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50 High Frequency Electronics
High Frequency Design
PULSED MEASUREMENTS
Techniques for Pulsed
S-Parameter Measurements
By David Vondran
Anritsu Company
M
any devices, particularly power
devices, are not
designed to operate continuously or with CW signals. This is especially
true when devices are
being tested on-wafer,
where the thermal resistance is greatly increased
[1]. In these cases, S-parameter measurements are best performed in a pulsed test
environment.
The details of pulsed measurement are
greatly dependent on the pulse properties
being studied. At one extreme is the realm of
high pulse repetition frequencies (PRFs) and
fairly narrow pulses, as is common in radar
applications. At the other extreme is the communications arena where PRFs are quite low
and pulse widths fairly wide (e.g., GSM [2]).
These two extremes exemplify two techniques, termed bandwidth limited and trig-
gered, that are discussed in this note. To a certain degree, the two approaches overlap in
terms of allowed parameters, so most situations can be covered by one if not both of them.
The objective of this article is to provide an
understanding of general S-parameter measurements performed with a vector network
analyzer (VNA), over a range of pulsed conditions for both RF and microwave/mm-wave
measurement applications. Pulse profiling of
the detailed transient response is not covered
here although it is briefly discussed elsewhere
with regard to triggered measurements [3].
Certain rise/fall time behaviors can be studied
using time domain mode [4] but that will not
be discussed either.
The Spectra of Pulsed RF Signals
As the reader may be aware, a pulsed RF
signal will have a spectrum composed of a
series of spectral lines with an envelope
described by a sinc function [5]. The spacing of
the lines is set by the PRF while the envelope
shape is fixed by the pulse width (assuming
rise and fall times are small relative to the
width). The relationship is shown in Figure 1.
The ‘size’ of this spectrum (occupied frequency
range) with respect to the IF bandwidth
(IFBW) of the network analyzer determines
the measurement mode.
In the case of low PRF and wide pulses, the
entire spectrum can fit within an IFBW. In
this case, the measurement proceeds normally
without a significant reduction in dynamic
range. Some additional smoothing/averaging
may be needed to reduce effects of the outlying
Pulsed measurement is an
essential tool for measuring
the performance of power
amplifiers under low duty
cycle conditions, including
on-wafer test applications
and high peak-to-average
modulation formats
1/T0
RF
First null at
offset = 1/T1
Figure 1. The spectrum of a pulsed RF signal
is shown here. The center maximum is at the
RF frequency, the line spacing is equal to
one over the pulse period T0, and the first
envelope null offset at one over the pulse
width T1 (neglecting rise and fall times).
From March 2003 High Frequency Electronics
Copyright © 2003, Summit Technical Media, LLC
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52 High Frequency Electronics
High Frequency Design
PULSED MEASUREMENTS
portions of the distribution in Figure
1. A requirement in this measurement is that the VNA measurement
be aligned in time with the pulse,
hence the term triggered.
In the case of a high PRF, the line
spacing can be substantial relative to
the IFBW so the analyzer can just
pick off the center line (thus the term
bandwidth limited). The measurement of just this line is sufficient to
perform an S-parameter measurement—since it carries the magnitude
and phase of the envelope at the cen-
ter point—as long
as a calibration is
performed under
those same conditions. However,
since only a fraction of the total signal energy is used,
the dynamic range
may be limited.
Triggered
Measurements
Since the spectrum fits entirely
in an IFBW in this
case, the dependence of the measurement on the
pulse train would appear simple. In
the time domain sense, however, one
wants the sampling to occur during
the ‘on’ period of the pulse in order to
capture the desired information. This
is accomplished by triggering the
VNA to measure in the appropriate
points in time.
The details of this process (and its
application to other measurement
types) are covered in greater detail in
[3] but will be summarized here.
As shown in Figure 2, the idea is
for the trigger pulse to arrive at the
VNA sometime before the RF pulse in
order to account for instrument
latency although starting later is
allowed. The sampling can begin
sometime after the RF pulse has settled unless that process is of interest
as well. The sampling can continue
for a substantial portion of the pulse
but should not continue beyond the
end. As a gross limit, the IFBW must
be greater than 1/T1 (T1=pulse
width) to keep sampling from overrunning the pulse (30 kHz with averaging is preferred in the MS462xx
family; see the Appendix). Because of
pulse settling, internal filtering and
some other latency issues, some safety margin is required. This will vary
greatly depending on setup and may
require some experimentation. If too
small an IFBW is used (or with too
much averaging), the trace data will
become very noisy.
The pulse for the RF or the DUT
control often comes from a pulse generator. The external trigger pulse for
the VNA can come from another
channel of the same generator or it
can be derived from the main pulse
chain with a small delay circuit of the
user’s design. An example setup is
shown in Figure 3. Some power level
details for the various systems are
Figure 2 · An illustration of the timing in a triggered
measurement: the VNA is triggered sometime near
the start of the pulse so that data is sampled within the
duration of the pulse.
Figure 3 · An example setup for a triggered measurement is shown here. A dual channel pulse generator is used to create the trigger pulse for the VNA
as well as the RF pulse for the DUT. In other tests, a
control line to the DUT may be pulsed instead of the
RF itself.
Scorpion (MS462xx) and related systems
CW mode: trigger rates up to 900 Hz
NB swept: up to 900 Hz
WB swept: up to 200 Hz
Low frequency instruments may allow higher rates. Fast CW
(available from GPIB in firmware versions 1.16 and later)
allows higher rates.
Lightning (37xxx) and related systems
CW mode: up to 500 Hz
NB swept: up to 150 Hz
WB swept: up to 50 Hz for a 40 GHz instrument (less for 65
GHz and Panorama systems)
Fast CW mode (available from GPIB) allows higher rates
NB refers to a narrowband sweep that does not include
bandswitch points. Consult the factory for more details.
Trigger width must generally be at least 50 µs and is a TTL
level signal.
Table 1 · Triggering limits for two Anritsu Company VNA
instrument families.
Margin for
pulse settling
Safety margin before
end of burst
RF Burst
Time
Ext. trigger pulse Data sampling
Pulse
Generator
VNA
Switch
A B
Ext. trigger in
1 2
DUT
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54 High Frequency Electronics
High Frequency Design
PULSED MEASUREMENTS
given in the Appendix.
There are additional limitations
on the trigger frequency that can be
fed to the VNA. With lower PRFs as
in GSM, for example, sampling can
occur on every RF pulse. With higher
PRFs, it may be necessary to sample
only on some pulses which may
require a more elaborate timing
setup. The maximum triggering frequency is dependent on the instrument, the mode, the IFBW, the frequency range, and user-intervention
(button pushing or other interrupts)
among other issues. To give the user
some idea of maximum possible trigger rates, some limits are listed by
instrument in Table 1. These are not
guaranteed and will vary with setup
(all assume a 10 kHz IFBW, single Sparameter).
As a very simple example, the
gain of an amplifier was measured
non-pulsed and under GSM conditions at a pair of power levels. The
IFBW was quite wide in these examples (30 kHz) and only a frequency
response (i.e., normalization) calibra-
tion was performed. In
the low power case,
when the amplifier was
far from compression,
the pulsed and nonpulsed results agree reasonably well as shown in
Figure 4. When the
amplifier is in compression at the higher power
level (0 dBm in), the
results diverge as perhaps might be expected.
In this measurement
class, calibrations may
be performed without
pulsing. However, if a
pulsing switch (as in
Figure 3) is used, it
should be present so its
reflections and loss can
be calibrated out. For full calibrations
(other than just a normalization), any
pulsing switch should precede the
test coupler. Ideally, it should precede
the reference coupler as well. In the
373xx family of VNAs, the external
preamplifier loop allows the switch to
precede the test coupler. In the
MS462xx family, a version with direct
receiver access (MS462xC models) is
helpful for this. These models are
used with the various power amplifier test systems (HATS and PATS).
Bandwidth Limited Measurements
In the case of bandwidth limited
measurements, the spectrum of the
pulsed RF signal is very wide with
respect to an IFBW. The selected
IFBW must be small enough that it
does not capture significant energy
from other lines than the center lobe.
Clearly, this technique has the most
utility for PRFs of 10 kHz or higher
although lower rates are allowed if
the user is willing to use IFBWs
smaller than the typical few kHz. An
example measurement where too
wide an IFBW was used is shown in
Figure 5. Here the PRF was about 30
kHz and a 30 kHz IFBW was used;
although this might have worked if
the IF filters had perfect stop bands,
this is not true in practice. Note that
narrower IFBWs also have the advantage of increased dynamic range.
An important point about this
technique is that a fair amount of the
signal energy is thrown away by the
IF filter. The signal reduction (SR) is
given by
Thus the dynamic range will
become limited as the duty cycle
shrinks. For duty cycles of 1 percent
or more, the reduction is 40 dB or
less. This is not a problem in the
MS462xx family since 90-120 dB of
dynamic range is usually available t
begin with (at narrower IFBWs). In
the microwave and mm-wave VNAs,
Figure 4 · A comparison of pulsed (triggered under GSM conditions) and
non-pulsed measurements of an amplifier is shown here. At low power levels, the results basically agree (no match correction accounts for small differences). When the amplifier is under compression, the results start to
diverge.
Figure 5 · A bandwidth limited measurement with
a PRF of 30 kHz: The flat trace represents a measurement with an IFBW of 3 kHz and is correct;
the noisy, incorrect trace is a measurement with
a 30 kHz IFBW in which additional spectral lines
are admitted to the IF system.
Gain (dB)
Gain (dB)
Frequency (GHz)
Frequency (GHz)
SR (dB)
=
log
20
period
10
pulse width
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56 High Frequency Electronics
High Frequency Design
PULSED MEASUREMENTS
where less dynamic range is originally available, the reduction becomes
more important for some measurements and duty cycles below 1 percent become problematic. Narrower
IFBWs will enhance dynamic range
in all of these limited bandwidth
cases since less integrated noise is
sent to the receiver.
This measurement also requires
periodicity of the pulse. The triggered
measurement could be more lax in
this respect as long as the VNA was
triggered at the right time. The bandwidth limited measurement makes
assumptions about the spectral content as discussed previously so periodicity is implied.
These assumptions about spectral
content are also considered in the
method of generating the RF pulse. It
is assumed that the on/off ratio is
very high so if the switching is poor
(or the DUT control is not too effective), there will be additional uncertainties. While the spectral lines will
not move in frequency, some of the
amplitudes may change and a quality
measurement may require a smaller
IFBW than would be obvious. An
on/off ratio of 50 dB or
better is preferred.
As a simple measurement example, consider
an amplifier to be pulsed
with a PRF of 30 kHz and
a duty cycle of 3 percent.
The measurements will
be made with an IFBW of
1 kHz and compared to
non-pulsed measurements at a pair of power
levels. A one-path twoport calibration was used
to correct for input match
effects. Different calibrations were needed for the
pulsed and non-pulsed
cases since the calibration must occur under
the same signal condi-
tions for bandwidth limited measurements. The results for pulsed and
non-pulsed drives are about the same
when the RF level is low as shown in
Figure 6. At compressive levels, however, the results diverge. This might
be expected since the average signal
seen by the amplifier is quite different in the two cases.
In this measurement type, calibrations must be performed under the
same pulsed conditions. As discussed
in the previous section, any pulsing
switches should precede at least the
test coupler for full calibrations (other
than simple normalizations). In the
above example, a MS462xC direct
receiver access VNA was used together with a Handset Amplifier Test
System (HATS) test set
If simple normalization calibrations are all that are desired, then
the pulsing switch may be located
after the couplers. This was done for
the example measurement in Figure
7. In this case the DUT was a simple
W-band waveguide high pass filter
and it was measured both pulsed and
non-pulsed over approximately 71 to
80 GHz. The pulsed normalization
was done with the pulsing switch in
place and operating at a PRF of 10
kHz with a duty cycle of 10 percent.
The curves agree quite well—any differences are primarily due to differences in uncorrected match in the
two setups.
It is important to note the dynamic range well in excess of 40 dB evidenced by this plot. Even though the
pulsing reduces the dynamic range
by 20 dB (per the earlier equation)
and the pulsing switch in this case
only has about 50 dB of on/off ratio,
there is still sufficient range to make
this measurement.
Summary
Two different techniques for
pulsed S-parameter measurements
have been presented along with some
details on how to make successful
measurements. Table 2 is a summary
of conditions when each of these tech-
Figure 6 · Bandwidth limited example measurements for an amplifier are
shown here. NP denotes a non-pulsed measurement while P denotes a
pulsed measurement.
Pulsed and non-pulsed amplifier |S21|
Base RF drive = –10 dBm
Frequency (GHz)
S21 (dB)
S21 (dB)
Frequency (GHz)
Pulsed and non-pulsed amplifier |S21|
Base RF drive = 0 dBm
Figure 7 · The results of a W-band filter measurement, pulsed and non-pulsed, are shown here.
As expected for a passive device, the results
agree well. Some differences are present since
only a normalization calibration was used and
mismatches are not corrected.
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March 2003 57
niques is most appropriate.
The boxes highlighted in bold are
optimal while the other parts of the
parameter space are measurable
with some performance trade-offs.
Note that for the BW limited applications, a narrower IFBW may be
required as the PRF drops; this narrower IFBW tends to increase
dynamic range.
References
1. J. P. Teyssier, J. P. Viaud, J.J.
Raoux, and R. Quere, “Fully integrated nonlinear modeling and characterization system of microwave transistors with on-wafer pulsed measurements,” IEEE MTT-S Micr. Symp.
Dig., May 1995, pp. 1033-1036.
2. T. S. Rappaport, Wireless Com-
munications: Principles and Practice,
Prentice-Hall, 1996, Ch. 10.
3. “Burst Measurements,” Micro-
waves and RF, scheduled for publica-
tion in March or April 2003.
4. Anritsu Application Note, “Time
Domain for Vector Network Analyzers,” Rev. B, July 1998.
5. A. Oppenheim, A. Willsky and I.
Young, Signals and Systems,
Prentice-Hall, 1983, Ch. 4.
Author Information
David Vondran is Product
Marketing Engineer at Anritsu
Company, 490 Jarvis Drive, Morgan
Hill, CA 95037; tel: 1-800-ANRITSU
(267-4878). He can be reached by email at: david.vondran@anritsu.com
Pulse width >50 µs
N/A
Triggered (measure on only some
pulses)
Triggered
PRF >10 kHz
PRF 1-10 kHz
PRF <1 kHz
Duty cycle > 1%
BW limited
BW limited (lower
IFBW)
BW limited (lower
IFBW) or triggered
(if duty cycle >5%)
Duty cycle < 1%
BW limited
(reduced DR, harder at
high frequencies)
BW limited
(reduced DR and lower
IFBW, harder at high frequencies)
BW limited
(reduced DR and
lower IFBW)
difficult
Table 2 · A Summary of pulse measurement techniques and when they
should be used.
Appendix
External triggering:
In both the Scorpion and Lightning families, external
triggering must be enabled, either from the front panel or
via GPIB. The front panel paths are as follows:
Scorpion (MS462xx): Sweep (hard key)/More (soft
key)/Triggers (soft key)/External (soft key)
Lightning (37xxx): Options (hard key)/Triggers (soft
key)/External (soft key)
Power levels:
The instruments and their various optional test set combinations have different allowed input and output power
combinations. The following summarizes what is available
but these are subject to change:
Base Scorpion (MS462xx) 10 MHz to 3, 6 or 9 GHz
Maximum source power ranges from +5 to +10 dBm
depending on model. Maximum power into a port (to
avoid compression) is +10 dBm, 27 dBm damage level.
Scorpion with HATS test set, 10 MHz to 3, 6, or 9 GHz
Maximum power into port 2: +36 dBm. Preamplifiers
can be inserted in the drive chain.
Scorpion with PATS test set, selected frequency
bands (200 MHz to 9 GHz)
Maximum power into port 2: +50 dBm. Preamplifiers
can be inserted in the drive chain (higher power versions
available as special requests)
Lightning (37xxx) 40 MHz to 20, 40, 50 or 65 GHz
Maximum source power ranges from +5 to –7 dBm
depending on model. A preamplifier loop is available to
increase port power to 27 dBm. Maximum power into
port 2 is 30 dBm.
Panorama system (ME7808), 40 MHz to 110 GHz
Maximum source power is –10 dBm (0 dBm from 65 to
110 GHz). Maximum power into port 2 is 30 dBm to 65
GHz, 10 dBm from 65-110 GHz.
Averaging and IFBW:
The two instrument families perform averaging and IF
filtering functions differently and this has some timing
impact, particularly in triggered measurements. In both
cases, sweep-to-sweep averaging can be implemented off
line which will reduce data jitter but not interfere with measurement timing.
The Lightning family uses analog IF filters and increasing settling time is allocated for narrower filters. The wider
bandwidths are advised for triggered measurements.
Averaging causes additional data samples to be taken at
each frequency.
In the Scorpion family, IF filtering is done digitally and
accomplished by taking more data samples for narrower
IFBWs. It functions identically to averaging so one can compute an effective IFBW = labeled IFBW/(# of averages). In
addition, in the 30 kHz setting, gain ranging is disabled
which can introduce some timing anomalies when enabled.
For dynamic range needs of less than 70-80 dB, 30 kHz
IFBW plus some averaging will result in the most predictable timing for triggered measurements.
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