Lowpass RF Filter

Standard lowpass RF filters in baseband-equivalent complex form

Library

Mathematical

Note

To use this block, you must install DSP System Toolbox™ software. For more information, see the RF Blockset™ release notes.

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Description

The Lowpass RF Filter block lets you design standard analog lowpass filters, implemented in baseband-equivalent complex form. The following table describes the available design methods.

Design Method	Description
Butterworth	The magnitude response of a Butterworth filter is maximally flat in the passband and monotonic overall.
Chebyshev I	The magnitude response of a Chebyshev I filter is equiripple in the passband and monotonic in the stopband.
Chebyshev II	The magnitude response of a Chebyshev  II filter is monotonic in the passband and equiripple in the stopband.
Elliptic	The magnitude response of an elliptic filter is equiripple in both the passband and the stopband.
Bessel	The delay of a Bessel filter is maximally flat in the passband.

The block input must be a discrete-time complex signal.

Note

This block assumes a nominal impedance of 1 ohm.

Select the design of the filter from the Design method list in the dialog box. For each design method, the block enables you to specify the filter design parameters shown in the following table.

Design Method	Filter Design Parameters
Butterworth	Order, passband edge frequency
Chebyshev I	Order, passband edge frequency, passband ripple
Chebyshev II	Order, stopband edge frequency, stopband attenuation
Elliptic	Order, passband edge frequency, passband ripple, stopband attenuation
Bessel	Order, passband edge frequency

The Lowpass RF Filter block designs the filters using the Signal Processing Toolbox™ filter design functions buttap (Signal Processing Toolbox), cheb1ap (Signal Processing Toolbox), cheb2ap (Signal Processing Toolbox), ellipap (Signal Processing Toolbox), and besselap (Signal Processing Toolbox).

Note

Some RF blocks require the sample time to perform baseband modeling calculations. To ensure the accuracy of these calculations, the Input Port block, as well as the mathematical RF blocks, compare the input sample time to the sample time you provide in the mask. If they do not match, or if the input sample time is missing because the blocks are not connected, an error message appears.

Parameters

The parameters displayed in the dialog box vary for different design methods. Only some of these parameters are visible in the dialog box at any one time.

Parameters that are tunable can be changed while the model is running.

Design method

Filter design method. The design method can be Butterworth, Chebyshev I, Chebyshev II, Elliptic, or Bessel. Tunable.

Filter order

Order of the filter.

Passband edge frequency (Hz)

Passband edge frequency for Butterworth, Chebyshev I, elliptic, and Bessel designs. Tunable.

Stopband edge frequency (Hz)

Stopband edge frequency for Chebyshev II designs. Tunable.

Passband ripple in dB

Passband ripple for Chebyshev I and elliptic designs. Tunable.

Stopband attenuation in dB

Stopband attenuation for Chebyshev II and elliptic designs. Tunable.

Finite impulse response filter length

Desired length of the baseband-equivalent impulse response for the filter.

Center frequency (Hz)

Center of the modeling frequencies.

Sample time (s)

Time interval between consecutive samples of the input signal.