Electronic Engineering Dictionary

Passive Filters

Filter. A selective network of resistors, capacitors, and inductors that offers comparatively little opposition to certain frequencies, while blocking or attenuating other frequencies. In electronics, a device that transmits only part of the incident energy and may thereby change the spectral distribution of energy: (a) high-pass filters transmit energy above a certain frequency; (b) low-pass filters transmit energy below a certain frequency; (c) bandpass filters transmit energy of a certain bandwidth; (d) band-stop filters transmit energy outside a specific frequency band. A group of passive components that reduce the effects of an undesired signal(s). A filter that does not use integrated circuits, transistors, or FETS for amplification or as part of the filter.

Filter Response. The output response of an active or passive filter. Refer to the definition of Filter for the different types. The graph shows the different orders of a Butterworth Filter, 1st order filter to fifth order.

Butterworth Filter

All-Pass filter. A filter that does not change or attenuate the signal magnitude. A filter that changes or shifts the phase of the incoming signal with attenuating different frequencies, all frequencies pass equally well through the filter.

Anti-Aliasing filter. A low-pass filter designed to attenuate high frequency components so the effect of Aliasing does not occur.

Butterworth filter. A type of filter that is flat in the pass band [maximally-flat], but less than ideal roll-off.

BandPass Filter. A filter that allows a narrow band of frequencies to pass through the circuit. Rejects or attenuates frequencies that are either higher or lower than the desired band of frequencies. A filter that ideally passes all frequencies between two non-zero finite limits and bars all frequencies not within the limits. The cutoff frequencies are normally taken to be the 3-dB points.

Band-Reject Filter. A tuned circuit that does not pass a specified band of frequencies. A filter that attenuates or rejects a certain range of frequencies.

Band-Stop Filter. Same as Band-Reject.

ByPass Capacitor. A Decoupling Capacitor used to filter the power supply pin of an IC.

Chebyshev filter. A type of filter that has a small amount of ripple in both the pass-band and stop-band but has a very steep roll-off.

Common Mode Choke Filter. An LC Filter that uses a Common Mode Choke [consisting of two equal windings about a common core]. Common Mode Choke Filters are used for AC lines or paired DC lines as in AC Power Line Filters. A common mode coil is shown to the right, 4-wires with 2 per inductor [winding] wound around a common core. Companies that make Choke Inductors.

Diplexer Filter. A filter that consists of both a High-Band filter and a Low-Band filter.

EMI Filter. Electro-Magnetic Filter. A filter that attenuates frequencies that could interfere with a circuits operation. A AC Inlet Filter is an example of an EMI filter.

High-Pass Filter. A filter that passes a majority of the high frequencies on to the next circuit and rejects, or attenuates, the lower frequencies. Also called a Low-frequency discriminator.

Ladder Filter. A passive filter similar to a pi filter, but having an additional series element in the return section of the filter. Refer to the graphic in the ride side-bar.

LC Choke Filter. See LC Filter. Normally an LC filter that filters both positive and negative lines, line line could be ground or reference. An example of an LC Choke filter may be found on the Chassis Power Line Filters page.

LC Choke-Input Filter. This filter is used in power supplies where voltage regulation is important and where the output current is relatively high and subject to varying load conditions. Related topic; Choke Manufacturers.

Low Pass PI Filter

LC Filter. A filter that uses one or more capacitors and inductors. LC Filters could be a low pass filter in the form of a pi filter or a high-pass filter in a 'T' configuration.

Low-Pass Filter. A filter that passes a majority of the low frequencies on to the next circuit and rejects, or attenuates, the higher frequencies. Also called a high-frequency discriminator, or Anti-Aliasing filter [Aliasing]. Refer to the graphic to the right.

Notch Filter. Same as a Band-Reject Filter.

Passive filter. A filter that does not active components in the circuit.

Phase-Shift filter. An All-Pass filter.

Pi Filter. A passive filter with one series element and two parallel elements [see above].

RC Filter. A filter used in applications where load current is low and constant, and voltage regulation is not necessary. An RC filter is comprised of some number of Resistors and Capacitors.

RC Low-Pass Filter Styles

Any number of combinations of resistors or capacitors are possible. The graphic above shows a few other possible RC combinations that would form a Low-Pass filter.

Root Cosine Filter. A low-pass Filter.

T Filter. A Filter in the shape of the letter 'T'. The example shows an LC Feed-Through Capacitor circuit.

LC T-Filter