[Scilab-users] Problems Bode Plot z-plane

[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