13 Filters

Filters are linear circuits that are devised to modify the frequency distribution of a signal spectrum and are available to cover the whole of the electromagnetic spectrum, from dc through to light frequencies. They are described as linear, because the principles of superposition and proportionality apply and the excitation (input) and response (output) are related by a linear differential equation. Filters are also time invariant, that is, their characteristics do not change with time. A common feature of all filters is that they have at least one pass band with zero attenuation and at least one stop band in which a finite attenuation is produced. This leads to the concepts of low-pass (LPF), high-pass (HPF), bandpass (BPF) and ...

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