
From:  Jonathan P Jacky 
Subject:  Re: [Discussgnuradio] MRFM (Magnetic Resonance Force Microsocopy) andGNU Radio 
Date:  Sat, 3 May 2008 09:02:46 0700 (PDT) 
Jeff 
I'm curious .. what do use the 'biquad filters' for? I assume you mean that you've implemented a 4th order IIR filter? If so that would mean somewhat nonlinear phase depending on the IIR design type. What type are you using? Elliptic? Other?
The pair of cascaded biquadratic filters form an optimal cantilever controller. Our experiment software calculates the controller transfer function, a ratio of polynomials with four numerator and four denominator coeffcients (floating point), using the methods described in the papers linked to our page at "Cantilever controller design is discussed _here_ ..." Our software refactors and scales that transfer function into two biquad filter stages with 24bit integer coefficients. So I guess the filter type is "other". Some phase compensation is included in the transfer function, as described in the papers.
Jon
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