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Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

(1) a filter that has an energy gain greater than one, that is, a filter that outputs more energy than it absorbs.
(2) a form of power electronic converter designed to effectively cancel harmonic currents by injecting currents that are equal and opposite to, or 180. out of phase with, the target harmonics. Active filters allow the output current to be controlled and provide stable operation against AC source impedance variations without interfering with the system impedance.
The main type of active filter is the series type in which a voltage is added in series with an existing bus voltage. The other type is the parallel type in which a current is injected into the bus and cancels the line current harmonics.

active RC filter

an electronic circuit made up of resistors, capacitors, and operational amplifiers that provide well-controlled linear frequency-dependent functions, e.g., low-, high-, and bandpass filters.

band stop filter

filter that exhibits frequency selective characteristic such that frequency components of an input signals pass through unattenuated from input to output except for those frequency components coincident with the filter stop-band region, which are attenuated. The stop-band region of the filter is defined as a frequency interval over which frequency components of the input signal are attenuated.

coupled line filter

a type of microstrip or stripline filter that is composed of parallel transmission lines. Bandwidth is controlled by adjusting the transmission line spacing. Wider bandwidths are obtained by tighter coupling. A two-port circuit is formed by terminating two of the four ports in either open or short circuits, which leaves ten possible combinations. Different combinations are used to synthesize low-pass, bandpass, all pass, and all stop frequency responses.

electromagnetic interference filter

a filter used to reduce or eliminate the electromagnetic interference (EMI) generated by the harmonic current injected back onto the input power bus by switching circuits. The harmonic current is caused by the switch action that generates switch frequency ripple, voltage and current spikes, and high-frequency ringing. Generally called an EMI filter.

EMI filter

See electromagnetic interference filter

filter

(1) a network, usually composed of inductors and capacitors (for lumped circuit), or transmission lines of varying length and characteristic impedance (for distributed circuit), that passes AC signals over a certain frequency range while blocking signals at other frequencies. A bandpass filter passes signals over a specified range (flow to fhi),
and rejects frequencies outside this range. For example, for a DBS receiver that is to receive satellite transmitted microwave signals in a frequency range of 11 GHz to 12 GHz, a band-pass filter (BPF) would allow signals in this frequency range to pass through with minimum signal loss, while blocking all other frequencies. A low-pass filter (LPF)
would allow signals to pass with minimum signal loss as long as their frequency was less than a certain "cutoff frequency" above which significant signal blocking occurs.

(2) an operator that transforms image intensity I_x of pixel x into a different intensity î_x, depending on the values of a set of (usually neighboring) pixels (which may or may not include x). Filtering is performed to enhance significant features of an image or to remove nonsignificant ones or noise. filter bank a set of filters consisting of a bank of analysis filters and a bank of synthesis filters. The analysis filters decompose input signal spectra into a number of directly adjacent frequency bands for further processing, and the synthesis filters recombine the signal spectra from different frequency bands.

filtering

(1) an estimation procedure in which the present value of the state vector (see the definition) is estimated based on the data available up to the present time.

(2) the process of eliminating object, signal or image components which do not match up to some pre-specified criterion, as in the case of removing specific types of noise from signals. More generally, the application of an
operator (typically a linear convolution) to a signal.

high pass filter

filter exhibiting frequency selective characteristic that allows high-frequency components of an input signal to pass from filter input to output unattenuated; all lower frequency components are attenuated.

high-pass filter

a filter that has a transfer function, or frequency response, whose values are small for frequencies lower than some intermediate frequency. A filter whose impulse response is a high-pass signal.

transmission line filter

a microwave device that is made up of sections of transmission lines so as to act as a filter in the microwave frequency range.