M850-00
T855 Fault Finding
B5.5
Copyright TEL 31/09/98
5.5 RF Checks
5.5.1 VCO Frequency
Check that the VCO is phase locked (refer to Section 5.4.2).
Connect a frequency counter (level +20dBm) to the VCO input to the mixer
(IC410).
Monitor the local oscillator frequency and check that it is 45MHz
below the
required receive frequency.
Refer to the synthesiser fault finding charts (Section 5.7.3) for further information.
5.5.2 RF Sensitivity
Ensure that the VCO is on the correct frequency and the receiver correctly aligned.
Check that the 12dB sinad sensitivity into the front end is as follows:
-117dBm (de-emphasised response)
-111dBm (flat response).
If the sensitivity is poor, you can trace the fault by measuring the sensitivity into
successive circuit blocks. Prepare a test cable by connecting a 1nF capacitor to the
end of a length of coax cable as shown in Figure 5.1.
Figure 5.1 RF Test Cable
Note:
Before using the test cable, ensure the coax braid is connected to an earth
point on the PCB.
Using the RF test cable, apply a modulated 45MHz signal to the test breaks in the
IF section, or an on-channel RF signal to the front end test breaks.
Check that the sensitivity at each test break is within 2dB of the levels shown on
the circuit diagram.
Poor sensitivity indicates a fault in one of the circuit blocks following the test break.
Note:
Poor sensitivity into the mixer can be caused by lack of drive level from the
VCO (the drive level should be >+17dBm).
Refer to the receiver fault finding charts (Section 5.7.6) for further information.
1nF
Coax
To Test Cable
Connect To Earth
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