162 E8257D/67D, E8663D PSG Signal Generators Service Guide
Troubleshooting
RF Path Description (Frequency Generation, Level Control, and Modulation)
When internally leveled, the ALC circuit is temperature compensated to
improve level accuracy over the specified temperature range. Temperature
compensation for the A25 Highband Detector consists of a temperature
sensing circuit on the A24 Highband Coupler. A resistor pack calibrates the
A25 Highband Detector; if the A25 Highband Detector is replaced, a new
resistor pack is provided with the new detector, and must be installed on the
A24 Highband Coupler. Although rare, unstable level performance above
2 GHz can result from poorly soldered resistors on the resistor pack, or with
other components in the temperature compensation circuit.
The A23 Lowband Coupler Detector temperature correction circuit is inside the
A23 Lowband Coupler Detector package. Because the lowband detector and
coupler are in one package, they cannot be replaced independently.
Modulation (Option UNT–AM, FM, ΦM, UNU–Standard Pulse, and
UNW–Fast Pulse)
Modulation features apply only to certain options of signal generators. The A11
Pulse/Analog Modulation Generator provides modulation signals for AM, FM,
pulse, and phase, and SYNC OUT and VIDEO OUT. For phase synchronization
and modulation accuracy, a dual function generator on the A11 Pulse/Analog
Modulation Generator is tied to the 10 MHz reference from A7 Reference. Pulse
Input, EXT1, and EXT2 provide external modulation input connections. A front
panel low frequency output connector (LF Output) can be selected to output
the signal generated by the A11 Pulse/Analog Modulation Generator and can
be used to drive other instruments.
EXT 1 and EXT 2 inputs contain modulation drive level detection circuits that
generate error messages if the signal applied to the input is too small or too
large. The input impedance of EXT 1 and EXT 2 can be set for 50 or 600 ohms,
and can be configured for dc or ac coupling.
AM (Option UNT)
The AM signal from the A11 Pulse/Analog Modulation Generator is routed to
the A10 ALC, where it is summed with the ALC reference voltage where it is
used to drive the A30 Modulation Filter, A36 Quadraplier, A37 44 GHz
Upconverter, and A8 Output ALC diodes.
FM/ΦM (Option UNT)
The FM/ΦM signal is routed from the A11 Modulation Generator to the A6
Frac–N. On the A6 Frac–N:
— FM rates < 4 kHz are applied to the pre–scale control circuit and the A6
Frac–N VCO. This generates a change in the pre–scaled drive (feedback
from the VCO) to the VCO’s phase detector, causing a VCO frequency
change.
— FM rates 4 kHz to 100 kHz are applied only to the A6 Frac–N VCO.
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