June 15, 2005 6815854H01-A
4-18 Troubleshooting Procedures: UHF Range 2 (450–520 MHz) Band Main Board Troubleshooting
NOTE: The UHF band radios use high-side injection with an IF frequency of 109.65 MHz; therefore,
the receive VCO frequencies are 109.65 MHz higher than the selected radio channel
frequencies.
NOTE: If a failure is indicated in any of the VCOs, whose operation is defined in the table above, then
the respective circuit should be checked to ensure proper bias is being applied to the
transistor. Specifically, the transistor collector pin should be at 8.2 V and base-emitter voltage
should be on the order of 0.8 V. The steering line voltage should be in the range of 2 V for the
lower frequency end of operation, and on the order of 11V at the upper frequency end of
operation. There could be typically +/- 1 V deviation from the above quoted numbers, which
allow for tolerances in the components. A clean, steady, frequency spike should be observed
on a spectrum analyzer of typically -18 dBm at the end of the resistive pad in the collector
circuitry of a properly working VCO.
3. Continue troubleshooting by using the Fail 001 troubleshooting chart in Chapter 5.
Table 4-11. VCO Frequency Calculation and Switching Logic (UHF Range 2)
VCO AUX1 AUX2 AUX3 VCO Frequency
RX VCO Q5905
001
559.65 <= f < 582.65
RX VCO Q5903
110
582.65 <= f < 605.65
RX VCO Q5901
010
605.65 <= f < 629.65
TX VCO Q5826
100
450 <= f < 485
TX VCO Q5825
000
485 <= f <= 520
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