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Electronic filter

An electronic filter eliminates unwanted frequencies 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. A notch filter is a type of band-stop filter that acts on a particularly narrow range of frequencies.

Band-stop and band-pass filters can be constructed by combining low-pass and high-pass filters.

A popular form of 2 pole filter is the Sallen-Key type. This is able to provide low-pass, band-pass, and high pass versions.

Table of contents
1 Passive Filters
2 Active Filters
3 Other filters

Passive Filters

The simplest electronic filters are based on combinations of resistors, inductors and capacitors. Since resistance has the symbol R, inductance the symbol L and capacitance the symbol C, these filters exist in so-called RC, RL, LC and LCR varieties. All these types are collectively known as passive filters, because they are activated by the power in the signal and not by an external power supply.

Here's how passive filters work: inductors block high-frequency signals and conduct low-frequency signals, while capacitors do the reverse. A filter in which the signal passes through an inductor, or in which a capacitor provides a path to earth, therefore transmits low-frequency signals more strongly than high-frequency signals and is a low-pass filter. If the signal passes through a capacitor, or has a path to ground through an inductor, then the filter transmits high-frequency signals more strongly than low-frequency signals and is a high-pass filter. Resistors on their own have no frequency-selective properties, but are added to inductors and capacitors to determine the time-constants of the circuit, and therefore the frequencies to which it responds.

At very high frequencies (above about 100 megahertz), sometimes the inductors consist of single loops or strips of sheet metal, and the capacitors consist of adjacent strips of metal.

Other components can be added to LC filters to make them more precise.

Filters are measured by their quality or "Q" factor. A filter is said to have a high Q if it selects or rejects a narrow range of frequencies compared with the absolute frequency at which it operates. Quality can be measured by the precision of a harmonic oscillator implemented with that type of device.

Active Filters

Filters can also be implemented using a combination of passive components and amplifiers to create active filters. These can have high Q, and achieve resonance without the use of inductors. However, their upper frequency limit is lower than that of a passive filter. Further detail is available in the digital filter section.

Other filters

In the late 1930s, engineers realized that small mechanical systems made of rigid materials such as quartz would acoustically resonate at radio frequencies, i.e. from audible frequencies (sound) up to several hundred megahertz.

Some early resonators were made of steel, but quartz quickly became favored. The biggest advantage of quartz is that it is piezoelectric. This means that quartz resonators can directly convert their own mechanical motion into electrical signals. Quartz also has a very low coefficient of thermal expansion. This means that quartz resonators produce stable frequencies over a wide temperature range.

Quartz crystal filters have much higher quality factors than LCR filters. When higher stabilities are required, the crystals and their driving circuits may be mounted in a "crystal oven" to control the temperature. For very narrow filters, sometimes several crystals are operated in series.

Engineers 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 introduced 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 megahertz 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.