Amplitude Modulation

ALC loop open, the minimum level is limited by the modulator’s range to approximately -50 dBm.

p out

_____-_--_--_--_--_ I Maximum Swcified Power - - - - - -

Deep PM On \

With AM BW Cal

Deep M( On

Without AM BW Cal

Deep AM Off

Figure M-2. Power Accuracy Over the AM Dynamic Range

Calibrating the Linear Modulator

The AM bandwidth calibration feature calibrates the linear modulator gain at the current CW frequency. This results in more accurate performance in deep AM mode when the ALC loop is opened. Figure M-2 shows the calibrated response of the modulator compared to the uncalibrated response. If you choose to calibrate “Always”, the synthesizer will automatically perform the calibration whenever you change the CW frequency. Although this feature provides more accurate performance, note that it also slows the frequency switching time by 20 ms.

ALC Bandwidth

Since the ALC loop is open at power levels less than -13 dBm in deep AM mode, power levels at very slow AM rates are subject to integrate-and-hold drift of typically 0.25 dB/s. Setting the ALC bandwidth to low reduces drift by a factor of 10 by switching a larger capacitor into the integrator circuit. The larger capacitor reduces the effects of leakage on the integrator. The ALC bandwidth defaults at factory preset to the auto selection which normally selects the appropriate bandwidth (high or low) for your application. However, in this case (modulating with deep AM at a slow rate), auto mode would have set the ALC bandwidth to high where a setting of low would decrease drift. To make the bandwidth selection, the synthesizer determines which functions are activated such as frequency list mode, step sweep mode, search leveling mode, sweep frequency mode, AM or pulse modulation, among others. (For a complete explanation of the selection sequence, see “Getting Started, Advanced”.)

M-16 Operating and Programming Reference

HP 8360

User’s

Handbook