HP 8560A, Service Manual

Page: 383 / 450

Unlocked Offset Lock Loop (Sampling Oscillator)
Operation
The Offset Lock Loop drives the Sampler. The Offset Lock Loop’s sampling oscillator
tunes to one of fifteen discrete frequencies between 280 MHz and 298 MHz. Refer to Al5
Schematic (sheet 3 of 3). M ixer
mixes the oscillator’s output with 300 MHz
from the Reference PLL, producing a 2 MHz to 20 MHz IF signal. The 2 MHz to 20 MHz
signal is divided down to 2 MHz or 2.5 MHz and compared in the phase/frequency detector
with the divided-down 10 MHz from the Reference PLL. The phase/frequency detector
drives a voltage-to-current (V/I) de switch which drives the loop integrator. Loop
bandwidth switches vary the loop bandwidth to minimize noise sidebands. The sampling
oscillator must produce low noise because the
Sampler multiplies noise by a factor of
approximately 24.
Table lists the divide ratios and control-line settings for each of the 15 discrete
frequencies to which the Offset Lock Loop may be set.
Troubleshooting
1. Use an active probe and spectrum analyzer to confirm the presence of the following
references to the Offset Lock Loop’s input.
300 MHz at
10 MHz TTL-level
2. If either of these signals is not correct, refer to “Unlocked Reference PLL” in this chapter.
3. Force the PLL to unlock by shorting pin 1 to pin 5 with a short length
of wire.
4. Connect a dc power supply to pin 16.
5. Monitor with an active probe/spectrum analyzer combination. Vary the dc
supply until the frequency of the sampling oscillator is 280 MHz.
6. The voltage required to tune the oscillator should measure between Vdc and +8 Vdc.
If the voltage is out of this range, perform the Sampling Oscillator Adjustment in
Chapter 2.
7. Vary the voltage to tune the sampling oscillator to 280 MHz.
8. Use an active probe/spectrum analyzer combination to measure the 300 MHz LO signal at
the following test point:
+7
9. If the signal is not measured near the indicated power, troubleshoot the Offset Lock Loop
Buffer (function block AM of Al5 RF Schematic sheet 3 of 3).
Synthesizer Section