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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 RF sampler (the RF is the YIG oscillator signal).
The A5 VCO output frequency (LO input) is set between 618 and 905 MHz. 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 integrated and summed with the pre-tune voltage on
the A9 YIG Driver, fine tuning the YIG oscillator to the desired frequency.
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 can set the YIG Oscillator to any frequency by using a dividing technique that can
set the A6 Frac–N VCO signal to any frequency. The A6 Frac–N VCO output is the phase reference
for the comparator on the A5 Sampler, so small changes in the divide number result in small
changes in the A6 Frac–N VCO output frequency, which result is small changes in the YIG
Oscillator frequency.
FM Mode
FM rates up to 230 Hz are accomplished by combining the FM signal and the A6 Frac–N VCO tune
signal to drive the YIG Driver main coil.
FM rates above 230 Hz are accomplished using the FM drive circuit on the A9 YIG Driver to drive
the FM coil in the YIG Oscillator.
Sweep Mode
The A6 Frac–N maintains phase lock in sweep mode by providing the frequency correction voltage;
in CW mode the A5 Sampler provides the frequency correction voltage.
The A6 Frac–N downconverts the RF signal from the YIG oscillator, and phase compares it to the
phase of a reference signal generated on the A6 Frac–N. The output of the phase comparator is
integrated and applied to main or FM coil drive paths using a crossover circuit on the A9 YIG
Driver.
On the A6 Frac–N, the RF signal is downconverted to 5 MHz using fixed and programmable
dividers. The RF 5 MHz is compared to a 5 MHz reference developed by dividing 10 MHz from the
A7 Reference by 2. The difference in phase is integrated and a correction voltage is applied to the
crossover circuit on the A9 YIG Driver. By using programmable dividers, the RF side of the
comparator can be kept at 5 MHz while the RF frequency sweeps. Because the instrument can
maintain phase lock while sweeping over a finite frequency range, the A6 Frac–N divide numbers
are changed at filter switch points and band crossings.
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