Introduction
High-performance vector network analyzers are used to
characterize the frequency responses of various RF and
microwave components. While most network analyzers
are used to measure continuous signals, some users have
a need to pulse the RF signal on and off. The need for
pulsed signals arises for various reasons. It might be
because the total power input to the device under test
must be limited to prevent overheating, or because the
component will only operate under pulsed conditions, or
because the component behaves differently in response
to pulses than it does to steady state inputs. Whatever
the reason, pulsed RF creates a unique set of conditions
that must be addressed in order to produce reliable
measurement results.
It is commonly presumed that only network analyzers
such as the Agilent 8510C or the 85108A that incorpo-
rate sophisticated timing circuits, dual reference chan-
nels, and wide IF bandwidths are suitable for
measurements of pulsed systems. However, the aim of
this product note is to show that under certain condi-
tions, network analyzers such as the 8720ES and 8753ES
can produce good results at a much lower cost. The
principles in this product note apply to either model,
and to some earlier models as well; for brevity we will
refer only to the 8720ES throughout the rest of this
product note.
In general, for a standard network analyzer to be able to
make pulsed measurements, there are three conditions
that must be met. The first condition is that the RF
source must be externally modulated, since a standard
network analyzer lacks any modulating circuitry. The
second condition is that if the pulse is a single or ran-
domly timed pulse, it must exceed a width of 400
microseconds. Also for this case, the user must supply
external triggering circuitry with variable delay between
the trigger signal and the RF pulse. The third condition
is that if the RF signal is a continuous pulse train, the
minimum pulse width must exceed 500 nanoseconds and
the pulse repetition frequency must be 20 Hz or greater.
The reasons for these constraints will be explained later.
(Note: the techniques described in this product note
cannot determine the impulse response of a device.)
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