# 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

### Example 1

To evaluate the discrete-time transfer function

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

at z = 1 + j, type

```H = tf([1 -1],[1 1 1],-1); z = 1+j; evalfr(H,z) ```

to get the result:

```ans = 2.3077e-01 + 1.5385e-01i ```

### Example 2

To evaluate the frequency response of a continuous-time IDTF model at frequency `w = 0.1 rad/s`, type:

```sys = idtf(1,[1 2 1]); w = 0.1; s = 1j*w; evalfr(sys, s)```

The result is same as `freqresp(sys, w)`.

## Limitations

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