Operating instructions
OPERATING MANUAL 2945A
46882-311D 3-61
Input filtering
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 right hand soft keys change, to allow selection of AF filters as
shown above. Press the [Return] key after selecting the required filter.
The 50 kHz LP filter can be considered to be the ‘unfiltered’ selection as this represents the
normal full bandwidth of the audio circuits.
Distortion measurement
The distortion measuring facilities of the monitor are available within the AF TEST mode and
are implemented by selecting the [Dist/S-N] key. This will cause the menu shown in fold out
diagram (7-4) to be displayed.
Pressing each of the soft keys will have the following effect:-
Off
Disables any active distortion measuring function.
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. The AF generator is disabled and
enabled alternately by the monitor software and the total output level from the circuit under test is
measured by the AF voltmeter at each state. See Fig. 3-8 a, & d Distortion level and signal to
noise level measurements, on page 3-33.
Calculations are performed by the software to calculate the signal to noise ratio using the
formula:-
S/N
= 20
1
2
log
V
V
dB
=
++
20 log
()SND
N
dB
The distortion bar chart ranges available are 0 to 30 dB and 0 to 100 dB. The signal to noise
ratio is displayed as a digital read-out above the bar chart. The AF test menu is recalled
automatically after pressing this key.
SINAD
Selects SINAD as the measurement method. The AF generator is set to 1 kHz and the 1 kHz
distortion notch filter is switched alternatively in and out of circuit, with the signal level being
measured by the AF voltmeter in each state. See Fig. 3-8 a, b, & c, Distortion level and signal to
noise level measurements, on page 3-33.