Instruction manual

8. Low Pass Filter
To eliminate disturbing noise or to comply with the Shannon theorem: “Signal frequency should
be less than half of the sampling frequency”, it can be advantageous, to use a low pass filter. For
higher accuracy in the time domain it is recommended to set the low pass at 1/10 the sampling
frequency.
The instruments of M68 series have 6 internal low pass filters. The scale at the positions of the
filter switch “LOW PASS” is shows the 3 dB limiting frequencies in kHz. The following table
shows the 3 dB and the 10 % limiting frequencies of the low pass filters:
3 dB Frequency
100 Hz
300 Hz
1 kHz
10 kHz
20 kHz
50 kHz
10 % Frequency
70 Hz
200 Hz
700 Hz
7 kHz
14 kHz
35 kHz
The slope of the low pass filters is 40 dB per decade.
11
180°
-180°
-90°
90°
0
0,1 1 10 100
kHz
0 dB
-20 dB
-40 dB
0,1 1 10 100
kHz
1 kHz
1 kHz
10 kHz
10 kHz
20 kHz
50 kHz
20 kHz
50 kHz
Figure 6: Typical frequency and phase response of the low pass filters
180°
-180°
-90°
90°
0
0 dB
-20 dB
-40 dB
10 100 1000 10000
Hz
10 100 1000 10000
Hz
100 Hz
300 Hz
100 Hz
300 Hz