E8257D/67D, E8663D PSG Signal Generators Service Guide 149
Troubleshooting
Reference/Synthesis Loop Description
A5 Sampler (CW mode only)
The A5 Sampler contains a microwave sampler used to convert a portion of the
YIG oscillator output frequency to an IF frequency for phase comparison.
The A5 Sampler VCO provides the LO input to the sampler and the YIG
oscillator provides the RF input. The VCO is phase locked to the 1 GHz
reference board output. The 1 GHz signal is split into 2 paths. One path is
divided by 4 and mixed with the other path to generate a 750 MHz signal. The
750 MHz signal is then split into 2 paths, one of which is divided down and
input to a phase detector. The other path goes through a low pass filter and
becomes the LO side of the mixer. The RF side of the mixer is the sampler VCO
output. The mixer IF output then passes through a 155 MHz low pass filter and
into the sampler phase detector. If the divided down 750 MHz path and the
mixer output do not agree, a sampler lock is reported.
The A5 VCO output frequency (LO input) is set between 618 and 905 MHz, the
RF input is between 3.2 and 10 GHz and the sampler output is an IF signal
between 30 and 64 MHz. The IF signal passes through an 80 MHz low pass
filter, eliminating all signals above 80 MHz that might pass through the
sampler.
The IF signal is one input to a phase comparator. The second input to the
phase comparator is 10 to 80 MHz, and is the result of dividing or mixing the
500 to 1000 MHz signal from the A6 Frac–N VCO. The output of the phase
comparator is a voltage proportional to the difference in phase that is
integrated and summed with the pre–tune voltage on the A9 YIG Driver, fine
tuning the YIG oscillator to the desired frequency.
The sampler contributes to the phase noise at offsets between 10 KHz and
100 KHz. Phase noise is better in FM off mode than in FM on mode. In off mode
the A6 Frac–N VCO signal on the A5 Sampler is divided down to get the phase
comparator reference frequency of 10 and 80 MHz. In FM on mode, and for FM
rates above 230 Hz, phase noise performance degrades because the
A6 Frac–N VCO uses a mixer (required for higher FM rates) to get to the
reference frequency.
A6 Frac–N
CW Mode
The A6 Frac–N uses a dividing technique to set the YIG Oscillator to the
desired frequency. Because the A6 Frac–N VCO output is the phase reference
for the comparator on the A5 Sampler, small changes in the Frac–N
programmable divider number result in small changes in the A6 Frac–N VCO
output frequency, which result in small changes in the YIG Oscillator
frequency.
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