Operating instructions

LOCAL OPERATION
3-42 46882-311D
that of the input to the Service Monitor. With the input impedance set at High, the indicated
level is that of the input to the Service Monitor with an assumed 600 Ω load.
AF filters
The AF signal to the AF INPUT connector on the front panel can be filtered using the following
audio filters:-
0.3 - 3.4 kHz
band pass
50 kHz
low pass
15 kHz
low pass
300 Hz
low pass
CCITT
†
or
C-MESS
‡
†
Only if option 23 (CCITT filter) is fitted.
‡
Only if option 24 (C-MESS filter) is fitted.
Press the [AF Filter] key. The left hand soft keys change, to allow selection of AF filters shown
above. Press the [Return] key after selecting the required filter.
*OSCILLOSCOPE*
The input to the oscilloscope is taken directly from the AF input connector on the front panel, not
through any of the AF filters. Therefore measurements obtained on the AF voltmeter will show
the effects of filtering, whereas those made using the oscilloscope will not.
Audio distortion
The Service Monitor is able to measure signal noise and distortion against the following
parameters:-
Signal to
noise ratio
SINAD
level
Percentage
distortion.
Distortion levels introduced into the audio signal path can be measured using the distortion
measuring module.
Pressing the [Dist/S-N] key will display the screen and menu shown in fold out diagram (3-4).
The function of each soft key on this menu is as follows:-
Off
Disables any active distortion measuring function and removes the distortion bar chart from the
display.
Hint
By disabling this function, the time taken to measure the remainder of the measurement functions
is shortened thereby reducing the update time.
S/N
Selects Signal to noise ratio as the measurement method. Compares the level of the demodulated
signal together with the system noise, against the level of the system noise only, using the
formula:-
S/N
= 20
1
2
log
V
V
dB
=
++
20 log
()SND
N
dB