HP 8360 B Series User Manual page 274

Leveling Narrow Pulses
For narrow pulses of less than 1 ps 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 swept signal generator 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 ALCoff'). 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 passband is centered on the RF output. The pulse envelope
changes with frequency and changes slightly with power level. Swept
signal generators with Option 006 pulse capability vary little with
frequency.
The pulse envelope produced by the swept signal generator has finite
rise time and overshoot. Below 2.0 GHz, the rise time and overshoot
are essentially independent of frequency, but above 2.0 GHz, in swept
signal generators without Option 006, they are strongly influenced by
the passband shape and centering of the tracking YIG filter.
Source Match
The best source match is obtained at the swept signal generator's
operating frequency. In addition, swept signal generators with certain
RF components at the output provide improved broadband source
match. These include swept signal generators with Option 006,
with high power output (HP 83623B and HP 83624B), or with the
Option 001 step attenuator set to 210 dB.
Performance can be improved by padding between the reflections. At
the source, for output power above -10 dBm, setting the leveling
mode to normal results in 0 dB attenuation. If enough power is
available, uncoupled operation can be used to improve the swept
Operating and Programming Reference M-21
Pulse Modulation