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From: charlie@oakhill.UUCP (Charlie Thompson)
Newsgroups: sci.electronics,rec.ham-radio
Subject: State Variable Filters
Keywords: filters
Message-ID: <2376@radio.oakhill.UUCP>
Date: 6 Sep 89 20:01:44 GMT
Organization: Motorola Inc. Austin, Tx
Lines: 20

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