal 2 have almost the same carrier
frequency, their spectra overlap. Signal 2 will dominate in the frequency domain due
to its greater amplitude. Without gating, you won't see the spectrum of signal 1; it is
masked by signal 2.
To measure signal 1, the gate must be on only during the pulses from signal 1. The
gate will be off at all other times, thus excluding all other signals. To position the
gate, set the gate delay and gate length, as shown in Figure 14-11, so that the gate is
on only during some central part of the pulse. Carefully avoid positioning the gate
over the rising or falling pulse edges. When gating is activated, the gate output signal
will indicate actual gate position in time, as shown in the line labeled “Gate.”
Figure 14-11
Timing Relationship of Signals During Gating
Once the signal analyzer is set up to perform the gate measurement, the spectrum of
signal 1 is visible and the spectrum of signal 2 is excluded, as shown if Figure 14-13
In addition, when viewing signal 1, you also will have eliminated the pulse spectrum
generated from the pulse edges. Gating has allowed you to view spectral components
that otherwise would be hidden.
185
Concepts
Time Gating Concepts
Figure 14-12
Signal within pulse #1 (time-domain view)
Figure 14-13
Using Time Gating to View Signal 1 (spectrum view)
Moving the gate so that it is positioned over the middle of signal 2 produces a result
as shown in Figure 14-15 Here, you see only the spectrum within the pulses of signal
2; signal 1 is excluded.
Figure 14-14
Signal within pulse #2 (time-domain view)
186
Concepts
Time Gating Concepts
Figure 14-15
Using Time Gating to View Signal 2 (spectrum view)
Measuring a complex/unknown signal
NOTE
The steps below help to determine the signal analyzer settings when using time
gating. The steps apply to the time gating approaches using gated LO and gated
video.
This example shows you how to use time gating to measure a very specific signal.
Most signals requiring time gating are fairly complex and in some cases extra steps
may be required to perform a measurement.
Step 1. Determine how your signal under test appears in the time domain and how it is
synchronized to the trigger signal.
You need to do this to position the time gate by setting the delay relative to the
trigger signal. To set the delay, you need to know the timing relationship between the
trigger and the signal under test. Unless you already have a good idea of how the two
signals look in the time domain, you can examine the signals with an oscilloscope to
determine the following parameters:
•
Trigger type (edge or level triggering)
•
Pulse repetition interval (PRI), which is the length of time between trigger events
(the trigger period).
•
Pulse width, or τ
•
Signal delay (SD), which is the length of time occurring between the trigger
event and when the signal is present and stable. If your trigger occurs at the same
time as the signal, signal delay will be zero.
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Concepts
Time Gating Concepts
Figure 14-16
Time-domain Parameters
In Figure 14-16, the parameters are:
•
Pulse repetition interval (PRI) is 5 ms.
•
Pulse width (τ) is 3 ms.
•
Signal delay (SD) is 1 ms for positive edge trigger (0.6 ms for negative edge
trigger).
•
Gate delay (D) is 2.5 ms.
•
Setup time (SUT) is 1.5 ms.
Step 2. Set the signal analyzer sweep time:
Gated LO: Sweep time does not affect the results of gated LO unless the sweep time
is set too fast. In the event the sweep time is set too fast, Meas Uncal appears on
the screen and the sweep time will need to be increased.
Gated Video: Sweep time does affect the results from gated video. The sweep time
must be set accordingly for correct time gating results. The recommended sweep
time is at least the number of sweep points – 1 multiplied by the PRI (pulse
repetition interval). Measurements can be made with sweep times as fast as (sweep
points–1)*(PRI–τ).
Step 3. Locate the signal under test on the display of the signal analyzer. Set the center
frequency and span to view the signal characteristics that you are interested in
measuring. Although the analyzer is not yet configured for correct gated
measurements, you will want to determine the approximate frequency and span in
188
Concepts
Time Gating Concepts
which to display the signal of interest. If the signal is erratic or intermittent, you may
want to hold the maximum value of the signal with Max Hold (located under the
Trace/Detector menu) to determine the frequency of peak energy.
To optimize measurement speed in the Gated LO case, set the span narrow enough so
that the display will still show the signal characteristics you want to measure. For
example, if you wanted to look for spurious signals within a 200 kHz frequency
range, you might set the frequency span to just over 200 kHz.
Step 4. Determine the setup time and signal delay to set up the gate signal. Turn on the gate
and adjust the gate parameters including gate delay and gate length as shown below.
Generally, the gate should be positioned over a part of the signal that is stable, not
over a pulse edge or other transition that might disturb the spectrum. Starting the gate
at the center of the pulse gives a setup time of about half the pulse width. Setup time
describes the length of time during which that signal is present and stable before the
gate comes on. The setup time (SUT) must be long enough for the RBW filters to
settle following the burst-on transients. Signal delay (SD) is the length of time after
the trigger, but before the signal of interest occurs and becomes stable. If the trigger
occurs simultaneously with the signal of interest, SD is equal to zero, and SUT is
equal to the gate delay. Otherwise, SUT is equal to the gate delay minus SD. See
Figure 14-17
Figure 14-17
Positioning the Gate
There is flexibility in positioning the gate, but some positions offer a wider choice of
resolution bandwidths. A good rule of thumb is to position the gate from 20 % to
90 % of the burst width. Doing so provides a reasonable compromise between setup
time and gate length.
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Concepts
Time Gating Concepts
Figure 14-18
Best Position for Gate
As a general rule, you will obtain the best measurement results if you position the
gate relatively late within the signal of interest, but without extending the gate over
the trailing pulse edge or signal transition. Doing so maximizes setup time and
provides the resolution bandwidth filters of the signal analyzer the most time to settle
before a gated measurement is made. “Relatively late,” in this case, means allowing
a setup time of at least 3.84/resolution bandwidth (see step 5 for RBW calculations).
As an example, if you want to use a 1 kHz resolution bandwidth for measurements,
you will need to allow a setup time of at least 3.84 ms.
Note that the signal need not be an RF pulse. It could be simply a particular period of
modulation in a signal that is continuously operating at full power, or it could even
be during the off time between pulses. Depending on your specific application,
adjust the gate position to allow for progressively longer setup times (ensuring that
the gate is not left on over another signal change such as a pulse edge or transient),
and select the gate delay and length that offer the best representation of the signal
characteristics of interest on the display.
If you were measuring the spectrum occurring between pulses, you should use the
same (or longer) setup time after the pulse goes away, but before the gate goes on.
This lets the resolution bandwidth filters fully discharge the large pulse before the
measurement is made on the low-level interpulse signal.
Figure 14-19
Setup Time for Interpulse Measurement
Step 5. The resolution bandwidth will need to be adjusted for gated LO and gated video. The
video bandwidth will only need to be adjusted for gated video.
Resolution Bandwidth:
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Concepts
Time Gating Concepts
The resolution bandwidth you can choose is determined by the gate position, so you
can trade off longer setup times for narrower resolution bandwidths. This trade-off is
due to the time required for the resolution-bandwidth filters to fully charge before the
gate comes on. Setup time, as mentioned, is the length of time that the signal is
present and stable before the gate comes on.
Figure 14-20
Resolution Bandwidth Filter Charge-Up Effects
Because the resolution-bandwidth filters are band-limited devices, they require a
finite amount of time to react to changing conditions. Specifically, the filters take
time to charge fully after the analyzer is exposed to a pulsed signal.
Because setup time should be greater than filter charge times, be sure that:
3.84
SUT > ------------RBW
where SUT is the same as the gate delay in this example. In this example with SUT
equal to 1.5 ms, RBW is greater than 2.56 kHz; that is, RBW is greater than 1333
Hz. The resolution bandwidth should be set to the next larger value, 2.7 kHz.
Video Bandwidth:
For gated LO measurements the VBW filter acts as a track-and-hold between sweep
times. With this behavior, the VBW does not need to resettle on each restart of the
sweep.
Step 6. Adjust span as necessary, and perform your measurement.
The analyzer is set up to perform accurate measurements. Freeze the trace data by
activating single sweep, or by placing your active trace in view mode. Use the
markers to measure the signal parameters you chose in step 1. If necessary, adjust
span, but do not decrease resolution bandwidth, video bandwidth, or sweep time.
191
Concepts
Time Gating Concepts
“Quick Rules” for making time-gated measurements
This section summarizes the rules described in the previous sections.
Table 14-1
Determining Signal Analyzer Settings for Viewing a Pulsed RF Signal
Signal
Analyzer
Function
Signal Analyzer Setting
Comments
Sweep Time
(gated video
only)
Set the sweep time to be equal to
or greater than
Because the gate must be on at least once
per trace point, the sweep time should be
set such that the sweep time for each trace
point is greater than or equal to the pulse
repetition interval.
Gate Delay
The gate delay is equal to the
signal delay plus one-fourth the
pulse width:
(number of sweep points - 1) ×
pulse repetition interval (PRI):
The gate delay must be set so that the
gating captures the pulse. If the gate delay
is too short or too long, the gating can miss
the pulse or include resolution bandwidth
transient responses.
Gate Delay = Signal Delay + τ/5
Gate Length
Resolution
Bandwidth
The gate length minimum is equal
to one-fourth the pulse width
(maximum about one-half):
If the gate length is too long, the signal
display can include transients caused by
the signal analyzer filters.
Gate Length = 0.7 x τ/4
The recommendation for gate placement
can be between 20 % to 90 % of the pulse
width.
Set the resolution bandwidth:
The resolution bandwidth must be wide
enough so that the charging time for the
resolution bandwidth filters is less than the
pulse width of the signal.
RBW > 19.5/τ
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Concepts
Time Gating Concepts
Figure 14-21
Gate Positioning Parameters
Most control settings are determined by two key parameters of the signal under test:
the pulse repetition interval (PRI) and the pulse width (τ). If you know these
parameters, you can begin by picking some standard settings. Table 14-2 summarizes
the parameters for a signal whose trigger event occurs at the same time as the
beginning of the pulse (in other words, SD is 0). If your signal has a non-zero delay,
just add it to the recommended gate delay.
Table 14-2
Suggested Initial Settings for Known Pulse Width (τ) and Zero Signal Delay
Pulse width (τ)
Gate Delay
(SD + τ/5)
Resolution
Bandwidth (>19.5/τ)
Gate Length
(0.7 x τ/4)
4 μs
0.8 μs
4.875 MHz
0.7 μs
10 μs
2 μs
1.95 MHz
1.753 μs
50 μs
10 μs
390 kHz
8.75 μs
63.5 μs
12.7 μs
307 kHz
11.11 μs
100 μs
20 μs
195 kHz
17.5 μs
500 μs
100 μs
39 kHz
87.5 μs
1 ms
200 μs
19.5 kHz
0.175 μs
5 ms
1 ms
3.9 kHz
0.875 ms
10 ms
2 ms
1.95 kHz
1.75 ms
16.6 ms
3.32 ms
1.175 kHz
2.905 ms
33 ms
6.6 ms
591 Hz
5.775 ms
50 ms
10 ms
390 Hz
8.75 ms
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Concepts
Time Gating Concepts
Table 14-2
Table 14-3
Suggested Initial Settings for Known Pulse Width (τ) and Zero Signal Delay
Pulse width (τ)
Gate Delay
(SD + τ/5)
Resolution
Bandwidth (>19.5/τ)
Gate Length
(0.7 x τ/4)
100 ms
20 ms
195 Hz
17.5 ms
≥130 ms
26 ms
151 Hz
22.75 ms
If You Have a Problem with the Time-Gated Measurement
Symptom
Possible Causes
Suggested Solution
Erratic analyzer trace with
dropouts that are not
removed by increasing
analyzer sweep time;
oscilloscope view of gate
output signal jumps
erratically in time domain.
Gate Delay may be greater
than trigger repetition
interval.
Reduce Gate Delay until it is less than
trigger interval.
Gate does not trigger.
1) Gate trigger voltage may
be wrong.
2) Gate may not be activated.
3) Gate Source selection
may be wrong.
With external gate trigger: ensure that
the trigger threshold is set near the
midpoint of the waveform (view the
waveform on and oscilloscope using
high input impedance, not 50 Ω).
With RF Burst Gate Source: ensure
that the start and stop frequencies are
within 10 MHz of the center
frequency of the carrier.
Check to see if other connections to
trigger signal may be reducing
voltage. If using an oscilloscope,
check that all inputs are high
impedance, not 50 Ω.
Display spectrum does not
change when the gate is
turned on.
Insufficient setup time.
Increase setup time for the current
resolution bandwidth, or increase
resolution bandwidth.
Displayed spectrum too low
in amplitude.
Resolution bandwidth or
video bandwidth filters not
charging fully.
Widen resolution bandwidth or video
bandwidth, or both.
194
Check Gate View to make sure the
gate delay is timed properly.
Concepts
Time Gating Concepts
Using the Edge Mode or Level Mode for triggering
Depending on the trigger signal that you are working with, you can trigger the gate in
one of two separate modes: edge or level. This gate-trigger function is separate from
the normal external trigger capability of the signal analyzer, which initiates a sweep
of a measurement trace based on an external signal.
Edge Mode
Edge mode lets you position the gate relative to either the rising or falling edge of a
trigger signal. The left diagram of Figure 14-22 shows triggering on the positive
edge of the trigger signal while the right diagram shows negative edge triggering.
Example of key presses to initiate positive edge triggering:
Press Sweep, Gate, More, Polarity (Pos).
Figure 14-22
Using Positive or Negative Edge Triggering
Level Mode
In level gate-control mode, an external trigger signal opens and closes the gate.
Either the TTL high level or TTL low level opens the gate, depending on the setting
of Trig Slope. Gate delay affects the start of the gate but not the end. Gate length is
applicable when using level mode triggering. Level mode is useful when your trigger
signal occurs at exactly the same time as does the portion of the signal you want to
measure.
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Concepts
Time Gating Concepts
Noise measurements using Time Gating
Time gating can be used to measure many types of signals. Noise and noise-like
signals are often a special case in spectrum analysis. With the history of gated
measurements, these signals are especially noteworthy.
The average detector is the best detector to use for measuring noise-like signals
because it uses all the available noise power all the time in its measurement. The
sample detector is also a good choice because it, too, is free from the peak biases of
the peak detector, normal and negative peak detectors.
When using the average or sample detector, noise density measurements using the
noise marker or band/interval density marker can be made without any consideration
of the use of gating--gated measurements work just as well as non-gated
measurements. Thus, the average detector is recommended for noise density
measurements.
Older analyzers only had the gated video version of gating available, and these only
worked with the peak detector, so the rest of this section will discuss the trade-offs
associated with trying to replicate these measurements with an X-Series analyzer.
Unlike older analyzers, X-Series analyzers can make competent measurements of
noise density using the noise marker with all detectors, not just those that are ideal
for noise measurements. Thus, X-Series analyzers can make noise density
measurements with peak detection, compensating for the extent to which peak
detection increases the average response of the analyzer to noise. When comparing a
gated video measurement using the noise marker between an X-Series and an older
analyzer where both use the peak detector, the X-Series answer will be
approximately correct, while the older analyzer will need a correction factor. That
correction factor is discussed in Agilent Technologies Application Note 1303,
Spectrum Analyzer Measurements and Noise, in the section on Peak-detected Noise
and TDMA ACP Measurements.
When making measurements of Band/Interval Power or Band/Interval Density, the
analyzer does not make compensations for peak detection. For best measurements
with these marker functions, average or sample detection should be used.
196
Concepts
AM and FM Demodulation Concepts
AM and FM Demodulation Concepts
Demodulating an AM signal using the analyzer as a fixed tuned receiver
(Time-Domain)
The zero span mode can be used to recover amplitude modulation on a carrier signal.
The following functions establish a clear display of the waveform:
•
•
•
•
Triggering stabilizes the waveform trace by triggering on the modulation
envelope. If the modulation of the signal is stable, video trigger synchronizes the
sweep with the demodulated waveform.
Linear display mode should be used in amplitude modulation (AM)
measurements to avoid distortion caused by the logarithmic amplifier when
demodulating signals.
Sweep time to view the rate of the AM signal.
RBW and VBW are selected according to the signal bandwidth.
Demodulating an FM signal using the analyzer as a fixed tuned receiver
(Time-Domain)
To recover the frequency modulated signal, a spectrum analyzer can be used as a
manually tuned receiver (zero span). However, in contrast to AM, the signal is not
tuned into the passband center, but to one slope of the filter curve as Figure 14-23
Figure 14-23
Determining FM Parameters using FM to AM Conversion
Here the frequency variations of the FM signal are converted into amplitude
variations (FM to AM conversion) utilizing the slope of the selected bandwidth filter.
The reason we want to measure the AM component is that the envelope detector
responds only to AM variations. There are no changes in amplitude if the frequency
changes of the FM signal are limited to the flat part of the RBW (IF filter). The
resultant AM signal is then detected with the envelope detector and displayed in the
time domain.
197
Concepts
IQ Analysis Concepts
IQ Analysis Concepts
Purpose
IQ Analysis (Basic) mode is used to capture complex time domain data from wide
bandwidth RF signals. This mode preserves the instantaneous vector relationships of
time, frequency, phase and amplitude contained within the selected digitizer span or
analysis BW, at the analyzer's center frequency, for output as IQ data. This IQ data
can then be utilized internally or output over LAN, USB or GPIB for use with
external analysis tools. Each measurement description specifies the types of data
available remotely for that measurement.
Within the IQ Analyzer mode, basic frequency domain, time domain and IQ
measurements are available as initial signal and data verification tools in preparation
for deriving the IQ data output. This is accomplished using the Complex Spectrum
and IQ Waveform measurements. Although Complex Spectrum and IQ Waveform
are defined as measurements in the IQ Analyzer (Basic) mode, they act primarily as
tools to verify the signals and data as stated above.
Complex Spectrum measurement
Purpose
This measurement is FFT (Fast Fourier Transform) based. The FFT-specific
parameters are located in the Advanced menu. The Complex Spectrum
measurement provides a display in the upper window of power versus frequency
with current (yellow trace) and average (blue trace) data. In addition, an IQ
waveform of voltage versus time is provided in the lower window. One advantage of
having an I/Q view available while in the spectrum measurement is that it allows you
to view complex components of the same signal without changing settings or
measurements.
Measurement method
The measurement uses digital signal processing to sample the input signal and
convert it to the frequency domain. With the instrument tuned to a fixed center
frequency, samples are digitized at a high rate, converted to I and Q components with
DSP hardware, and then converted to the frequency domain with FFT software.
Troubleshooting hints
Changes made by the user to advanced spectrum settings, particularly to ADC range
settings, can inadvertently result in spectrum measurements that are invalid and
cause error messages to appear. Care needs to be taken when using advanced
features.
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Concepts
IQ Analysis Concepts
IQ Waveform Measurement
Purpose
The IQ Waveform measurement provides a time domain view of the RF signal
envelope with power versus time or an IQ waveform with the I and Q signal
waveforms in parameters of voltage versus time. The RF Envelope view provides the
power verses time display, and the I/Q Waveform view provides the voltage versus
time display. One advantage of having an I/Q Waveform view available while
making a waveform measurement is that it allows you to view complex components
of the same signal without changing settings or measurements.
The waveform measurement can be used to perform general purpose power
measurements in the time domain with excellent accuracy.
Measurement method
The instrument makes repeated power measurements at a set frequency, similar to
the way a swept-tuned signal analyzer makes zero span measurements. The input
analog signal is converted to a digital signal, which then is processed into a
representation of a waveform measurement. The measurement relies on a high rate
of sampling to create an accurate representation of a time domain signal.
199
Concepts
Spurious Emissions Measurement Concepts
Spurious Emissions Measurement Concepts
Purpose
Spurious signals can be caused by different combinations of signals in the
transmitter. The spurious emissions from the transmitter should be minimized to
guarantee minimum interference with other frequency channels in the system.
Harmonics are distortion products caused by nonlinear behavior in the transmitter.
They are integer multiples of the transmitted signal carrier frequency.
This measurement verifies the frequency ranges of interest are free of interference by
measuring the spurious signals specified by the user defined range table.
Measurement method
The table-driven measurement has the flexibility to set up custom parameters such as
frequency, span, resolution bandwidth, and video bandwidth. Up to the top 40 spurs
can be viewed
For each range that you specify and activate, the analyzer scans the band using the
specified Range Table settings. Then using the Peak Excursion and Peak Threshold
values determines which spurs to report.
As each band is swept, any signal which is above the Peak Threshold value and has a
peak excursion of greater than the Peak Excursion value will be added to a list of
spurs displayed in the lower results window. A total of 200 spurs can be recorded for
one measurement, with a limit of 10 spurs per frequency range. To improve
repeatability, you can increase the number of averages.
From the spurs in the list, those with peak amplitude greater than the Absolute Limit
for that range will be logged as a measurement failure and denoted by an 'F' in the
'Amplitude' column of the table. If no spurs are reported, but the measured trace
exceeds the limit line for any range, the fail flag is set to fail.
This measurement has the ability to display two traces using different detectors on
the display simultaneously. All spur detection and limit line testing are only applied
to the trace associated with Detector 1, which will be colored yellow. The trace
associated with Detector 2 will be colored cyan.
If the sweep time for the range exceeds 2 seconds, a flashing message
“Sweeping...Please Wait” will appear in the annunciator area. This advises you that
the time to complete the sweep is between 2 and 2000 seconds, and is used as
without it the display would appear stagnant and you may think the measurement is
not functional.
200
Concepts
Spectrum Emission Mask Measurement Concepts
Spectrum Emission Mask Measurement Concepts
Purpose
The Spectrum Emission Mask measurement includes the in-band and out-of-band
spurious emissions. As it applies to W-CDMA (3GPP), this is the power contained in
a specified frequency bandwidth at certain offsets relative to the total carrier power.
It may also be expressed as a ratio of power spectral densities between the carrier
and the specified offset frequency band.
This spectrum emission mask measurement is a composite measurement of
out-of-channel emissions, combining both in-band and out-of-band specifications. It
provides useful figures-of-merit for the spectral regrowth and emissions produced by
components and circuit blocks, without the rigor of performing a full spectrum
emissions mask measurement.
Measurement method
The spectrum emission mask measurement measures spurious signal levels in up to
five pairs of offset/region frequencies and relates them to the carrier power. The
reference channel integration bandwidth method is used to measure the carrier
channel power and offset/region powers. When Offset is selected, spectrum
emission mask measurements are made, relative to the carrier channel frequency
bandwidth. When Region is selected, spurious emission absolute measurements are
made, set by specifying start and stop RF frequencies. The upper frequency range
limit is 3.678 GHz.
This integration bandwidth method is used to perform a data acquisition. In this
process, the reference channel integration bandwidth (Meas BW) is analyzed using
the automatically defined resolution bandwidth (Res BW), which is much narrower
than the channel bandwidth. The measurement computes an average power of the
channel or offset/region over a specified number of data acquisitions, automatically
compensating for resolution bandwidth and noise bandwidth.
This measurement requires the user to specify the measurement bandwidths of
carrier channel and each of the offset/region frequency pairs up to 5. Each pair may
be defined with unique measurement bandwidths. The results are displayed both as
relative power in dBc, and as absolute power in dBm.
201
Concepts
Occupied Bandwidth Measurement Concepts
Occupied Bandwidth Measurement Concepts
Purpose
Occupied bandwidth measures the bandwidth containing 99.0 of the total
transmission power.
The spectrum shape of a signal can give useful qualitative insight into transmitter
operation. Any distortion to the spectrum shape can indicate problems in transmitter
performance.
Measurement method
The instrument uses digital signal processing (DSP) to sample the input signal and
convert it to the frequency domain. With the instrument tuned to a fixed center
frequency, samples are digitized at a high rate with DSP hardware, and then
converted to the frequency domain with FFT software.
The total absolute power within the measurement frequency span is integrated for its
100% of power. The lower and upper frequencies containing 0.5% each of the total
power are then calculated to get 99.0% bandwidth.
202
Concepts
Baseband I/Q Inputs (Option BBA) Measurement Concepts
Baseband I/Q Inputs (Option BBA) Measurement Concepts
The N9020A Option BBA Baseband I/Q Inputs provides the ability to analyze
baseband I/Q signal characteristics of mobile and base station transmitters. This
option may be used only in conjunction with the following modes:
•
IQ Analyzer (Basic)
•
802.16 OFDMA (WiMAX/WiBro)
•
cdma2000
•
GSM/EDGE
•
TD-SCDMA
What are Baseband I/Q inputs?
Option BBA consists of a Baseband Input module, four input connectors, and a
calibration output connector. The connectors are at the left side of the front panel.
The two ports labeled “I” and “Q” are the “unbalanced” inputs.
An unbalanced or “single-ended” baseband measurement of an I or Q signal is made
using a probe connected to the I or Q connector. A simultaneous I/Q unbalanced
single-ended measurement may be made using two probes connected to the I and Q
input connectors.
If “balanced” signals are available, they may be used to make a more accurate
measurement. Balanced signals are signals present in two separate conductors, are
symmetrical about ground, and are opposite in polarity, or out of phase by 180
degrees.
Measurements using balanced signals can have a higher signal to noise ratio
resulting in improved accuracy. Noise coupled into each conductor equally in a
“common mode” to both signals may be separated from the signal. The measure of
this separation is “common-mode rejection”.
To make a balanced measurement, the two connectors labeled “I” and “Q” are used
in conjunction with the I and Q inputs. The terms “I-bar” and “Q-bar” may be
applied to the signals, as well as the inputs themselves. Probes (customer provided)
must be used to input balanced baseband I/Q signals. This may be referred to as a
balanced measurement.
Balanced baseband measurements are made using the I and connectors for I only
signal measurements, while the Q and connectors are used for a Q only signal
measurement. Balanced measurements of I/Q require differential probe connections
to all four input connectors. For details of probe selection and use, refer to “Selecting
input probes for baseband measurements” on page 205.
What are Baseband I/Q signals?
In transmitters, the term baseband I/Q refers to signals that are the fundamental
products of individual I/Q modulators, before the I and Q component signals are
combined, and before upconversion to IF or RF frequencies.
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Concepts
Baseband I/Q Inputs (Option BBA) Measurement Concepts
In receivers, baseband I/Q analysis may be used to test the I and Q products of I/Q
demodulators, after an RF signal has been downconverted and demodulated.
Why make measurements at baseband?
Baseband I/Q measurements are a valuable means of making qualitative analyses of
the following operating characteristics:
•
I/Q signal layer access for performing format-specific demodulation
measurements (for example, CDMA, GSM, W-CDMA).
•
Modulation accuracy – that is,. I/Q plane metrics:
— rho
— error vector magnitude; rms, peak, or 95%
— carrier feed-through
— frequency error
— magnitude and phase errors
•
Code-domain analysis (including code-specific metrics)
•
CCDF of I + Q
•
Single sideband (SSB) metrics for assessing output quality
•
Basic analysis of I and Q signals in isolation including: DC content, rms and peak
to peak levels, CCDF of each channel
2
2
Comparisons of measurements made at baseband and RF frequencies produced by
the same device are especially revealing. Once signal integrity is verified at
baseband, impairments can be traced to specific stages of upconversion,
amplification, or filtering by RF analysis. In addition, impairments to signal quality
that are apparent at RF frequencies may be traceable to baseband using baseband
analysis.
204
Concepts
Baseband I/Q Inputs (Option BBA) Measurement Concepts
Selecting input probes for baseband measurements
The selection of baseband measurement probe(s) and measurement method is
primarily dependent on the location of the measurement point in the circuit. The
probe must sample voltages without imposing an inappropriate load on the circuit.
The system supports a variety of 1 MΩ impedance input passive probes as well as
the Agilent 1153A active differential probe using the InfiniMax probe interface.
The Agilent 1153A active probe can be used for both single-ended and differential
measurements. In either case a single connection is made for each channel (on either
the I or Q input). The input is automatically configured to 50 Ω single-ended type
measurement and the probe power is supplied through the InfiniMax interface. The
probe can be configured for a variety of input coupling and low frequency rejection
modes. In addition, a wide range of offset voltages and probe attenuation accessories
are supported at the probe interface. The active probe has the advantage that it does
not significantly load the circuit under test, even with unity gain probing.
With passive 1 MΩ probes, the probe will introduce a capacitive load on the circuit,
unless a higher attenuation is used at the probe interface. Higher attenuation helps
isolate the probe, however, it reduces the signal level and degrades the
signal-to-noise-ratio of the measurement. Passive probes are available with a variety
of attenuation values for a moderate cost. Many Agilent passive probes can be
automatically identified by the system, setting the input impedance required as well
as the nominal attenuation. For single-ended measurements a single probe is used for
each channel. Other passive probes can be used, after manually setting the
attenuation and probe impedance configurations.
For full differential measurements, the system supports probes on each of the four
inputs. The attenuation for each of the probes should be the same for good common
mode rejection and channel match.
Supported probes
The following table lists the probes currently supported by Option BBA:
Probe Type
Model Number
Description
Active
1130A
1.5 GHz differential probe amp
(No probe head)
1131Aa
InfiniMax 3.5 GHz probe
1132Aa
InfiniMax 5 GHz probe
1133Aa
InfiniMax 7 GHz probe
1161A
Miniature passive probe, 10:1, 10 MΩ,
1.5 m
Passive
205
Concepts
Baseband I/Q Inputs (Option BBA) Measurement Concepts
a. Probe heads are necessary to attach to your device properly. Probe connectivity kits such as the E2668A, E2669A or E2675A are needed. For
more details, refer to the Agilent probe configuration guide,
5968-7141EN and 5989-6162EN.
Probes without stored calibration
The following 115xA active probes may be used with the MXA’s baseband IQ inputs
and may use the same probe calibration utility software. However, the probe
calibration data is not stored in the MXA and will be lost if power is cycled. Use of
the E2655B de-skew and calibration kit, including the calibration fixture, is required
because of the different physical configuration of the probes. (The physical
connections are different mechanically, not electrically.)
Probe Type
Model Number
Description
Active
1153A
200MHz differential probe
1156A
Active probe, 1.5 GHz
1157A
Active probe, 2.5 GHz
1158A
Active probe, 4 GHz
Refer to the current Agilent probe data sheet for specific information regarding
frequency of operation and power supply requirements.
Baseband I/Q measurement views
Measurement result views made in the IQ Analyzer (Basic) mode are available for
baseband signals if they relate to the nature of the signal itself. Many measurements
which relate to the characteristics that baseband I and Q signals have when mixed
and upconverted to signals in the RF spectrum can be made as well. However,
measurements which relate to the characteristics of an upconverted signal that lie
beyond the bandwidth available to the Baseband I/Q Input circuits can not be
measured (the limit are dependent on the installed options: Standard – 10 Hz to 20
MHz, Option B25 – 10 Hz to 50 MHz, and Option S40 – 10 Hz to 80 MHz).
At RF frequencies, power measurements are conventionally displayed on a
logarithmic vertical scale in dBm units, whereas measurements of baseband signals
using Baseband I/Q inputs may be conveniently displayed as voltage using a linear
vertical scale as well as a log scale.
Spectrum views and 0 Hz center frequency
To view the Spectrum display of I only or Q only signals, use the Complex Spectrum
measurement capability in IQ Analyzer (Basic) Mode.
I only and Q only Spectrum views are conventional, displayed with 0 Hz at the left
side of the horizontal axis. When upconverted or multiplied, an I only or Q only
signal could ultimately lie above or below the carrier center frequency, but in either
case it will only occupy half the bandwidth.
206
Concepts
Baseband I/Q Inputs (Option BBA) Measurement Concepts
Waveform Signal Envelope views of I only or Q only
To view the Signal Envelope display of I only or Q only signals, use the Waveform
measurement capability in IQ Analyzer (Basic) Mode.
The I and Q Waveform of an I/Q signal is very different from the complex signal
displayed in the RF Envelope view. That is because the RF Envelope is a product of
both the I and Q modulation waveforms.
However, an I and Q Waveform measurement of an I only or Q only signal is exactly
the same signal displayed in the RF Envelope view. That is because an I only or Q
only waveform determines the I only or Q only signal envelope. Thus, the RF
Envelope view can be used to measure an I only or Q only waveform directly.
207
Concepts
Baseband I/Q Inputs (Option BBA) Measurement Concepts
208
Index
Numerics
50 ohm load, 18
50 ohm/75 ohm minimum loss pad, 18
75 ohm matching transformer, 18
A
AC probe, 18, 19
Accessories, 18
accessories
50 ohm load, 18
50 ohm/75 ohm minimum loss pad, 18
75 ohm matching transformer, 18
AC probe, 18, 19
broadband preamplifiers, 19
GPIB cable, 19
power splitters, 20
RF limiters, 19
transient limiters, 19
ACP key
MEAS key, 118
ACP measurement, 116
Adjacent Channel Power
measurement, 116
AM demodulation
time-domain demodulation, manually
calculating, 197
AM signal demodulation, 140
amplifiers, 19
amplitude calibration for external
mixing, 161
analyzer distortion products, 64
arrow keys, using, 12
attenuation
input, reducing, 44
setting automatically, 46
setting manually, 45
averaging
description, 52
types, 52
B
band power marker, 83
Bluetooth power measurement, 126
broadband preamplifiers, 19
Burst Power key
Meas key, 127
Burst Power measurement, 126
C
cable
GPIB, 19
CCDF measurement, 121
CCDP key
Meas key, 122
Channel Power key
Meas key, 112
Channel power measurement, 112
using, 81
noise marker, 74
overview, 72
signal to noise, 72
channel power measurement
noise-like signals, 81
comparing signals
two signals, 22
two signals not on the same screen, 25
complex spectrum measurement, 198
Concepts
AM demodulation, 197
FM demodulation, 197
concepts
gated FFT (PSA), 183
gated LO (PSA), 182
gated video (ESA), 182
IF filter, defined, 174
resolving signals of equal
amplitude, 174
resolving small signals hidden by large
signals, 174
time gating, 179
resolution bandwidth, reducing, 47
signals
low-level, overview, 44
off-screen,
comparing,
resolving,
equal
amplitude, 28, 33
signals, viewing, 14
time gating
ESA-E time gate, 104
PSA gated FFT, 108, 100
D
data, entering from front panel, 12
DC probes
use of, 18, 19
delta band marker function, 39
delta marker, 22
demodulating
AM, 140
AM overview, 140
detectors, average, 50
distortion measurements
identifying distortion products, 64
identifying TOI distortion, 67
overview, 64
distortion products, 64
drifting signals, 61
25,
22
TOI distortion, 67
trace averaging, 52
tracking a signal, 61
F
factory preset, description, 13
finding hidden signals, 174
FM demodulation
time-domain demodulation, manually
calculating, 197
frequency accuracy, increasing, 15
frequency readout resolution
increased, 56
front panel
entering data, 12
E
G
Enter key, using, 12
equipment, 17
ESD safety accessories, 20
examples
AM demodulation, 140
amplitude calibration, 161
average detector, using, 50
averaging, trace, 52
external mixing, 160
frequency accuracy, 15
frequency drift, 58
identify distortion products, 64
input attenuation, reducing, 44
manual demodulation, 140
marker counter, 56
measuring
low-level signals, 50
noise
band power marker, 78
gate delay
setting the gate delay, time gating, 190
gate length
setting the gate length, time gating, 190
gated FFT (PSA), concepts, 183
gated LO (PSA), concepts, 182
gated video (ESA), concepts, 182
GPIB cable, 19
Spectrum Analyzer Measurement Guide 209
channel
power,
H
harmonic distortion
measuring low-level signals, 25
I
identifying distortion products, 64
initial setting for time gating, 193
input attenuation, reducing, 44
intermodulation distortion, third
order, 67
Index
interval power marker function, 39
iq waveform measurement, 199
band power marker,
channel power, 81
noise marker, 74
overview, 72
signal to noise, 72
K
keypad, using, 12
knob, using, 12
78
time gating, 93
L
limiters
RF and transient, 19
load, 50 ohm, 18
low-level signals
harmonics, measuring, 25
input attenuation, reducing, 44
resolution bandwidth, reducing, 47
sweep time, reducing, 50
trace averaging, 52
M
marker
frequency and amplitude, reading, 14
moving
to reference level, 14
with knob or arrow
key, 14
marker annotation
location, 14
marker counter example
marker frequency resolution, 56
marker function
delta band, 39
interval power, 39
markers
delta, 22
markers, advanced
band power, 83
markers, advances
noise marker, 83
MEAS key
ACP key, 118
Meas key
Burst Power key, 127
CCDP key, 122
Channel Power key, 112
Spectrum Emission Mask key, 136
measure complex modulation power, 39
measurement
ACP or spurious emissions mask, 200
occupied bandwidth, 202
spectrum shape, 202
spectrum emission mask, 201
measurements
distortion, 64
external mixing, 160
frequency drift, 58, 61
identifying distortion, 64
210
R
noise
ESA-E time gate, 104
PSA gated FFT, 108, 100
TOI distortion, 67
moving signals, 61
N
noise marker, 83
noise measurements
band power marker, using, 78
channel power, using, 81
noise marker, using, 74
overview, 72
signal to noise, 72
sweep time, reducing, 50
noise power measurement near noise
floor, 88
numeric keypad, using, 12
S
O
occupied bandwidth
99.0% bandwidth, 202
measurement method, 202
purpose, 202
total absolute power, 202
Occupied Bandwidth measurement, 114
overviews
distortion, 64
low-level signal, 44
noise, 72
resolving signals, 174
time gating, 179
P
positioning the gate, time gating, 189
power amplifiers, 19
power diff
trace math function, 88
power splitters, 20
preamplifiers, 19
preset
factory, 13
types, 13
user, creating, 13
probes
AC and DC, 18, 19
RBW selections, 49
reference level, setting, 14
resolution bandwidth
adjusting, 47
resolving signals, 174
resolving signals
small signals hidden by large
signals, 174
resolving two signals
equal amplitude, 28, 174
resolving, equal amplitude, 174
RF limiters, 19
RPG, using, 12
rules for time gating, 192
signal connection, 167
signal parameters for a time-gated
measurement, 187
signal tracking
example, 61
marker tracking, 37
using to resolve signals, 37
signals
low-level, overview, 44
off-screen, comparing, 25
on-screen, comparing, 22
resolving, overview, 174
separating, overview, 174
signals, increasing accuracy, 15
signals, viewing, 14
softkeys, auto and man mode, 13
softkeys, basic types, 12
Spectrum (Frequency Domain) key, 198
Spectrum analysis measurement
application, 111
channel power, 81
spectrum emission mask
in-band and out-of-band spurious
emissions, 201
integration bandwidth method, 201
measurement method, 201
offset or region frequency pairs, 201
purpose, 201
reference channel integration
bandwidth, 201
spectral regrowth, 201
Spectrum Emission Mask key
Meas key, 136
Spectrum Emission Mask
measurement, 135
spectrum measurement
method, 198
splitters, 20
static safety accessories, 20
Statistical Power measurement, 121
sweep time and sensitivity trade off, 49
sweep time for a time-gated
measurement, 104, 188
Spectrum Analyzer Measurement Guide
Index
sweep time, changing, 50
T
test equipment, 17
third order intermodulation distortion
example, 67
time gating
description, 179
ESA-E time gate, using, 104
example, 93
gated FFT (PSA), concepts, 183
gated LO (PSA), concepts, 182
gated video (ESA), concepts, 182
how time gating works, 181
initial settings, 193
keys, 189
positioning the gate, 101, 189
PSA gated FFT, using, 108
PSA gated sweep, using, 100
rules, 192
setting sweep time, 193
setting the gate length, 190
setting the resolution bandwidth, 190,
191
setting the span, 189, 191
setting the video bandwidth, 190
signal parameters, 187
steps for measuring unknown
signals, 187
sweep time, 104, 188
triggering
edge mode, 195
level mode, 195
negative edge, 195
positive edge, 195
troubleshooting, 193
time gating measurement, 93
trace math function
power diff, 88
tracking unstable signals, 61
transient limiter, 19
troubleshooting
time-gated measurements, 193
U
unit softkeys, using, 12
unstable signals, 61
user preset
creating, 13
description, 13
W
waveform
method, 199
Waveform (Time Domain) key, 199
Spectrum Analyzer Measurement Guide 211
Index
212
Spectrum Analyzer Measurement Guide
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