Operating instructions

LOCAL OPERATION
3-49
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.
Return to the
RECEIVER TEST screen and adjust the RF output level to that required by the test
specification.
Switch the RF generator output
ON by pressing the orange [ON OFF] function key.
The audio output from the receiver is measured and the level and frequency displayed.
For measuring sensitivity levels, rather than checking against settings, the various parameters on
the monitor can be adjusted in steps using the LEVEL [×] and [Ø] keys, or by precision
adjustment using the variable control.
AF bandwidth
The audio bandwidth of a receiver should be complementary to that of the transmitter or
transmitters associated with it.
The test for audio bandwidth is made by establishing a reference frequency, usually 1 kHz, and
reference level, then varying the modulating frequency until the measured audio output level
reduces by 3 dB. The 3 dB points above and below the reference frequency will be the limits of
the AF bandwidth.
When carrying out audio bandwidth tests the monitor audio filtering should be set to the widest
possible, i.e. 50 kHz.
Note that most FM transceivers apply pre-emphasis filtering to the transmitter modulation and
de-emphasis filtering to the demodulated signal.
AF distortion
The two methods used for quoting distortion within radio receivers are Distortion Factor and
SINAD.
SINAD tests are usually made in conjunction with sensitivity testing (see above), while distortion
test results are often quoted within the power output parameter. For example ‘3 Watts output with
better than 10% THD’ (total harmonic distortion).
The signal to noise level is also quoted but is a measure of receiver quality, not a true measure of
distortion.
The AF distortion measuring facility performs continuous reading and display metering using the
selected function, (SINAD or distortion factor). Therefore a distortion test can be carried out by
ensuring that the RF generator is tuned to the correct frequency, select the distortion measuring
function required and set the levels of the RF and AF generators. The distortion factor or SINAD
level is then read from the screen.
To obtain sets of readings relating to differing levels of RF or AF, the levels are reset and the new
reading made.