E8257D/67D, E8663D PSG Signal Generators Service Guide
Troubleshooting
Reference/Synthesis Loop Description
1-96
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
A28 YIG Oscillator (YO)
The A28 YIG Oscillator output frequency (3.2 to 10 GHz) is controlled by the amount of current through the main and
FM coils. The main coil is used to set the frequency and for FM ≤230 Hz. The FM coil is used for FM rates >230 Hz.
Both the A9 YIG Driver and the A28 YIG Oscillator contribute to phase noise between 100 KHz and 1 MHz.
A29 20 GHz Doubler
The A29 20 GHz Doubler provides:
• a thru path for output frequencies from 3.2 to 10 GHz
• two internal couplers that provide a 3.2 to 10 GHz signal to both the A6 Frac–N for phase locking during sweep
operation, and to generate frequencies ≤3.2 GHz; and the A5 Sampler to provide the feedback needed for fine tune
correction in CW mode
• a doubler path to generate frequencies from 10 to 20 GHz
• RF signal amplification and filtering
Both the A29 20 GHz Doubler and the A30 Modulation Filter contribute to phase noise >3.2GHz.
A38 Switch Filter and A43 Lowband Amp Filter (Option 1EH)
The A38 and A43 assemblies provide reduced harmonics below 3.2 GHz. The assemblies contain low pass filters that
are switched in at different frequencies. The switch filter has several modes. In the CW and analog modulation modes,
filters are on between 250 MHz and 2 GHz, with reduced power between 10 MHz and 2 GHz. In ramp, step and list
sweep modes, filters are on between 250 MHz and 2 GHz, with reduced power between 250 MHz and 2 GHz. In IQ
mode, filters are on between 10 MHz and 2 GHz, with reduced power between 10 MHz and 2 GHz, and degrade IQ
bandwidth.
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