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evalfr

Evaluate frequency response at given frequency

Syntax

frsp = evalfr(sys,f)

Description

frsp = evalfr(sys,f) evaluates the transfer function of the TF, SS, or ZPK model sys at the complex number f. For state-space models with data (ABCD), the result is

H(f) = D + C (fI – A)–1B

evalfr is a simplified version of freqresp meant for quick evaluation of the response at a single point. Use freqresp to compute the frequency response over a set of frequencies.

Examples

collapse all

Create the following discrete-time transfer function.

$$H\left( z \right) = {{z - 1} \over {{z^2} + z + 1}}$$

H = tf([1 -1],[1 1 1],-1);

Evaluate the transfer function at z = 1+j.

z = 1+j;
evalfr(H,z)
ans =

   0.2308 + 0.1538i

Create the following continuous-time transfer function model:

$$H\left( s \right) = {{1} \over {{s^2} + 2s + 1}}$$

sys = idtf(1,[1 2 1]);

Evaluate the transfer function at frequency 0.1 rad/second.

w = 0.1;
s = j*w;
evalfr(sys,s)
ans =

   0.9705 - 0.1961i

Alternatively, use the freqresp command.

freqresp(sys,w)
ans =

   0.9705 - 0.1961i

Limitations

The response is not finite when f is a pole of sys.

See Also

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Introduced in R2012a

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