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 ERR 333 600 UNLK 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 A15J301 and verify that the reference frequency is 10 MHz f40 Hz after a 5 minute 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).

lo-42 Synthesizer Section