Agilent Technologies 8564EC Service Manual page 430

Synthesizer Section
Unlocked Fractional N PLL
4. Check the frequency at A14TP1. It should equal the value found by
5. Check the tune voltage at R240 in function block AQ.
6. Look up the expected problem area in
Table 11-9 Unlocked Fractional N Troubleshooting Areas
Measured VCO
Frequency
Relative to
Expected Value
Measured > expected
Measured < expected
Measured, not
oscillating
7. VCO clamp troubleshooting: Q131, Q132 and the associated
8. VCO troubleshooting: Check the dc biases in the VCO function block.
562
pressing ,
CAL MORE 1 OF 2
the information from steps 4 and 5. Go to the appropriate
troubleshooting steps.
Below −4
V
VCO
clamp
VCO
clamp
VCO
clamp
components should limit the tune voltage at R240 to about −3.3 V to
+11 V . If the integrator (its output voltage is
on TP13) tries to produce a voltage outside this range, excess current
is shunted through CR131 and Q131 for positive excursions or
CR132 and Q132 for negative excursions. The base of Q131 should
be at about +9.60 V, and the base of Q132 should be at about −2.09 V
for proper operation.
The bias voltages, for some points in the VCO, are indicated in
Figure 11-8 on page 564
, , and
FREQ DIAGNOSE
Table 11-9 on page 562
Tune Voltage
About Between
−3.3 V −2 V and
+10 V
VCO Divider or
integrator
Divider or Divider or
integrator integrator
VCO VCO
.
RAW OSC FREQ
with
About Above
+11 V +12.5 V
Divider or VCO
integrator clamp
VCO VCO
clamp
VCO VCO
clamp
Chapter 11