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 (A, B, C, D), 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

Evaluate Discrete-Time Transfer Function

Open Live Script

Create the following discrete-time transfer function.

$H (z) = \frac{z - 1}{z^{2} + z + 1}$

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

Evaluate Frequency Response of Identified Model at Given Frequency

This example uses:

System Identification Toolbox

Open Live Script

Create the following continuous-time transfer function model:

$H (s) = \frac{1}{s^{2} + 2 s + 1}$

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.

Documentation