1-98
Troubleshooting
Reference/Synthesis Loop Description
A9 YIG Driver
CW Mode
The A9 YIG Driver provides the YIG oscillator tune voltage. Coarse tuning of the YIG Oscillator
occurs when the A18 CPU sets the pre-tune DACs 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 A6 Frac–N and A5 Sampler compare the phase of a reference signal and the YIG oscillator
frequency, and generate a correction voltage that is summed with the pre-tune voltage on the A9
YIG Driver, which fine tunes the 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 YIG oscillator.
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. 10 MHz 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
frequency to either the stop frequency or a band crossing. The Sweep DAC sets the start frequency,
and the sweep DAC output voltage directly relates to the YIG output frequency:
• If sweep DAC output = 5V, YIG frequency = 5 GHz
• If sweep DAC output = 10V, YIG frequency = 10 GHz
Signals that are summed with the sweep DAC voltage include:
• delay compensation, which is calculated by the A18 CPU and is dependent on the sweep rate and
frequency band
• YIG pre-tune (the pre-tune DAC is always set to 0 volts in sweep mode)
• frequency correction voltage from the A6 Frac–N, which is dependent on the amount of
frequency correction required
To Next Page
Loading...
Need help?
General
