7. Level Indicators
The red LED “Overload” warns of signal clipping by high magnitudes. The indicator switches
on at around 9 V peak voltage. At 9 V the output signal is still without distortion, however, it is
close to the limit.
The overload detector monitors the signal level at both the output and before the filters and inte-
grators (see Figure 3). This ensures that high signal components occurring beyond the filter pass
band are indicated, even though they do not appear at the output.
In addition, the M72Ax and M72S1 instruments display the modulation level via the digital sen-
sitivity display if neither ▲ nor ► have been pressed for more than 5 seconds.
The modulation
is displayed as a percentage of the peak value (0 – 99 %). The display is updated every 5 sec-
onds. For display values of over 80 % the gain should be decreased and for under 6 % it should
be increased to yield an undistorted low-noise signal.
8. Filters and Integrators
8.1. Low Pass Filter
To eliminate disturbing noise or to comply with the Shannon theorem, it can be advantageous to
use a low pass filter.
signal frequency
<
sampling frequency
2
Furthermore, for higher accuracy in the time domain the following is recommend:
signal frequency
<
sampling frequency
10
The M72 Series instruments have four internal low pass filters. To select the filter press the
“low pass” button consecutively. The selected -3 dB frequency limits are indicated by four green
LEDs in kilohertz.
The corresponding 10 % - limits are shown in the following table:
3 dB Frequency
100 Hz
1 kHz
10 kHz
50 kHz
10 % Frequency
80 Hz
800 Hz
8 kHz
45 kHz
The M72 has fourth-order Butterworth filters. The slope of the low pass filters is over 75 dB per
decade. The frequency response is shown in 6.
10