Tuned Frequency-Selective FiltersTuned filters are widely used to separate signals in relatively-narrow bands of frequencies from signals in wider spectrums.
Tuned filters utilize one or more tuned circuits (resonant or anti-resonant circuits) to separate signals in relatively-narrow bands of frequencies from signals in wider frequency spectrums. Most transmitters and receivers incorporate many tuned filters. External tuned filters also are sometimes used as pre-selectors to greatly attenuate strong undesired signals received by antennas before they reach receivers, and between transmitters and antennas to greatly attenuate harmonics and other emissions that could cause interference.
LCR tuned filters (also known as RLC tuned filters) are probably the most flexible of all frequency-selective or frequency-nonlinear circuits. An LC circuit made from lossless components would be a perfect device for taking the second integral or second derivative of input waveforms. However, practical L and C components always dissipate energy due resistance, so practical LCR circuits are used primarily for their frequency-selection properties.
LCR tuned filters can be parallel-resonant or series-resonant circuits. Each of those circuits can be excited in a variety of ways and each way causes the circuit to appear to have its own special frequency of maximum response.
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