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Presuming that a device will not overheat during
a single sweep, pausing between sweeps can keep
the average temperature within bounds. A test-
sequencing program that contains a user-defined
pause between each measurement will do this auto-
matically. More information on test sequencing is
available in your network analyzer's user's guide.
Pausing between sweeps dramatically slows down
testing time and might not be practical in some
situations. Pulsed measurements can be a better
solution to the heating problem.
In some cases pulsed RF and DC-bias measure-
ments might be needed rather than the usual CW
measurements made with a network analyzer.
Pulsed measurements are used for several reasons.
As noted above, pulses can be configured so that
an isothermal measurement is achieved. A pulsed
signal might also be used because it is representative
of the signals that the DUT encounters in actual
use (radar is a good example of this), or if there
is interest in the transient response of a device
stimulated with a pulse, etc.
20
Pulsed measurements typically requires a test set
designed for this purpose. The Agilent 85108 is an
example of a network analyzer system designed for
pulsed measurements.
While not common, it is possible to make pulsed
RF measurements with a standard network analyzer
under certain conditions. The ability to make pulsed
measurements depends on the pulse repetition
frequency (PRF) of the pulse used, relative to the
IF bandwidth and the sampling rate of the network
analyzer and the duty cycle of the signal. There
are three cases to consider when determining if
pulsed measurements are possible using a network
analyzer. (Note that we are referring to finding a
DUT's steady-state frequency response under pulse
conditions, not a DUT's transient time-variant
response to a pulse. A standard network analyzer
cannot be used to measure the transient time-variant
response to a pulse.)
1. The first case is when the PRF of the pulse is
less than the IF BW of the analyzer. When the PRF
is less than the receiver's IF BW, the pulse side-
bands pass through the IF filter and the modulated
RF (the pulse) cannot be measured (Figure 6). So
for pulsed measurements with a PRF less than the
IF BW of the analyzer, a standard network analyzer
will give inaccurate results.
Pulsed RF
PRF
PRF
PRF
t
Figure 6. When the PRF is less than the IF bandwidth,
pulse sidebands are measured in addition to the center
spectral line.
IF Filtering
f
IF BW
Modulated IF
t
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