Modulation (Am, Fm, Pm, And Pulse) - Agilent Technologies PSG Series Service Manual

Troubleshooting
RF Path Block Description (Frequency Generation, Level Control, and Modulation)
ALC ON operation is referred to as a closed loop operation because the output signal level is continually
monitored and used to adjust the modulator drive resulting unleveled output power.
In ALC OFF mode, the feedback path, the detected voltage path is opened and only the reference voltage is
used to set the output power level. Because the feedback path is open, ALC OFF mode is referred to as open
loop operation. With the modulator level is set to a fixed drive level and without the output level being
monitored, the RF output level will vary.
In external leveling operation, internal detector voltages are replaced with an external detector voltages. The
external detector must be a negative detector to provide the correct signal to the A10 ALC. Leveling accuracy
is a function of the detector and instrument calibration. Since the instrument is calibrated with internal
detectors, leveling accuracy in external mode is dependent on the detector being used.
The ALC circuit is temperature compensated to provide leveled performance over a specified temperature
range, The high band diode temperature sensing circuit is located on the RF deck on the A24 High Band
Coupler and A25 High Band Detector. A resistor pack is used to calibrate the high band detector performance
over temperature. If the A25 Detector is replaced, the resistor pack provided with the detector must also be
installed to maintain maximum performance. The low band detector can not be replace independently of the
coupler, so temperature compensation is contained within the assembly. Although rare, unstable level
performance can be the result of problems with poorly soldered resistors on the resistor pack or with other
components in the temperature compensation circuit.

Modulation (AM, FM, PM, and Pulse)

Modulation features apply only to analog versions of signal generators. The A11 Pulse/Analog Modulation
Generator provides modulation signals for AM, FM, pulse, and phase. A dual function generator is built onto
the A11 Pulse/Analog Modulation Generator and tied to the 10 MHz reference from A7 Reference for phase
synchronization. Pulse Input, EXT1, and EXT2 provide external modulation input connections. A low
frequency output (LF Output) connector located on the front panel can be selected to output the signal
generated by the A11 Pulse/Analog Modulation Generator to drive other instruments. SYNC OUT and
VIDEO OUT signals are also generated on the A11 Pulse/Analog Modulation Generator.
EXT 1 and EXT 2 inputs have modulation drive level detection circuits and generate error messages if too
small or too large of a signal is applied to the input. EXT 1 and EXT 2 can be set for 50 or 600 ohms input
impedance and can be configured for dc or ac coupling. The FM/PM modulation signal is routed to the
A6 Frac-N. FM rated of less than 4 kHz are applied to both prescale circuit for the A6 Frac-N VCO and to the
A6 Frac-N VAC's tune voltage to achieve FM. The prescale circuit modifies the VCO feedback signal to the
VCO phase detector causing a change in the VCO frequency. FM rates of 4 kHz to 100 kHz are applied to the
A6 Frac-N VCO. FM rates above 100 kHz are routed from the A6 Frac-N to the A9 YIG Driver and applied to
the FM coil in the YIG Oscillator. Phase modulation is achieved by applying the same signal used for FM to
the A6 Frac-N, where the signal goes through an added LCR circuit for PM. The modulation signal is then
routed in the same manner as when FM is used.
The AM signal is routed to the ALC where it is summed with the ALC reference voltage and applied to the
Modulation Filter and Output ALC diodes. Pulse modulation signals are routed directly to the Output and the
Modulation Filter pulse diodes. Pulse diodes must be biased on for RF to pass through. Because the ALC
bandwidth is limited, the ALC cannot properly level the RF output with pulse widths of <1 microsecond. To
compensate for leveling limitations at faster pulse rates, Search Mode operation has been added. In Search
Mode, the signal generator turns off all modulation, levels the power using closed loop operation (feedback),
then opens the loop and levels using the leveling voltage determined during closed loop operation. Changes in
power level, frequency, or pulse settings will cause the signal generator to turn modulation off, level the power
with the new settings, then open the loop and turn modulation back on. Because drift in level circuits with out
feedback control can occur over time, it is not recommended to leave Search node on for extended periods
without closing the loop and resetting the leveling voltage.
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