228 Receiver Fault Finding TM9100 Service Manual© Tait Electronics Limited August 200510.1 Faulty Receiver SensitivityIntroduction This section covers the determination of the extent of the receiver’s loss ofsensitivity. Depending on the nature of the fault, a reduction in receiversensitivity of 1dB is often due to a reduction in receiver gain of manydecibels. It is therefore easier to measure gain loss rather than sensitivity loss.Consequently, if the receiver sensitivity is too low, first check the receivergain. The procedure is given in Task 1 below.Task 1 —Determine Extentof Sensitivity LossDetermine the receiver gain as follows. The corresponding loss of sensitivitycan then be deduced. Depending on the extent of the loss, continue with“Excessive Loss of Sensitivity” on page 230 or “Moderate or Slight Loss ofSensitivity” on page 234 to rectify the fault.1. Input an RF signal (not necessarily modulated) of –90 dBm (or –84dBm 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. Dependingon the frequency band in which the radio operates, the value shouldbe:Note that a change in the input level of 10mdB 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 xare:With an unmodulated RF signal the frequency should be 64.000kHz,provided that the LO1, FCL and LO2 are locked and on the correctfrequency.5. 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” onpage 234 (sensitivity is low)receiver output level x: normally between 500 000 and 6000 000x = 500 000: 12mV ppx = 6000 000: 120mVpp
