[Scilab-users] Problems Bode Plot z-plane

[Florian Augustin]

the Problem is ,that when the filter parameters for the high and lowpass 
filter in the discrete case are of some king, silab plots the bode 
diagram only from 6x10^-1 even if it was called by:

bode(lisys_lowpass_disc,0.001,100);

and also the scaling of the y-axis is not fitting at the beginning. Any 
suggestions?

Am 15.07.2014 22:48, schrieb Florian Augustin:
> Hello,
>
> I got a first order high and lowpass. Transfer funtion in the s-plane:
>
> tau = (0.98*0.002)/(1-0.98);
> deltat = 1/500;
> s = poly(0, 's');
> h_lowpass_cont = (1)/(1+s*tau);
> h_highpass_cont = (tau*s)/(1+s*tau);
> lisys_lowpass_cont = syslin('c', h_lowpass_cont);
> ...
>
> And if I convert it using the backward difference method the 
> corresponding transfer functions in the z-plane are:
>
> h_lowpass_disc=(1)/(1+tau/deltat*(1-%z^-1));
> h_highpass_disc=(tau/deltat*(1-%z^-1)/(1+tau/deltat*(1-%z^-1)));
> lisys_highpass_disc = syslin(deltat,h_highpass_disc);
> ...
>
> Doing a bode plot of this works fine for bothe the s and z case, but 
> if I cange the paramter tau to: tau = (0.998*0.002)/(1-0.998);
> which is implemented in a system and should be stable in the z-plane 
> the bode plot just looks weird but the s-plan plot looks fine.
>
> Does anyone know why?
>
> Thanks for any help!!!
> Florian
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