IEEE Std 1549-2011, IEEE Standard for Microwave Filter ... Standard for Microwave... · This introduction is not part of IEEE Std 1549-2011, IEEE Standard for Microwave Filter Definitions

IEEE Standard for Microwave Filter Definitions

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IEEE 3 Park Avenue New York, NY 10016-5997 USA 20 May 2011

IEEE Microwave Theory and Techniques Society

IEEE Std 1549™-2011

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IEEE Std 1549™-2011


Sponsor

Standards Coordinating Committee of the IEEE Microwave Theory and Techniques Society

Approved 31 March 2011

IEEE-SA Standards Board


Abstract: Standardized definition of terms commonly used in the field of electrical networks and filters are defined. Keywords: definitions, filters, IEEE 1549, multiplexers, networks, resonators, terms

•

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Introduction

This introduction is not part of IEEE Std 1549-2011, IEEE Standard for Microwave Filter Definitions.

This document is an initial release of standardized definitions for terms commonly used in the field of electrical filter networks. The initial release is primarily, but not exclusively, focused on terms used in passive microwave filter networks. It is anticipated that future revisions to this standard will include additional terms, particularly in the field of active filter networks.

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iv Copyright © 2011 IEEE. All rights reserved.


Interpretations

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v Copyright © 2011 IEEE. All rights reserved.


Participants

At the time this standard was submitted to the IEEE-SA Standards Board for approval, the Filter Term Definitions Working Group had the following membership:

Sridhar Kanamaluru, Chair Ming Yu, Vice Chair

Smain Amari Ali Atia Clark Bell Pierre Blondy Vicente E. Boria-Esbert Richard Cameron Har Dayal Wael Fathelbab Xun Gong

Jia-Sheng Hong Ian Hunter Chandra Kudsia Jen-Tsai Kuo Ralph Levy Pino Macchiarella Raafat Mansour George Matthaei Jeff Pond

David Rhodes Fabien Seyfert Sanghoon Shin Richard Snyder Daniel Swanson Chi Wang Robert Wenzel Ke-Li Wu Kawthar Zaki

The following members of the individual balloting committee voted on this standard. Balloters may have voted for approval, disapproval, or abstention. Suresh Channarasappa Carlo Donati James Gilb Randall Groves Werner Hoelzl Michael Janezic

Sridhar Kanamaluru Greg Luri Roger Marks Jon Martens Edward McCall Michael S. Newman Satoshi Oyama

Ulrich Pohl R. K. Rannow Bartien Sayogo Gil Shultz Richard Snyder Walter Struppler

vi Copyright © 2011 IEEE. All rights reserved.


When the IEEE-SA Standards Board approved this standard on 31 March 2011, it had the following membership:

Richard H. Hulett, Chair John Kulick, Vice Chair

Robert M. Grow, Past Chair Judith Gorman, Secretary

Masayuki Ariyoshi William Bartley Ted Burse Clint Chaplin Wael Diab Jean-Philippe Faure Alexander Gelman Paul Houzé

Jim Hughes Joseph L. Koepfinger* David J. Law Thomas Lee Hung Ling Oleg Logvinov Ted Olsen

Gary Robinson Jon Walter Rosdahl Sam Sciacca Mike Seavey Curtis Siller Phil Winston Howard L. Wolfman Don Wright

*Member Emeritus

Also included are the following nonvoting IEEE-SA Standards Board liaisons:

Satish Aggarwal, NRC Representative Richard DeBlasio, DOE Representative Michael Janezic, NIST Representative

Michael Kipness

IEEE Standards Program Manager, Document Development

Catherine Berger IEEE Standards Project Editor

vii Copyright © 2011 IEEE. All rights reserved.


Contents

1. Overview .................................................................................................................................................... 1 1.1 Scope ................................................................................................................................................... 1 1.2 Purpose ................................................................................................................................................ 1

2. Definitions .................................................................................................................................................. 1

viii Copyright © 2011 IEEE. All rights reserved.



IMPORTANT NOTICE: This standard is not intended to ensure safety, security, health, or environmental protection. Implementers of the standard are responsible for determining appropriate safety, security, environmental, and health practices or regulatory requirements.

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1. Overview

1.1 Scope

The scope of this standard is to provide the standard definitions for microwave filter terms.

1.2 Purpose

The purpose of this standard is to standardize the definitions of microwave filter terms.

2. Definitions

For the purposes of this document, the following terms and definitions apply. The IEEE Standards Dictionary: Glossary of Terms & Definitions should be referenced for terms not defined in this clause.1

absorptive filter: A frequency selective network where the attenuation in the stop band is obtained through power dissipation rather than by means of reflection as in classical passive filters, thereby allowing a degree of matching at input and output ports in the pass and stop band(s).

1 The IEEE Standards Dictionary: Glossary of Terms & Definitions is available at http://shop.ieee.org.

1 Copyright © 2011 IEEE. All rights reserved.


IEEE Std 1549-2011 IEEE Standard for Microwave Filter Definitions

acoustic-wave filter: Filter devices that rely on acoustic wave propagation and interference for pass and stop band(s) characteristics, along with conversion from acoustic to electrical energy (or vice-versa) by piezoelectric material for access by electrical terminals. Acoustic filter types include bulk acoustic wave (BAW), film bulk acoustic resonator (FBAR), and surface acoustic wave (SAW). See also: BAW filter, FBAR filter, SAW filter.

active filter: A frequency selective network that uses resonating elements implemented by simulated inductance or capacitance, using feedback, feed-forward, or impedance rotation in an active circuit. Such circuits present a low loss, high-Q reactance through the use of equivalent negative resistance elements that offset or cancel the losses of passive elements in the network.

NOTE—The so-called “impedance rotation” type of element is typified by the inverted common collector or inverted common drain configurations in transistors. The operating principle for such devices is the presentation of negative resistance in series with a lossy reactive element, at the base or drain terminal. When the input reactance is inductive, a capacitive reactance is presented at the emitter or source circuit, hence the name “impedance rotation”. The effective loss of the input reactance is reduced due to the negative series resistance. The input reactance is then resonated to form a high-Q input resonator. Several such resonators are coupled to form the active filter network.2

admittance inverter: A circuit having a characteristic admittance, often symbolized by J, and a phase shift of plus or minus 90 degrees. Contrast: impedance inverter, K-inverter. See also: J-inverter, direct coupled filter.

NOTE—Admittance inverters are usually comprised of lumped or semi-lumped elements, or their waveguide equivalents, and used for coupling between resonators in band pass filters when the resonators are all of the same type.

asynchronous tuning: A tuning method for multiple-resonator filters, where each resonator has a different self-resonant frequency. The tuning involves the phasing of reflections from successive resonators to provide a desired performance, e.g., minimal return loss over a desired frequency range. Contrast: synchronous tuning.

attenuation: The ratio of the available power from the generator to the output power from a network at a given frequency, usually expressed in decibels (dB). See also: attenuation function; transmission function.

NOTE—Attenuation is the same as “insertion loss” if the network terminations are equal. Note the S-parameter magnitudes in dB such as are measured by network analyzers are by nature “transmission” ratios (i.e., output/input) in dB and are the negatives of attenuation expressed in dB.

attenuation function: The ratio of generator voltage or current to the load voltage or current in a network expressed as a function, often of complex frequency. See also: transmission function.

NOTE—The zeros of the attenuation function are the “natural frequencies” and the poles are the “frequencies of infinite attenuation” for the network.

balanced filter: A filter type that provides frequency selectivity for balanced signals, and is usually used in systems where the signals are processed in a balanced manner.

balun: A type of network that converts signals that are balanced (with respect to ground) to signals that are unbalanced or vice versa, and may also include an impedance transformation.

balun filter: A network that performs the functions of both a balun and a filter. A balun filter comprises of three ports; one is the unbalanced port and the other two are the balanced ports. Ideally, the transfer functions from the unbalance port to each of the other two ports have the same magnitude, but have a 180 degree phase difference. See also: balun.

2 Notes in text, tables, and figures are given for information only and do not contain requirements needed to implement the standard.





band pass filter: A frequency selective network that has a low attenuation frequency range called the pass band, which is interposed between a lower and upper stop band, both of which have higher attenuation as compared to the pass band. Contrast: band reject filter; band stop filter. See also: pass band and stop band.

band reject filter: A frequency selective network that has a high attenuation frequency range called the stop band, which is interposed between a lower and upper pass band, both of which have lower attenuation as compared to the stop band. Contrast: band pass filter. Syn: band stop filter. See also: pass band and stop band.

band stop filter: See: band reject filter.

bandwidth: The range of frequencies within which the performance of the network complies with the specified limits.

Bessel filter: A filter derived using Bessel functions with a goal of linear phase, i.e., maximally flat group delay. Syn: maximally flat delay filter, Thomson filter.

NOTE—The term “Bessel filter” is due to the fact that low pass filters of this type can be synthesized starting from Bessel polynomials.

BAW filter: A filter that utilizes piezoelectric material transducers to convert electrical energy to acoustic wave energy which travels within the bulk of the material achieving pass band or stop band characteristics through constructive and destructive interference. The acoustic energy is converted back to electrical energy at the filter output. See also: acoustic-wave filter, FBAR filter, SAW filter.

Butterworth filter: See: maximally flat filter.

Cauer filter: See: elliptic filter.

ceramic filters: Filters that employ high dielectric constant materials, typically ceramics, in a comb-line or inter-digital configuration to reduce the filter size.

NOTE—Also referred to as “block filters” or “ceramic block filters”, although the usage of either term is discouraged.

channel dropping filter: A three-port network where a band stop characteristic is observed between (for example) ports 1 and 2, while a band pass characteristic is observed between ports 1 and 3; the band pass and band stop frequency ranges are identical. The common port (e.g., port 1) of the channel dropping filter is always matched over the entire frequency range, irrespective of the port 2 and port 3 frequency bands. The channel dropping filters can therefore be cascaded without reflective interactions to form multiplexers, with the port 2 of each channel dropping filter being connected to the port 1 of successive filter.

characteristic impedance: Most commonly applied to two-port symmetrical circuits such as transmission lines, and is defined as the input impedance seen looking into an infinite cascade of copies of the given network so there is never a reflection from the far end. It can be computed as the square root of the impedance seen looking into one end of the two-port network with the other end open circuited multiplied by the input impedance when that end is short circuited. Historically “characteristic impedance” has also been defined for unsymmetrical networks as the input impedance of an infinite cascade of copies of the network where End 1 of each copy is connected to End 2 of the next copy and vice-versa. The characteristic impedances Z01 and Z02 seen at ports 1 and 2 of the network can be computed from the following equations:

CDADA

Z2

)(4)(201

−+−+=

CDADA

Z2

)(4)(202

−−−+=

where A, B, C, and D are the general circuit parameters of the two-port network. See also: image impedance.




Chebyshev filter: A filter derived from Chebyshev polynomials that has an equal-ripple attenuation variation in its pass band with monotonically increasing attenuation as the frequency moves away from the pass band. Contrast: inverse Chebyshev filter.

Chebyshev rational function filter: A generalized type of elliptic filter whose attenuation characteristics are represented by the elliptic rational function (also called the Chebyshev rational function). The elliptic rational function characteristics are controlled by two parameters called the ripple factor and selectivity factor.

NOTE—Elliptic function filters are a subset of Chebyshev rational function filters. Elliptic function filters have a maximum number of stop band transmission zeros and the stop band rejection level is equal ripple. Chebyshev rational function filters may or may not have the maximum number of stop band transmission zeros and the stop band rejection level may or may not be equal ripple.

comb-line filter: A filter type with all resonator lines arranged in a parallel array with one end of each resonator shorted to ground and with the other end loaded with capacitance to ground. The capacitive loading on the resonators may result in their length being less than quarter wavelength at the mid pass band frequency, yielding a compact filter.

common-mode rejection filter: A filter type, usually a balanced filter that attenuates the signal common to both input lines; that signal is called the common-mode signal.

common resonator multiplexer: A multiplexer that uses a resonator (called the common resonator) as an input junction to connect the common input port to every channel filter in the multiplexer. If the common resonator has a parallel-type resonance, it may also be known as a “susceptance annulling network”; while if it is of the series-resonant type, it may be known as a “reactance annulling network.” See also: multiplexer filter.

NOTE—The resonant frequency of the common resonator is typically around the center of the overall frequency range of the multiplexer.

constant reactance element: A type of electrical network, usually a combination of inductances and capacitances that collectively provide an approximately constant reactance with respect to frequency, over a limited frequency range.

corrugated waveguide filter: A waveguide filter type that uses corrugations in the upper, lower, or both waveguide walls to obtain a pseudo low pass filter characteristic. The filter has a lower cutoff frequency due to the waveguide cutoff. See also: waffle iron filters.

NOTE—Corrugated waveguide filters typically have very wide stop bands.

coupled line filter: A filter type comprising of a cascade of parallel transmission line pairs with two of each set of four ports per pair being open or short-circuited.

coupling coefficient: A measure of the degree of coupling between adjacent resonators, particularly in direct-coupled band pass filters that have small to moderate bandwidth.

If the adjacent resonators exhibit series-type resonance, the coupling coefficient (k1,2) can be obtained from the ratio of the impedance-inverter parameter (K1,2) associated with the coupling between the resonators divided by the square root of the product of the reactance slope parameters (x1, x2) of the two resonators.

( )21

1,22,1

xxKk

×=



If the resonators exhibit parallel-type resonance, the same relations apply except in terms of the admittance inverter parameter (J1,2) associated with the coupling and the susceptance slope parameters (b1, b2) of the resonators.

( )21

1,22,1

bbJk

×=

coupling matrix: The coupling matrix M of an N-resonator band pass filter is an N-by-N matrix of real numbers whose (i,j) element, Mi,j for i≠j is the normalized, frequency independent coupling between resonators i and j of the N resonator filter. The values of the diagonal elements Mi,i are equal to –2 Δi/ωo where Δi = ωoi - ωo , ωoi is the resonant angular frequency of the i-th resonator and ωo is center angular frequency of the filter. See also: coupling coefficient.

NOTE—The actual values (un-normalized) of the coupling ki,j are given by : ki,j = BW × Mi,j/fo where BW is the filter (equal ripple) band width and fo is the center frequency of the filter.

cross-coupled filter: A band pass filter composed of a number of resonators (N) with couplings existing among adjacent and non-adjacent resonators. The existence of the non-adjacent couplings makes it possible to realize transfer functions that have finite transmission zeros or zeros at complex frequencies in the complex frequency plane.

cross coupling: A coupling beyond nearest neighbor resonators while the intervening resonators have couplings between nearest neighbors. Cross coupling is used to achieve transmission zeros in the complex plane.

delay: The term “delay” is often (imprecisely) used interchangeably with the term “group delay.” In some situations, the term may refer to the amount of time a signal such as a pulse is delayed when passing through a filter or other circuit. However, this usage may be imprecise because the shape of the pulse may have changed when coming out of the circuit. See: group delay.

delay equalized filter: Delay equalized filters, also referred to as linear phase filters, fall in the category of non-minimum phase filters, and are characterized by transfer functions (S21) that have at least one zero in the right half of the complex frequency plane and have typically more linear phase at the expense of poorer amplitude response. Contrast: minimum phase filter.

NOTE—Unlike minimum phase filters, the magnitude and phase of delay equalized filters is not uniquely related. It is therefore necessary to optimize both the amplitude and phase characteristics to achieve the desired filter function. This inherent tradeoff between amplitude and phase (or group delay) is the defining feature of linear phase filters, making them attractive for many applications. It should be noted that delay equalized filters achieve better phase and group delay response at the expense of poorer amplitude response. However, it is always possible to choose a higher order delay equalized filter to attain the desired amplitude and phase response at the expense of increased complexity.

dielectric resonator (DR) filter: A filter type that uses high dielectric constant blocks of rectangular, parallelepiped, cylindrical, or other shaped resonators, whose fundamental resonances confine the fields within the dielectric, thereby greatly reducing housing conductor losses.

NOTE—Coupling between multiple dielectric resonators may be achieved as between any pair of resonators, i.e., through free-space, iris, or other. The DR filters differ from ceramic or block filters in that the resonant modes in dielectric resonators are dispersive.

diplexer: A multiplexer with only two channels. See: multiplexer filter.

direct coupled filter: A band pass filter that uses lumped or semi-lumped reactance elements for coupling between resonators. Contrast: quarter wavelength coupled filter.




NOTE—The coupling reactances in direct coupled filters can be viewed as comprising part of “impedance inverters” or “admittance inverters” that have the same electrical properties as a quarter wavelength line (except that their phase shift may be either plus or minus 90 degrees).

directional filter: A four-port filter type where the direction of flow of power is dependent on the frequency of operation and, if the ports are terminated appropriately, will ensure an impedance match for all ports at all frequencies.

NOTE—A directional filter will have the power flow from (for example) port 1 to port 2 or vice versa at the resonant frequencies, while the power will flow from port 1 to port 3 and from port 2 to port 4 at non-resonant frequencies. A directional filter can be used as a channel-dropping filter.

dual band filter: A filter capable of operating on two different frequency bands, with the frequency and guard bands specified precisely in terms of center frequencies and bandwidths.

dual mode filter: A band pass filter whose resonators have physical symmetry permitting at least two simultaneous orthogonal modes at the same frequency per resonator.

NOTE—In this type of dual mode filter, the two orthogonal modes in a resonator structure are coupled by introducing a small perturbation in the structure; thus having, for example, a three resonator structure exhibiting the response of a six-resonator filter. It is also possible to achieve designs using three orthogonal modes per resonator structure.

electrical coupling: A type of resonator coupling that is dominated by electric fields or electric charges, with the induced charges on the second resonator having an opposite sign to those on the first one. Contrast: magnetic coupling.

elliptic filter: A filter that has attenuation characteristics with equal-ripple variation in the pass band and stop band. The poles of attenuation are so distributed in the stop band (or stop bands) so that the points of minimum attenuation between the poles are all at the same level.

NOTE—The transfer function can be defined in terms of elliptic functions, hence the term “elliptic filter.”

E-plane filter: A type of waveguide filter in which the waveguide is split in half along the center E plane, and a thin metal “fin” is clamped between the two halves. Portions of the metal fin are cut out in order to realize shunt-inductive or shunt-capacitive discontinuities.

NOTE—The E-Plane filter technique is particularly attractive for mm-wave applications as it facilitates fabrication of very small filters.

equalizer: A network whose transmission function characteristic is transmitting all the incident power and reflecting none, and whose phase can be manipulated to provide or improve the phase or attenuation response. An equalizer is also referred to as an all-pass filter or network.

equiripple: A function is said to have an equiripple characteristic if all its minima or maxima have the same functional value.

evanescent mode filter: Compact filter networks using circuit elements with significant energy storage in evanescent fields, usually lengths of waveguides below cutoff.

NOTE—Evanescent mode filter designs are usually based on quasi-lumped equivalents to lengths of below cutoff waveguide. The inductive equivalent circuit elements are resonated by capacitive elements and are coupled via inductive or capacitive proximity effects. The filters are similar in structure to comb-line filters, but have different field distributions leading to wider stop bands and higher Q. The coupling between resonators in these filters is essentially due to coupling between evanescent fields. The considerable use of evanescent fields in these structures enables them to be quite compact.




external quality factor (Q): A measure of the coupling between resonators at the ends of the filter and their adjacent terminations, usually defined for band pass filters of narrow to moderate bandwidth. The term external Q is also used for single-resonator structures that have either one or two terminations. Syn: loaded-Q.

NOTE—External Q is the Q of an end resonator when it is coupled to its adjacent termination, while ignoring any losses within the resonator itself.

extracted pole filter: A filter network that generates a transmission zero at a finite frequency by a shunt-connected, series-type, resonant branch shorting out transmission, or by series-connected, parallel-type, resonant branch blocking transmission. This technique can only create zeros of transmission on the jω axis of the complex frequency plane (or to the left of that axis if loss is included). Contrast: cross-coupled filter.

FBAR filter: Film bulk acoustic resonator (FBAR) is a high-Q resonator that uses the elastic wave of the thickness mode (bulk wave) of a piezoelectric thin film sandwiched between two electrodes. When a RF signal is applied across the device, it produces a mechanical motion in the piezoelectric layer. When the thickness of the film is equivalent to half the wavelength of the RF signal in the piezoelectric material, fundamental resonance occurs. A filter characteristic of a FBAR filter is achieved by appropriate coupling topologies of the resonators, such as ladder configuration. See also: acoustic-wave filter, BAW filter, SAW filter.

filter bank: An electrical device comprising a group of filters constructed hierarchically to selectively combine or divide the frequency spectrum, commonly used for analysis of a signal’s frequency content in a wide band of the electromagnetic spectrum.

filter section: The smallest section of a filter that provides a pole, zero, or both. In a low pass filter, a filter section may be a shunt capacitor plus half of each adjacent series inductor. In a direct coupled band pass filter, it may be a resonator plus half of the coupling structure on the left and on the right.

fractional bandwidth: A ratio of pass band or stop band bandwidth to the mid-band frequency of the pass band or stop band, respectively, of the filter. See also: percentage bandwidth.

group delay: The group delay is a function of frequency and may be computed from the derivative with respect to radian frequency of the phase of an attenuation function or the negative of the derivative of the phase of a transmission function. See also: attenuation function, transmission function, and phase.

group delay distortion: The deviation of the group delay vs. frequency characteristic from a value prescribed at a reference (usually the filter mid-band) frequency. Group delay distortion results in the distortion of the shape of the signal envelope, as the signal passes through the network.

helical resonator filter: A filter utilizing the inductive or capacitive coupling between resonators formed from wire helices, usually in the frequency range of 100 MHz to 1000 MHz.

high pass filter: A filter that strongly attenuates signals from DC to some specified cutoff frequency and passes all frequencies above the cutoff frequency with minimal attenuation. Contrast: low pass filter.

NOTE—In practice, the extent of the pass band of high pass filters is limited due to the emergence of higher order modes.

HTS filters: Filters that utilize high temperature superconductor (HTS) material for their circuitry, resulting in very high resonator Qs and very low filter loss.

NOTE—Most HTS filters operate at a relatively high temperature (e.g., 77 K), which is high compared to the very low temperature (4 K) required for non-high temperature superconductors.





image impedance: The impedance seen looking into an infinite cascade of copies of the network where port 1 of a copy is connected to port 1 of its neighbor and a port 2 is connected to port 2 of its neighbor. The image impedance for a given end of the two-port network can be computed as the square root of the product of the impedances seen looking in that end with the other end short circuited and with the other end open circuited. The definition applies to both symmetrical and unsymmetrical two-port networks. If the network is symmetrical, image impedance and characteristic impedance are the same. See also: characteristic impedance.

image transfer function: The transfer function of a two-port network when terminated in its image impedances. See also: transmission function.

impedance inverter: A circuit that has a characteristic impedance, often symbolized by K, and a phase shift of plus or minus 90 degrees. Such circuits are usually comprised of lumped or semi-lumped elements and are used for coupling between resonators in band pass filters. Contrast: admittance inverter, J-inverter. See also: direct coupled filter, K-inverter.

iris coupled filter: A filter formed of multiple waveguide resonators, with pairs of resonators coupled to each other by small windows. See also: inductive window filter.

inductive window filter: A filter formed of multiple waveguide resonators, with pairs of resonators coupled to each other by inductive windows, which are thin conducting plates that partially cover the cross section to form a waveguide of smaller width (a-dimension) and identical height (b-dimension) as the resonator. See also: iris coupled filter.

interdigital filter: A band pass filter formed of a multiple transmission line resonators arranged parallel to each other, with each resonator short circuited at one end and open circuited at the other, with the open and short-circuited locations alternating from one resonator to the next. At the mid-band frequency, the resonators are nominally a quarter-wavelength long.

inverse Chebyshev filter: A filter that has a maximally flat pass band characteristic, while the poles of attenuation are so distributed that it is equiripple in the stop band (or stop bands). See: maximally flat filter.

J-inverter: A J-inverter is an idealized admittance representation of a two-port network that operates like a quarter-wavelength line of characteristic admittance J. Contrast: K-inverter, impedance inverter. See also: direct coupled filter, admittance inverter.

K-inverter: A K-inverter is an idealized impedance representation of a two-port network that operates like a quarter-wavelength line of characteristic impedance K. Contrast: J-inverter, admittance inverter. See also: direct coupled filter, impedance inverter.

ladder filter: A filter that has a cascade or tandem connection of alternating reactance elements in series and shunt arms.

lattice filter: A filter that has at least four resonators connected in series to form a mesh, with two non-adjacent junction points as input terminals and the remaining two junction points as output terminals.

loaded quality factor (Q): The Q of a resonator including its internal losses plus the loss of any termination connected to the resonator. See: external quality factor (Q).

low pass filter: A filter that passes frequencies from DC up to some specified cutoff frequency with minimal attenuation while, ideally, strongly attenuating all signals above this frequency. In practice, the extent of the stop band is limited due to the emergence of higher order modes.

LTCC filter: A filter implemented using multilayer low temperature cofired ceramic technology.



magnetic coupling: A type of resonator coupling that is dominated by magnetic fields. Contrast: electrical coupling.

manifold multiplexer: A multi-port frequency selective network, formed by connecting multiple filters (for different multiplexer channels) to a common transmission line called the manifold (usually waveguide) at strategic points using lengths of transmission line that minimize interaction effects between channels. Contrast: channel dropping filter, directional filter. See also: multiplexer filter.

NOTE—Well-designed manifold multiplexers have the filter connections points on the manifold and the line lengths from the filters to the manifold chosen strategically, with the elements of each filter that are nearest to the manifold modified, so the many interaction effects between the filters and manifold will work together harmoniously. This is in contrast to multiplexers using “channel-dropping” or “directional” filters that can be cascaded without interactions as a result of their matched impedances.

matrix rotation: Matrix rotation(s) is a sequence of similarity transformations applied to the coupling matrix of a network, to yield a matrix with preferred coupling arrangement and/or elements with the same transfer and reflection characteristics as the original matrix.

maximally flat delay filter: See: Bessel filter.

maximally flat filter: A filter that has a monotonic increasing attenuation in the pass band as the frequency moves away from the center of the pass band. Filters most commonly referred to as “maximally flat” have attenuation that increases monotonically as the frequency moves away from the pass band, although variants such as the inverse Chebyshev filters may have other types of attenuation characteristics. Syn: Butterworth filter. See also: inverse Chebyshev filter.

minimum phase filter: A filter whose transfer function has all of its zeros of transmission in the left-half of the complex frequency plane or on the jω axis. The phase is minimum in the sense that that if one or more of the transmission zeros were moved to the corresponding points in the right half of the complex frequency plane the transfer function would have the same amplitude but an increased amount of phase shift. Contrast: non-minimum phase filter.

mixed-mode filter: A filter class in which more than one principal electromagnetic modes are employed in realizing a filter function. The filter is designed so that the modes resonate at potentially different frequencies to achieve the desired performance.

multiplexer filter: A group of n filters, each of which has a different pass band range, that are designed and interconnected so as to provide a common input port and n output ports, each covering a different frequency range. Thus, an n-channel multiplexer will separate a broad band of signals at the common input channel into n isolated frequency bands. Conversely, a multiplexer can be used to efficiently sum the signals in n separate channels onto one single wideband channel. An order n multiplexer can be multiplexed at both ends, providing a two-port transfer function with specific pass bands or stop bands.

non-minimum-phase filter: A filter that has one or more zeros of transmission in the right half of the complex frequency plane. The phase is non-minimum in the sense that if any of the transmission zeros in the right half of the complex frequency plane were moved to corresponding points in the left half of the plane, the amplitude characteristic of the frequency response would be unchanged but the phase shift would be reduced. Filters with non-minimum phase are commonly used in order to make the phase characteristic more nearly linear. Contrast: minimum phase filter.

non-resonating node: A node in a network that is connected to ground by a (ideally) frequency-independent reactive element. At microwave frequencies, the reactive element can be approximated by a capacitive or inductive entity over a narrow frequency band, simulating an approximate frequency independent characteristic.




notch filter: A two-port network that has very high attenuation at a specific frequency or across a frequency band in the input frequency spectrum, and passes the rest of the spectrum with negligible attenuation. The signals within the reject band are either reflected to the input port or absorbed within the two-port network. In the case where the rejected signals are reflected to the input port, the notch filter behaves exactly as a band stop filter. See also: band stop filter.

pass band: The range of frequencies within which the attenuation of a filter or network is lower than at frequencies outside the range.

percentage bandwidth: Fractional bandwidth expressed as a percentage. See also: fractional bandwidth.

phase: A representation of the completed fraction of a sinusoid relative to a reference time; more commonly refers to the phase of an attenuation or transmission (e.g., S21) function as it varies with frequency. The phase of an attenuation function or the phase of a transmission function (e.g., S21) differ by a negative sign. The phase slope vs. frequency will be positive at most frequencies for an attenuation function, while it will be negative at most frequencies for a transmission function.

pre-distorted filter: A filter type where the poles of the transmission function are moved towards the jω axis, to increase the selectivity and bandwidth of a low Q filter at the expense of increased insertion loss. When losses are added to a filter designed as a lossless filter, there is a tendency for the poles and zeros of the transfer function to move to the left in the complex frequency plane. In anticipation of this, a filter design may be “pre-distorted” so that, where possible, the poles and zeros are placed a distance to the right in the complex frequency plane so that when losses are added their positions will move back to the desired locations. Of course, this is not possible for zeros of transmission on the jω axis.

pseudo-elliptic: Filters with poles of attenuation arrayed in their stop bands in a manner similar to the poles of attenuation in an elliptic filter, but not necessarily requiring the minima of attenuation between poles to be equal.

NOTE—Pseudo-elliptic filters can be based on general rational Chebychev functions, with an arbitrary number of and arbitrarily positioned poles of attenuation. The responses can also be generated using optimization, with the only real requirement being that the minima of attenuation between the poles are not necessarily at the same level, in contrast to filters based on elliptic functions.

quadruplet section: A two-port network comprising of four resonators labeled 1 through 4, with resonator 1 being the input and resonator 4 being the output resonators. Couplings exist only between resonators (1 & 2); (2 & 3); (3 & 4), and (1 & 4).

quarter wavelength coupled filter: A filter type that uses same type of resonators separated by quarter wavelength sections of transmission lines to simulate equivalent dual-type resonators, a cascade of which perform like a ladder filter type with alternating series- and parallel-type resonators.

NOTE—Since a resonator when seen through a quarter wavelength section of transmission line looks like its dual (for example, a resonator exhibiting parallel-type resonance will look like a resonator that has a series-type resonance), a cascade of resonators of the same type coupled by quarter-wavelength lines will perform like a filter consisting of a ladder of alternating series-and parallel-type resonators.

reactance slope parameter: The derivative of a series-type resonator reactance with respect to frequency evaluated at the resonant frequency, multiplied by the resonant frequency divided by 2. See also: susceptance slope parameter.

NOTE—The above reactance definition is convenient as the reactance slope parameter divided by any resistance in series with the resonator will give the Q-factor of the combination.




recursive filter: Networks implemented primarily but not exclusively as active circuits, employing feedback to provide transmission poles and zeros through cancellation and interference effects. See also: active filter.

NOTE—Recursive filters use feedback technique in contrast to transversal filters that normally use feed forward technique and impedance rotation-based filters explained in the notes for active filters.

ridged waveguide filter: A filter whose resonant elements are ridge waveguides used to achieve very wide spurious free stop bands, of which different sections are coupled by evanescent mode waveguide sections or ridge waveguide sections of different cross section.

NOTE—Ridge waveguide filters may be easily implemented in LTCC technology.

ripple: The variation of insertion loss within the specified bandwidth of transmission characteristics.

shape factor: A measure of the rate of cutoff of a filter.

NOTE—For low pass filters, shape factor is the ratio of the frequency at which the attenuation has risen to a specified high level to the frequency of the pass band edge, while it is the reciprocal of this ratio for a high pass filter. For band pass filter, it is the ratio of the bandwidth between points where the attenuation has risen to a specified high level to the bandwidth between the pass-band edges, while it is the reciprocal of this ratio for band stop filters.

surface acoustic wave (SAW) filter: A filter that utilizes piezoelectric material transducers to convert electrical energy to acoustic wave energy, which travels along the surface of a substrate achieving pass band or stop band characteristics through constructive and destructive interference. The acoustic energy is converted back to electrical energy at the filter output. See also: acoustic-wave filter, BAW filter, FBAR filter.

NOTE—Since acoustic surface waves travel very slowly compared to electromagnetic waves, very small metallic arrays of strips deposited on the substrate surface can be used to realize taps for transversal-type filters. Very sophisticated filter characteristics can be realized in this manner.

stepped impedance resonator (SIR) filter: A filter comprised of resonators formed by two or more transmission line sections with different impedances.

stop band: The range of frequencies within which the attenuation of a filter or network is higher than at frequencies outside the range.

superconductive filter: Filters formed of superconducting materials that exhibit very low DC and RF resistance characteristics below a so-called transition temperature; thereby achieving very large values of unloaded Q and low insertion loss.

susceptance slope parameter: The derivative of a parallel-type resonator susceptance with respect to frequency evaluated at the resonant frequency, times the resonant frequency divided by 2. See also: reactance slope parameter.

NOTE—The above susceptance definition is convenient as the susceptance slope parameter divided by any conductance in parallel with the resonator will give the Q-factor of the combination.

synchronous tuning: A tuning method for multi-section resonator filters or transmission lines with discontinuities, whereby successive reflections are phased to provide maximum cancellation usually at one specified frequency. Contrast: asynchronous tuning.

Thomson filter: See: Bessel filter.




transitional filter: Filters (e.g., Gaussian-Chebychev) with transfer functions between those defined by the classical polynomials such as Chebychev or Butterworth.

transmission function: The ratio of load voltage or current to the generator voltage or current in a network expressed as a function, often of complex frequency. See also: attenuation function.

NOTE—The zeros of the transmission function are the “transmission zeros or zeros of transmission” and the poles are the “natural frequencies” for the network.

transmission zeros: A filter’s transmission zero is the frequency at which there is ideally infinite attenuation of the input signal.

transversal filter: A filter type that relies on constructive and destructive types of interferences between weighted and delayed samples of the input signal to achieve a specified performance at the output where all the weighted and delayed samples are usually summed. See also: active filter, recursive filter.

NOTE—A transversal filter is a filter using a delay mechanism such as a delay line or low pass network, with a matched load at its end and with taps connected at various points along the line. A signal applied to the input of the delay line is viewed by the taps as the signal passes by, and the coupling of the taps to the line may be “weighted” to vary the strength and polarity of the signals picked up at the various taps. The tap outputs are all summed at a single port. In the idealized situation it is easily shown that if an impulse is applied at the input of the delay line, the output at the sum port will be a sequence of impulses spaced by the delay between tap points on the line with impulse amplitudes and polarities fixed by the tap weights. Thus, with proper choice of tap weights and output smoothing using bandwidth limiting, an arbitrarily prescribed impulse response can be obtained. Many such selective networks employ feed forward to provide transmission poles and zeros through cancellation and interference effects. Used primarily, but not exclusively, in active implementations, they are contrasted to both recursive (feedback based) and impedance rotation based active selective networks.

triplet section: A two port network comprising of three resonators labeled 1 through 3, with resonator 1 being the input and resonator 3 being the output resonators. Couplings exist only between resonators (1 & 2); (2 & 3), and (1 & 3).

triplexer: A specific case of a multiplexer where the number of channels is 3. See also: multiplexer.

Tschebycheff Filter: See: Chebyshev Filter.

tunable filter: A filter with some parameter such as center frequency and/or bandwidth being variable by means of tuning mechanism such as electric field, voltage, current, temperature, or mechanically.

Unloaded-Q: Unloaded-Q is defined as 2π times the ratio of the stored energy (W) of an uncoupled resonator and the energy loss (WT) of the resonator within one period (T), i.e., Q0 = 2π W/WT.

waffle iron filter: A pseudo low pass waveguide filter that has an internal structure like that of a waffle iron and operates similarly to a conventional corrugated waveguide filter, except the additional corrugations in the width direction of the waveguide make the structure particularly immune to spurious pass bands that might otherwise occur in the presence of higher-order modes.

Yttrium-Iron-Garnet (YIG) filter: A band pass filter that utilize magnetically tunable yttrium-iron garnet ferrimagnetic resonators, whose pass band center frequency can be varied widely by altering the biasing magnetic field produced by an electromagnet.

Zolotarev filter: An equiripple pass band filter, typically quasi low pass or dual band, having an attenuation characteristic obtainable through Zolotarev function that is a generalized type of Chebychev function whereby the normalized low frequency does not extend down to zero.



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