c is the velocity of the P wave or S wave in the seismic source region, and [f.sub.c] is the
corner frequency. Its formula [32] is
Microphones were chosen to exhibit flat frequency response in bandwidth of respiratory sounds (i.e., 100 to 1000 Hz) and accelerometers featuring a resonant frequency well above the upper
corner frequency.
The active signal filter is formed by using an AC op-amp integrator with DC gain control with the
corner frequency of 15.92 kHz.
It can be realized by increasing
corner frequency of first-order inertial element so as to increase the flow-pressure coefficient of the load sensing valve or reducing the spring stiffness of the load sensing valve and the piston area of variable hydraulic cylinder.
To remove the low frequency noise caused by the motion artifact, a study has been conducted to ascertain a reasonable value for the high-pass
corner frequency filter.
The MOSFETs are biased in the subthreshold region and this will provide high output impedance and hence enhance the lower
corner frequency. All biased currents are designed using simple current mirrors.
For the 85 % of rock bursts, the
corner frequency does not exceed 23 Hz.
The highpass
corner frequency was determined from De Luca.
A
corner frequency [f.sub.injRC] is defined in (9).
The output current filter may easily be designed by choosing both a desired filter
corner frequency and a small inductor value (one freedom degree).
According to Figure 5 a pessimistic estimation of the OTA
corner frequency is around 100 kHz.
For all measurements, the highpass filter
corner frequency is set 8 % lower than the measurement frequency, and the lowpass filter
corner frequency is set 8 % higher than the measurement frequency.