Distributed-element filter

A circuit designed to operate in the band 10.7 GHz to 11.8 GHz featuring many of the filter structures described in this article. The rows of holes in some tracks are not for filtering but are via fences for suppressing non-TEM modes of transmission.

A distributed element filter is an electronic filter in which the elements of the circuit cannot be regarded as being localised at one place. Conventional filters are constructed from inductors and capacitors and the circuits so built are described by the lumped element model. This model is simple in conception, but becomes less and less applicable as the frequency of the signal increases (or, equivalently, as the wavelength decreases). The distributed element model, on the other hand, applies at all frequencies. It is used in transmission line theory and many distributed element components are physically made of short lengths of transmission line.

There are many component forms used to construct distributed element filters, but all of them have the common property of causing a discontinuity on the transmission line. These discontinuities present a reactive impedance to a wavefront travelling down the line and the reactances can be chosen by design to function, to first order, like lumped inductors, capacitors or resonators as required by the filter.

Distributed element filters may be constructed that have any of the bandforms possible with lumped elements (low-pass, band-pass etc.), with the exception of high-pass which is usually only approximated. Likewise, all the well known filter classes used in lumped element designs (Butterworth, Chebyshev etc.) can equally be implemented with a distributed element approach. Distributed element filters are used for all of the many functions to which lumped element filters are applied: selectivity of radio channel, bandlimiting of noise and multiplexing of many signals into one channel to name but a few.

The development of distributed element filters was spurred on by the wartime need of the military for radar and electronic counter measures. After the end of World War II, the technology found applications in microwave links used by telephone companies and other organisations with large fixed communication networks, such as television broadcasters. Nowadays the technology can be found in several mass-produced consumer items - the device illustrated opposite is an example of the converter found on the back of satellite television dishes.

Overview

A parallel-coupled lines filter in microstrip construction.

Throughout this article, the symbol λ is used to mean the wavelength of the signal being transmitted on the line or a section of line of that electrical length.

Distributed element filters are mostly used at frequencies above the VHF (Very High Frequency) band (30 to 300 MHz). At these frequencies, the physical length of passive components is a significant fraction of the wavelength of the operating frequency, and it becomes difficult to use the conventional lumped element model.

For clarity of presentation, the diagrams in this article are drawn with the components implemented in stripline format. This does not imply an industry preference, although planar formats are popular because they can be implemented using established printed circuit board manufacturing techniques. The structures shown can also be implemented using microstrip or buried stripline techniques (with suitable adjustments to dimensions) and can be adapted to coaxial cables, twin leads and waveguides, although some structures are more suitable for some implementations than others. The open wire implementations, for instance, of a number of structures are shown in the second column of figure 1 and open wire equivalents can be found for most other stripline structures. Planar transmission lines are also used in integrated circuit designs.

The most noticeable difference in behaviour between a distributed element filter and its lumped-element approximation is that the former will have multiple passband replicas of the lumped-element prototype passband because transmission line transfer characteristics repeat at harmonic intervals. These spurious passbands are undesirable in most cases.

History

Development of distributed element filters was underway in the years prior to World War II. A major paper on the subject was published by Mason and Sykes in 1937. Mason had filed a patent much earlier, in 1927, and that patent may contain the first published design which moves away from a lumped element analysis. Mason and Sykes' work is focused on the formats of coaxial cable and balanced pairs of wires – the planar technologies were not yet in use. Much development was carried out during the war years driven by the filtering needs of radar and electronic countermeasures. A good deal of this was at the MIT Radiation Laboratory, but other laboratories in the US and the UK were also involved.

Some important advances in network theory were needed before filter design could be advanced much further. One of these was the commensurate line theory of Paul Richards. Commensurate lines are networks in which all the elements are the same length (or in some cases multiples of the unit length), although they may differ in other dimensions to give different characteristic impedances. Richards' transformation allows a lumped element design to be taken "as is" and transformed directly into a distributed element design using a very simple transform equation.

The difficulty with Richards' transformation from the point of view of building practical filters was that the resulting distributed element design invariably included series connected elements. This was not possible to implement in planar technologies and was often inconvenient in other technologies. This problem was solved by K. Kuroda who used impedance transformers to eliminate the series elements. He published a set of transformations known as Kuroda's identities in 1955, but his work was written in Japanese and it took several years to reflect his ideas in English-language literature.

Following the war, one important research avenue was trying to increase the design bandwidth of wide-band filters. The approach used at the time (and still in use today) was to start with a lumped element prototype filter and through various transformations arrive at the desired filter in a distributed element form. This approach appeared to be stuck at a minimum Q of five. In 1957, Leo Young at Stanford Research Institute published a method for designing filters which started with a distributed element prototype. This prototype was based on quarter wave impedance transformers and was able to produce designs with bandwidths up to an octave, corresponding to a Q of about 1.3. Some of Young's procedures in that paper were empirical, but later, exact solutions were published. Young's paper specifically addresses directly-coupled cavity resonators, but the procedure can equally be applied to other directly-coupled resonator types such as those found in modern planar technologies and illustrated in this article. The capacitive gap filter (figure 6) and the parallel coupled lines filter (figure 7) are examples of directly-coupled resonators.

Fig. 1. Some simple planar filter structures are shown in the first column. The second column shows the open-wire equivalent circuit for these structures. The third column is a semi-lumped element approximation where the elements marked K or J are impedance or admittance transformers respectively. The fourth column shows a lumped-element approximation making the further assumption that the impedance transformers are λ/4 transformers. (a) A short-circuit stub in parallel with the main line. (b) An open-circuit stub in parallel with the main line. (c) A short-circuit line coupled to the main line. (d) Coupled short-circuited lines. (e) Coupled open-circuited lines.

The introduction of printed planar technologies greatly simplified the manufacture of many microwave components, filters included, and microwave integrated circuits then became possible. It is not known when planar transmission lines originated, but experiments using them are recorded as early as 1936. The inventor of printed stripline, however, is known; this was Robert M. Barrett who published the idea in 1951. This caught on rapidly, and Barrett's stripline soon had fierce commercial competition from rival planar formats, especially triplate and microstrip. The generic term stripline in modern usage usually refers to the form then known as triplate.

Early stripline directly coupled resonator filters were end-coupled but the length was reduced and the compactness successively increased with the introduction of parallel-coupled line filters, inter-digital filters, and comb-line filters. Much of this work was published by the group at Stanford led by George Matthaei, and also including Leo Young mentioned above, in a landmark book which still serves as a reference to designers today. The hairpin filter was first described in 1972. By the 1970s most of the filter topologies in common use had already been described. More recent research has concentrated on new or variant mathematical classes of the filters, such as pseudo-elliptic, while still using the same basic topologies, or with alternative implementation technologies such as suspended stripline and finline.

The initial non-military application of distributed element filters was in the microwave links used by telecommunications companies to provide the "backbone" of their networks. These links were also used by other industries with large, fixed networks, notably television broadcasters. Such applications are part of large capital investment programs. However, mass-production manufacturing made the technology cheap enough to incorporate in domestic satellite television systems. An emerging application is in superconducting filters for use in the cellular base stations operated by mobile phone companies.

Basic components

The simplest structure that can be implemented is to introduce a discontinuity by a sudden change in characteristic impedance of the line. This is done in planar technologies by a change in the width of the transmission line. Figure 2(a) shows a step up in impedance (narrower lines have higher impedance). A step down in impedance would be the mirror image of figure 2(a). The discontinuity can be represented approximately as a series inductor, or more exactly, as a low-pass T circuit as shown in figure 2(a). The discontinuities are often used in conjunction with impedance transformers to produce a multiple-element filter. These can be just a short (often quarter wavelength) length of transmission line. Impedance transformers are also used in hybrid mixtures of lumped and distributed element filters (the so-called semi-lumped structures). These multiple element filters can implement any of the filter families (Butterworth, Chebyshev, etc.) just as can be done with lumped elements.

Fig. 2. More stripline elements and their lumped-element counterparts. (a) An abrupt stepped impedance. (b) A line coming to an abrupt end. (c) A hole or slit in a line. (d) A transverse half-slit across the line. (e) A gap in the line.

Another very common component of distributed element filters is the stub. Over a narrow range of frequencies, a stub can be used as a capacitor or an inductor (its impedance is determined by its length) but over a wide band it behaves as a resonator. Short-circuit, nominally quarter wavelength stubs (figure 1(a)) behave as shunt LC (anti-)resonators, and an open-circuit nominally quarter wavelength stub (figure 1(b)) behaves as a series LC resonator. Stubs can also be used in conjunction with impedance transformers to build more complex filters and, as would be expected from their resonant nature, are most useful in bandpass applications. While open-circuit stubs are easier to manufacture in planar technologies, they have the drawback that it is impossible to create an ideal open circuit (see figure 2(b)) often leading to a preference for short-circuit stubs (one can always be used in place of the other by adding or subtracting λ/4 to or from the length). A helical resonator is similar to a stub, in that it requires a distributed element model to represent it, but is actually built using lumped elements. They are most useful in the upper VHF and lower UHF bands whereas stubs are more often applied in the higher UHF and SHF bands.

Coupled lines (figures 1(c-e)) can also be used as filter elements; like stubs, they can act as resonators and likewise be short-circuit or open-circuit. These tend to be preferred in planar technologies whereas stubs tend to be preferred elsewhere, although this is by no means a strict rule. Implementing a true open circuit in planar technology is not feasible because of the dielectric effect of the substrate which will always ensure that the equivalent circuit contains a shunt capacitance. Numerous element types can be classified as coupled lines and a selection of the more common ones is shown in the figures.

Some common structures are shown in figures 1 and 2, along with their lumped-element counterparts. These lumped-element approximations are not to be taken as equivalent circuits but rather as a guide to the behaviour of the distributed elements over a certain frequency range. Figures 1(a) and 1(b) show a short-circuit and open-circuit stub, respectively. When the stub length is λ/4, these behave, respectively, as anti-resonators and resonators and are therefore useful, respectively, as elements in band-pass and band-stop filters. Figure 1(c) shows a short-circuited line coupled to the main line. This also behaves as a resonator, but is commonly used in low-pass filter applications with the resonant frequency well outside the band of interest. Figures 1(d) and 1(e) show coupled line structures which are both useful in band-pass filters. The structures of figures 1(c) and 1(e) have equivalent circuits involving stubs placed in series with the line. Such a topology is straightforward to implement in open-wire circuits but not with a planar technology. These two structures are therefore useful for implementing an equivalent series element.

Low-pass filters

Fig. 3. Stepped-impedance low-pass filter formed from alternate high and low impedance sections of line.

A low-pass filter can be implemented quite directly from a ladder topology lumped-element prototype with the stepped impedance filter shown in figure 3. The filter consists of alternating sections of high-impedance and low-impedance lines which correspond to the series inductors and shunt capacitors in the lumped-element implementation. Low-pass filters are commonly used to feed direct current (DC) bias to active components. Filters intended for this application are sometimes referred to as chokes. In such cases, each element of the filter is λ/4 in length (where λ is the wavelength of the main-line signal to be blocked from transmission into the DC source) and the high-impedance sections of the line are made as narrow as the manufacturing technology will allow in order to maximise the inductance. Additional sections may be added as required for the performance of the filter just as they would for the lumped-element counterpart. As well as the planar form shown, this structure is particularly well suited for coaxial implementations with alternating discs of metal and insulator being threaded on to the central conductor.

Fig. 4. Another form of stepped-impedance low-pass filter incorporating shunt resonators.

A more complex example of stepped impedance design is presented in figure 4. Again, narrow lines are used to implement inductors and wide lines correspond to capacitors, but in this case, the lumped-element counterpart has resonators connected in shunt across the main line. This topology can be used to design elliptical filters or Chebyshev filters with poles of attenuation in the stopband. However, calculating component values for these structures is an involved process and has led to designers often choosing to implement them as m-derived filters instead, which perform well and are much easier to calculate. The purpose of incorporating resonators is to improve the stopband rejection. However, beyond the resonant frequency of the highest frequency resonator, the stopband rejection starts to deteriorate as the resonators are moving towards open-circuit. For this reason, filters built to this design often have an additional single stepped-impedance capacitor as the final element of the filter. This also ensures good rejection at high frequency.

Fig. 5. Low-pass filters constructed from stubs. (a) Standard stubs on alternating sides of main line λ/4 apart. (b) Similar construction using butterfly stubs. (c) Various forms of stubs, respectively, doubled stubs in parallel, radial stub, butterfly stub (paralleled radial stubs), clover-leaf stub (triple paralleled radial stubs).

Another common low-pass design technique is to implement the shunt capacitors as stubs with the resonant frequency set above the operating frequency, so that the stub impedance is capacitive in the passband. This implementation has a lumped-element counterpart of a general form similar to the filter of figure 4. Where space allows, the stubs may be set on alternate sides of the main line as shown in figure 5(a). The purpose of this is to prevent coupling between adjacent stubs which detracts from the filter performance. However, a structure with all the stubs on the same side is still a valid design. If the stub is required to be a very low impedance line, the stub may be inconveniently wide. In these cases, a possible solution is to connect two narrower stubs in parallel. That is, each stub position has a stub on both sides of the line. A drawback of this topology is that additional transverse resonant modes are possible along the λ/2 length of line formed by the two stubs together. For a choke design, the requirement is simply to make the capacitance as large as possible, for which the maximum stub width of λ/4 may be used with stubs in parallel on both sides of the main line. The resulting filter looks rather similar to the stepped impedance filter of figure 3, but has been designed on completely different principles. A difficulty with using stubs this wide is that the point at which they are connected to the main line is ill defined. A stub that is narrow in comparison to λ can be taken as being connected on its centre-line and calculations based on that assumption will accurately predict filter response. For a wide stub, however, calculations that assume the side branch is connected at a definite point on the main line leads to inaccuracies as this is no longer a good model of the transmission pattern. One solution to this difficulty is to use radial stubs instead of linear stubs. Two radial stubs in parallel (one either side of the main line) is called a butterfly stub and a filter constructed from these elements is shown in figure 5(b). Three radial stubs in parallel, which can be achieved at the end of a line, is called a clover-leaf stub.

Band-pass filters

A band-pass filter can be constructed using any kind of elements that can resonate. Filters using stubs can clearly be made band-pass; numerous other structures are possible and some are presented below.

An important parameter when discussing band-pass filters is the fractional bandwidth. This is defined as the ratio of the bandwidth to the geometric centre frequency and the inverse of this quantity is called the Q-factor, Q. If ω₁ and ω₂ are the frequencies of the passband edges then,

bandwidth $\Delta \omega =\omega _{2}-\omega _{1}\,$,

geometric centre frequency $\omega _{0}={\sqrt {\omega _{1}\omega _{2}}}$ and

$Q={\frac {\omega _{0}}{\Delta \omega }}$

Capacitive gap filter

Fig. 6. Capacitive gap stripline filter.

The capacitive gap structure consists of sections of line around λ/2 in length which act as resonators and are coupled "end-on" by gaps in the transmission line. It is particularly suitable for planar formats, is easily implemented with printed circuit technology and has the advantage of taking up no more space than a plain transmission line would. The limitation of this topology is that performance (particularly insertion loss) deteriorates with increasing fractional bandwidth, and acceptable results are not obtained with a Q less than about 5. A further difficulty with producing low-Q designs is that the gap width is required to be smaller for wider fractional bandwidths. The minimum width of gaps, like the minimum width of tracks, is limited by the resolution of the printing technology.

Parallel-coupled lines filter

Fig. 7. Stripline parallel-coupled lines filter. This filter is commonly printed at an angle as shown to minimize the board space taken up, although this is not an essential feature of the design. It is also common for the end element, or the overlapping halves of the end two elements to be a narrower width for matching purposes (not shown in the diagram but see photograph at the top of the article).

Parallel-coupled lines is another popular topology for printed boards, for which open-circuit lines are the simplest to implement since the manufacturing consists of nothing more than the printed track. The design consists of a series of λ/2 resonators side by side but coupling over only λ/4 to each of the resonators either side in a staggered line as shown in figure 7. Wider fractional bandwidths are possible with this filter than with the capacitive gap filter, but a similar problem arises on printed boards as dielectric loss reduces the Q. Lower-Q lines require tighter coupling and smaller gaps between them which is limited by the accuracy of the printing process. One solution to this problem is to print the track on multiple layers with adjacent lines overlapping but not in contact because they are on different layers. In this way, the lines can be coupled across their width, which results in much stronger coupling than when they are edge-to-edge, and a larger gap becomes possible for the same performance. For other (non-printed) technologies, short-circuit lines may be preferred since the short-circuit provides a mechanical fixing point for the line and Q reducing dielectric insulators are not required for mechanical support. Other than for mechanical and assembly reasons, there is little preference for open-circuit over short-circuit coupled lines. Both structures can realize the same range of filter implementations with the same electrical performance. Both types of parallel-coupled filters, in theory, do not have spurious passbands at twice the centre frequency as seen in many other filter topologies (e.g. stubs). However, suppression of this spurious passband requires perfect tuning of the coupled lines which is not realized in practice, so there is inevitably some residual spurious passband at this frequency.

Fig. 8. Stripline hairpin filter.

The hairpin filter is another structure that uses parallel-coupled lines. In this case, each pair of parallel-coupled lines is connected to the next pair, not directly, but via a λ/4 impedance transformer. The "U" shapes so formed give rise to the name hairpin filter. The angled bends seen in figure 8 are common to stripline designs and represent a compromise between a sharp right-angle, which produces a large discontinuity, and a smooth bend, which takes up more real estate on the board which can be at a premium. Such bends are often seen in long stubs where they could not otherwise be fitted into the space available. The lumped-element equivalent circuit of this kind of discontinuity is similar to a stepped-impedance discontinuity. Examples of such stubs can be seen on the bias inputs to several components in the photograph at the top of the article.

Interdigital filter

Fig. 9. Stripline interdigital filter.

Interdigital filters are another form of coupled-line filter. Each section of line is around λ/4 in length and is terminated in a short-circuit at one end only, the other end being left open-circuit. The end which is short-circuited alternates on alternate line sections. This is perfectly straightforward to implement in planar technologies, but also particularly lends itself to a mechanical assembly of lines fixed inside a metal case. The lines can be either rods or bars, and interfacing to a coaxial format line is easy. As with the parallel-coupled line filter, the advantage of a mechanical arrangement that does not require insulators for support is that dielectric losses are eliminated. The spacing requirement between lines is not so onerous as the parallel line structure; so higher fractional bandwidths can be achieved, and Q values down to 1.4 are possible.

The comb-line filter is similar to the interdigital filter in that it lends itself to mechanical assembly in a metal case without dielectric support. In the case of the comb-line, all the lines are short-circuited at the same end rather than alternate ends. The other ends are terminated in capacitors to ground, and the design is consequently classified as semi-lumped. The chief advantage of this design is that the upper stopband can be made very wide, that is, free of spurious passbands at all frequencies of interest.

Stub bandpass filters

Fig. 10. Stripline stub filter composed of λ/4 short-circuit stubs.

As mentioned above, stubs lend themselves to bandpass designs. General forms of these are similar to stub low-pass filters except that the main line is no longer a narrow high impedance line. There are a great number of topologies of stub filters to choose from, a representative example is shown in figure 10. This particular filter consists of a row of λ/4 short-circuit stubs coupled together by λ/4 impedance transformers. The stubs in the body of the filter are double paralleled stubs while the stubs on the end sections are only singles, an arrangement that has impedance matching advantages. The impedance transformers have the effect of transforming the row of shunt anti-resonators into a ladder of series resonators and shunt anti-resonators. A filter with similar properties can be constructed with λ/4 open-circuit stubs placed in series with the line and coupled together with λ/4 impedance transformers, although this structure is not possible in planar technologies.

Fig. 11. Konishi's 60° butterfly stub.

Yet another structure available is λ/2 open-circuit stubs across the line coupled with λ/4 impedance transformers. This topology has both a low-pass and band-pass characteristics. Because it will pass DC, it is possible to transmit biasing voltages to active components without the need for blocking capacitors. Also, since short-circuit links are not required, no assembly operations other than the board printing are required when implemented as stripline. The disadvantages are (i) the filter will take up more board real estate than the corresponding λ/4 stub filter, since the stubs are all twice as long; (ii) the first spurious passband is at 2ω₀, as opposed to 3ω₀ for the λ/4 stub filter.

Konishi describes a wideband 12 GHz bandpass filter, which uses 60° butterfly stubs and also has a low-pass response (short-circuit stubs are required to prevent such a response). As is often the case with distributed element filters, the bandform into which the filter is classified largely depends on which bands are the desired bands and which are to be considered the spurious bands.

High-pass filters

Genuine high-pass filters are difficult, if not impossible, to implement in distributed elements. The usual design approach is to start with a band-pass design, but make the upper stopband occur at a frequency that is so high to be of no interest. Such filters are described as pseudo-high-pass and the upper stopband is described as a vestigial stopband. Even structures that seem to have an "obvious" high-pass topology, such as the capacitive gap filter of figure 6, turn out to be bandpass when their behaviour for very short wavelengths is considered.

See also

• Spurline
• RF and microwave filter

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