Pulse Modulation
Leveling Narrow Pulses
For narrow pulses of less than 1
s width, either use search leveling
mo de or use unleveled op eration. (If you do not, you will see the
output level continue to rise as the swept signal generator tries to
correct for the o portion of the cycle.)
In search leveling mode, the RF amplitude is set with pulse
mo dulation o and the ALC lo op closed. Then the lo op integrator
output is measured. Next, the integrator is disconnected and the
mo dulator is driven directly with a DC voltage which has b een set to
the value that was provided by the lo op integrator. Any AM signal
present is added to this DC voltage. This pro cedure is automatic
with search leveling mo de engaged. The level setting procedure is
automatically repeated whenever the carrier frequency or p ower level
is changed and takes approximately 250 ms. This pro cedure should
also be repeated perio dically to correct for the eects of temp erature
drift.
Unleveled op eration can b e used for very narrow pulses by op ening
the ALC lo op (see \Leveling Mo de ALCo"). The p ower level is set
in CW op eration, with pulse mo dulation o, using an external p o
wer
meter. With Option 006, pulses as narrow as 20 ns can be pro duced
in this mo de. Changes due to temp erature drift can be exp ected in
this mo de also.
Pulse Envelop e
The best pulse envelopes are obtained with the p eak RF function
(see \Peak RF Always"). This feature aligns the output lter so
that its passband is centered on the RF output. The pulse envelope
changes with frequency and changes slightly with p ower level. Swept
signal generators with Option 006 pulse capabilityvary little with
frequency.
The pulse envelope pro duced by the swept signal generator has nite
rise time and oversho ot. Below 2.0 GHz, the rise time and oversho ot
are essentially independent of frequency, but ab ove 2.0 GHz, in swept
signal generators without Option 006, they are strongly inuenced by
the passband shap e and centering of the tracking YIG lter.
Source Match
The best source match is obtained at the swept signal generator's
operating frequency. In addition, swept signal generators with certain
RF comp onents at the output provide improved broadband source
match. These include swept signal generators with Option 006, with
high p ower output (Agilent 83623B and Agilent 83624B), or with the
Option 001 step attenuator set to
10 dB.
Performance can be improved by padding between the reections. At
the source, for output p ower ab ove
0
10 dBm, setting the leveling
mo de to normal results in 0 dB atten
uation. If enough p ower is
available, uncoupled op eration can b e used to impro
ve the swept
Operating and Programming Reference M-21
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