Filters

A filter is a circuit that passes certain frequencies while suppressing others. This property isuseful for eliminating unwanted frequencies and separating wide frequency bands into multiple channels. A passive filter does not require a power source, but because it dissipates input power it cannot provide either current or voltage gain. Moreover, it has a limited frequency range. Signal loss caused by filtering with a passive filter is called insertion loss.

By contrast, an active filter can perform the same functions as a passive filter, but it can perform those functions over a wider frequency range, and it can provide current or voltage gain. Although an active filter requires a power source, it does not need a bulky inductor.

BASIC FILTER TYPES

There are four basic types of filter:

1. A low-pass filter can pass all frequencies from zero to its cutoff frequency, and block all frequencies above the cutoff.
2. A high-pass filter can block all frequencies below its cutoff frequency, and pass all frequencies above the cutoff. Its response is the inverse of the low-pass filter.
3. A bandpass filter can pass all frequencies within a band defined by lower and upper cutoff frequencies, and block all frequencies above and below that band.
4. A band-reject or notch filter can block all frequencies between its lower and upper cutoff frequencies, and pass all frequencies above and below that band. Its response is the inverse of the bandpass filter.