Electronic filter
Electronic filters are circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal, to enhance wanted ones, or both. Electronic filters can be:
passive or active
analog or digital
high-pass, low-pass, band-pass, band-stop (band-rejection; notch), or all-pass.
discrete-time (sampled) or continuous-time
linear or non-linear
infinite impulse response (IIR type) or finite impulse response (FIR type)
The most common types of electronic filters are linear filters, regardless of other aspects of their design. See the article on linear filters for details on their design and analysis.
History
The oldest forms of electronic filters are passive analog linear filters, constructed using only resistors and capacitors or resistors and inductors. These are known as RC and RL single-pole filters respectively.
More complex multipole LC filters have also existed for many years, and their operation is well understood.
Hybrid filters are also possible, typically involving a combination of analog amplifiers with mechanical resonators or delay lines. Other devices such as CCD delay lines have also been used as discrete-time filters. With the availability of digital signal processing, active digital filters have become common.