From: mark sam on 16 Apr 2010 18:16 Hi I try to design a 60Hz digital notch filter with 120kHz sampling rate. The z transform function is: hd= (0.9986 z^2 - 1.997 z + 0.9986)/( z^2 - 1.997 z + 0.9971) After that I run following: t=0:ts:0.09 in=sin(60*2*pi*t); lsim(hd,'b',in,t); In time domain, it shows perfect result that 60Hz signal gets removed. With bode(hd), the it has correct frequency response in frequency domain too. But furthermore, when I run following, the filter output 60hz signal is as big as half of the 1V 60Hz input signal. This is very confused. Ts=1/120000; t=0:Ts:0.05; ypd=[0;0] ynotch=[0;0] wu=2*pi*60; u=sin(wu*t) %input sig for n = 2:0.05/Ts ypd(n+1)=u(n); ynotch(n+1)=1.997*ynotch(n)-0.9871*ynotch(n-1)+0.9986*ypd(n+1)-1.997*ypd(n)+0.9986*ypd(n-1); end plot(t,u,t,ynotch); Thank you for help, Mark
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