zplane - don't the zeros and poles need to be complex?

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Tom 2012-5-22

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I'm just getting my head around the zplane function. I'm not how the values of z and p are mapped onto the pole-zero plot. I thought zeros and poles were supposed to have both real and imaginary parts - as these are the axes of the pole-zero plot. But in the examples of zplane code given in the help documentation the z and p values are just single real values. Also, for a 4 pole/zero filter there are 5 values for z and also for p. Shouldn't this be four?

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Matt Fig 2012-11-2

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Honglei Chen 2012-5-22

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zplane is a little subtle. When B and A are rows, they represent transfer functions. However, if B and A are columns, they are zeros and poles.

Compare

    [b,a] = ellip(4,.5,20,.6);
    zplane(b,a)
    zplane(roots(b),roots(a))

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Tom 2012-5-24

great thanks :)

Honglei Chen 2012-5-24

Hi Tom, in your code, b and a are coefficients. So if you want to use zplane, you need to keep them in the row form and zplane will automatically calculate zeros and poles for you and display in the figure.

On the other hand, if you want to pass in column vectors, then zplane treat them as just zeros and poles. In this case, you need to compute zeros and poles yourself before passing it into the function. So you need to do zplane(roots(b),roots(a)).

HTH

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