Fundamentals of Microwave Frequency

Fundamentals of Microwave Frequency
This literature was published years prior to the establishment of Agilent Technologies as a company independent from Hewlett-Packard
and describes products or services now available through Agilent. It may also refer to products/services no longer supported by Agilent.
We regret any inconvenience caused by obsolete information. For the latest information on Agilent’s test and measurement products go to:
H
www.agilent.com/find/products
Or in the US, call Agilent Technologies at 1-800-452-4844 (8am–8pm EST)
Fundamentals of Microwave
Frequency Counters
Application Note 200-1
Electronic Counters Series
1
Table of Contents
Down-Conversion Techniques for Automatic
Microwave Frequency Counters ................................................. 3
Prescaling .................................................................................................... 3
Heterodyne Converter ............................................................................... 4
Transfer Oscillator ..................................................................................... 5
Harmonic Heterodyne Converter ............................................................ 6
Comparing the Principal Microwave Down-Conversion
Techniques .................................................................................... 8
Measurement Speed .................................................................................. 8
Accuracy ...................................................................................................... 8
Sensitivity and Dynamic Range ............................................................... 10
Signal-to-Noise Ratio ................................................................................. 11
FM Tolerance .............................................................................................. 12
AM Tolerance ............................................................................................. 14
Amplitude Discrimination ........................................................................ 14
Summary of the Comparison ................................................................... 15
Additional Considerations in Choosing a
Microwave Counter ...................................................................... 16
Signal Inputs ............................................................................................... 16
Systems Interface ....................................................................................... 16
IF Offsets ..................................................................................................... 16
Microwave Power Measurements ........................................................... 16
Some Applications of the HP 5342A Microwave
Frequency Counter ...................................................................... 17
2
Down-Conversion Techniques for Automatic
Microwave Frequency Counters
A frequency counter, being a digital instrument, is limited in its frequency range by the speed of its logic circuitry. Today the state of the
art in high-speed logic allows the construction of counters with a
frequency range of around 500 MHz. Continuing advances in IC technology should extend this range beyond 1 GHz in the not-too-distant
future.
The designer of an automatic microwave counter must look to some
form of down-conversion in order to extend frequency measurement
beyond 500 MHz. Four techniques are available today to provide this
down-conversion:
1. Prescaling, with a range of 1.5 GHz;
2. Heterodyne Converter, allowing measurements as high as 20 GHz;
3. Transfer Oscillator, used in counters with ranges to 23 GHz;
4. Harmonic Heterodyne Converter, a new technique which can
provide measurements to 40 GHz.
Prescaling
Prescaling involves simple division of the input frequency resulting in a
lower frequency signal which can be counted in digital circuitry. The
frequency measured by the counter section is related to the input
simply by the integer N. A display of the correct frequency is accomplished either by multiplying the counter’s contents by N or by increasing the counter’s gate time by a factor of N. Typically, N ranges from 2
to 16.
Figure 1 shows the block diagram of a high frequency counter using
prescaling as its down-conversion technique. The input signal is
conditioned to interact correctly with the prescaling circuit, and then it
is divided by N before entering the main gate. Beyond this point the
block diagram looks like a conventional counter, with the main gate
being opened and closed (by the main gate flip-flop) in timing precisely
determined by the crystal time base of the instrument. The decade
counting assembly (DCA) now accumulates the under-500 MHz frequency measurement, which is multiplied by N and transmitted to the
display.
Modern frequency counters using this technique are capable of measuring above 1.0 GHz. Recent developments in solid-state technology
promise to extend this range into the low microwave range within a
few years.
3
Input Signal Conditioning
÷ N Prescaler
fx
Main
Gate
DCA
Display
Time
Base
÷N
Time Base
Dividers
Main Gate
FF
Figure 1. Block
diagram of a highfrequency counter
using the prescale
down-conversion
technique.
Heterodyne Converter
Heterodyne down-conversion centers about a mixer which beats the
incoming microwave frequency against a high-stability local oscillator
signal, resulting in a difference frequency which is within the conventional counter’s 500 MHz bandwidth.
Figure 2 is the block diagram of an automatic microwave counter using
the heterodyne down-conversion technique. The down-converter
section is enclosed by the dotted line. Outside the dotted line is the
block diagram of a conventional counter, with the addition of a new
block called the processor. The decision-making capability of a processor is necessary here in order to lead the counter through its measurement algorithm. The high stability local oscillator of Figure 2 is generated by first digitally multiplying the frequency of the instrument’s time
base to a convenient fundamental frequency (designated f in ), typically
100 to 500 MHz. This f in is directed to a harmonic generator which
produces a “comb line” of frequencies spaced at f in extending to the
full frequency range of the counter. One line of this comb, designated
Kf in , is then selected by the microwave filter and directed to the mixer.
Emerging from the mixer is a video frequency equal to f x –Kf in . This
video frequency is amplified and sent to the counter. The display
contains the sum of the video frequency and Kfin, which is provided by
the processor. (The processor stores the value of K, since it is in
control of the microwave filter.)
The signal detector block in Figure 2 is necessary for determining the
correct K value. In practice, the processor will begin with K=1 and will
“walk” the value of K through the comb line until the signal detector
determines that a video frequency is present. At this point the acquisition routine is terminated and measurement can begin.
The remaining block in Figure 2 which has not been discussed is the
automatic gain control (AGC) circuit. This circuit provides a degree of
noise immunity by desensitizing the video amplifier such that only the
strongest frequency components of the video signal will enter the
Schmitt trigger and be counted.
4
A key ingredient in automating the heterodyne down-conversion
process is the microwave filter. Two filters used for this purpose are
(1) a YIG filter, and (2) an array of thin-film filters which are selected
by PIN diode switches.
Mixer
Unknown Input (fx)
Kfin
fvideo
fx ± Kfin
YIG/PIN
Switch Filter
Video Amp
Time
Base
Schmitt
Trigger
Main
Gate
DCA
dc
Amp
AGC
Amp
Signal
Detector
fin
Harmonic
Generator
fx – Kfin
Main Gate
FF
Display
Multiplier
Filter
Control
Processor
Decade
Dividers
Figure 2. Block diagram of the heterodyne down-conversion technique.
Transfer Oscillator
The transfer oscillator uses the technique of phase locking a low
frequency oscillator to the microwave input signal. The low frequency
oscillator can then be measured in a conventional counter, and all that
remains to be accomplished is to determine the harmonic relationship
between that frequency and the input.
Figure 3 is the block diagram of a microwave counter using the transfer
oscillator technique. Once again, the down-conversion circuitry is
contained within the dotted line. A processor is not necessarily included in the block diagram, although some decision-making ability is
necessary in the acquisition process, just as with the heterodyne
converter above.
In Figure 3 the input signal is shown being phase locked to a voltage
controlled oscillator (VCO #1) in the upper portion of the converter
section. Once phase lock is achieved, the relationship between the
input and the VCO frequency is fx =N f , where N is an integer. The
remainder of the down-converter circuitry is devoted to the task of
determining N. The counter can now measure f 1 , (typically 100–200
MHz) and multiply by N for a display of the microwave frequency. As in
the case of prescaling above, this multiplication is usually accomplished by extending the gate time of the counter by a factor of N
(which takes values from 1 to 200).
5
The quadrature detector in the phase lock loop of the automatic
transfer oscillator insures that the output of VCO #1 bears the correct
phase relationship with respect to the input signal.
F if 1
Sampler
From Time Base
Video Amp
fx
F if
F1
Phase
Detector
VCO 1
Quadrature
Detector
F0
VCO 2
fx
F if
F if 2
F2
Main
Gate
Main Gate
FF
Time
Base
Decade
Dividers
REF
NF0
Video
Amp 2
Sampler
Schmitt
Trigger
Reference
Oscillator
Amp
Power
Divider
fx
REF
Mixer
DCA
Display
N Counter
Figure 3. Block
diagram of the
transfer oscillator
down-conversion
technique.
Harmonic Heterodyne Converter
The harmonic heterodyne converter, as its name implies, is a hybrid of
the previous two techniques. A counter using this block diagram
(Figure 4) will acquire the input microwave frequency in the manner of
the transfer oscillator, but it will then make frequency measurements
like a heterodyne converter.
Figure 4 shows the input f x being directed to a sampler, with the
resulting down-converted video signal f if = f x –Nf s amplified and sent
to the counter. The sampling frequency fs is created by a processorcontrolled synthesizer.
The acquisition routine for this down-converter consists of tuning the
synthesizer f s until the signal detector finds a video signal f if of the
appropriate frequency range (defined by the band-pass filter). Next, the
harmonic number N must be determined, as in the transfer oscillator.
One method of finding N is to use a second sampler loop, as with the
transfer oscillator (Figure 3), or similar technique. A second method is
6
to step the synthesizer back and forth between two closely-spaced
frequencies and observe the differences in counter readings; it is then a
simple task for the processor to calculate N.
A frequency measurement is accomplished by the processor’s multiplying the known synthesizer frequency fs by N, adding the result to the
video frequency fif measured in the DCA, and displaying the answer:
fx=Nfs+fif. In this process the harmonic heterodyne converter resembles the heterodyne converter, since the sampler is effectively
mixing the Nth harmonic of a very stable source with the input to
produce a video difference frequency.
The harmonic heterodyne converter has the potential to be constructed at a lower cost than the previous two techniques because it
can be designed with just one microwave component (the sampler)
and the control, decisions, and calculations can be performed by a
low-cost microprocessor.
fif
fx
Sampler
fs
Synthesizer
Video Amp
Band-Pass
Filter
Signal
Detector
Schmitt
Trigger
Main Gate
FF
DCA
Decade
Dividers
Display
Time
Base
Processor
Figure 4.
Block diagram
of the harmonic
heterodyne
down-conversion
technique
7
Comparing the Principal Microwave
Down-Conversion Techniques
In this chapter we will examine the performance trade-offs between
the three down-conversion techniques which allow measurements over
1.5 GHz: heterodyne converter, transfer oscillator, and harmonic
heterodyne converter.
Measurement Speed
The time required for a microwave counter to perform a measurement
may be divided into two parts:
1. Acquisition — The time necessary for the counter to detect a
microwave signal and prepare to make a measurement; and
2. Gate Time — The duration of the counter’s gate required to measure
to a given resolution.
Each of the three down-conversion techniques we have discussed
offers trade-offs in the area of measurement speed.
The heterodyne converter, using the YIG filter, has an acquisition time
ranging from 40 milliseconds to over 200 milliseconds. A design using
thin film filters has an impressively short acquisition time of less than
1 millisecond. The gate time for heterodyne converter counters is 1/R,
where R is the desired resolution in Hertz.
A microwave counter using the transfer oscillator technique will typically
have an acquisition time of about 150 milliseconds, which is comparable
to the heterodyne converter. Gate times for the transfer oscillator are
longer since, as with the prescaler, they must be set to N/R in order to
effectively multiply the counter’s contents by N. This factor of N can cause
the transfer oscillator counter to measure much more slowly than the
heterodyne converter for high resolution (100 Hz or less) measurements of
microwave frequencies. For typical measurements with resolution 1 kHz
or greater, the difference in measurement speed between the three
techniques will not be noticed by the operator.
The harmonic heterodyne converter has an acquisition time of 350 to
500 milliseconds, the slowest of the three techniques. Gate time can
range from 1/R to 4/R, much better than the transfer oscillator technique and close to the heterodyne technique.
Accuracy
The accuracy of microwave counter measurements is limited by two
factors:
1. The plus/minus one-count quantization error; and
2. Time base errors.
Time base errors may further be looked at in two different ways: shortterm stability, which generally limits the repeatability from one measurement to the next; and long-term stability, which limits the absolute
accuracy of a measurement.
8
It should be noted that, in a typical user environment, a counter’s
accuracy capabilities will be masked by the effects of temperature
fluctuations on the time base. Depending on the crystal oscillator used,
these fluctuations will generally limit both repeatability and absolute
accuracy to at least parts in 10 9 and perhaps as much as parts in 10 6 . It
is best to carefully examine temperature stability specifications before
attempting high-precision measurements with a microwave counter.
Assuming that a stable temperature environment exists, let us compare
the short-term stability (repeatability) of microwave measurements for
each of the three down-conversion techniques. Figure 5 illustrates this
comparison. For a gate time of one second, it is clear that the transfer
oscillator is limited to about 1×10 –8 resolution. The heterodyne and
harmonic heterodyne converters are limited to about 1×10 –9, where
short-term instabilities of common crystal oscillators become the
limiting factor. With the high stability of an oven oscillator, these two
converters are capable of resolving 1×10 –10 at microwave frequencies.
σ ( ∆f )
f
10 –11
Harmonic Heterodyne Converter
and
Heterodyne Converter with
direct count counter
Oven (1s Avg)
10 –10
TCXO (1s Avg)
10 –9
t
un
co
±1
10 –8
RTXO (1s Avg)
Transfer Oscilltor followed by 100 MHz
direct count counter
fx
100 MHz
1 GHz
10 GHz
100 GHz
Figure 5. Shortterm stability:
Resolution limits
of the three downconversion
techniques for a
measurement with
1-second gate
time. Three types
of crystal
oscillator are
considered. Room
temperature
(RTXO),
temperature
compensated
(TCXO), and highstability ovenized
(“oven”).
Of considerably more importance to the user, however, are the longterm effects which limit the accuracy of microwave counter measurements. Figure 6 graphs the combined effects of inaccuracies due to time
base aging and the ±1 count uncertainty. In this figure it is assumed that
the time base was calibrated to a high degree of accuracy one month
ago. Clearly, even with the best time bases available, the long-term
instability of the time base becomes the accuracy limitation, no matter
which down-conversion technique is used. It may therefore be concluded that accuracy is not a consideration in choosing between
microwave down-conversion techniques for a particular application.
9
∆f
f
10 –10
Harmonic Heterodyne Converter
or
Heterodyne Converter with
direct count counter
Transfer Oscillator
followed by 100 MHz
direct count counter
10 –9
nt
10 –8
ou
c
±1
±1 count
Oven
TCXO
RTXO
10 –7
fx
100 MHz
1 GHz
10 GHz
100 GHz
Figure 6. Longterm stability:
Accuracy limits
of microwave
counters for a
1-second gate
time, assuming
one month since
last calibration.
Sensitivity and Dynamic Range
As illustrated in Figure 7, there is little difference in sensitivity specifications among the three down-conversion techniques. A good microwave
counter will have sensitivity of about –25 dBm for most measurements.
The lower dashed line in Figure 7 indicates the true sensitivity of a
typical HP 5340A Frequency Counter, which uses the transfer oscillator technique. The transfer oscillator is capable of exceptional sensitivity since the input signal into the down-converter enters a narrow-band
(about 200 kHz) phase lock loop. The counter is therefore relatively
insensitive to noise on the input and may be designed for high sensitivity without too much concern for triggering on low-level noise. The
harmonic heterodyne converter, with a relatively narrow input bandwidth, also has the potential of outstanding sensitivity. The heterodyne
converter, on the other hand, has an effective input bandwidth of
200 to 500 MHz; great care must be exercised in the design of these
counters to avoid false readings due to broadband noise.
The dynamic range of a microwave counter is a measure of the separation of the sensitivity specification and the highest level input signal
which can be counted reliably. A typical value for this upper level is
+7 dBm, which is also graphed in Figure 7. It should be noted that
some microwave counters allow measurements of inputs to +20 dBm
and beyond; for example, the HP 5341A (Heterodyne Converter) and
the HP 5342A Option 003 (Harmonic Heterodyne Converter) both
measure +20 dBm inputs.
10
Input Level
+20 dBm
Maximum Measured Input
0 dBm
–20 dBm
HP 5340A Typical
–40 dBm
5 GHz
10 GHz
15 GHz
20 GHz
Sensitivity:
Harmonic Heterodyne Converter
Heterodyne Converter
Transfer Oscillator
Figure 7. Available
microwave counter
sensitivity
specifications.
Maximum
measured input
(regardless of
down-conversion
technique) is
typically +7 dBm,
although many
counters allow
measurements to
+20 dBm.
Signal-to-Noise Ratio
An important consideration in choosing a microwave counter is the
signal-to-noise environment of the measurement. As mentioned in the
above paragraph, the apparent amplifier bandwidth at the counter’s
input limits the amount of noise which the counter can tolerate on the
measured signal.
Consider a microwave frequency to be measured which has a good deal
of noise surrounding the carrier. Such a situation is illustrated in Figure
8a. A transfer oscillator or harmonic heterodyne converter counter will
be capable of measuring the signal if the peak carrier exceeds the noise
floor by 20 dB. A typical heterodyne converter counter, however, will
require 40 dB or greater separation to allow accurate measurement.
A common situation wherein broadband noise surrounds a signal to be
measured is in the monitoring of solid state microwave sources. Figure
8b shows the typical output of a solid state sweeper. With this type of
spectrum to be measured, a transfer oscillator or harmonic heterodyne
converter will provide reliable readings up to the maximum sweeper
output power (about +10 dBm). The typical heterodyne converter
counter will encounter noise interference at sweeper output levels near
–10 dBm.
11
a. Bandlimited Noise
b. Broadband Noise
16 MHz
Transfer Oscillator 20 dB S/N
Heterodyne Converter 40 dB S/N
Harmonic Heterodyne Converter 20 dB S/N
Transfer Oscillator +10 dBm
Heterodyne Converter –10 dBm
Harmonic Heterodyne Converter +10 dBm
Figure 8. Spectral
display of tests on
microwave counters
to determine
signal-to-noise
requirements. Tests
included (a) bandlimited AM noise,
and (b) broadband
noise generated
at the output of
a solid-state
microwave sweeper.
FM Tolerance
All modern microwave counters are capable of measuring today’s
microwave sources with their inherent incidental frequency modulation. There are applications, however, in which it is desired to measure
a microwave communications carrier with frequency modulation
present. In these cases the FM tolerance of microwave counters becomes a consideration for choosing the appropriate instrument.
A heterodyne converter may be thought of as dividing microwave
frequency space into distinct bands, of a width equal to the comb line
spacing. The design of these instruments is such that the video counting
capability of the conventional counter is somewhat greater than the
comb line spacing. It is this resulting overlap between adjacent bands
that is the measure of the FM tolerance of the counter. Figure 9 illustrates the FM tolerance of the HP 5341A Frequency Counter. In this
counter the comb line spacing is 500 MHz, but the video bandwidth of
the counter is 530 MHz. (As seen in Figure 9a, a frequency measurement
band begins 15 MHz above the comb line and ends 45 MHz above the
next comb line.) As Figure 9b indicates, the FM tolerance of this
particular design is over 500 MHz with the carrier located mid-band,
and diminishes to 30 MHz at band edges. These are typical values for
the heterodyne converter technique.
a.
Max p-p
Deviation
(MHz)
LO = 4000
LO = 3500
3515
LO = 2500
3015
2515
2545
3045
500
4015
LO = 3000
b.
4545
4045
3545
Band
Overlap
400
300
Center of
Video
Pass Band
200
100
265
30
50
100 150 200 250 300 350 400 450
Difference Frequency into Video Amplifier
MHz
500 550
Figure 9. Analysis of the FM tolerance of the heterodyne down-conversion technique. The HP 5341A Frequency Counter is
used as an example.
12
The transfer oscillator’s tolerance of frequency modulation is more
complex. As seen in Figure 10a, the maximum allowable peak-to-peak
deviation is a function of modulating frequency and carrier frequency.
In general, this tolerance is at a minimum at the point where modulating frequency is equal to the bandwidth of the input phase lock loop. If
more than one tone modulates the carrier simultaneously (as in the
case of the multichannel communication modulation), the analysis of
Figure 10a is no longer applicable. A typical response to multichannel
FM is shown in Figure 10b; this is a graph illustrating the capabilities of
the HP 5340A Frequency Counter. On this chart tolerance of FM is
indicated by the number of voice channels which are modulated onto
the carrier. It can be seen that the FM tolerance of the transfer oscillator is in this case dependent upon carrier frequency and the per-channel
loading of the radio. Since most microwave radios operate between
86 kHz rms and 140 kHz rms per-channel loading, it can be seen that a
transfer oscillator like the HP 5340A is capable of measuring just about
all fully loaded microwave communications carriers in use today.
The FM tolerance of the harmonic heterodyne converter is much easier
to analyze. Since the down-converter automatically centers the video
frequency f if in the video amplifier’s passband (see Figure 4), then the
bandwidth of the video amplifier determines the FM tolerance. In the
case of the HP 5342A Microwave Frequency Counter, FM tolerance is
equal to or better than 50 MHz peak-to-peak.
In summary, although the transfer oscillator is capable of measuring
microwave frequencies with all common forms of FM modulation, the
heterodyne converter and harmonic heterodyne converter have a clear
advantage in the area of FM tolerance.
b.
a.
100 KHz
RMS/CH
40 KHz
RMS/CH
3600
1000
Typical HP 5340A
FM Capability
at –20 dBm
100
18 GHz
10
8 GHz
4 GHz
2 GHz
1 GHz
500 MHz
250 MHz
1.0
0.1
10
Number of Channels
∆f — Peak-to-Peak Deviation — MHz
3000
100
1K
10K
200 KHz
RMS/CH
2400
3 Master Groups
1800
2 Master Groups
1200
Typical Allowable Channel Loading
for FDM Communication Carriers
Measured by the HP 5340A
600
100K
1M
fm — Modulating Frequency — Hz
10M
0
2
4
6
8
10 12 14 16
Carrier Frequency — GHz
18
20
Figure 10. Graphical representation of the FM tolerance of a transfer oscillator counter — the HP 5340A Frequency Counter.
13
AM Tolerance
A second form of modulation encountered during microwave measurements is amplitude modulation. Few microwave radios use AM for
communications transmissions, but nearly all microwave sources
provide signals with incidental AM. Also, in many R&D and maintenance environments a time-varying attenuation of the signal is commonly encountered.
The heterodyne converter’s tolerance to amplitude modulation is
limited by its AGC circuitry when such a circuit is employed in the
counter design. In Figure 2 we saw that the AGC circuit is used to
provide a variable attenuation of the input according to the signal
strength entering the counter. If the signal amplitude at the input varies
due to AM, it is possible that the AGC circuitry will be unable to track
the changing level and will prevent operation of the counter. A practical limitation of AM tolerance for the heterodyne converter is less than
50% AM.
The transfer oscillator and harmonic heterodyne converter suffer no
such limitations with respect to AM. Essentially the only requirement
of these down-converters when measuring an amplitude modulated
signal is that the lowest amplitude point of the waveform be strong
enough that the counter can continue to measure. For example, the
HP 5340A or HP 5342A can easily measure a carrier at a level of
–10 dBm with 95% AM.
Amplitude Discrimination
Frequently a microwave counter will be called upon to measure a signal in
the presence of other lower level signals. The ability to perform this
measurement directly is referred to as amplitude discrimination.
All modern microwave counters incorporate amplitude discrimination
in their designs. This capability is one of the key features of the
transfer oscillator and harmonic heterodyne converter. These counters
are typically capable of always finding the most prominent component
of the spectrum, provided that it is at least 2 dB above nearby signals
and at least 10 dB above signals at the far end of the counter’s frequency range. Figure 11 is an illustration of these measurement
capabilities.
The heterodyne converter is somewhat more difficult to analyze.
Although this technique allows amplitude discrimination of widely
separated signals (through the use of a variable attenuator), the
questions remains: What happens when two significant frequency
components are within the same band? The answer to this situation is
that the AGC circuitry (see Figure 2) must be able to differentiate
between the two signals. Typical AGC circuits found in heterodyne
converters provide discrimination between signals which lie from 4 dB
to 30 dB apart, located in the same band.
14
2 dB (typical)
200 MHz
5 dB (typical)
Figure 11. Amplitude
discrimination
capabilities of the
transfer oscillator
and harmonic
heterodyne
converter. Each
drawing indicates
the required level
separation in order
for the counter to
distinguish the
greater signal
1 GHz
10 dB (typical)
18 GHz
Summary of the Comparison
Thus far in this note we have examined the performance trade-offs
among the three down-conversion techniques used in microwave
counters. A summary of these trade-offs is listed in Figure 12. Bold type
indicates that the technique enjoys a significant performance advantage. It should be noted that these comparisons are made on the basis
of typical specifications; a comparison of individual instruments may
produce different results in some categories.
Heterodyne Converter
Transfer Oscillator
Harmonic
Heterodyne Converter
20 GHz
23 GHz
40 GHz
Measurement Speed
150 ms acquisition
1/R gate
150 ms acquisition
N/R gate
350 ms acquisition
4/R gate
Accuracy
Time base limited
Time base limited
Time base limited
Sensitivity/
Dynamic Range
–30 dBm/35–50 dB
–35 dBm/40 dB
–30 dBm/35–50 dB
40 dB
20 dB
20 dB
FM Tolerance
30–40 MHz peak-peak
1–10 MHz peak-peak
10–50 MHz peak-peak
AM Tolerance
Less than 50%
Greater than 90%
Greater than 90%
Amplitude
Discrimination
4–30 dB
2 –10 dB
2 –10 dB
Characteristic
Frequency Range
Signal-to-Noise Ratio
Figure 12. Summary of the performance of the three principal microwave counter down-conversion techniques.
15
Additional Considerations
in Choosing a Microwave Counter
Signal Inputs
The first check of the inputs to a microwave counter is to insure that
the frequency ranges covered by the various input connectors satisfy
the requirements of the application. At times, it can be burdensome to
be continually changing the input connector from one spigot to
another. Of course, the ideal situation in a systems application is for
one connector to cover the full frequency range of the counter.
A consideration of great importance in microwave counters is that of
damage level limitations on the input signal level. Most microwave
counters today can tolerate up to +25 dBm inputs without damage. An
overload indicator which warns of an input signal level approaching
the damage level can also be very useful; most modern microwave
counters incorporate such a feature.
Systems Interface
A feature of great importance in microwave counters is systems
compatibility. Specifying an input/output interface compatible with
IEEE Standard 488-1975 insures that the counter will be able to
interact readily with printers, computers, and desk-top controllers in a
well-defined fashion. The Hewlett-Packard Interface Bus, which is
HP’s implementation of IEEE-488, provides data output and programming of all front panel controls of microwave counters using the ASCII
code via an 8-bit bidirectional bus.
IF Offsets
In some communications applications of microwave counters, it is
convenient to have the counter’s display offset by some constant. This
is a feature which is available in most microwave counters today. The
most elegant provision for offsets is found in the HP 5342A, which
allows arbitrary offsets (to a resolution of 1 Hz) to be entered via a
front panel keyboard; this feature is made possible by the use of a
microprocessor in the instrument.
Microwave Power Measurements
A common microwave instrument application is the measurement of
frequency and signal level in the same set-up. A microwave counter
which has the ability to measure and display signal level along with
frequency can be quite convenient in these situations. The HP 5342A
has an amplitude measurement option for just this purpose.
16
Some Applications of the HP 5342A
Microwave Frequency Counter
1. Problem: A maintenance technician needs to measure the local
oscillator of a microwave radio receiver. This L.O. is 70 MHz less than
the 3850 MHz transmitted frequency, but for convenience he wishes to
reference his measurement to the transmitted frequency.
Probe
Radio
IF
HP 5342A
RF
Solution: Measure the L.O. in the HP 5342A’s high-frequency input
with an offset of +70 MHz entered on the keyboard. The display will
read 70 + 3780 = 3850 MHz. Note that the 70 MHz IF oscillator can be
measured in the low-frequency input of the HP 5342A. If an output
connector for the IF is not available, an oscilloscope probe can be
used to probe the interior of the radio.
2. Problem: A simple, automated test set-up is needed to measure the
tuning linearity of microwave VCOs in a production area.
HP Interface BUS
DAC
HP 5342A
Controller
VCO
Solution: Construct a small system using the HP Interface Bus, with a
desktop computing controller such as the HP 9825A. A programmable
Digital-Analog Converter such as the HP 59303A provides the voltage
stimulus, and the HP 5342A provides the high-accuracy frequency
measurement. The computing controller can print the results, or it can
be configured to graph the output on an XY plotter.
17
3. Problem: A microwave R&D engineer needs to measure the effect of
small frequency changes on the output power of a microwave VCO.
HP 5342A
Power Supply
VCO
Solution: The HP 5342A with Option 002 measures both parameters
simultaneously. The engineer can fine tune the VCO’s output frequency
with a power supply and observe amplitude changes with frequency on
the HP 5342A’s display.
4. Problem: A researcher needs to characterize the drift of a microwave oscillator over a long time.
Oscillator
HP 5342A
Recorder
Solution: The HP 5342A with Option H01 provides analog output of
operator-selected digits in the HP 5342A’s display. With the HP 5342A
measuring the oscillator’s output, connect the analog output from the
HP 5342A’s rear panel to a strip chart recorder for an unattended set-up.
18
19
H
For more information about HewlettPackard test and measurement products,
applications, services and for a current
sales office listing, visit our web site,
http://www.hp.com/go/tmdir.
You can also contact one of the following
centers and ask for a test and measurement sales representative.
United States:
Hewlett-Packard Company
Test and Measurement Call Center
P.O. Box 4026
Englewood, CO 80155-4026
1 800 452 4844
Canada:
Hewlett-Packard Canada Ltd.
5150 Spectrum Way
Mississauga, Ontario
L4W 5G1
(905) 206-4725
Europe:
Hewlett-Packard
European Marketing Centre
P.O. Box 999
1180 AZ Amstelveen
The Netherlands
(21 20) 547 9900
Japan:
Hewlett-Packard Japan Ltd.
Measurement Assistance Center
9-1, Takakura-Cho, Hachioji-Shi,
Tokyo 192, Japan
Tel: (81-426) 56-7832
Fax: (81-426) 56-7840
Latin America:
Hewlett-Packard
Latin American Region Headquarters
5200 Blue Lagoon Drive
9th Floor
Miami, Florida 33126
U.S.A.
(305) 267 4245/4220
Australia/New Zealand:
Hewlett-Packard Australia Ltd.
31-41 Joseph Street
Blackburn, Victoria 3130
Australia
1 800 629 485
Asia Pacific:
Hewlett-Packard Asia Pacific Ltd.
17-21/F Shell Tower, Time Square,
1 Matheson Street, Causeway Bay,
Hong Kong
Tel: (852) 2599 7777
Fax: (852) 2506 9285
Data Subject to Change
Printed in U.S.A. May 1997
Hewlett-Packard Company
Copyright © 1997
5965-7661E
20
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement