Agilent Technologies 856290216 service manual vco

Table 1 I-1 1. Unlocked Fractional N Troubleshooting Areas
Measured VCO
Frequency Relative
to Expected Value
Measured > expected
Measured < expected
Measured, not
oscillating
Below -4 V
VCO clamp
VCO clamp
VCO clamp
Tune Voltage
*“..:‘”
vco
Divider or
integrator
vco
Divider or
integrator
Divider or
integrator
vco
About +ll V Above
+12.5 V
Divider or
integrator
vco
vco
VCO clamp
VCO clamp
VCO clamp
7. VCO clamp troubleshooting:
Q131, Q132 and the associated components should limit the tune voltage at R240 to
about fll 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 bases of these transistors
should be at about f9.6 V for proper operation.
8. VCO troubleshooting:
Check the dc biases in the VCO function block. The bias voltages, for some points in the
VCO, are indicated in Figure 11-8.
9. Divider and integrator troubleshooting:
Measure the frequency of the pulses at TP6 in block AO. Look up the expected problem
area in Table 11-12 and go to the appropriate troubleshooting steps.
Synthesizer Section 11-33