Operating instructions

LOCAL OPERATION
3-44 46882-311D
As there is no direct/linear relationship between RF signal level and audio output with FM
receivers, most manufacturers normally quote receiver sensitivity in one of the following four
ways:-
12 dB SINAD sensitivity.
20 dB weighted SINAD sensitivity.
20 dB signal to noise sensitivity.
20 dB quieting sensitivity.
When defined as part of a specification, the sensitivity will be given in the form:-
‘Sensitivity (12 dB SINAD) 0.25 µV’
or
‘Sensitivity (20 dB SINAD) 1.0 µV EMF’
or
‘12 dB (minimum) SINAD for 0.31 µV (pd) signal input (at 60% deviation and 1 kHz
modulation).’
The test method for each of the above sensitivity specifications varies, but the common features
are that each requires a signal of accurately defined level, modulation frequency and modulation
level to be injected into the receiver, the AF output level referenced, the modulation or a portion
of it, to be removed and the audio level re-referenced.
To carry out each of these tests using Service Monitor proceed as follows:-
12 dB SINAD sensitivity
This test determines the level of the modulated RF signal at which the SINAD measurement is
12 dB. The level of modulation is given as part of the test specification. The standard
modulation frequency is 1 kHz.
As the RF signal level is reduced, the ‘noise’ component of the AF output will become
proportionally greater. This will result in the SINAD dB reading becoming closer to zero.
To use this Service Monitor to make a 12 dB SINAD sensitivity test, the Service Monitor is set up
to make a SINAD distortion measurement and the RF level progressively reduced until the
SINAD level reads 12 dB. The RF level of the signal generator will equal the 12 dB SINAD
sensitivity level of the receiver.
For a GO/NO GO test the monitor is set up in the same manner as above. The RF level is set to
the specified sensitivity level and the SINAD distortion level observed. A reading higher than
12 dB would indicate GO, while a reading of less than 12 dB (i.e. closer to zero) would indicate
NO GO.
20 dB weighted SINAD sensitivity
This test is very similar to the 12 dB SINAD sensitivity test
The differences are firstly that the SINAD level specified is 20 dB and secondly that a
psophometric weighting filter is included in the test path. This filter imposes a frequency
response characteristic to the signal similar to that of the human ear response.
There are two specifications of filter in common use. One is the European standard CCITT and
the other is the American C-MESS. The specification of the receiver will state the correct filter
to select.
The Service Monitor can have a filter meeting one of these specifications fitted as an option.
The test procedure is identical to that for 12 dB SINAD sensitivity except that the SINAD
distortion pass level is 20 dB.