Faulty Receiver Sensitivity; Determine Extent Of Sensitivity Loss - Tait TM9100 Service Manual

10.1

Faulty Receiver Sensitivity

This section covers the determination of the extent of the receiver's loss of
Introduction
sensitivity. Depending on the nature of the fault, a reduction in receiver
sensitivity of 1dB 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
Determine the receiver gain as follows. The corresponding loss of sensitivity
Task 1 —
Determine Extent
can then be deduced. Depending on the extent of the loss, continue with
of Sensitivity Loss
"Excessive Loss of Sensitivity" on page 230
Sensitivity" on page 234
1.
2.
3.
4.
5.
228 Receiver Fault Finding
to rectify the fault.
Input an RF signal (not necessarily modulated) of –90 dBm (or –84
dBm with a trigger-base radio) at the RF connector.
Enter the CCTM command 378 to measure the receiver output level.
Note the value x returned for the receiver output level. Depending
on the frequency band in which the radio operates, the value should
be:
receiver output level x: normally between 500 000 and 6000 000
Note that a change in the input level of 10mdB should result in a ten-
fold change in x.
If necessary, measure the RF voltage at the
(see Figure 10.1). (There is access through a hole in the
For comparison, the voltages corresponding to the above values of x
are:
x = 500 000: 12mV
pp
x = 6000 000: 120mV
pp
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.
Given the value of x, go to the relevant section as follows:
x < 1500, go to "Excessive Loss of Sensitivity" on page 230
(sensitivity is very low)
x < 500 000, go to "Moderate or Slight Loss of Sensitivity" on
page 234 (sensitivity is low)
Task 1 below.
or "Moderate or Slight Loss of
test point
QN
© Tait Electronics Limited August 2005
can.)
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TM9100 Service Manual