Operating instructions

LOCAL OPERATION
3-50
Selectivity
Coupler
2041
Signal Generator
Service Monitor
Rx
Under
Test
DE MODULATED AF TEST SIGNAL
'ADJACENT CHANNEL' SIGNAL
'ON CHANNEL' SIGNAL
C3375
Fig. 3-12 Selectivity test setup
Selectivity tests are measurements of the capability of a receiver to differentiate between a wanted
signal and an unwanted signal, usually that transmitted on an adjacent channel.
This test is carried using the Service Monitor as for previous tests but with a second RF signal
generator simulating the adjacent channel transmitter. See Fig. 3-12, Selectivity test setup, on page
3-50.
A specification for receiver selectivity requires parameters to be set down to measure against.
Within the specification of a typical mobile radio a typical adjacent channel selectivity figure
might read:- ‘12 dB adjacent channel selectivity; better than 80 dB’, and the 20 dB weighted
SINAD sensitivity is quoted as ‘better than 1 µV’.
To confirm this selectivity figure proceed as follows:-
The equipment is connected as in Fig. 3-12, Selectivity test setup. The level of modulation for the
‘wanted’ signal is set to 60% of the maximum deviation for FM receivers or 30% modulation for
AM receivers. The modulation frequency is 1 kHz unless specified differently.
The generator being used to provide the adjacent channel signal is set to the frequency of the
adjacent channel with a modulation frequency of 400 Hz unless specified differently. The
modulation level is set to 60% of the maximum deviation for FM receivers or 30% modulation for
AM receivers. The carrier is switched off at the commencement of the test.
The level of the Service Monitor providing the ‘wanted’ signal is adjusted until a SINAD reading
of 20 dB is obtained as for sensitivity testing. The RF output level of the generator should be
noted. This should be the same level as that established in the 20 dB sensitivity test (above).
The carrier from the signal generator providing the ‘adjacent channel’ signal is switched on and
the level of its output increased until the SINAD level is reduced to 12 dB. The RF output level
should be noted.
The difference in output level of the two RF generators, expressed in dB, is the ‘12 dB Adjacent
Channel Selectivity’ level.
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.