Pulsed Measurement Capability of
Copper Mountain Technologies VNAs
Dec. 2015
Introduction
Pulsed S-parameter measurements are important when testing a DUT at a higher power than it can
handle without damage in the steady state, or when the normal operating mode of the DUT involves
RF pulses. Examples include amplifier chips or circuits without their thermal packages and radar
components or systems.
This application note demonstrates how the minimum measurement window and trigger timing
behavior of CMT VNAs can be experimentally verified. It was developed based on the 8 GHz Planar
804/1 VNA using v3.49 software. To ensure the measurement settings and pulse timing match the
descriptions in this application note, please install v3.49 software or later.
System Overview
Copper Mountain Technologies VNAs are capable of measuring externally generated or modulated pulsed
signals. When the pulse signal is generated externally from the VNA, the system is setup as shown below.
The 10 MHz reference signal will synchronize the two instruments to eliminate any frequency offset
between the pulsed RF signal generators and the VNA receivers. The trigger output from the function
generator is connected to the Planar 804/1 trigger input to initiate the VNA’s measurement synchronous
with the pulse occurring on the function generator output.
Connected as such, the system is not much useful for measuring S-parameters of a DUT, but rather is
configured for experimental measurements of the trigger timing and measurement window behavior.
Figure 1 - Block diagram of the test setup.
The signal to be measured is produced by the function generator, and is specified by its pulse duration
and pulse rate:
RF Signal
Pulse
duration
Figure 2 - Pulse duration and interval/rate.
Pulsed Measurement Capability of
Copper Mountain Technologies VNAs
Dec. 2015
Figure 3 - Photograph of the test setup.
Since the signal is generated by the function generator instead of the VNA itself, several instrument
configuration steps are required in order to obtain a proper measurement:
Turn off the VNA’s RF output (Stimulus -> Power -> RF Out -> Off)
Change the measurement type from S-parameter to absolute
(Measurement -> Absolute -> Receiver A (Source port 1) or Receiver B (Source port 2))
 Change the Trigger settings
o Stimulus -> Trigger -> Continuous
o Stimulus -> Trigger -> Ext Trigger -> Position -> Before Sampling
 Change the reference source
o System -> Misc Setup -> Ref source -> External
The behavior of each of these settings, as well as alternative configuration options, are detailed in
the VNA’s operating manual.


Pulsed Measurement Capability of
Copper Mountain Technologies VNAs
Dec. 2015
IF bandwidth setting
Setting the IF bandwidth (IFBW) properly is essential for obtaining reliable measurements. The IFBW
setting will determine the length of the VNA’s measurement window. The relationship is
1.2
approximately 𝑇𝑖𝑚𝑒𝑚𝑒𝑎𝑠𝑢𝑟𝑒 =
. e.g 1 kHz IFBW -> 1.2 msec; 10 kHz IFBW -> 120 usec; 30 kHz
𝐼𝐹𝐵𝑊
IFBW -> 40 usec; etc. And some overhead time that relates to the trigger jitter time and uncertainty
of measurement time needs to be added as well, which is about 10 usec.
The length of measurement window must be shorter than the pulsed signal itself in order to obtain
a stable measurement result. Otherwise, the VNA’s measurement will include the edge of the pulsed
signal and some portion of time with no signal present, which contributes only noise to the
measurement result.
External Trigger Delay
External trigger delay allows for introducing a delay between the occurrence of trigger event and
initiation of the actual measurement. It can be useful when trying to avoid the rising edge, or when
measuring a specific portion of a pulsed signal.
The timing is as shown below:
Signal being measured:
Trigger Signal:
Measurement Window:
Trigger
Delay
Figure 4 - Timing diagram of signals in the system.
Figure 6 shows the trigger and RF signals as captured by oscilloscope.
Measurement Delay
Measurement delay is applied between Setup and Sampling. Measurement delay can be setup by
Stimulus -> Meas Delay. Measurement delay is useful when extra time is need for the DUT to be
ready, e.g. when measuring a DUT with a long internal time delay or slow time constant.
Setup Time
Measurement
Delay
Measurement
Window
Figure 5 - Measurement sequence for each point in the sweep.
Note that measurement delay will only shift the measurement window relative to the trigger event
if the trigger is set to “Before Setup” (Stimulus -> Trigger -> Ext Trigger -> Position -> Before Setup).
Pulsed Measurement Capability of
Copper Mountain Technologies VNAs
Dec. 2015
In this case, the analyzer will start Setup (retuning and levelling of its generator output) after
receiving the trigger signal. The total amount of time (Setup+Delay+Measurement) will vary a little
bit, since the time needed for setup varies.
Figure 6 - Oscilloscope screen capture of the pulsed 30 MHz signal, 300 usec duration; blue trace,
along with the trigger signal (Red trace).
Example Measurement
The following example shows measurements of a pulsed 30 MHz sinusoidal signal where the
duration of each pulse is 130 usec. Each setting addressed here can be customized for any particular
measurement.
In this measurement, we only want to measure the power at 30 MHz, so the stimulus setting is set
to Start frequency = Stop frequency = 30 MHz.
If we want to measure 201 points, just set “Points” to 201. Under this configuration, 201 external
trigger events will complete a full cycle of measurements (a complete sweep in the plot window).
Suppose we want to measure power at 3 particular frequencies, for example 30 MHz, 70 MHz and
100 MHz. We can setup a segment sweep for those three frequencies (Stimulus -> Sweep Type ->
Segment) and configure the segment table (Stimulus -> Segment Table).
For each trigger signal received, we want to measure one point. Therefore, set the trigger event to
on point (Stimulus -> Trigger -> Ext Trigger -> Event -> On point).
Since the pulse duration is 130 usec, we want to set the measurement window to be shorter than
that. So we set the IFBW to be 30 kHz, which corresponds to approximately 40 usec. And to make
Pulsed Measurement Capability of
Copper Mountain Technologies VNAs
Dec. 2015
sure it’s not measuring the rising edge of the pulsed signal, the external trigger delay is set to 50
usec. Now we are ready to begin measurements. The measurement timing diagram can be shown
as follows:
Signal being measured:
Trigger Signal:
Measurement Window:
50 us Trigger
Delay
Figure 7 - Timing diagram with 50 usec trigger delay. The pulse and trigger signals begins
simultaneously; Trigger Delay shifts the measurement window to a later portion of the pulse.
VNA measurement results at 30 MHz, 70 MHz and 100 MHz (3 points in total) are shown below. It
shows that the first point (30 MHz) measures about +0.2 dBm signal strength and the following two
points (70 MHz and 100MHz) do not measure much power. This is what we would expect, since the
generated signal only contains energy at 30 MHz.
Figure 8 - Measurement of 30 kHz pulse with external trigger on point.
The following screenshot shows the same VNA measurement when the external delay is set to 400
usec. When the external trigger delay is too long, the pulsed signal will be missed by the
measurement window, so we will see little or no signal energy measured at 30 MHz.
Pulsed Measurement Capability of
Copper Mountain Technologies VNAs
Dec. 2015
Figure 9 - Same test configuration but with trigger delay too large to catch the pulse.
Here is the approximate shortest pulse duration measureable for several CMT VNA models,
corresponding primarily to each instrument’s maximum IF Bandwidth:
Planar 304/1, 8x4/1, 808/1
S5048, S7530
Cobalt C1209
Cobalt C1220
Max IF BW
Minimum Measurable Pulse Duration
(including overhead time; typ.)
30 kHz
50 usec
1 MHz
<20 usec
Conclusion:
Copper Mountain Technologies VNAs are capable of measuring externally-generated or externallymodulated pulsed signals, provided the pulse duration is at least as long as the measurement
window plus timing jitter/overheads. By adjusting the external trigger delay setting, the position of
the measurement window with respect to an externally-generated trigger signal can be optimized.
Planar series VNAs from CMT are able to measure pulses as short as 50 usec in duration, while Cobalt
series instruments can measure pulses 20 usec in duration or shorter.