TROUBLESHOOTING AND REPAIR
FREQUENCY SYNTHESIS
The steering voltage from the DAC on the Sub-Synthesizer PCA (J1-16), is amplified
and filtered (U207) before it is sent to the Coarse Loop VCO (J15). This stage is a
500-Hz active Bessel filter.
The discriminator "measures" the frequency noise of the VCO and generates a
correction signal to reduce the noise. A portion of the RF output from the Coarse
Loop VCO (P2) is amplified (U402) and fed into a two stage limiting power amplifier
(Q405-6). The output of the amplifier is filtered (C422-6, CR402-3). This filter is
switched at 712 MHz, the same point as a VCO band change. An op-amp (U209) drives
the pin diode switches (CR402-3). The output power from the filter is approximately
+20 dBm to the 90-degree power splitter on the Discriminator Board (A25). The
outputs of the power splitter are connected to the LO input of the mixer (U404) and to
the delay cable. The other end of the delay cable is connected to the RF input of the
mixer. The delay cable is trimmed so that when the coarse loop steps in 8-MHz
increments, the phase is such that the IF output is approximately at 0V DC. A filter
(C418-421, L403-4, R419-420) is connected to the IF port. This filter provides both
UHF and HF terminations for the mixer. The output of the filter is AC-coupled (C101,
R106) to a low noise amplifier (Q101-106). The output of this amplifier is resistively
summed (R123, R125) with the phaselock voltage from U206. Diodes (CR101-2) limit
the output of the discriminator amplifier while in the acquisition mode. To ensure
stability, the gain of the low noise amplifier is rolled off (C104, R119).
COARSE LOOP TROUBLESHOOTING 6C-16.
The Coarse Loop (A2) can be broken up into two sections: the reference section and
the main coarse loop. These sections can be treated independently.
REFERENCE SECTION
A status code 246 indicates a problem in the reference section. If the unit is in external
reference and there is no reference supplied, or if the reference is the wrong frequency
or is outside of the lock range specification, a status code 246 will be displayed. If the
unit is in internal reference and the high or medium stability option is installed, check
to see that the 10-MHz output from the option is correct.
Measure the voltage at TP19. If this voltage is approximately -13V, this indicates a
problem with the internal TCXO, the internal/external reference circuitry, or the
phase detector/loop amplifier. If the voltage is approximately 13V, this indicates a
problem in the 40-MHz oscillator, buffers, dividers, or phase detector/loop amplifier.
If the voltage at TP19 is approximately 13V, first check the 40-MHz oscillator. When
measuring signals with an oscilloscope all voltages are approximate. Use a 10-Meg, 8pf
probe. Make a ground connection at the probe tip with less than 1 inch of lead. There
should be a 40-MHz signal, 1.4V p-p at TP4. All frequencies are not exact because the
reference loop is not locked and will typically be slightly higher than indicated. After
the buffers at the L605, C624, C625 and L605, C617, C625 junctions, this signal should
be 630 mV p-p. After the ECL buffer, at U602 pin 2, there should be a 40-MHz ECL
signal. At TP17, which connects to the Sub-Synthesizer (A4), there should be a 700 mV
p-p signal. The output of the first divide-by-2, U506 pin 15, should be a 20-MHz ECL
square wave. Following the buffer, U513, that supplies the modulation oscillator (A6),
there should be a 20-MHz, 1.2V p-p signal at TP17. There should be a 10-MHz ECL
square wave at U506 pins 2 and 3. At the collector of Q505, there should be a 10-MHz
TTL signal. At J7, the 10-MHz Reference Out, there should be a 10-MHz 5V p-p sine
wave. When in internal or external 10-MHz reference, REFSEL should be a logic high.
There should be a 10-MHz TTL signal at the phase detector input, U503 pin 11. When
6C-22
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