E8257D/67D, E8663D PSG Signal Generators Service Guide 151
Troubleshooting
Reference/Synthesis Loop Description
A9 YIG Driver
CW Mode
The A9 YIG Driver provides the A28 YIG Oscillator tune voltage. Coarse tuning
of the A28 YIG Oscillator occurs when the A18 CPU sets the pre–tune DAC’s
output voltage to a value that sets the YIG oscillator frequency close to the
desired frequency. 3.2 volts out of the pre–tune DAC results in a 3.2 GHz YIG
oscillator output. The A28 YIG Oscillator output (in GHz) is proportional to the
pre–tuned DAC output (in volts).
The A6 Frac–N and A5 Sampler compare the phase of a reference signal and
the YIG oscillator frequency. A correction voltage is generated and summed
with the pre–tune voltage on the A9 YIG Driver, which fine tunes the A28 YIG
Oscillator to the desired frequency. For higher rate FM operation, the FM signal
is scaled and the gain calibrated on the A9 YIG Driver before being used to
drive the FM coil in the A28 YIG Oscillator.
Both the A9 YIG Driver and the A28 YIG Oscillator contribute to phase noise
between 100 KHz and 1 MHz.
Ramp Sweep Mode (Option 007)
In sweep mode the A9 YIG Driver provides the tune voltage for the YIG
oscillator, generates the sweep voltage, and provides the ALC Reference
correction used during sweep.
The A9 YIG Driver has two similar circuits that generate:
— rear panel sweep out signal
— YIG sweep voltage
The hardware in these two circuits is identical, and includes:
— a sweep rate clock that is a digitally developed synthesis (DDS) chip with an
output frequency of 0 to 25 MHz. A 10 MHz signal from the A7 Reference
maintains the DDS frequency accuracy, and its output frequency clocks the
programmable counter at its output rate.
— a programmable counter that can count either up or down; its output
increments the sweep DAC.
—a sweep DAC
In the rear panel Sweep Out circuit, the DDS frequency is set so the
programmable counter output increments the sweep DAC at the desired sweep
rate. The programmable counter in the Sweep Out circuit always increments
the sweep out DAC from 0 to +10V.
Because the output frequency can be equal to, a fraction of, or a multiple of
the YIG oscillator frequency, the frequency of the DDS in the YIG Drive circuit
changes during a sweep so the RF output frequency always sweeps at the
desired sweep rate. The amount the programmable counter increments the
sweep DAC depends on the required tune voltage. The YIG tunes from the start
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