Agilent Technologies PSG Series Service Manual page 84

Troubleshooting
Overall Block Description
on the A8 Output by mixing down 1000-750 MHz with a 1 GHz LO from the A7 Reference. The signal
(100 kHz - 3.2 GHz) is then routed to the A23 Low Band Coupler/Detector and then is merged into the high
band path by a switch in the A30 Modulation Filter microcircuit. The signal path to the from panel is dictated
by model of signal generators as detailed above in the highband description.
Frequency Control
Frequency accuracy and stability are established with the A9 YIG Driver, A18 CPU, A7 Reference,
A5 Sampler, and A6 Frac-N. This process is commonly referred to as a Phase Lock Loop or PLL. The A18 CPU
instructs the A9 YIG Driver to coarsely tune the YIG with a voltage provided by the pre-tune DAC. The
A5 Sampler compares the signal from the YIG to a reference signal and generates an error voltage
proportional to the frequency error. This voltage is summed with the pre-tune DAC voltage and corrects
(fine tunes) the YIG's output signal.
The reference signal is provided by the A6 Frac-N VCO (Voltage Controlled Oscillator) and may be controlled
by an external 10 MHz signal, an internal standard 10 MHz OCXO (Oven Controlled Crystal Oscillator) on
the A7 Reference, or and optional high-stability 10 MHz OCXO. The A5 Sampler must convert the GHz range
YIG frequencies to MHz range IF frequencies to make the comparison with the reference. The comparison is
done by a phase detector which generates an output voltage proportional to the difference in frequency/phase.
This voltage is to tune the YIG to a frequency so there is no difference in frequency/phase.When the A18 CPU
coarse tunes the YIG, it also sets the A5 Sampler VCO frequency and the A6 Frac-N frequency. These VCO
frequencies are not fixed and vary according to the frequency of the YIG. The A6 Frac-N's VCO is further
divided by the A5 Sampler.
Output Power Level/Automatic Leveling Control
Output power control circuitry consists of two detectors, an ALC assembly, and two ALC modulators. This
circuitry is commonly referred to as the ALC loop. The two detectors are the A23 Low Band Coupler/Detector
and the A24 High Band Coupler and A25 High Band Detector. The A23 Low Band Coupler/Detector is used
for frequencies up to 2 GHz and the A24 High Band Coupler and A25 High Band Detector for frequencies
above 2 GHz. Note that this is not the same frequency breakpoints as defined by the Lowband and Highband
paths. The couplers provide a small portion of the RF signal to the detectors. The detectors convert the RF
signal to a dc voltage that is fed to the A10 ALC. In closed loop operation, a comparison is made between a
reference voltage and the detected voltage. If the detected and reference voltage levels agree, the modulator
drive current remains constant. If the detected and reference levels do not agree, the modulator drive current
changes causing the RF output power to increase or decrease until the reference and detected voltages agree.
In open loop operation or ALC OFF mode, only the reference voltage is used to control the modulation drive
current. The reference voltage is determined by the desired power setting and includes the stored calibration
data used to make up for any losses that occur after the coupler/detector. The reference voltage is generated
on the A10 ALC by a DAC and is controlled by the A18 CPU.
Analog Modulation
The A11 Pulse/Analog Modulation Generator includes a pulse generator and a numeric synthesizer to provide
AM, FM, PM, and pulse modulation capabilities. The numeric synthesizer can also be used to provide LF
output signals up to 1 MHz. Modulation signals are either externally generated or internally generated using
an internal waveform generator. The internal generator is tied to the 10 MHz from the A7 Reference for
better phase modulation control. There is also a feature that allows internally generated signals to be
switched to the LF Output BNC on the front panel. Modulation signals are provided to the A10 ALC (AM),
A30 Modulation Filter and A8 Output (pulse), and Frac-N (FM and PM).
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