E8257D/67D, E8663D PSG Signal Generators Service Guide
Troubleshooting
Reference/Synthesis Loop Description
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Reference/Synthesis Loop Description
This description covers generating CW and swept signals at frequencies from 3.2 to 10 GHz. For information on
generating other frequencies, see the “Overall Description” on page 1- 83.
The Reference/Synthesis Loop establishes the frequency characteristics of the signal generator. Frequency requirements
include the following:
• fundamental frequency range generation
• phase noise
• frequency accuracy
• sweep rate (in sweep mode, Option 007)
• swept frequency accuracy (in ramp sweep mode, Option 007)
Reference/Synthesis Loop Hardware
In CW operation, the hardware used includes the A7 Reference, A5 Sampler, A6 Frac–N, A7 Reference, A9 YIG Driver,
A28 YIG Oscillator (YO), and A29 20 GHz Doubler.
In sweep mode, the A5 Sampler is not used.
CW Mode
The following two basic functions are used to generate an accurate frequency:
• coarse and fine tuning
The Pre–tune DACs on the A9 YIG Driver provide coarse tuning; the A5 Sampler and A6 Frac–N assembly provide
fine–tuning.
• frequency monitoring or feedback
Example (5 GHz CW)
The A18 CPU sets the A9 YIG Driver Pre–tune DAC to a value such that the YIG oscillator output is approximately
5 GHz (within approximately 3 MHz of the set frequency). The A18 CPU also sets the A5 Sampler VCO and the
A6 Frac–N VCO frequencies to the values necessary to generate a 5 GHz YIG Oscillator output.
The A29 20 GHz Doubler receives the output of the YIG oscillator, and couples off and routes a portion of the signal
to the A5 Sampler.
On the A5 Sampler, the VCO output samples the 5 GHz from the A29 20 GHz Doubler and converts it to an IF
frequency between 30 and 64 MHz. The IF signal is then filtered and routed to a phase detector.
The output of the VCO on the A6 Frac–N is also routed to the A5 Sampler, where a divide circuit generates the phase
detector reference frequency.
At this point, the reference IF and the sampled IF are approximately at the same frequency/phase. When the two
signals are in phase, the output of the phase comparator is 0 volts. If the two signals are not in phase, the phase
detector generates a voltage proportional to the phase (frequency) difference or error.
The phase detector error voltage is integrated and summed with the YIG oscillator pre–tune voltage on the A9 YIG
Driver, fine tuning the YIG oscillator output to exactly 5 GHz.
Ramp Sweep (Option 007)
The following basic functions are required to generate an accurate sweep in ramp sweep mode:
• coarse and fine tune
• sweep rate control
• sweep generation
• output frequency monitoring or feedback
Example (start 3.2 GHz, stop 10 GHz, sweep rate 50 ms)
The A18 CPU sets the A9 YIG Driver Sweep DAC for a +3.2V output, which results in a YIG oscillator output of
approximately 3.2 GHz. To sweep the instrument from 3.2 to 10 GHz, a programmable counter on the A9 YIG Driver
increments the Sweep DAC’s output to +10V.
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