Operating instructions

OPERATING MANUAL 2945A
46882-311D 3-45
20 dB signal to noise sensitivity
This test determines the level of RF carrier which, when unmodulated will produce an AF output
level due only to receiver noise, 20 dB below the AF level produced when modulated at the
specified level and at the specified frequency.
As the RF signal level is reduced, the ‘noise’ component of the AF signal will become
proportionally greater. This will result in the Signal to Noise dB reading becoming closer to
zero.
To use this Service Monitor to make a 20 dB S/N sensitivity test, it is set up to make signal to
noise measurements through the distortion meter menu of the Rx TEST mode. The modulation
frequency and level are set according to the receivers test specification requirements.
The RF level is adjusted until the S/N level indicated on the Service Monitor reads 20 dB. The
RF level will then equal the 20 dB S/N sensitivity level.
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 setting and the S/N level observed. A reading higher than 20 dB would
indicate GO, while a reading of less than 20 dB (i.e. closer to zero) would indicate NO GO.
20 dB quieting sensitivity
This test determines the level of unmodulated RF signal that will produce an AF output from the
receiver that is 20 dB lower than the noise level from the receiver when no carrier is being
received.
The reduction in AF output when a carrier is being received is due to the action of the automatic
gain control circuits in the RF amplifiers of the receiver.
The gain of the receiver will reduce as the level of the RF signal increases. Therefore the noise
level will decrease with an increase in RF signal level.
To use this Service Monitor to make a 20 dB quieting sensitivity test the unmodulated RF signal
is applied to the receiver at the frequency called for in the test specification. If no frequency is
specified, a convenient mid band frequency is selected.
Select [RF Gen] from the soft key options. Turn the RF signal off by use of the orange [ON/OFF]
function key and note the indicated AF level.
Turn the RF signal
ON and adjust the RF level until the indicated AF level is 20 dB less than the
previously noted level. The indicated RF level is the 20 dB quieting sensitivity level of the
receiver.
For a GO/NO GO test proceed as above until the AF level with no RF input is noted. Then turn
the RF signal on and adjust the RF level to the figure specified as the 20 dB quieting sensitivity
level. Now observe the AF level indication. A reading greater than 20 dB will indicate GO. A
reading lower than 20 dB, (i.e. closer to zero), will indicate NO GO.
Basic sensitivity test
To carry out a basic sensitivity test on an AM receiver:
Set the RF generator frequency to that required by the test program. This setting can be entered
using the data input keys or set using the Rx=Tx facility discussed under Tx mode. The RF
generator can be switched off during the setup operation if required, by pressing the orange [ON
OFF] function key.
Select the ‘Mod Gen’ menu from the
RECEIVER TEST screen.
Set the modulation frequency and level. It is usual to use a single sinewave tone when carrying
out sensitivity tests and essential for SINAD tests.
Select the ‘pre-emphasis’ facility if required, by pressing the [Pre-emph] key.
Return to the
RECEIVER TEST screen and select the ‘AF filter’ menu. Select the required AF
filtering.