1-95
Troubleshooting
Reference/Synthesis Loop Description
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 1-85.
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, 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). 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. 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.
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