Main Content

Frequency response of analog filters

`freqs(___)`

with no output arguments plots the magnitude
and phase responses as functions of angular frequency in the current figure window. You can
use this syntax with either of the previous input syntaxes.

`freqs`

returns the complex frequency response of an analog filter
specified by `b`

and `a`

. The function evaluates the
ratio of Laplace transform polynomials

$$H(s)=\frac{B(s)}{A(s)}=\frac{b(1){s}^{n}+b(2){s}^{n-1}+\cdots +b(n+1)}{a(1){s}^{m}+a(2){s}^{m-1}+\cdots +a(m+1)}$$

along the imaginary axis at the frequency points *s* = *jω*:

s = 1j*w; h = polyval(b,s)./polyval(a,s);