E8257D/67D, E8663D PSG Signal Generators Service Guide
Troubleshooting
Troubleshooting Unlocks
1-64
2802 ALC Mod System Test
• Replace the A8.
2803 Pulse Mod System Test
• Replace the A8.
2804 Clamp Detector Test
• Replace the A8.
2805 Bias Switch Test
• Replace the A8.
2806 Switch Filters Test
• Replace the A8.
2807 DAC Functionality Test
• Replace the A8.
Troubleshooting Unlocks
The phase lock loop refers to part of the frequency generating circuit used to maintain the phase relationship between
the frequency generation loop and a reference frequency. By maintaining the phase relationship, the frequency
accuracy of the signal generator is guaranteed.
To maintain frequency accuracy the phase lock loop compares the phase of a signal from the reference assembly to
the phase of the frequency loop signal. If the signals are slightly out of phase, the phase comparator circuit adjusts
the frequency loop signal until the two signals are in phase. If the phase comparator circuit cannot adjust the phase
of the frequency loop signal to agree with the reference signal, the phase comparator rails and generates an unlocked
message. The unlock message is immediately displayed on the front panel display.
There are four phase lock circuits in the frequency generation loop that can generate error messages. These phase lock
loops are located on the A6 Frac–N, A5 Sampler, and A7 Reference. There is also one other phase lock loop on the
A11 Pulse/Analog Modulation Generator that can generate an error message when the signal generator is in phase
modulation mode.
CAUTION To avoid damage to the signal generator, turn it off and disconnect it from the power source before
installing or reinstalling any assembly.
508 A6 Frac–N Loop Unlock
This error can indicate a failure in other than the A6 Frac–N Loop board. To determine which assembly is defective,
check whether the error occurs in both the CW and sweep modes, or in just the sweep mode.
• Occurs in both the CW and sweep modes:
Check P33–15 on the A31 Motherboard for the 10 MHz sync. approximately 2 V
pp
.
— If the signal is present replace the A6 Frac–N.
— If the signal is not present, troubleshoot back to the A7 Reference.
• Occurs in only the sweep mode:
1. Start narrowing the sweep frequency range and increasing the sweep time.
2. If making the sweep adjustments resolves the error, replace the A28 YIG Oscillator.
3. If the A28 YIG Oscillator does not resolve the error, replace the A9 YIG Driver.
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