A survey of coaxial and strip-line microwave components

Transcription

1 , 1967, No. 5/6/7 A survey of coaxial and strip-line microwave components Introduetion S. J. Robinson and P. T. SaaIer 211 The universal demand for components having a wide bandwidth and small size in the microwave field has led to a move away from waveguide to coaxial and strip transmission line, and so although the work described is based on specialized wideband requirements, it has followed recent passive microwave component development. In particular, considerable recourse has been made to balanced circuits incorporating hybrid junctions and to the use of double-comb filters (usually called interdigital filters), and in these areas a number of innovations have been introduced. In surveying the work carried out over the last few years in these Laboratories no attempt is made to list the components studied but rather a few items which typify the general trends are described. Due to close tolerance requirements and the existence of enclosed conductors, the constructional techniques play an important part in the design of transmission line components. These aspects will, therefore, be considered briefly, following the discussion of the electrical aspects. At the end of World War 11, waveguide was by far the most universally used form of transmission line for conveying signals at microwave frequencies. The frequency bands used were usually around 1 GHz, 3 GHz and 10 GHz and these are still popular frequencies. The chief application for microwaves in those days was radar, and a simple radar system worked in general at a spot frequency. There was therefore little requirement for a transmission line which would propagate a wide frequency range. In fact, rectangular waveguide is capable of working over a fractional band of approximately 40 % of the centre frequency without higher modes being set up and waveguides are still used extensively in systems requiring a fairly wide band, e.g. communication links. Nevertheless, waveguides have some severe disadvantages. The working bandwidth is limited by the phenomenon of cut-off at the lower frequency and overmoding (i.e. the setting up of higher modes) at the upper. Waveguide is large and heavy, especially at low microwave frequencies; a guide capable of propagating at 1 GHz (wavelength 30 c~) has to be approximately 20 cm by 10 cm. For these reasons alternative formsof transmission line were investigated and three basic types became popular; these were coaxialline, strip- line (or slab-line) and microstrip. Strip-line consists of two flat ground planes with a rectangular section inner conductor (fig. la), in slab-line the inner conductor has a circular cross-section (fig.lh), and microstrip consists of one ground plane and an exposed "hot" line (fig. le). These lines, when operated in their dominant mode, have no cut-off frequency although the upper working frequency is limited by overmoding. They are capable ofworking over much greater bandwidths than waveguides and are considerably smaller and lighter. The disadvantages are their high attenuation compared with that of a waveguide and the difficulty of making /ij/ij//// ///ij///(/ o )77/ ))77))777, / Q Fig. 1. The three types of transmission line used for making microwave components. a) Strip-line consists oftwo flat ground planes with a rectangular section inner conductor. b) Slab-line - the inner conductor has. a circular cross-section. c) Microstrip consists of one ground plane and an exposed "hot" line. connections between one component and thenext. ln addition, micro strip has a drawback which is not present in coaxial or strip-line. Since it is not entirely enclosed, microstrip tends to radiate unless the line is supported on a material having a high dielectric constant. This causes power loss and interference with adjacent circuits. For this reason microstrip has not been studied extensively at the Mullard Research Laboratories and no further mention will be made ofit. Coaxialline makes an excellent transmission medium for frequencies at least up to 10 GHz, although flexible cable has a high loss at this frequency. Air-dielectric coaxial line has been extensively used for building microwave components, in particular the hybrid ring and devices based upon it, e.g. mixers and ph asediscriminators. Work has also been done on microwave filters which use coaxialline; mainly on low-pass, bandpass and band-rejection types. Strip transmission line did not become popular until Bates [1] in 1956 resolved the difficulty of calculating s S. J. Robinson, M.A., and P. T. Saaier, Ph.D., B.Sc.(Eng.), are with Mullard Research Laboratories, Redhill, Surrey, England. [1] R. H. T. Bates, The characteristic impedance of the shielded slab line, IRE Trans. on microwave theory and techniques MTT-4, 28-33, 1956.

2 212 PHILlPS TECHNICAL REVIEW VOLUME 28 the characteristic impedance of the line from its dimensions. Strip-line in itself has few advantages over coaxial line but it is much more convenient for making a class of microwave components which depends on electromagnetic coupling for its operation. These devices require the close proximity of two or more transmission-line inner conductors so that the electromagnetic field set up on the input line couples across to the nearby conductors. Using this technique a variety of directional couplers (hybrids) and microwave filters can be designed. Coaxialline, being totally enclosed, does not lend itself to this arrangement whereas a strip-line configuration allows a number of inner conductors to be placed close together between a pair of common ground planes. Strip-line has one further important advantage over waveguide or coaxial line. This lies in the facility to print the inner conductor as a thin copper strip on a sheet of solid dielectric. A second sheet of the same thickness is added, both the sheets being clad with copper on their outer faces to form the ground planes. This technique, which is often simpler to apply than its "thick-line", air-dielectric, counterpart, is particularly useful in the production of complicated structures such as filters and phase-discriminators, especially where a fi large number of the same device is req uired. In the past few years a number of microwave components using the above techniques have been built at Mullard Research Laboratories and a few typical examples will be described in more detail. Hybrid junctions and couplers A simple hybrid junction is essentially a four port device in which the power input at one port splits (usually equally) between two ports, the fourth port being isolated from the input. The addition of a reversed-polarity pair of microwave diodes to the isolated ports of such a device forms a balanced mixer. The simple hybridjunction in coaxialline or in waveguide consists of a closed ring with four ports spaced at quarter-wavelength and three-quarter-wavelength intervals. A hybrid with a broader bandwidth is obtained if a phase reversal section is inserted between two ports, the four ports being then symmetrically placed around the ring (fig.2). A signal input at port Pi divides equally between ports P3 and P4, the outputs being in antiphase. This device has been used as the basis of a range of microwave mixers marketed by the M.E. L. Equipment Company Ltd. A mixer in the band GHz is shown in fig. 2. A more recent mixer design employs the electromagnetic directional coupler, a solid-line version of which is shown in jig. 3. This type of coupler has the property that a signal at port PI divides equally (for 3 db coupling) between ports P3 and P4, port P2 being Fig. 2. Coaxial phase reversal ring balanced mixer. Band GHz, isolation 15 db. The signal input al port PI divides equally between ports P3 and P4, the outputs being in antiphase. Cr; and Cr2 are crystal diodes. R phase reversal hybrid ring. Fig. 3. Solid-line ninety-degree coupler ( GHz). This coupler divides a signal at PI equally (for 3 db coupling) between P3 and P4, P2 being theoretically isolated. The unwanted signal in P2 is typically 25 db down on the input signal. The output signals at P3 and P4 are in phase quadrature at all frequencies - hence ninety-degree coupler. R

3 1967, No. 5/6/7 MICROWAVE COMPONENTS 213 theoretically isolated, although the unwanted signal in port P2 is typically 25 db down on the input signal. In respect of isolation or directivity, coaxial and strip-line hybrids are inferior to their waveguide counterparts. The output signals at ports P3 and P4 (fig. 3) are in phase quadrature at all frequencies and this device is therefore known as a ninety-degree coupler. ]t has a broader working bandwidth than the phase reversal ring and can easily cover a 3 : 1 or 4 : I frequency range. The application of one microwave diode to such a coupler yields a single-ended mixer. Such a device employing a printed coupler for use in the band GHz is shown in fig. 4. A balanced mixer is obtained by fitting a 3 db coupler with two diodes. Fig. 5. Slab-line re-entrant ninety-degree coupler ( GHz). Coupling 3 db, isolation 23 db. The two inner conductors pass through a metallic block forming a re-entrant section. Microwave bridge circuits as frequency and phasediscriminators Fig. 4. Printed single-ended mixer within the band GHz. At 9.6 GHz the local oscillator signal isolation is 20 db and the voltage standing wave ratio is 1.3. The 3 db ninety-degree coupler or the phase reversal hybrid can be used as a phase comparator. Interconnecting five couplers or hybrids with transmission lines produces a twin-bridge circuit having four outputs from one input. The relative amplitudes of the outputs depend on the line lengths and it can therefore be arranged that the outputs yield the input signal frequency without ambiguity. Such bridge circuits find applications for frequency and phase measurement in frequency metering and interferometer systems. High incremental accuracy «1%) can be obtained over A variation of the solid-line ninety-degree coupler is shown infig. 5. The two inner conductors pass through a metallic block forming a re-entrant section. This configuration was devised by Cohn in America [2J and has been developed in printed form at Mullard Research Laboratories. In this case the surrounding metallic block is replaced by two printed copper fiat sheets which are not earthed. The advantage of the arrangement is that the small coupling gap between the lines which is otherwise required to achieve 3 db coupling is avoided, hence the mechanical tolerances are less stringent. The solid-line re-entrant coupler uses slabline for the feed arms, and a manufacturing method which takes advantage of this is shown by the power splitter in fig. 6. Here the slab-line inner conductors are made from ordinary copper wire and are sealed into the dielectric by electric current heating. [2] S. B. Cohn, The re-entrant cross section and wide-band 3-dB hybrid couplers, IEEE Trans. on microwave theory and techniques MTT-ll, , o cm Fig. 6. Power splitter consisting ofthree re-entrant couplers. Here the slab-line inner conductors are sealed into the dielectric by electric current heating.

4 214 PHILIPS TECHNICAL REVIEW VOLUME 28 I! I I I o Scm have achieved a rejection of 20 db with good input match over the frequency range 2-6 GHz. Microwave filters Very broad band filters are difficult to construct in waveguide, whereas strip and coaxial line lend themselves well to filter design. A low-pass filter can be formed as a cascade of transmission-line sections which have alternatively high and low characteristic impedance (stepped-impedance filter). Band-pass filters are formed by coupling a number of microwave cavities or resonant sections together. In these components strip and slab-line find their most powerful applications. The most popular forms of transmission-line filters are: 1) Half-wave resonant sections, quarter-wave coupled. 2) Double-comb (interdigital) transmission line. 3) Comb transmission line. Fig. 7. Coaxial phase-discriminator ( GHz) using solid-line, coaxial phase reversal ring hybrids. wide frequency ranges. Fig. 7 shows a discriminator for the 2 GHz band which uses solid-line, coaxial phase reversal ring hybrids. Such a discriminator is extremely difficult and expensive to produce and in recent years printed techniques and ninety-degree couplers have therefore been employed. Fig. 8 shows a printed discriminator (within the band GHz) with the ground plane removed. The inner conductors of the re-entrant couplers can be seen, as the auxiliary planes have also been removed. Modulators, switches and limiters The association of diodes with directional couplers leads to a num ber of other wideband balanced circuits. Among these are carrier suppressed modulators, microwave limiters and switches. A three-element passive limiter can be made by fitting diodes to six ports of the power splitter arrangement shown in fig. 6. The diodes are heavily reflecting at low signal levels but become matched to the incident signal power at a level of about I mw. Thus the insertion loss through the device is low at low level but will rise to about 20 db at high level. This type of component may be used to protect a sensitive receiver from overload by large signals. A P-I-N diode does not rectify microwave signals but acts as an electrically variable impedance. Circuits incorporating a pair of such diodes and a 3 db coupler may be designed to operate as switches, attenuators and modulators. Such circuits have been constructed and Fig. 8. Printed phase-discriminator ( GHz), shown with the ground plane removed. The inner conductors of the reentrant couplers can be seen, as the auxiliary planes have also been removed. The dou ble-cornb (interdigital) filter was derived from the half-wave filter by Matthaei [3] and is now extensively used. It consists of two sets of resonant fingers (combs), each finger being capacitively coupled to its neighbour and ideally short-circuited at one end and open-circuited at the other (jig. 9a). This type of filter can be made [3] G. L. Matthaei, lnterdigital band-pass filters, IRE Trans. on microwave theory and techniques MTT-IO, ,1962.

5 1967, No. 5/6/7 MICROWAVE COMPONENTS 215 i! i i~ ~ +- ~~ ~ ~.o cm I l..... a b Fig. 9. a) Solid-line double-comb (interdigitaj) filter. Pass-band 4-8 GHz. Insertion loss 0.5 db. Voltage standing wave ratio 2.0. It consists of two sets of resonant fingers (combs), each finger being capacitively coupled to its neighbour and ideally short-circuited at one end and open-circuited at the other. b) Typical printed double-comb filter. broadband (3 : 1 frequency range) or narrowband (less than 10% ofcentre frequency). The limits are imposed by the narrowness of the coupling gaps between the fingers and the width of the fingers themselves. The length of the fingers is approximately a quarter-wavelength at midband and this at high frequencies becomes very short. Since for good operation the length-towidth ratio of the fingers should be high (3 or more) the ground plane spacing has to be made small, which scales down all the dimensions except the fingerlength. Thus it is difficult to make doublecomb filters work satisfactorily much above 10 GHz. The situation is improved by printing on high dielectric-constant material since for the same finger impedance the width is reduced more than the length (fig. 9b). Matthaei's design equations are approximate and it has been found useful to make these filters (when printed) as separate combs of beryllium-copper foil which are mounted between sheets of unclad dielectric. It is then fairly simple to move one comb relative to the other to provide some experimental adjustment of the coupling gaps. The foreshortening of the fingers at their open-circuit ends has to be found experimentally; for narrowband filters, especially above 3 GHz, this is very critical. Also due to Matthaei and closely allied to the dou blecomb filter is the comb filter. This has only one comb, all the fingers being short-circuited at the same end. The other end of each finger must be loaded by capacitance in order to produce a pass-band so that comb filters are really only suitable for narrow bandwidths. They have the advantage over double-comb filters that they can be tuned over a wide frequency range by varying the capacitance. One comb filter can, for example, be made to tune to anyone of twenty UHF television channels. The comb filter is particularly well suited to the UHF range since the fingers can be made quite short, usually an eighth-wavelength at midband. Other forms of coaxial and strip-line filter have been studied. A line with shunt open- or short-circuit stubs at quarter-wavelength intervals along its length forms a band-pass filter which is particularly useful for braadband operation. For narrowband performance the impedance values required for the stubs may be too high to be mechanically practicable. A printed bandpass filter ( GHz) of this form is shown infig. 10, together with a printed low-pass stepped-impedance filter of the type mentioned at the beginning of this section. o 2 J 4 5cm Fig. 10. Printed stubs and line band-pass filter (left) and printed stepped-impedance low-pass filter (right). Pass-band GHz. Insertion loss 1 db.

6 216 PHILlPS TECHNICAL REVIEW VOLUME 28 Construction techniques High dimensional tolerances can lead to construetional difficulties with small microwave components, and careful mechanical design is necessary if they are to be made using normal machine shop practice. As far as printing is concerned, the main problem is that the dimensional tolerances are a smaller fraction of the pattern size than is usual in printed wiring. The optical systems in the printing process and dielectric material stability impose undesirable limitations on what can be easily achieved. Three techniques for printing strip-line microwave components have been useful. In the first method, most conventional but least accurate, a photographic negative is made of a drawing and the print then produced from the negative on copper-clad dielectric which has been coated with photoresist material. In the second method, which is used for the most accurate work, a negative is cut by a precision machine in a thick acrylic sheet which has been coated with a photo-opaque material. An accuracy of a few microns can be achieved but the master negative may suffer dimensional changes with time and temperature. The third method is very simple; a speciallaminated material is used in which the negative is prepared by cutting the top layer which is opaque and peeling it off. The accuracy obtained is of the order of 100 (.Lm. For all methods, considerable experience in etching the copper pattern is required for good and consistent results. It has been found that the copper patterns for double-comb filters can be reproduced to an accuracy better than 25 (.Lmwhere the combs are made as a free foil and not on clad dielectric. In this case berylliumcopper is used to give the necessary mechanical strength. The dielectrics are low-loss irradiated cross-linked polystyrene or polyolefin. These materials have dissipation factors of the order of in the microwave region. For narrowband filters the polyolefins are preferred. as, apart from their somewhat lower loss, they are softer, a feature which is useful if small tuning screws have to be inserted. Future objectives Good performance much above 10 GHz is difficult to obtain in coaxial and strip-line devices. The reason for this lies in the manufacturing techniques since the size (diameter or ground-plane spacing) has to be made small to prevent overmoding and the reduction in size increases the power loss in the device. In addition, any small discontinuities present have a greater effect at high frequencies than at low since they are not usually scaled down in proportion. The situation is aggravated by the poor length-to-width ratios ofthe line which are inevitable in components in which the line is resonant, i.e. when it is a quarter-wavelength long. One further difficulty, which is inherent in the use of conventional strip and coaxial line, is the comparatively poor performance of coaxial to strip-line transitions and coaxial connectors which have to be used. Even subminiature connectors become highly reflective at frequencies around 20 GHz. Nevertheless, some work at these frequencies is now being carried out using conventional printing techniques. Some of these basic difficulties are now being tackled by building miniaturized components on ceramic material. Alumina materials having very low losses and dielectric-constants of about 10 are now available and on these strip-line can be laid down by evaporation. Using this technique the hybrid for a mixer within the band GHz measures approx. 2.5 mmê. It is intended to build arrays of integrated components in this way, i.e. mixers, filters etc. laid down on one piece of dielectric material without the need for separate connectors. It is a logical further step to adopt a high-resistivity semiconductor substrate as the dielectric and to add devices such as diodes and transistors to the circuit by standard integrated circuit methods. Work is now going on, in co-operation with the microwave device groups of Associated Semiconductor Manufacturers Limited, to exploit this approach. Summary. A short survey is presented of work on coaxial and strip-line passive microwave components. Most interest has been centred on the design ofhybrid junctions and directional couplers, microwave mixers, filters and phase-discriminators. Some work has also been carried out on microwave modulators, switches and limiters. General considerations for good design technique are presented and reference is made to methods of manufacture. Results show that with modern printing techniques it is possible to build smaller and cheaper strip-line components comparable in performance with their coaxial counterparts.