# Lowpass Filter

## Overview

This is a recursive low-pass filter that supresses signal frequencies higher than the cut-off frequency for all channels at the input seperately.## Operator ports

**Input S**: Floating point values

**Output LP(S)**: Floating point values

## Properties

Find more information about changing properties here: "Properties Viewer"CutOffFrequency

*type: See description*

The cut-off frequency in Hz (must be greater than 0)

TimeConstant

*type: See description*

The time constant T of the filter in seconds.

Caption

*type: Word or phrase*

The name of the object in the project. This name must not contain '.', '$' nor '@' characters.

For more information about the rules and usage of the Caption property, please refer to "Caption property - background and usage".

Documentation

*type: See description*

Optional documentation of this object. If this object is an operator, the Documentation text is displayed below the operator symbol.

## Variable Parameters

Find more information about Variable Parameters here: "Variable Parameters"CutOffFrequency

*type: See description*

The cut-off frequency in Hz (must be greater than 0)

TimeConstant

*type: See description*

The time constant T of the filter in seconds.

## Details

This filter uses the following formula to supress high frequencies:*y(n) = k * y(n-1) + (1 - k) * x(n)*

where k = e ^ ( -1 / ( Fs / (2 * PI * Fc) ) )

with:

Fs is the sample frequency,

PI is 3.1415...,

Fc is the cut-off frequency setting,

x(n) is the input sample value,

y(n) is the output sample value,

y(n-1) is the previous output sample value.

where k = e ^ ( -1 / ( Fs / (2 * PI * Fc) ) )

with:

Fs is the sample frequency,

PI is 3.1415...,

Fc is the cut-off frequency setting,

x(n) is the input sample value,

y(n) is the output sample value,

y(n-1) is the previous output sample value.

The time constant

*Tc*and the cut-off frequency

*Fc*have the following relation:

*Fc = 1 / (2 * PI * Tc)*

If the filter setting contains a wrong value (not a number or a wrong number), then the filter symbol will indicate this and will not output a valid signal anymore.

It is not allowed to connect a constant to the input of the filter (since a constant does not have a time component).

## Examples

Example: Recursive Filter Demo

Demonstrates the High Pass and Low Pass standard filters

*Examples\DF0204020_001_LowpassFilter_Demo.xmc*