E8257D/67D, E8663D PSG Signal Generators Service Guide 161
Troubleshooting
RF Path Description (Frequency Generation, Level Control, and Modulation)
ALC Loop
The ALC loop maintains power level control and power level accuracy. Level
accuracy 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 the
difference, 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 A25
Highband 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 be
used to control the RF output level. On the A10 ALC assembly, the dc is
compared to a reference voltage and integrated. The integrated voltage is
used to drive the modulator diodes on the A8 Output, A30 Modulation Filter,
A36 Quadraplier, and A37 44 GHz Upconverter. Temperature–sensing circuits
maintain level correction over the specified temperature range. In sweep mode,
additional compensation is applied to the ALC from the A9 YIG Driver to
maintain power flatness during sweep.
A10 ALC
The A10 ALC assembly provides circuitry for level control and amplitude
modulation. In ALC On mode (closed–loop operation), the output signal level is
continually monitored and used to adjust the modulator diode output to
produce the desired RF output level. In ALC On mode, the A10 ALC integrates
the difference between the detected voltage and the reference voltage to
adjust the modulator diode drive until both voltages agree. The A23 Lowband
Coupler/Detector, A24 Highband Coupler, and A25 Highband Detector provide
the feedback (detected DC) voltage to the A10 ALC. On the A10 ALC, the
reference voltage is generated by DACs that are controlled by the CPU. The
DAC’s output voltage is calibrated to the RF output power level.
In ALC Off mode (open–loop operation), the feedback path (the RF detected
voltage path) is opened, and only the reference voltage is used to set the
output 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 vary
from the set power level. In open–loop operation, the power level can also drift
over time due to changes in temperature.
In external leveling operation, the internal detectors are replaced with an
external detector, so the level control voltage is provided by the external
detector. The external detector must be a negative detector to provide the
correct polarity for the A10 ALC. In standard instruments, the external leveling
operating range is option–dependent, but is either –20 dBm or –15 dBm to
maximum power.
Level accuracy is a function of the detector and instrument calibration.
Because the instrument is calibrated with internal detectors, external level
accuracy depends on the difference in characteristics between the internal and
external detectors.
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