Anritsu HFE0303 Vondran
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
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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