Tait TB7100, Service Manual

Page: 206 / 370

208 Receiver Fault Finding TB7100 Service Manual
© Tait Electronics Limited October 2005
12.3 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 500 000, but not as low as 1500. With a gain less than 40 000, the loss
of sensitivity will be moderate — about 15 dBm 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 12.2 .)
3. For each of the frequencies
FE TUNE0
to
FE TUNE4
in turn, carry out
the following procedure: Enter the CCTM command 101 a a 0 ,
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 12.2 .
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 123 . 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 40 000; otherwise go to Task 9 .