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.

- One-click synthesis from a given specifications
- Chebyshev, Elliptic, Butterworth, Bessel or Legendre filter types
- Multiple topologies
- Arbitrary input and output impedances
- Option to use closest capacitor and inductor standard values

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:

- Low-pass filter - passes frequencies below a cutoff frequency fc while attenuating frequencies above it.
- High-pass filter - passes frequencies above a cutoff frequency fc while attenuating frequencies below it.
- Band-pass filter - passes frequencies between a lower and upper cutoff frequency fl and fh while attenuating all other frequencies.
- Band-stop filter - attenuates frequencies between a lower and upper cutoff frequency fl and fh while passing all other frequencies.

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.

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, \[ \mathbf{S}= \begin{bmatrix} S_{11} & S_{12} \\ S_{21} & S_{22} \end{bmatrix} \]

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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