234 Receiver Fault Finding TM9100 Service Manual
© Tait Electronics Limited August 2005
10.3 Moderate or Slight Loss of Sensitivity
Introduction This section covers the case where the receiver has suffered a moderate or
slight loss of sensitivity. As measured in Task 1, the receiver gain will be less
than 500000, but not as low as 1500. With a gain less than 40000, the loss
of sensitivity will be moderate — about 15mdB too low; otherwise it will
be slight — just a few decibels too low. There are three tasks:
■ Task 7: front-end calibration and tuning voltages
■ Task 8: moderately low receiver sensitivity
■ Task 9: slightly low receiver sensitivity
The fault-diagnosis procedures of Task 8 and Task 9 are similar; although
the differences are minor they are important.
Task 7 —
Front-end
Calibration
and Tuning Voltages
If the loss of sensitivity is moderate or slight, the fault is probably in the
front-end tuning circuitry.
1. Using the calibration application, check the calibration of the front-
end tuning circuitry: Open the “Raw Data” page and click the
“Receiver” tab.
2. Record the values listed in the “Rx FE Tune BPF Settings” field — these
are the DAC values of the FE (front-end) tuning voltages for the five
frequencies FE TUNE0 to FE TUNE4.
(FE TUNE0 is the lowest frequency and FE TUNE4 the highest fre-
quency in the radio’s frequency band; the values are given in
Table 10.2.)
3. For each of the frequencies FE TUNE0 to FE TUNE4 in turn, carry out
the following procedure: Enter the CCTM command 101 aa0,
where a is the frequency in hertz.
Enter the CCTM command 376 and record the value returned — this
is the front-end tuning voltage in millivolts.
4. Compare the values measured in Step 2 and Step 3 with the nominal
DAC and voltage values listed in Table 10.2.
5. If the DAC and voltage values are correct, go to Step 8. If they are
not, go to Step 6.
6. Recalibrate the receiver using the calibration application, and check
the DAC and voltage values again.
7. If the DAC and voltage values are now correct, the fault has been
rectified; go to “Final Tasks” on page 147. If they are not, go to
Step 8.
8. Go to Task 8
if the receiver output level x measured in Task 1 was less
than 40000; otherwise go to Task 9
.
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