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An electronic filter eliminates unwanted frequencies from an electronic signal. There are several kinds of electronic filters:
This article will consider the design and construction techniques common to all these filter types.
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.
1 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 RLC varieties. All these types are collectively known as passive filters, because they are activated by the powerMechanical power In physics, power (symbol: P is the amount of work W done per unit of time t''. This can be modeled as an energy flow, equivalent to the rate of change of the energy in a system, or the time rate of doing work, as defined by : where P is 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 megahertzHertz is also the name of a car rental company. See The Hertz Corporation''. The hertz (symbol Hz is the SI unit of frequency. It is named in honour of the German physicist Heinrich Rudolf Hertz who made some important contributions to science in the fiel), sometimes the inductors consist of single loops or strips of sheet metal, and the capacitors consist of adjacent strips of metal. These are called stubs. 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 oscillatorA harmonic oscillator is a mechanical system in which there exists a returning force F directly proportionate to the displacement x i. where k is a constant. It can also refer to any physical system that is analogous to this mechanical system, in which so implemented with that type of device.
2 Active filters
Active filterActive filters are a special kind of electronic filters. They use additional energy than that of the signal to filter the signal. Commonly they are constructed using an operational amplifier. The topologies correspond to R and C components allong to a rets are implemented using a combination of passive and active components. Operational amplifierAn operational amplifier or op-amp is an electronic circuit module (normally built as an integrated circuit, but occasionally with discrete transistors or vacuum tubes) which has a non-inverting input (+), an inverting input and one output. The output vols are frequently used in active filter designs. These can have high Q, and achieve resonance without the use of inductors. However, their upper frequency limit is limited by the bandwidth of the amplifiers used.
