charlie@oakhill.UUCP (Charlie Thompson) (09/07/89)
I have a question for the netland filter jocks out there... For years I have seen a 3 op-amp filter structure known as a 'state variable' or 'biquad' filter. The filter provides simultaneous highpass, lowpass, and bandpass outputs. I understand how the filter works but WHY is it called a 'state variable' / 'biquad' filter??? State variables come from linear system theory ... why does this filter attract the name? can't any filter have state variables?? Biquadratic relates to the fact that the filter has 2 integrators making it 2nd order...o.k. lots of filter structures can be 2nd order and they don't even need 2 op-amps to be 2nd order! So why the name biquad? Trivially yours, Charlie -- WB4HVD