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What is an LC Filter?

Date:2019-05-02 13:18:08 Hits:1359

An LC filter combines inductors (L) and capacitors (C) to form low-pass, high-pass, multiplexer, band-pass, or band-reject filtering in radio frequency (RF) and many other applications. Passive electronic LC filters block, or reduce, noise (EMI) from circuits and systems, and separate, or condition, desired signals.

While ideal filters would pass desired signal frequencies with no insertion loss or distortion, and completely block all signals in the stop-band, real filters have DC and AC resistances that contribute to insertion loss, requiring careful component selection. Selecting the exact values of the parts for a particular application requires high quality components as well as complete specifications and performance models. The simplest to design and implement are the low-pass and high-pass types.

Coilcraft high-Q, tight-tolerance, surface-mount RF chip inductors and air-core inductors help you achieve top performance in all of these LC filter categories.

How do you design LC filters?

The alignment (type) of the filter determines the flatness of frequency behavior and the sharpness of the cut-off. There are many types of alignments, including those with the most commonly desired characteristics such as Butterworth, Bessel, Chebyshev, and elliptic.

The simplest LC filter consists of one inductor and one capacitor. Higher-order filter alignments use more components to give a sharper, more defined roll-off in attenuation of unwanted noise. For example, Elliptic (Cauer) filters give the sharpest roll-off and are the least sensitive to component variation. As a trade-off, there is more pass-band ripple and stop-band ripple in Elliptic LC filters.

For more details on the various filter alignments shown in Appendix A of this application note.

Appendix A: Passive LC Filter Design and Analysis
Filter Alignments and Properties

What is an LC Filter?What is an LC Filter?
What is an LC Filter?What is an LC Filter?

Modern circuit synthesis and analysis programs can quickly perform the otherwise tedious and time-consuming calculations for designing LC filters. Filter synthesis programs generate the required inductance (L) and capacitance (C) values. Analysis programs simulate the results after the user enters the appropriate values. Once the initial ideal values have been calculated, practical solutions are created using off-the-shelf components.

Ideally, one could simply define the band of frequencies to be passed and those to be blocked, and a program would generate standard component values resulting in the actual on-board performance. Realistically, a passive LC filter design starts with calculations and then a very iterative trial-and-error process is needed to match the actual performance to the required performance. To speed design time and improve accuracy of design calculations, models of real-world inductors are available.

For many designs, accurate inductance models based on actual component measurements are necessary, but ideal capacitors can be used for the simulation. Simulations of filters near the Gigahertz range may require non-ideal capacitor models as well.

Free programs for generating basic LC filter designs are available. Coilcraft's LC Low Pass Filter Designer software by Nuhertz uses real measurement-based s-parameter models of the inductors for improved filter simulations.

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