Almost all communication systems use filters. A filter passes one band
of frequencies while rejecting another. A filter can be either passive or
active. Passive filters are built with resistors, capacitors, and inductors.
They are generally used above 1 MHz, have no power gain, and are
relatively difficult to tune. Active filters are built with resistors,
capacitors, and op amps. They are useful below 1 MHz, have power
gain, and are relatively easy to tune. Filters can separate desired
signals from undesired signals, block interfering signals, enhance
speech and video, and alter signals in other ways.
