An electronic filter eliminates undesired frequencies (usually) from an electronic signal.
A low-pass filter passes low frequencies. A high-pass filter passes high frequencies. A band-pass filter passes a limited range of frequencies. A band-stop filter passes all frequencies except a limited range.
Band-stop and band-pass filters can be constructed by combining low-pass and high-pass filters.
The earliest electronic filters were based on combinations of inductors and capacitors. Since the conventional component numbering systems call coils L1, L2... and capacitors C1, C2.., these are known as L/C filters.
Here's how L/C filters work: Inductors with more windings pass lower-frequency signals. Capacitors with more capacitance can never pass a current flow, but as capacitance increases, they can pass lower frequencies. Except in high-power filters, the undesired frequencies are shorted to chassis ground. A high-pass filter uses inductors to short the lower-frequencies, and capacitors to conduct the higher frequencies. A low-pass filter uses inductors to pass the lower frequencies on, and capacitors to short the high frequencies.
At very high frequencies, sometimes the inductors consist of single loops or strips of sheet metal, and the capacitors consist of adjacent strips of metal.
To make L/C filters more precise, more components must be added.
Filtering systems are measured by their quality factor, usually called their "Q." Higher Q devices can achieve a more precise selection of frequencies with fewer devices. Qaultiy is determined by the precision of a harmonic oscillator implemented with that type of device.
In the late 1930s, engineers realized that small mechanical systems made of rigid materials (like quartz) would acoustically resonate at frequencies that would make good radio waves, that is, from audible frequencies up to several hundred megacycles.
Some early resonators were made of steel, but quartz quickly became favored. The biggest advantage is that quartz is piezoelectric. This means that quartz resonators can directly convert their own mechanical motion into electrical signals. Quartz also has a very low rate of change in its size when temperatures change. This means that quartz resonators produce stable frequencies over a wide temperature range.
Quartz crystal filters have much higher qualifty factors than LC filters. When higher stabilities are required, the crystals and their driving circuits may be mounted in a "crystal oven" to control the temperatures. For very narrow filters, sometimes several crystals are operated in series.
Engineers some realized that a large number of crystals could be collapsed into a single component, by mounting comb-shaped evaporations of metal on a quartz crystal. In this scheme, a "tapped delay line" reinforces the desired frequencies as the sound waves flow across the surface of the quartz crystal.
The tapped delay line has become a general scheme of making high-Q filters in many different ways.
Lately, for lower frequencies, digital signal processing has been able to inexpensively construct very high Q filters. In this scheme, a computer program simulates a tapped delay line. An analog to digital converter turns the signal into a stream of numbers. The computer program stores the numbers in a list in the computer's memory. Then, the program selects numbers from this list, at a spacing that simulates the comb of a tapped delay line. These numbers are multiplied by constants, and added together to make the output of the filter. The filter's output becomes a signal by passing it through a digital to analog converter. There are problems with noise instroduced by the conversions, but these can be controlled and limited for many useful filters. Digital signal processing is especially useful for audio.
Another method of filtering, at frequencies from 800 meghertz to about 5 gigahertz is to use a synthetic single-crystal garnet sphere made of a chemical combination of Titanium, Iron and Nitrogen. The garnet sits on a strip of metal driven by a transistor, and a small loop antenna touches the top of the sphere. An electromagnet changes the frequency that the garnet will pass. The advantage of this method is that the garnet can be tuned over a very wide frequency by varying the strength of the magnetic field.
For even higher frequencies and greater precision, the electrons of atoms must be used. Atomic clocks use Cesium masers as ultra-high Q filters to stabilize their primary oscillators. Another method, used at high, fixed frequencies with very weak radio signals, is to use a ruby maser tapped delay line.