TROUBLESHOOTING AND REPAIR
FREQUENCY SYNTHESIS
With a 500-ohm, 10X probe connected to the spectrum analyzer, check the levels in the
RF section against those in Table 6C-3. These levels are as measured on the spectrum
analyzer. The actual level is 20 dB higher.
NOTE
The levels in Table 6C-3 are approximate and can vary as much as ±3 dB.
Table 6C-3. Coarse Loop RF Voltage Levels
CONNECTION POINT
P2
U401 output
U405 output
U402 output
LEVEL
-14 dbm
-18 dbm
-16 dbm
-16 dbm
At the output of the divide-by-4 amplifier, there should be a 160-MHz, -16 dBm signal
as measured with the 500-ohm, 10X probe. The N-divider programs the coarse loop in
8-MHz steps. The output of the N-divider, TP1, should be approximately 40 MHz,
ECL level. For coarse loop frequencies of 640, 800 and 960 MHz, there should not be
any signal at TP2. If the rate multiplier divider (U305, etc.) is working correctly, there
should be a 2-MHz ECL signal at U305 pin 14. The programming to the rate multiplier
(128, 64, 32, 16, 8 MHz bits) is active low. There are 20 steps in the rate multiplier
programming. The logic states for the N-divider are given in Table 6C-4.
If the N-divider is functional, check the mixer amplifier Q205 and Q206. The collector
bias at C273 is approximately 8V. At TP5 there should be a 40-MHz 1.2V p-p signal.
The output of the filter, TP11, should be a 500-mV p-p slightly triangular signal. The
frequency will be a function of how close the RF signal is to 640 MHz, but less than 125
kHz.
Reenable the search oscillator by removing the ground from TP28-TP13. Ground
TP11. There should be a 100-Hz 10V p-p sine wave (~0V DC) at TP6. Program the
UUT to SPCL 943. This programs the DACs to full scale, which turns Q201-Q204 on.
There should be a 100-Hz 650-mV p-p sine wave at TP7. Clear the UUT. Remove the
jumper between TP7 and ground. The loop should be locked. If the loop still does not
lock, ground TP9. If the loop locks, the problem is in the delay line discriminator
section. If the loop is locked and there still is a status code 243, check the unlock
detector, U204.
If the phase noise of the UUT is not within specified limits, the problem could be in the
discriminator section. First measure the DC voltage at TP1 on the Discriminator PCA
(A25). It should be ± 100 mV as the RF frequency is changed. If the voltage is nearly
zero, remove the semi-rigid cable connecting to J10 and measure the power with the
spectrum analyzer. It should be between +9 and +14 dBm depending on the RF
frequency. If there is no power, or the power is low, check the RF power amplifier
levels (Q404, Q405) against Table 6C-5 using the 500-ohm, 10x probe. The collector
bias voltages should be 10.4 and 5.1, respectively.
6C-24
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