Operating instructions

LOCAL OPERATION
3-47
Making measurements
(Examples of receiver tests).
The procedure for making each of the tests listed in the receiver testing overview, above, follows.
Some of the tests may require separate operations and others will only require a reading to be
made.
Sensitivity
Receiver sensitivity is defined as the minimum RF level required at the antenna to produce an
intelligible output of the modulated information.
The problem with the above definition is that of defining ‘intelligible output’. As this is a
subjective decision, sensitivity specifications require certain parameters to be set down. The
nature of FM and phase transmissions makes sensitivity measurements less straightforward than
with AM transmissions. A sensitivity test on an AM receiver can be stated as ‘X watts audio
output from Y µV RF input at Z% modulation’.
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% of maximum system 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.