HP 8560E, Service Manual

Page: 530 / 665

Chapter 10 529
Synthesizer Section
Unlocked Reference PLL (600 MHz SAWR)
Unlocked Reference PLL (600 MHz SAWR)
NOTE
The following information is for A15 RF assemblies earlier than
08563-60054, 08563-60055, or 08563-60056. For A15 RF assemblies
with the aforementioned HP part numbers or later, refer to Unlocked
Reference PLL (100 MHz VCXO) earlier in this chapter.
Operation (600 MHz SAWR)
The reference PLL 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 A15 RF
schematic in the
HP 8560 E-Series Spectrum Analyzer Component Level
Informatio
n.
Troubleshooting (600 MHz SAWR)
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 100-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 8560E/EC, press
AUX CTRL
,
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 A15J200 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.