HP 83620A Pulse Modulation, User’s Handbook, Leveling Narrow Pulses, Pulse Envelope, Source Match

Pulse Modulation

Leveling Narrow Pulses

For narrow pulses of less than 1 /JS 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 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. 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 GHz, the rise time and overshoot are essentially independent of frequency, but above 2.3 GHz, in synthesizers 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 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 83623A and 83624A), 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 synthesizer’s source match by inserting 10 dB attenuation and using a 10 dB higher ALC level.

HP 8360	Operating and Programming Reference M-21
User’s Handbook