Reflection examples
Up to this point our examples have focused on transmis-
sion measurements, using a pulse modulator attached to
port 1 of the network analyzer as shown in Figure 3.
This is simple to set up and easy to use, but unfortu-
nately it is often not valid for reflection measurements.
In a pulsed transmission measurement, the magnitude of
the received signal at port 2 depends only upon the
characteristics of the DUT and the pulse itself. In a
reflection measurement, the pulse modulator is present
in the reflected signal path. When the modulator is on,
we see the reflection characteristics of the DUT. When
the modulator is off, the network analyzer source is still
on, so we measure the reflection characteristics of the
pulse modulator. Recalling from our earlier examples
that high PRFs appear to the network analyzer as a sin-
gle continuous frequency, the net effect of a high PRF
using the setup of Figure 3 is that we measure some
“average” response, which reflects the characteristics of
both the DUT and the pulse modulator. Such a response
makes this configuration unsuitable for high PRF reflec-
tion measurements.
Measurement Conditions:
Frequency: 9 to 11.5 GHz
Number of points: 201
IF bandwidth: 1000 Hz
Pulse width: 5 µs
Pulse period: 200 µs
Trigger: internal
Delay: 0 ms
Figure 7. Invalid reflection result for high PRF
If the PRF is low, we can use the setup of Figure 3 with
the previously described techniques for low PRF trans-
mission measurements to make valid measurements in
the point-on-pulse mode.
Figure 8 shows a pulsed and non-pulsed measurement in
this mode. The results are indistiguishable.
Measurement Conditions:
Frequency: 9 to 11.5 GHz
Number of points: 201
IF bandwidth: 1000 Hz
Pulse width: 5 ms
Pulse period: 200 ms
Trigger: external on point
Delay: 2 ms
Figure 8. Valid reflection measurement in point-on-pulse mode
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