Tait TB7100, Руководство по обслуживанию

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202 Receiver Fault Finding TB7100 Service Manual
© Tait Electronics Limited October 2005
12.1 Faulty Receiver Sensitivity
Introduction
This section covers the determination of the extent of the receiver’s loss of
sensitivity. Depending on the nature of the fault, a reduction in receiver
sensitivity of 1 dB is often due to a reduction in receiver gain of many
decibels. It is therefore easier to measure gain loss rather than sensitivity loss.
Consequently, if the receiver sensitivity is too low, first check the receiver
gain. The procedure is given in
Task 1 below.
Task 1 —
Determine Extent
of Sensitivity Loss
Determine the receiver gain as follows. The corresponding loss of sensitivity
can then be deduced. Depending on the extent of the loss, continue with
“Excessive Loss of Sensitivity” on page 204 or “Moderate or Slight Loss of
Sensitivity” on page 208 to rectify the fault.
1. Input an RF signal (not necessarily modulated) of –90 dBm (or –84
dBm with a trigger-base radio) at the RF connector.
2. Enter the CCTM command 378 to measure the receiver output level.
3. Note the value
x
returned for the receiver output level. Depending
on the frequency band in which the radio operates, the value should
be:
Note that a change in the input level of 10 dBm should result in a ten-
fold change in
x .
4. If necessary, measure the RF voltage at the
QN
test point
(see Figure 12.1 ). (There is access through a hole in the
IF TOP
can.)
For comparison, the voltages corresponding to the above values of
x
are:
With an unmodulated RF signal the frequency should be 64.000kHz,
provided that the LO1, FCL and LO2 are locked and on the correct
frequency.
5. Given the value of x , go to the relevant section as follows:
■
x
< 1500, go to “Excessive Loss of Sensitivity” on page 204
(sensitivity is very low)
■
x
< 500 000, go to “Moderate or Slight Loss of Sensitivity” on
page 208 (sensitivity is low)
receiver output level x : normally between 500 000 and 6000 000
x = 500 000: 12mV
pp
x = 6000 000: 120mV
pp