E8257D/67D, E8663D PSG Signal Generators Service Guide 163TroubleshootingRF Path Description (Frequency Generation, Level Control, and Modulation)— FM rates >100 kHz are routed from the A6 Frac–N to the A9 YIG Driver andapplied to the FM coil in the YIG Oscillator.— For phase modulation, the FM drive signal is routed through an LCR circuitOther than the LCR circuit, ΦM signals are routed the same as FM signals.Pulse (Options UNU and UNW)Pulse modulation signals from the A11 Pulse/Analog Modulation Generatordepends on model and option can be routed directly to the A8 Output and theA30 Modulation Filter or A12 Pulse Modulation (UNW) pulse diodes. Pulsediodes must be biased on to pass RF signals. Circuitry on the A8 Output, A30Modulation Filter, A36 Quadraplier, and A37 44 GHz Upconverter keep thepulse diodes turned on in instruments that do not have modulation capability.Because ALC bandwidth is limited, the ALC circuit can only level pulse widths> 1 microsecond. To level pulse widths < 1 microsecond, use search mode.Search mode calibration sequencing is as follows:1. Modulation is turned off.2. The ALC feedback loop is closed and the RF power leveled.3. The ALC feedback loop is opened and the integrated output voltage usedto drive the ALC diodes is maintained at the level determined duringclosed loop operation (step 2).Changes in power level, frequency, or pulse settings cause the signalgenerator to repeat the search mode leveling procedure and determine a newALC diode drive level. Because drift in that leveling circuit can occur over timein open–loop operation, it is not recommended to stay in search mode forextended periods of time without closing the loop and refreshing the ALC diodedrive voltage.Digital ModulationDigital modulation is produced using portions of the following majorassemblies:— A8 Output— A13 I/Q Multiplexer— A14 Baseband Generator (Option 601/602)— A35 I/Q ModulatorTogether, these assemblies generate and route the baseband signals to the I/Qmodulators located on the A8 Output and the A35 I/Q Modulator. The A8Output modulates carrier signals form 250 kHz to 3.2 GHz, while theA35 I/Q Modulator modulates carrier signals form 3.2 to 20 GHz. TheA14 Baseband Generator (Option 601/602) comprises data generation, burstcontrol, and symbol building circuitry. The output section of the A14 BasebandGenerator consists of two 16–bit DACs: one for the I signal and one for the Qsignal. The data produced from these I/Q signals is converted to analog,
