HP 8360

User’s Handbook

Amplitude Modulation

Uncoupled mode can also be used for the following:

nTo increase the available AM depth if you are modulating near the minimum power range of the ALC loop.

nTo offset the power sweep range.

nTo reduce AM noise by operating at a higher ALC level.

AM Rate

The maximum AM rate available is limited by the bandwidth of the components in the RF path. At rates of about 100 kHz, the integrator can no longer respond so the ALC loop is eflectiwely opened. The feedforward path provides the capability to modulate at much faster rates.

RF components in the ALC loop limit the ALC bandwidth to

250 kHz. High power and Option 006 synthesizers are also limited to 100 kHz by the components in the RF path. Synthesizers with frequency doublers (A32) are limited by the 100 kHz bandwidth of the doubler for carrier frequencies greater than 20 GHz. Note that due to the feedforward scheme, AM bandwidth is not affected when amplitude and pulse modulation are simultaneously activated.

Deep AM

Deep AM mode is a means of reducing distortion when the desired AM depth is very deep (greater than 90%) or when modulating below an ALC level of -20 dBm. Amplitude modulation is summed with the reference level signal. The detected signal is compared to the reference. Therefore, the ALC loop should follow the AM input. However, the detector’s ability to sense low power levels limits the maximum AM depth. When the modulation signal reduces the output power level to a level which is below the detector’s range limit, the error signal generated sends the integrator to rail, resulting in gross AM distortion. This is where deep AM mode should be used.

Deep AM engages a comparator circuit (see Figure M-l) to sense the power level of the detected signal. When the signal level is out of the detector’s range, the loop integrator switch opens (opening the ALC loop). The output of the integrator is frozen, applying a constant drive to the modulator. Since the modulator’s most linear range is at low power levels, the AM envelope distortion is minimal. When the comparator senses a signal that is within the detector’s range, the integrator switch is closed, re-engaging ALC loop leveling.

Figure M-2 shows the leveled AM characteristics in the different modes. The maximum leveled output with ALC engaged is shown as the synthesizer’s maximum leveled output specification. (Individual synthesizers may have more power; watch for an IJHLG’LED message.) The minimum level is limited by the detector’s range (approximately -20 dBm.) With deep AM engaged, the minimum level (where the ALC loop is opened) is set to -13 dBm. This guarantees that the detector can still sense the signal level with no distortion. With the

Operating and Programming Reference M-15