# zpk

Converts rffilter to zero-pole-gain representation

## Syntax

``[z,p,k] = zpk(filter)``

## Description

example

````[z,p,k] = zpk(filter)` returns zero-pole-gain representation of S-parameters, Sij contained in z{i,j}, p, and k{i,j} of the filter. This method only works for the `'Transfer function'` implementation of `rffilter` object.```

## Examples

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Generate the zpk of a high-pass fourth-order Chebyshev filter for cut-off frequency of 1 rad/sec.

Create the rffilter object.

```filtobj = rffilter('FilterType',"Chebyshev",'ResponseType','Highpass', ... 'FilterOrder',4,'Implementation',"Transfer function", ... "PassbandFrequency",1/(2*pi),"Zin",50,"Zout",50);```

Use `zpk` function to generate the zeroes, poles, and gain.

```[zeros,poles,gain] = zpk(filtobj); zeros{1,1}```
```ans = 4×1 complex 0.0000 + 1.0824i 0.0000 - 1.0824i 0.0000 + 2.6131i 0.0000 - 2.6131i ```
`poles`
```poles = 4×1 complex -0.0941 + 1.0482i -0.0941 - 1.0482i -1.0482 + 2.0022i -1.0482 - 2.0022i ```
`gain{1,1}`
```ans = 0.1250 ```

## Input Arguments

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RF filter, specified as an `rffilter` object.

## Output Arguments

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Zeroes of the filter, returned as a 2-by-2 cell array. Each cell contains zeros corresponding to its S-parameter.

Poles of the filter, returned as a 1-D array of doubles or a 2-D array of doubles.

Gain of the filter, returned as a 2-by-2 cell array. k{i,j} corresponds to the gain of the Sij S-parameter.

## Version History

Introduced in R2019b