Low frequency vibration data is often required presented in displacement format, which more clearly relates to parameters such as dynamic strain and movement. The double integration process inherent in deriving displacement from accelerometer signals produces a 1/f ² weighting of the frequency spectrum and thus the following properties : progressive attenuation of data vs. frequency thus filtering high frequency signal components which may be spurious ; gain increases asymptotically to infinity @ zero frequency - clearly a minimum frequency constraint must be imposed. The hardware implementation of the process (DM/04) therefore comprises a double integrator proceeded by a high pass filter, confining displacement conversion to above the minimum frequency as defined by the filter. The transient response of the filter generates ripple, limiting conversion to quasi steady state periodic signals such as emanate from rotating machinery.

NOISE
Double integration of f ^½ and (bandwidth limited) white noise, i.e. as inherent to charge amplifiers, is characterised by an output noise voltage proportional to f_o^-3/2 where f_o is the corner frequency as defined by the high pass filter.
Thus a 100 : 1 increase in corner frequency is accompanied by a 60dB noise level reduction in displacement converter output, 20dB less than may be expected by virtue of the f ^-2 transfer characteristic of the DM/04.

IMPLEMENTATION
Two switched high pass filter corner frequencies are provided, together with six switched, 10dB increment displacement scalings. Table 1 lists available filter corner frequencies, each with its maximum realizable displacement scaling assuming a CA/O4 10pC/g, 10mV/g input noise source. Optimisation for a specific application may call for tradeoffs between the various parameters governing noise and phase/ amplitude error, as extending the LF response to improve the latter increases noise. Separate phase/gain data for the filter and integrator sections is provided to help in this subjective area.
The dependence of noise level on filter corner frequency may render the higher sensitivity displacement scalings unuseable at the lower of the two filter frequencies.

STANDARDSCALING
Stock availability as opposed to custom production, thus reduced cost and leadtime. HP filter frequencies 5 & 20Hz, displacement range 0.0316, 0.1, 0.316, 1, 3.16, 10mm pk. pk./volt pk. for 10mV pk./g input.