Operating instructions

LOCAL OPERATION
3-37
The SINAD level is then calculated according to the formula:-
SINAD
= 20
1
2
log
V
V
dB
=
++
+
20 log
()
()
S
N
D
ND
dB
To obtain the distortion factor of a signal at the output of a transmitter the output level is measured
and noted as for SINAD level measurement.
A notch filter as used for SINAD measurement is used to make a second reading which will
consist of the same values as for the second SINAD measurement.
The distortion factor is then calculated according to the formula:-
Distortion
= 100
2
1
V
V
%
=
+
++
100
()
()
%
N
D
SND
For signal to noise level measurements a reading is taken of the output level as for SINAD level
and distortion factor measurements. The AF input signal is then disabled and a second level
reading taken. A notch filter is not included in the signal output path. The second measurement
will only consist of the value of any noise present on the demodulated signal. See page 3-36,
Fig. 3-10 a, & d, Distortion level and signal to noise level measurements.
The signal to noise level is then calculated according to the formula:-
S/N
= 20
1
2
log
V
V
dB
=
++
20 log
()S
N
D
N
dB
The distortion measuring system within the Service Monitor has the capability to make all three
measurements. Pressing the [Audio Meter] key, to change the soft keys as shown in fold out
diagram (3-3), then pressing the [Dist/S-N]soft key, accesses the distortion measurement menu
shown in fold out diagram (3-3-2)
The action of selecting a measurement system returns the
TRANSMITTER TEST main (top level)
menu to the display.
The result of the selected measurement will be displayed on a bar chart within the transmitter test
screen and a digital read-out given above it. When no distortion measurement system is selected,
by pressing the [Off] key within the distortion measuring menu, the distortion bar chart and digital
read-out are removed from the screen.
Hint. By disabling this function, the time taken to measure the remainder of the measurement functions is
shortened thereby reducing the update time.
Measurements of SINAD and distortion percentage carried out by the monitor use a frequency of
1 kHz for the tests. When either of these test methods is selected, the system software sets the
frequency of the modulation generator to this frequency to avoid erroneous results.
The input signal level influences the readings obtained by the distortion measuring system and is
usually specified as part of the test specification.
*OSCILLOSCOPE*
With the oscilloscope facility selected, the distortion level bar chart is removed from the display
but the digital read-out is retained.
*DUPLEX*