To Next Page

To Previous Page

240 Receiver Fault Finding TM8100/TM8200 Service Manual

10.1 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 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 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 242 or “Moderate or Slight Loss of

Sensitivity” on page 246 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 10dBm should result in a ten-

fold change in x.

4. If necessary, measure the RF voltage at the

QN test point

(see Figure 10.1). (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 242

(sensitivity is very low)

■ x < 500 000, go to “Moderate or Slight Loss of Sensitivity” on

page 246 (sensitivity is low)

receiver output level x: normally between 500 000 and 6000 000

x = 500 000: 12mV

x = 6000 000: 120mV

Tait TM8110 Service Manual

Other manuals for Tait TM8110