Operating instructions

OPERATING MANUAL 2945A
46882-311D 3-49
difference in the level of the RF generator readings for the two frequencies, expressed in dB, is
the ‘image frequency rejection ratio’.
The procedure for measuring the IF rejection ratio is similar to that used to measure image
frequency rejection and uses the same setup. The RF reference level is found at the RF frequency
to which the receiver is tuned, then the RF generator is retuned to the IF frequency of the receiver
under test. The RF output level is increased until a 20 dB SINAD level is obtained. The output
level of the RF generator is noted. The difference in level of the RF generator readings, expressed
in dB, is the ‘IF rejection ratio’ of the receiver.
AGC response
Automatic Gain Control is used in AM receivers to stabilise the level of the audio output signal
against changes in the level of RF input. This is necessary as the output voltage from the detector
circuit with a constant % depth of modulation is proportional to the level of the RF signal applied
to the detector.
To plot the AGC response of a receiver, set up the instrument and receiver as described for
receiver audio frequency response tests. The test can be made at one RF frequency only or at
various points throughout the receiver tuning range. This will depend on the reason for making
the test and the specification of the receiver.
Set the RF generator frequency of the Service Monitor to the selected tuning point of the receiver
with the modulation level set to 30% at 1 kHz. Increase the RF output level until an AF output is
registered on the Service Monitor display, either bar chart or oscilloscope.
Reduce the RF output level until the AF output level of the receiver reaches the noise level.
Record the RF signal level and the AF output level.
Increase the RF generator level by suitable increments and record the level of AF output at each
point.
Continue until the RF input to the receiver reaches the maximum allowed by the receiver
specification or until the AF output level no longer increases, whichever occurs at the lowest RF
level.
FM receivers incorporate limiting circuits which reduce the level of the RF signal by clipping the
extremities of the signal rather than by compression which is necessary with AGC circuits for
AM receivers.