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Re: c2d output Interpretation

From: Sergei Steshenko
Subject: Re: c2d output Interpretation
Date: Mon, 20 Feb 2017 21:51:40 +0000 (UTC)

From: Nicholas Oleksinski <address@hidden>
To: address@hidden 
Sent: Monday, February 20, 2017 7:00 PM
Subject: c2d output Interpretation

I am trying to design a digital phase lead compensator using the c2d function 
in the Octave 4.0.0 control package.
The output of the b & a coefficients I fed in looks like this:

      7.474 z^2 - 14.29 z + 6.829
 y1:  ---------------------------
         z^2 - 1.52 z + 0.5772

I noticed a couple things about this output:
1) The powers of the 'a' coefficients start positive.  Does that not make this 
system non-causal?
2) Why is this discrete time LTI model represented as "y1" and not "H(z)"?  I 
fed in the coefficients of H(s) by running c2d this way:
  comp_tf = tf(gain*numc,denc); %create the s domain LTI transfer function
  comp_tf_d = c2d(comp_tf,0.001); % sample at 1kHz



By dividing both the numerator and denominator by z^2 you'll get an equivalent 
expression with non-positive powers of z.


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