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