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

Evaluate Discrete-Time Transfer Function

Create the following discrete-time transfer function.

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

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

Create the following continuous-time transfer function model:

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

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.