540 Chapter11
Synthesizer Section
Unlocked Reference PLL (100 MHz VCXO)
NOTE The 300 MHz CAL OUTPUT signal comes from the tripled 100 MHz
which is passed through a leveling loop. The 300 MHz signal passes
through a low-pass filter for reducing higher harmonics. These
harmonics can fool the detector. The 300 MHz signal passes through a
variable attenuator controlled by PIN diode CR503 which is controlled
by the feedback loop. Diode CR504 is the detector diode (the same type
as CR505). Diode CR504 provides temperature compensation between
the reference voltage and the detected RF voltage.
• Measure the level of 300 MHz at A15 TP505 with an active
probe/spectrum analyzer combination. If the signal is less than
+2 dBm, repeat the first 27 steps of this procedure.
• If the signal at this point is correct, place a short across the PIN
diode CR503.
• If the signal level at the CAL OUTPUT is still less than −10 dBm
with CR503 shorted out, troubleshoot the RF forward path
through amplifier Q505. (The signal amplitude decreases.)
• If the CAL OUTPUT signal level is greater than −10 dBm,
troubleshoot the PIN diode attenuator, the detector, or the
feedback path.
29.Measure the detector voltage at A15J502 pin 14. The voltage should
measure approximately +0.3 Vdc when the CAL OUTPUT signal is
at −10 dBm. This voltage should change with adjustment of
A15R561, CAL AMPTD.
30.Check that the voltage at U507A Pin 3 is +1.7 Vdc. If this voltage is
not correct, there may be a problem with the +10 V reference.
31.Measure voltage at U507B pin 5 while adjusting R561. This is the
temperature-compensated adjustable voltage reference to which the
detected voltage is compared. It should vary between +0.15 V and
+0.6 V.
32.Adjust R561 to its limits and verify that the output U507B pin 7
measures approximately +1 Vdc at one limit and −12 Vdc at the
other limit.
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