E8257D/67D, E8663D PSG Signal Generators Service Guide 161TroubleshootingRF Path Description (Frequency Generation, Level Control, and Modulation)ALC LoopThe ALC loop maintains power level control and power level accuracy. Levelaccuracy is achieved by converting a portion of the RF signal to a dc voltage,comparing the measured dc voltage to a reference dc voltage, integrating thedifference, and using the integrated output to drive the ALC modulation diode.Level control is achieved by adjusting the reference voltage.The A23 Lowband Coupler/Detector, A24 Highband Coupler and A25Highband Detector couple off the RF signal and convert it to a dc voltage.Because the coupled signal is a known ratio of the RF output power, it can beused to control the RF output level. On the A10 ALC assembly, the dc iscompared to a reference voltage and integrated. The integrated voltage isused to drive the modulator diodes on the A8 Output, A30 Modulation Filter,A36 Quadraplier, and A37 44 GHz Upconverter. Temperature–sensing circuitsmaintain level correction over the specified temperature range. In sweep mode,additional compensation is applied to the ALC from the A9 YIG Driver tomaintain power flatness during sweep.A10 ALCThe A10 ALC assembly provides circuitry for level control and amplitudemodulation. In ALC On mode (closed–loop operation), the output signal level iscontinually monitored and used to adjust the modulator diode output toproduce the desired RF output level. In ALC On mode, the A10 ALC integratesthe difference between the detected voltage and the reference voltage toadjust the modulator diode drive until both voltages agree. The A23 LowbandCoupler/Detector, A24 Highband Coupler, and A25 Highband Detector providethe feedback (detected DC) voltage to the A10 ALC. On the A10 ALC, thereference voltage is generated by DACs that are controlled by the CPU. TheDAC’s output voltage is calibrated to the RF output power level.In ALC Off mode (open–loop operation), the feedback path (the RF detectedvoltage path) is opened, and only the reference voltage is used to set theoutput power level. With the modulator reference level set to a fixed drive level,and without the output level being monitored, the RF output level can varyfrom the set power level. In open–loop operation, the power level can also driftover time due to changes in temperature.In external leveling operation, the internal detectors are replaced with anexternal detector, so the level control voltage is provided by the externaldetector. The external detector must be a negative detector to provide thecorrect polarity for the A10 ALC. In standard instruments, the external levelingoperating range is option–dependent, but is either –20 dBm or –15 dBm tomaximum power.Level accuracy is a function of the detector and instrument calibration.Because the instrument is calibrated with internal detectors, external levelaccuracy depends on the difference in characteristics between the internal andexternal detectors.
