Guidelines For Choosing RF And Microwave Products: Lumped Element (LC) Filters

By Anatech Electronics Inc.

The LC, or lumped element, filter is perhaps the most common of all electromagnetic filter types. It takes its name from the inductors and capacitors upon which it is based. The resonant frequency of an LC filter circuit is given by:

LC filters can be specified in low-pass, bandpass, bandstop, or high-pass configurations and in diplexers and duplexers as well. Their task, like all types of electrical filters, is to modify the frequency- or time-domain characteristics of a transmission path to reject interference arising from various external sources. LC filters are useful over a frequency range of below 100 kHz to slightly above 3 GHz.

Capacitors and inductors arranged in either series or parallel resonant circuits result in filters ranging in size from about 0.5 in. at high frequencies to 26 in. at low frequencies. The actual size of a particular LC filter is dictated by the size of these capacitors, inductors, as well as the level of RF power it must withstand.

Their frequency limit of 3 GHz results from the fact that parasitic capacitance and lead length inductance issues make them impractical to fabricate at higher frequencies. In addition, at very low frequencies capacitance and inductance values of tuned circuits become so large that they become problematic from a packaging perspective.

The performance of LC filters has increased over the years thanks to the development of ferrite materials used to manufacture inductors and the dielectric materials used in capacitors. As a result, high inductor Q factors can be achieved in smaller sizes than with earlier designs.