1MA32_0E_noise_figure_meas

1MA32_0E_noise_figure_meas
Products: FSP, FSE, FSIQ, ESI, FS-K3
Noise Figure Measurements on Amplifiers
in Pulsed Mode
Power amplifiers in digital TDMA mobile phones are fed with current only during the active slot to save
power. Using conventional measuring equipment, the noise figure, an important parameter in power
amplifiers, can only be determined with the amplifier in continuous operation. However, continuous
operation can lead to amplifier overload and produce distorted results. Now there is a convenient way of
determining the noise figure of amplifiers with pulsed power supply: using a Spectrum Analyzer (FSP or
FSE or FSIQ or ESI), Noise Measurement Software FS-K3 and a function or pulse generator.
Subject to change – Minihold 12/2000 – 1MA32_0E
Noise Figure Measurements on Amplifiers in Pulsed Mode
Contents
1
2
3
4
Overview..................................................................................................2
Principle of Operation ..............................................................................3
Test Setup ...............................................................................................4
Setting of Parameters ..............................................................................5
Presettings of FS-K3 Noise Measurement Software.....................5
Presettings on FSP........................................................................5
Presettings on FSE/FSIQ/ESI .......................................................6
5 Measurement...........................................................................................6
6 References ..............................................................................................7
7 Ordering Information................................................................................7
1 Overview
Power amplifiers in digital TDMA mobile phones are fed with current only
during the active slot to save power. The bias voltage of the amplifier is
applied in pulsed form, for GSM with a pulse width of about 1/8 of the total
period of 4.615 ms. Using conventional measuring equipment, the noise
figure, an important parameter in power amplifiers, can only be determined
with the amplifier in continuous operation. However, continuous operation
can lead to amplifier overload and produce distorted results. Now there is a
convenient way of determining the noise figure of amplifiers with pulsed
power supply: using a Spectrum Analyzer (FSP or FSE or FSIQ or ESI),
Noise Measurement Software FS-K3 and a function or pulse generator.
1MA32_0E
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Rohde & Schwarz
Noise Figure Measurements on Amplifiers in Pulsed Mode
2 Principle of Operation
Noise Measurement Software FS-K3 determines the noise figure at each
frequency point in zero span mode. The noise source is switched on and off
at every point and the average power measured by the analyzer. From the
average values obtained, the software calculates the noise figure and the
gain.
To determine the noise figure of an amplifier for a given part of a periodic
pulse, the analyzer needs a corresponding periodic trigger signal. The
active phase of the amplifier can be displayed, starting from the lefthand
edge of the analyzer screen, by setting an appropriate sweep time (1 ms for
GSM applications) and a suitable trigger offset. Now the search limits have
to be activated and set so that only the trace section representing the active
phase of the amplifier is used for calculating the average power and thus
the noise figure. This is illustrated by Fig. 1.
Fig. 1:
1MA32_0E
Correct trigger setting and setting
sectionusing search limit lines (T1, T2)
3
the
required
trace
Rohde & Schwarz
Noise Figure Measurements on Amplifiers in Pulsed Mode
3 Test Setup
Function Generator
EXT.TRIGGER
Output
Sync
Spectrum
Analyzer
FSP
(FSE/FSIQ/
ESI)
TTL
VBIAS
Vcc
DUT
Noise Source NC346B
VBIAS
Fig. 2:
Test setup for determining the noise figure of an amplifierin
pulsed mode
The bias input of the amplifier under test is driven by a function or pulse
generator. The pulse width and repetition period are set for the standard
being tested, e.g. 577 µs or 4.615 ms for GSM. The pulse amplitude is set
depending on the bias voltage so that the required current flows in the
amplifier under test.
The external trigger input of the analyzer is driven by a TTL pulse from the
function generator, from the sync or trigger output, which is synchronous to
the pulse applied to the amplifier. This synchronizes the pickup of
measured values with the switch-on phase of the amplifier under test.
The noise source is connected to the noise source output of the analyzer.
The output signal of the noise source drives the input of the amplifier under
test. It is advisable to terminate the amplifier output with a power attenuator
(6 dB attenuation) that matches the output power capacity of the amplifier to
protect the RF input of the analyzer.
1MA32_0E
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Rohde & Schwarz
Noise Figure Measurements on Amplifiers in Pulsed Mode
4 Setting of Parameters
FS-K3 is set up as for conventional noise measurement, with only a few
very small changes to the defaults in the DEVICE menu to set the analyzer
up for noise figure measurement. See Fig. 2.
Presettings of FS-K3 Noise Measurement Software
• INIT BEFORE MEASUREMENT must be switched off.
• Set the Sweep Time to a suitable value (about twice the length of the
active time slot). In the example given in Fig. 2, a sweep time of 1 ms
has been selected to match the slot length of 577 µs (1 ms is the
shortest sweep time selectable with FS-K3). If shorter sweep times are
needed, they can be set by hand directly on FSP (FSE/FSIQ/ESI).
• To achieve reproducible results, taking into account the relatively short
sweep time for pulsed signals, a sufficient number of sweeps have to be
averaged. About 30 is usually sufficient for FSP, which uses an RMS
detector. For an FSE/FSIQ/ESI, which all use a sample detector, the
average number has to be increased (e.g. 100 for 1 ms sweep time).
• Only for FSE (FSIQ/ESI): Set the video bandwidth to 1 MHz (deviating
from the default setting of 100 Hz) to avoid any problems during settling
of the pulsed signals.
• Click the INIT button once, then make the following manual settings on
FSP or FSE/FSIQ/ESI:
Fig. 2:
Setup of FSP with FS-K3 Noise Measurement Software
Presettings on FSP
• Key TRIG:EXTERN: Set the trigger offset so that the switching pulse
(bias ON) just appears on the screen.
• Key MEAS: TIME DOMAIN:SEARCH LIMITS ON
1MA32_0E
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Rohde & Schwarz
Noise Figure Measurements on Amplifiers in Pulsed Mode
Set START LIMIT to the start of the pulse and STOP LIMIT to the end of
the pulse (see Fig. 1).
This ensures that only the trace section representing the active phase of the
amplifier will be evaluated.
Presettings on FSE/FSIQ/ESI
• Key TRIG:EXTERN: Set the trigger offset so that the switching pulse
(bias ON) just appears on the screen.
• Key MARKER SEARCH:SEARCH LIMITS ON:
• Key LINES: Set TIME LINE 1 to the start of the pulse and TIME LINE 2
to the end of the pulse.
5 Measurement
Further operation as well as graphical or tabular output of results with Noise
Measurement Software FS-K3 are the same as with conventional
measurements.
Fig. 4:
1MA32_0E
User interface and graphical display of results
with Noise Measurement Software FS-K3
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Rohde & Schwarz
Noise Figure Measurements on Amplifiers in Pulsed Mode
6 References
Data sheet PD757.2380.12 for Noise Measurement Software FS-K3
Noise Measurement Software FS-K3, News from Rohde & Schwarz No. 168
7 Ordering Information
Spectrum Analyzer
FSP3/7/13/30
FSEx20/30
FSIQ3/7/26.5/40
ESI7/26.5/40
9 kHz to 3/7/13/30 GHz
9 kHz to 3.5/7/26.5/40 GHz
9 kHz to 3.5/7/26.5/40 GHz
9 kHz to 3.5/7/26.5/40 GHz
Software
Noise Measurement Software FS-K3
1093.4495.xx
-1119.5005.xx
1088.7490.xx
1057.3028.02
ROHDE & SCHWARZ GmbH & Co. KG . Mühldorfstraße 15 . D-81671 München . P.O.B. 80 14 69 . D-81614 München .
Telephone +49 89 4129 -0 . Fax +49 89 4129 - 13777 . Internet: http://www.rohde-schwarz.com
This application note and the supplied programs may only be used subject to the conditions of use set forth in the
download area of the Rohde & Schwarz website.
1MA32_0E
7
Rohde & Schwarz
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