LC Filter Design Tool Description

LC Filter Design Tool is a web-based application for lumped LC filter synthesis. It is feature rich, user-friendly and available for free from any desktop or mobile device.



The RF filter is a two-port linear device used to attenuate certain unwanted frequencies of a signal while passing other wanted ones. The frequency band over which the filter passes through is called the passband, and the frequency band it rejects is called the stopband. The filter frequency response is classified according to its passband and stopband boundaries. The most common ones are:

Along having frequency selectivity, the RF filter is expected to have minimal influence on the pass band phase and amplitude response and maintain good impedance match at each port.

Types of Filter Responses


The passive RF filter is a linear device with matched ports, which is typically described in the frequency domain; it is therefore convenient to model its response using s-parameters. An overview on s-parameters is available in Wikipedia . In the present context, the passive RF filters s-parameters consists of a two-by-two complex and frequency dependent matrix, \[ \mathbf{S}= \begin{bmatrix} S_{11} & S_{12} \\ S_{21} & S_{22} \end{bmatrix} \]

Filter Two-Port Network Schematics

Because the device is passive (and non-magnetic) it is also reciprocal, meaning \(S_{21} = S_{12}\) and only three parameters are needed to describe the filter response, \(S_{11}\), \(S_{21}\), and \(S_{22}\). The magnitude and phase of these correspond to several frequency dependent measures important for filter analysis:

Parameter Relation
Insertion Loss (dB)\(IL = -20log_{10}(|S_{21}|)\)
Input Return Loss (dB)\(RL_{in} = -20log_{10}(|S_{11}|)\)
Output Return Loss (dB)\(RL_{out} = -20log_{10}(|S_{22}|)\)
Phase (rad)\(\phi = arg(S_{21})\)
Group Delay (sec)\(\tau_{d} = -\frac{1}{2\pi}\frac{d\phi}{df}\)

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