Unlocked Reference Pll; Operation; Troubleshooting - HP 8560A Service Manual

Unlocked Reference PLL

Operation

The Reference PLL's 600 MHz output is generated by a 600 MHz SAWR (Surface Acoustical
Wave Resonator) VCO. The SAWR provides a high Q feedback path in the oscillator ensuring
good phase noise. If the oscillator is off-frequency, the phase-lock circuitry is probably
at fault. If there is no signal present at A15J701, or if the level is less than -3 dBm, the
oscillator has failed. Transistor Q703 provides active bias for oscillator transistor Q701.
Transistor Q704 provides active bias for 600 MHz buffer amplifier Q702. Refer to function
blocks Q and R of Al5 RF Schematic (sheet 2 of 3).

Troubleshooting

1. If Q701 and Q703 are functioning, check the bias on varactors CR701 and CR702. The
varactors should be reverse-biased between 0 V and 18 V, depending on tune voltage.
2. If the active devices are functioning properly, check the SAWR by placing a loo-ohm
resistor across U701 pins 1 and 2. This bypasses the SAWR, but provides the equivalent
loss of a correctly functioning SAWR.
3. If the oscillator begins to oscillate, the SAWR is probably defective.
4. On the HP 8560A, press (j), REAR PANEL, and 10 MHz INT .
5. Measuring tune line voltage (LO3 ERR) indicates if the Reference PLL is locked. Measure
the voltage at A15J502 pin 3. Connect the ground lead to A15J502 pin 6.
6. If voltage is not between 0 V and 5.75 V the loop is unlocked and
should be displayed on the CRT.
7. If the 600 MHz oscillator is working, the reason for the unlocked condition is either a
problem in the 10 MHz Reference or a fault in the signal path around the loop.
8. On the HP 8560A, press [mCTRL), REAR PANEL, and 10 MHz INT .
9. Check the 10 MHz reference frequency-accuracy by connecting a frequency counter to
warm-up period.
10. If a 10 MHz TTL level signal is not present, refer to the "10 MHz Reference" in
Chapter 11.
11. Measure the signal on U504 pin 3 with an oscilloscope. Refer to function block 0 of Al5
RF Schematic (sheet 2 of 3).
12. Measure the signal at U504 pin 11 with an oscilloscope. Refer to function block 0 of Al5
RF Schematic (sheet 2 of 3).
13. If TTL-level signals (approximately 10 MHz) are not present, check signals backwards
through the loop to find a fault in the signal path.
14. Use an oscilloscope to check for 10 MHz ECL level at U503 pin 3. Refer to function block
X of Al5 RF Schematic (sheet 2 of 3).
ERR 333 600 UNLK