# tf

Convert digital filter to transfer function

## Syntax

• ```[num,den] = tf(d)``` example

## Description

example

``````[num,den] = tf(d)``` converts a digital filter, `d`, to numerator and denominator vectors.```

## Examples

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### Highpass Filter Transfer Function

Design a highpass FIR filter of order 8 with passband frequency 75 kHz and passband ripple 0.2 dB. Specify a sample rate of 200 kHz. Compute the coefficients of the equivalent transfer function.

```hpFilt = designfilt('highpassiir','FilterOrder',8, ... 'PassbandFrequency',75e3,'PassbandRipple',0.2, ... 'SampleRate',200e3); [b,a] = tf(hpFilt)```
```b = 1.0e-03 * 0.0128 -0.1024 0.3583 -0.7166 0.8958 -0.7166 0.3583 -0.1024 0.0128 a = 1.0000 5.7637 15.4847 25.1134 26.7644 19.1396 8.9554 2.5058 0.3212```

## Input Arguments

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### `d` — Digital filter`digitalFilter` object

Digital filter, specified as a `digitalFilter` object. Use `designfilt` to generate a digital filter based on frequency-response specifications.

Example: `d = designfilt('lowpassiir','FilterOrder',3,'HalfPowerFrequency',0.5)` specifies a third-order Butterworth filter with normalized 3-dB frequency 0.5π rad/sample.

## Output Arguments

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### `num` — Numerator coefficientsrow vector

Numerator coefficients, returned as a row vector.

Data Types: `double`

### `den` — Denominator coefficientsrow vector

Denominator coefficients, returned as a row vector.

Data Types: `double`