Operating instructions
LOCAL OPERATION
3-48 46882-311D
In the example above, with a ‘wanted’ signal level of −90 dB and a ‘adjacent’ signal level of
−7 dB, the performance of the receiver would be satisfactory. If the ‘adjacent’ signal level was
found to read −13 dB the performance of the receiver would be below specification.
Blocking (or desensitisation)
Blocking (or desensitisation) is an undesirable response by a receiver to a signal whose frequency
is spaced from the wanted signal by a frequency difference greater than the adjacent channel
spacing. The interfering signal will enter the IF pass band of the receiver with sufficient strength
to cause the automatic gain control to operate. This will reduce the gain of the receiver and
‘block out’ weak signals.
The test setup is the same as for selectivity testing.
The generator being used to provide the ‘wanted’ signal is set as for selectivity testing and the
level adjusted to produce a SINAD level of 20 dB.
The generator providing the ‘interfering’ signal should be unmodulated and set to an RF level of
between 80 and 90 dB above the level of the ‘wanted’ signal
The frequency of the ‘Interfering’ signal should be slowly swept over a range of 1 to 10 MHz
either side of the ‘wanted’ signal frequency and the SINAD level observed for any significant
variation. If any interaction is found, adjust the frequency of the ‘interfering’ signal for
maximum reaction. Vary the level of the ‘interfering’ signal until the SINAD level is 14 dB and
note the RF level. This is the blocking level of the receiver for the frequency concerned.
Spurious response
The two types of spurious response measurements normally carried out on a receiver are image
frequency rejection and IF rejection.
For a specific receiver type, the frequency of the image signal of a ‘wanted’ signal is twice the
receiver intermediate frequency above the wanted frequency if the receiver local oscillator runs at
a frequency above the wanted signal, or twice the receiver intermediate frequency below the
wanted frequency if the receiver local oscillator runs at a frequency below the wanted signal. See
Fig. 3-11, Spurious response ‘Image Frequency’ location.
Frequency
Local oscillator running
above
tuned frequency
885.7 MHz
(LOCAL OSCILLATOR)
875 MHz
(TUNED FREQUENCY)
896.4 MHz
(IMAGE FREQUENCY)
Frequency
Local oscillator running
below
tuned frequency
864
.
3MHz
(LOCAL OSCILLATOR)
875 MHz
(TUNED FREQUENCY)
853
.
6MHz
(IMAGE FREQUENCY)
For a 10.7 MHz Intermediate Frequency
C1614
Fig. 3-11 Spurious response ‘Image Frequency’ location
With the receiver and Service Monitor set up as for sensitivity testing, establish a reference RF
input level. Change the RF output frequency of the Service Monitor to the image frequency of
receiver tuning point, without altering the receiver tuning. Increase the RF output level of the
Service Monitor RF generator until the SINAD level reads 20 dB and note the RF level. The