TROUBLESHOOTING AND REPAIR
FREQUENCY SYNTHESIS
4. With J6 still grounded, examine the generator output with a spectrum analyzer as
frequency is stepped in the range from 512 to 1056 MHz.
The frequency should always be within about 2 MHz of programmed frequency.
Note that the output section can be bypassed by examining the signal at VCO output
J7 with a 500-ohm probe, grounding the probe nearby. The level at this point is about
-14 dBm. If the signal is good, the problem is likely in another PCA. If the signal is
faulty only over a frequency band corresponding to one of the VCO bands, the
associated VCO circuit is likely at fault. If the VCO appears to be faulty, DC voltages
can be measured at various circuit nodes with the UUT programmed to frequencies
corresponding to the four VCO bands. UUT frequencies of 600, 700, 800, and 900
MHz will enable each of the four bands. Refer to Table 6C-12 for expected
approximate voltage measurements. These measurements should help isolate the
faulty circuit.
Table 6C-12. A9 Sum Loop VCO PCA Expected DC Voltages
LOCATION
ON bias transistor collector (Q5, Q6, Q7 or Q8, depends on band)
OFF bias transistor collectors
ON oscillator transistor collector (Q1, Q2, Q3 or Q5, depends on band)
OFF oscillator transistor collectors
U1,2,3,4 outputs
CR9, CR10, CR11, CR12 node
VOLTS DC
-14.4
0
7.8
9.4
4.5
9.7
6C-46
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