HP 8560A, Service Manual

Page: 358 / 450

1. The loop integrator’s output voltage (1) should be attempting to tune the oscillator to the
correct frequency.
a. The voltage at (1) should increase as the frequency increases on the following
YTO PLL
pin 1 (YTO LOOP ERROR)
Reference PLL
pin 3 (LO3 ERR)
Sampler PLL pin 13 (OFL ERR)
b. The voltage at (1) should increase as the frequency decreases on the following
Main Roller PLL pin 7 (MAINSENSE)
Offset Roller PLL
pin 1 (OFFSENSE)
Transfer Roller PLL
pin 1 (XFRSENSE)
S K 1 1 0 5
Figure Unlocked PLL
2. If the integrator’s output voltage changes in the manner described in step 1, the problem
is external to the PLL. For example, the pretune DAC could be faulty. If the integrator’s
output voltage appears incorrect, confirm that the pulses out of the phase detector (2) are
attempting to tune the oscillator in the correct direction.
3. If the phase detector’s output is bad, check the inputs to the detector (3). One input
should be higher in frequency than the other; this should match the phase detector
outputs.
4. Confirm proper power levels for the signals at the input to the “N” dividers the
reference input and the loop’s feedback path (6).
Synthesizer Section