Faulty Receiver Sensitivity - Tait TM8100 mobiles Service Manual

10.1

Faulty Receiver Sensitivity

Introduction
Task 1 —
Determine Extent
of Sensitivity Loss
248 Receiver Fault Finding
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 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
can then be deduced. Depending on the extent of the loss, continue with
"Excessive Loss of Sensitivity" on page 250
Sensitivity" on page 255
1. 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.
2.
Note the value x returned for the receiver output level. Depending
3.
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 10dBm should result in a ten-
fold change in x.
4. 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
x = 6000 000: 120mV
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:
5.
x < 1500, go to
■
(sensitivity is very low)
x < 500 000, go to
■
page 255 (sensitivity is low)
Task 1 below.
or
to rectify the fault.
pp
pp
"Excessive Loss of Sensitivity" on page 250
"Moderate or Slight Loss of Sensitivity" on
© Tait Electronics Limited November 2007
"Moderate or Slight Loss of
test point
QN
IF TOP
TM8100/TM8200 Service Manual
can.)