Instruction manual

9. High Pass Filter

The M68 signal conditioners have a high pass filter with a lower limiting frequency of 3 Hz (-

3 dB). By means of this filter low frequency noise can be removed. Low frequency noise may

occur, for example, by the influence of temperature transients to piezoelectric compression type

accelerometers. The slope of the 3 Hz high pass filter is 40 dB / frequency decade. The 3 Hz

high-pass filter is switched on by turning the “INTEGRATOR HIGH PASS” switch into positi -

on “ACC 3 Hz”. If the high pass filter is switched off (switch position “0.1 Hz ACC”), the lower

limiting frequency of the amplifier is 0.1 Hz.

With the integrators the high pass filter is always switched on.

As the filters are located between the amplifier stages (see Figure 2) the instrument does not be-

come overloaded, even if the measured signal has higher spectral components outside the filter

range.

10. Integrators

The M68 can integrate the measuring signal one or two times. Integration can be useful for vi -

bration measurement by means of accelerometers on rotating machinery.

Single integration of vibration acceleration results in velocity, double integration in displace-

ment.

The „INTEGRATOR HIGH PASS“ switch activates the integrators. In position “ACC“ (accele-

ration) the measuring signal passes without integration. At the position “VEL” (velocity) the si-

gnal is integrated once, in position “DISP” (displacement) twice.

With switched on integrators the 3 Hz high pass filter is always activated.

The following calculations show how the M68 output u

out

corresponds to the three vibration

quantities. The selected gain range of the M68 is G and the accelerometer sensitivity (see trans-

ducer data sheet) is B

Figure 7: Frequency and phase response of the high pass filter

0 dB

-20 dB

-40 dB

0,1 1 10 100

180°

-180°

-90°

90°

0,1 1 10 100