Measurement theory
In general two types of pulse measurements may be
made: Those with a pulse width greater than the net-
work analyzer’s response time, and those with a pulse
width less than the network analyzer’s response time.
These will be designated as low PRF measurements and
high PRF measurements, where PRF stands for pulse
repetition frequency. Each of these conditions must be
treated as a separate case, since each has its own unique
requirements.
Low PRF measurements
Every network analyzer has a sample response time that
depends upon fixed internal characteristics such as CPU
speed, and variable characteristics such as the IF band-
width of the system. It is characteristic of the digital fil-
tering used in the 8720E and 8753E that wide IF
bandwidths result in faster sample times, while narrow
IF bandwidths improve dynamic range.
If a given pulse is wider than the minimum analyzer
response time, and the proper timing is established
between the external signal that triggers the analyzer
and the signal that triggers the pulse itself, we can be
guaranteed that a valid measurement point will result.
This is because during the entire time the analyzer is
sampling, the RF signal is on. We define this condition to
be a low PRF measurement. Because the 8720ES trig-
gers on a falling edge, the external trigger signal must
often be inverted to generate this edge before the RF
pulse appears. Also, it should be noted that the analyzer
does not begin sampling until some time has elapsed
after the trigger signal has been received. Table 1 shows
the relationship between the IF bandwidth and the mini-
mum pulse width and delay for the 8720ES when operat-
ing in the low PRF mode. Note that the trigger delay
varies slightly between instruments.
Table 1. Using external triggering for low PRF
IF Bandwidth Minimum pulse Trigger delay
width for a single (typical)
1
measurement point (milliseconds)
(milliseconds)
6000 Hz 0.40 0.70
3700 Hz 0.53 0.70
3000 Hz 0.60 0.70
1000 Hz 1.10 0.70
300 Hz 3.20 0.70
100 Hz 8.90 0.70
30 Hz 32.0 0.70
10 Hz 120 0.70
Two types of low PRF measurements are possible. The
first type is commonly referred to as the pulse-profile
mode of operation. In this mode the source is operating
at a CW frequency, and the analyzer is armed to trigger a
sweep. The RF signal is pulsed on after the sweep has
been triggered. Because the 8720ES does not contain
any circuits for controlling the delay between receipt of
a trigger and the start of a sweep, the user must provide
this delay externally. If the pulse is wide enough that
several measurement points can be taken during a single
pulse, then we can see the shape of the pulse over time
as it is transmitted through the device under test (DUT).
Each point represents the same frequency over time.
This mode is useful for determining such things as pulse
droop. The reason specialized network analyzers such as
the 8530A have an advantage over the 8720ES in this
mode is that their minimum pulse width is much narrow-
er, 1 microsecond or less.
The second type of low PRF measurement is commonly
referred to as point-on-pulse mode. In this mode the
source is tuned, the RF pulsed on, and the analyzer is
armed to trigger on each individual pulse after a prede-
termined time delay. Each measurement point repre-
sents a different frequency. This mode is useful for
determining the frequency response of the DUT when it
cannot be powered up for long periods. Because the
8720ES does not begin to sample instantly when a trig-
ger signal is received, the user must provide an
adjustable delay externally. Specialized network analyz-
ers such as the 85108A have built in triggering circuits
to adjust the delay between the time the RF pulses on
and the analyzer takes a measurement. The 85108A also
uses wideband detection instead of the narrowband
detection used on the 8720ES, giving it an effective IF
bandwidth of 1.5 MHz. This means the sample response
time is much faster, which allows the user to sample at
widely variable times within pulses as narrow as 5
microseconds.
There is a third type of measurement known as peak-
response mode. This is when the pulse is narrower than
the analyzer’s response time, but not so narrow that the
analyzer fails to register any signal. It may be considered
the transition between low PRF and high PRF measure-
ments. Because of the critical timing requirements, this
mode is not recommended for the 8720E or 8753E.
3
1. The digital IF takes several ADC samples before it starts taking the "real" data for the point. It uses these samples to compute the IF gain setting; it does this even if IF gain is
controlled manually in the service menu. The most common value for this delay is 0.70 milliseconds, but some instruments exhibit delays of 0.81 or 1.00 milliseconds. There is no
way to tell which is which, except by experiment. Once the trigger delay is determined, it is constant within a given instrument (i.e. there is no dependence on settings). The user
is always safe by presuming a shorter delay than given in the table, however the minimum pulse width may have to be increased correspondingly to compensate for any
difference between the presumed delay time and the actual delay time.
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