HP 8360 - Manual (Page 267)

Pulse Modulation
Leveling Narrow Pulses
For narrow pulses of less than 1 width, either use search leveling
mode or use unleveled operation. (If you do not, you will see the
output level continue to rise as the synthesizer tries to correct for the
off portion of the cycle.)
In search leveling mode, the RF amplitude is set with pulse
modulation off and the ALC loop closed. Then the loop integrator
output is measured. Next, the integrator is disconnected and the
modulator is driven directly with a DC voltage which has been set to
the value that was provided by the loop integrator. Any AM signal
present is added to this DC voltage. This procedure is automatic
with search leveling mode engaged. The level setting procedure is
automatically repeated whenever the carrier frequency or power level
is changed and takes approximately 250 ms. This procedure should
also be repeated periodically to correct for the effects of temperature
drift.
Unleveled operation can be used for very narrow pulses by opening
the ALC loop (see “Leveling Mode The power level is set
in CW operation, with pulse modulation off, using an external power
meter. With Option 006, pulses as narrow as 20 ns can be produced
in this mode. Changes due to temperature drift can be expected in
this mode also.
Pulse Envelope
The best pulse envelopes are obtained with the peak RF function
(see “Peak RF Always”). This feature aligns the output filter
so that its
is centered on the RF output. The pulse
envelope changes with frequency and changes slightly with power
level. Synthesizers with Option 006 pulse capability vary little with
frequency.
The pulse envelope produced by the synthesizer has finite rise
time and overshoot. Below 2.3
the rise time and overshoot
are essentially independent of frequency, but above 2.3 in
synthesizers without Option 006, they are strongly influenced by the
shape and centering of the tracking YIG filter.
Source Match
The best source match is obtained at the synthesizer’s operating
frequency. In addition, synthesizers with certain RF components at
the output provide improved broadband source match. These include
synthesizers with Option 006, with high power output (HP
and
or with the Option 001 step attenuator set to
Performance can be improved by padding between the reflections.
At the source, for output power above -10
setting the
leveling mode to normal results in 0 attenuation. If enough
power is available, uncoupled operation can be used to improve the
synthesizer’s source match by inserting 10
attenuation and using
a 10 higher ALC level.
HP 8360
User’s Handbook Operating and Programming Reference