GB2057196A - Microwave series-T junction - Google Patents

Abstract

A planar microwave series-T junction (3) comprises a strip conductor (5) crossing a slot line (4) formed in a ground plane (2) associated with the strip conductor (5), the slot (4) having a termination (6) which presents a high impedance where the strip (5) crosses the slot (4); energy fed into the slot (4) produces in the transmission line comprising the strip conductor (5) and the ground plane (2) signals in antiphase on opposite sides of the slot (4). An unbalanced transmission line, e.g. a microstrip conductor (7) short-circuited at 8, may be coupled to the slot line (4) to form a balun. Further conductors (14, 15) may be connected to the strip conductor (5) on opposite sides of the slot to form a balanced line, for example a slot line or an odd-mode microstrip line. The specification also describes a single-balanced mixer and a double-balanced ring mixer/modulator using such junctions. <IMAGE>

Description

SPECIFICATION Microwave series - T junction The invention relates to a microwave series - T junction. The invention further relates to a combination of such a junction with a balanced transmission line, and to a balun, to a single-balanced mixer, and to a double-balanced mixer using such junctions.

Microwave components and circuits embodying the invention are particularly suited to manufacture on a single dielectric substrate.

Microwave baluns of generally planar form are known from, for example, U.S. Patents 3,652,941, 3,784,933, and 4,125,810; the first and third of these patents also describe mixers using the baluns. The baluns described all have the disadvantages that they inherently cannot completely decouple an unbalanced mode of propagation from a balanced mode of propagation, and are liable to exhibit resonances if operated over a broad bandwidth.

According to a first aspect of the invention, a microwave series - T junction comprises a first transmission line formed by a slot in a conductive plane and a strip second transmission line, wherein the conductive plane is a ground plane of the second line, wherein a first strip conductor which is a strip conductor of the second line crosses the slot so as to be coupled therewith, the first line having at one end a termination which presents in the operating frequency range of the junction a high impedance at the region where the strip conductor crosses the slot, wherein a portion of the first transmission line extending from said region and away from said end constitutes a first arm of the junction and wherein two portions of the second transmission line extending from the slot on opposite sides thereof constitute second and third arms respectively of the junction, whereby microwave energy fed into the junction on the first arm produces signals leaving the junction on the second and third arms, the signals produced in the second and third arms being in antiphase with one another at points electrically equidistant from the slot.

The second line is suitably substantially symmetrical about the first line so as to provide equal signals in the second and third arms.

The first line may be a slot line formed on a dielectric substrate, or it may, for example, be a fin line that may similarly comprise a dielectric substrate, the substrate in that case being supported in a waveguide.

According to a second aspect of the invention, a series - T junction embodying the first aspect of the invention may be used in combination with a balanced third transmission line comprising said conductive plane and further comprising second and third strip conductors which are each connected at one end to the first strip conductor at the second and third arms of the junction respectively and at least parts of which extend closely adjacent to one another so as to be coupled together. Such a combination is useful for various embodiments of the invention.

According to a third aspect of the invention, a microwave balun comprises a series-T junction embodying the first aspect of the invention or comprises a combination embodying the second aspect of the invention, and further comprises an unbalanced fourth transmission line coupled to the first line, portions of the second line on opposite sides of the slot forming a balanced system. The unbalanced fourth transmission line may be a microstrip line formed by the conductive plane and a fourth strip conductor, and may be coupled to the first line by, for example, the fourth strip conductor crossing the slot at or immediately adjacent an open-circuit termination of the other end of the slot, or a transducer as claimed in any claim of U.K.

Patent Specification (U.K. Patent Application 42929/76) (the transducer is also described in IEEE Trans. MTT-24, pp 231-233, April 1976).

In a balun comprising a combination embodying the second aspect of the invention, the only parts of the second and third strip conductors which extend closely adjacent to one another may be remote from the slot. This may reduce radiation loss and may inhibit any tendency for even-mode propagation along the coupled conductors.

In a balun comprising a combination embodying the second aspect of the invention wherein the slot is terminated by an open-circuit at or immediately adjacent the region where the first strip conductor crosses the slot, the first second and third strip conductors are suitably substantially spaced from the open-circuit except at said region. The impedances of the transmission lines comprising the three strip conductors may thereby be kept substantially constant without altering the widths of the strip conductors, and radiation loss from the vicinity of the open-circuit may be reduced.

The coupled strip conductors may be juxtaposed to a portion of the conductive plane which has a termination so that in operation the electric fields between each of the coupled conductors and the conductive plane are mainly or wholly transformed into an electric field between the coupled conductors. Beyond the termination of the conductive plane, the coupled conductors may thus operate as a balanced line completely decoupled from any unbalanced modes of operation of the other lines.

Suitably, said portion of the conductive plane is tapered towards the termination thereof with a progressively decreasing width transverse to the longitudinal direction of the balanced third line so as progressively to restrict the extent of the electric fields between each one of said coupled conductors and the conductive plane in a direction transversely away from the other of said coupled conductors.

This avoids a sharp discontinuity, and assists the desired transformation of the electric fields; it is particularly suitable for operation over a large bandwidth.

As an alternative, the termination of the conductive plane may be an edge generally transverse to the longitudinal direction of the unbalanced third line, said edge having a plurality of slots each of which extends into the conductive plane to form a quater-wave choke; such an arrangement is of course adapted for operation over a relatively nar row bandwidth.

A portion of a dielectric substrate supporting the coupled conductors may extend longitudinally beyond the termination of the conductive plane and be disposed between and parallel to second and third conductive planes so that the coupled conductors and the second and third conductive plane form a suspended odd-mode microstrip line, as described in Netherlands Patent Application 7810942. As an alternative, each of the coupled conductors may be of substantially increased width beyond the termination of the conductive plane so that the closely adjacent edge regions of the widened conductors form a balanced slot transmission line.

The increase in width of each of the coupled conductors may be progressive from the region of the termination of the conductive plane. This can also assist in obtaining the desired transformation of the electric fields and in avoiding a sharp discontinuity. As an alternative, the width of each of the coupled conductors may increase at an edge which is generally transverse to the slot line and which has a plurality of slots each extending into the respective conductor to form a quarter-wave choke.

According to a fourth aspect of the invention, a microwave single-balanced mixer comprises a combination of a series - T junction and a balanced third transmission line embodying the second aspect of the invention, wherein the second and third strip conductors are of substantially equal electrical lengths and are closely adjacent over substantially their whole lengths, the mixer further comprising two diodes connected in opposite senses between a common input terminal and the other ends of the second and third strip conductors respectively, and wherein an effective substantial short-circuit to the conductive plane in presented to a first signal applied to the common terminal at each of said other ends of the second and third strip conductors, the mixer further comprising output means for deriving from the diodes a difference frequency signal produced when a second signal of a different frequency from the first signal is simultaneously applied to the first arm of the series - Tjunction.

Each of said other ends of the strip conductors may be electrically spaced m i. 2, where m = 0, 1, 2..., from an open-circuited stub having an electrical length (2n + 11 '.4, where n = 0, 1,2..., for presenting said effective substantial short-circuit, and wherein said output means comprises a stub short-circuited at one end to the conductive plane, having an electrical length (2p + 1) ,'. 4, where p = 0, 1,2..., and connected at its other end to the common terminal,,.

being a wavelength in the operating frequency range of the first signal.

The electrical length of each of the second and third strip conductors may be substantially a quarter wavelength in the operating frequency range of the first signal.

According to a fifth aspect of the invention, a microwave double-balarced ring mixer or modulator having three ports comprises two series - T junctions, in each of which the second line is substantially symmetrical about the first line, and a diode quad having two pairs of opposed terminals, wherein the two terminals of each pair of opposed terminals are respectively connected to the second and third arms of a respective one of said two junctions by two transmission lines of equal electrical lengths, wherein the first arms of said two junctions respectively constitute first and second of the three ports, and wherein coupling means to two points each electrically substantially mid-way between the second and third arms of a respective one of the two junctions constitute the third of the three ports.

The coupling means may comprise two wires respectively conductively connected to the first strip conductors of the two junctions at the regions where those strip conductors cross the respective slots of the two junctions.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a plan view of a balun embodying the first, second and third aspects of the invention; Figure 2 shows part of a modification of the balun of Figure 1; Figure 3 is a plan view of a single-balanced mixer embodying the first, second, third and fourth aspects of the invention; Figure 4 is a plan view of a double-balanced ring mixer embodying the first, second, third and fifth aspects of the invention, and Figures 5A and 5B are equivalent circuit diagrams of a diode quad used in the mixer of Figure 4.

The balun shown in Figure 1 is formed on a dielectric substrate 1 having conductive coatings on its upper and lower major surfaces. The coating 2 on the lower surface covers a substantial portion thereof and constitutes a ground plane; edges of the coating 2 are denoted by dashed lines, while the edges of the coating on the upper surface are denoted by continuous lines. A series - Tjunction 3 embodying the first aspect of the invention comprises a straight slot transmission line 4 of uniform width formed in the lower coating 2, and a strip conductor 5 of uniform width on the upper surface of the substrate.The strip conductor 5, which in combination with the ground plane formed by coating 2 forms a microstrip line, crosses the slot at right angles thereto, one end of the slot line 4 being terminated immediately adjacent the region wherein the strip conductor crosses the slot by an opencircuit 6 in the form of a circular aperture in the coating 2.

An unbalanced further transmission line, in this case also a microstrip line formed by a further strip conductor 7 in combination with the coating 2, is coupled to the slot line 4 by virtue of the strip conductor 7 crossing the slot at right angles thereto and being conductively connected to the coating 2 by a pin 8 extending through the substrate 1 immediately adjacent the slot, and bv the other end of the slot line being terminated immediately adjacent the region wherein the strip conductor 7 crosses the slot by another open-circuit 9 similar to the open-circuit 6. Pin 8 has a diameter similar to the width of strip conductor 7 so as to present as low a series inductance as possible.

Each open-circuit termination of the slot line presents in the operating frequency range of the balun a high impedance at the region where the adjacent strip conductor crosses the slot, this high impedance being in parallel with the load presented to the slot line by the microstrip line comprising the respective strip conductor; consequently, if the microstrip line is approximately matched to the slot line, there will be good coupling between the two lines. The bandwidth of such a transition is dependent on how the driving point impedance of the nominal "open-circuit" (i.e. the impedance presented by the "open-circuit" at the point where it is connected to the slot line) varies with frequency. In principle, the type of transition shown in Figure 1 has only a low-frequency cut-off dependent on the size of the open-circuit aperture and is thus inherently broad-band.

The portion of the slot line 4 extending from strip conductor 5 and away from the open-circuit termination 6 constitutes a first arm of the series - T junction, and portions of the transmission line comprising the ground conductor 2 and portions of strip conductor 5 extending from slot 4 or opposite sides thereof respectively constitute second and third arms of the junction.

When microwave energy is fed into the first arm of the series - Tjunction 3 from the unbalanced transmission line comprising strip conductor 7, the electric field across the slot 4 couples to the transmission line comprising strip conductor 5 in opposite senses on opposite sides of the slot, thus ,,roduing signals which are in antiphasewith one Aher at points electrically equidistant from the slot. The second and third arms appear to the first arm to be in series with one another, and for correct matching, the sum of the respective characteristic impedances of the second and third arms should therefore equal the characteristic impedance of the first arm. The junction is thus a direct analogue, in s#bstantially planar form, of a waveguide series - T junction.Since strip conductor 5 is symmetrical about the slot in both position and width, the antiphase signals are equal in magnitude. Although the microstrip line formed by strip conductor 5 and coating 2 is of course an unbalanced transmission line, portions of conductor Son opposite sides of the slot may be considered to form a balanced system in view of the voltages thereon which are equal in magnitude but opposite in polarity with respect to the ground plane.

In this embodiment, strip conductor 5 is extended on opposite sides of slot 4 by two further strip conductors 14 and 15 respectively extending transversely and longitudinally away from the slot and, like conductor 5. symmetrical thereabout. The conductors 14 and 15 are shaped so that they then extend towards one another until, at a region remote from slot 4, they are closely adjacent to one another, being separated by a narrow gap 16. The closely adjacent portions of the strip conductors 14, 15 are thereby coupled together to form a balanced oddmode microstrip line, the electric field between the two strip conductors increasing as the distance between them decreases and the respective electric fields between each conductor and the ground plane simultaneously decreasing.

Furthermore, in this embodiment the portion of the conductive coating 2 juxtaposed to conductors 14 and 15 is terminated at 17 a short distance beyond the region in which the conductors become strongly coupled together; the widths of the conductors 14 and 15 are simultaneously increased, as at 18, so that the closely adjacent edge regions of the widened conductors form a balanced slot transmission line. The termination of the ground plane ensures that the electric fields become substantially wholly transformed into an electric field between the conductor; (assuming no other grounded conductors to be nearby) and that the balanced mode of propagation in the slot line in completely decoupled from unbalanced modes of propagation associated with the ground plane, such as on the microstrip line comprising strip conductor 7.The ground plane is tapered towards its termination at 17 so that its progressively decreasing width progressively restricts the extent of the electric fields between each one of the conductors 14,15 and the ground plane in a direction transversely away from the other of those conductors, thereby assisting in transforming these electric fields into a field between the two conductors, as does the subsequent progressive increase in width (as at 18) of the two conductors.

It will be seen that the transformation of the electric field patterns involves a change from a field pattern mainly perpendicular to the substrate to one mainly parallel to the substrate.

The progressive changes in width of the portion of the ground plane juxtaposed to the conductors 14,15 and of those conductors are also adapted to reduce radiation losses by avoiding discontinuities, and are particularly suited for operation over a broad bandwidth. Other forms may be used for the termination of the ground plane and the increase in width of the conductors: for example one or both may comprise an edge generally transverse to the lines. Figure 2 shows by way of example an arrangement where both have edges generally perpendicularly transverse to the lines, each edge having a plurality of slots 19 each extending into the respective conductive coating to form a quarter-wave choke; the choke action is of course restricted to a relatively narrow bandwidth. The slotted transverse edges are shown in the Figure as being longitudinally well spaced for the sake of clarity, but need not be so separated in practice. As a further alternative, one or both conductive coatings may comprise open-circuit apertures, analogous to the apertures 6 and 9, adjacent to their generally transverse edges and to the balanced lines.

It will be seen that in the embodiment of Figure 1, the strip conductors 5, 14 and 15 are substantially spaced from the open-circuit 6 except where the conductor 5 crosses the slot 4. This helps to reduce radiation losses, and to reduce changes in impedance of the microstrip lines comprising these conductors which could be caused by an edge of the ground plane coming close to an edge of the strip conductor.

The abrupt large increase in width of the group in the coating 2 where each of the apertures 6 and 9 joins the slot 4, with the edges of the aperture extending substantially perpendiclarly away from the edges of the slot, provides a large increase in slot-line impedance at a well defined location, making these terminations particularly suitable for broad-band use. A balun of the form of Figure 1 exhibits a high-pass filter characteristic, its lowest suitable operating frequency being dependent on its physical dimensions, such as the diameters of the apertures 6 and 9.

Any of the sections of transmission line of the balun may of course be constructed to have impedance-transforming properties; they may for example comprise one or more quarter-wavelength transformers, for operation over fairly narrow bandwidths, or have progressive tapering for operation over broader bandwidths.

In an embodiment of the general form of Figure 1 constructed with gold coatings on a 1/4 mm thick alumina substrate, the strip conductor (7) of the unbalanced microstrip lines was 0.24mm wide; the slot line (4) of the series - T junction was curved with its ends mutually perpendicular, had a slot width of 50 Ftm and a length of about 151/2 mm, and was terminated with circular apertures (6,9) 1 Omm in diameter; the strip conductors (5, 14, 15) associated with the series - T junction all had a width of 0.9mm; the strip conductors (14,15) connected to the second and third arms of the junction had a maximum separation of about 23mm, and because closely adjacent to one another at a region some 25mm beyond the junction, being then separated by a gap (16) having a uniform width of 50 term; and the termination (17) of the ground plane was about 21/2 mm beyond that region. This embodiment has been tested over the range of 2 - 18 GHz (but is thought to have a greater bandwidth); over the tested range, the VSWR was typically about 1.4.

A balun embodying the invention may of course couple between an unbalanced line or system and a balanced line or system other than those of the embodiment of Figure 1. The slot line (4) of the series - Tjunction (3) may be directly or indirectly coupled to an unbalanced line other than a microstrip line (7): for example, it may be directly coupled to a coaxial line.The strip conductor (5) of the series - T junction may he connected, for example, to a pair of strip conductor s which are brought closely adjacent to one another so as to be coupled together, in an arrangement similar to that of Figure 1: the portion of the ground conductor (2) juxtaposed to the coupled conductors may be terminated and the coupled strip conductors may extend further along the substrate between and parallel to two planar ground conductors spaced from the substrate, the arrangement forming a suspended odd-mode microstrip line, as disclosed in Netherlands Patent Application 7810942. (The coupling of the two strip conductors to the planar ground conductors will depend on the spacings of the ground conductors from the substrate.) In this case (as in others), the slot line (4) and'or the microstrip line (7) may be curved so that the microstrip line extends in the same direction as the slot line extends in the region where the strip conductor (5) of the series - T junction crosses the slot.

Baluns embodying the invention may be particularly useful in applications requiring large bandwidth, such as the slot line antenna disclosed in U.S.

Patent Application 9554/78, and in applications for which substantially complete isolation from unbalanced modes of propagation is desirable, such as determining a null of a phase bridge.

Figure 3 is a plan view, analogous to Figure 1, of a single-balanced mixer embodying the invention. The mixer circuit is formed on a dielectric substrate 11 with conductive coatings on both major surfaces, the coating on the lower surface again being the ground plane. A series - T junction 31 comprises a slot line 41, formed in the ground plane, and a strip conductor 51 crossing the slot. The right-hand end (as drawn) of the slot is terminated at 60 by a shortcircuit, and the left-hand end by an open-circuit 91. A strip conductor 71 of a microstrip line, forming a local-oscillator signal port, crosses the slot at the open-circuit termination.Only relatively narrowband operation is required forthis particularembodiment: the short-circuit at 60 presents a high impedance at the region where strip conductor 51 crosses it by virtue of the short-circuit being a quarter-wavelength beyond that region, and the strip conductor 71 is coupled to slot line 41 by virtue of the strip conductor terminating in an open-circuit at 80 a quarter wavelength beyond the slot. The open-circuit termination 91 at the left-hand end of the slot line could of course be replaced by a short-circuited quarter-wavelength portion of slot line, as at the right-hand end. A pair of strip conductors 141,151 each a quarter wavelength long are connected to the strip conductor 51 on opposite sides of the slot and extend alongside the slot parallel and closely adjacent to one another so as to be coupled together.With this arrangement, in addition to the coupling between slot line 41 and the microstrip line comprising strip conductor 51, there may also be coupling between each of the strip conductors 141,151 and the short-circuited quarterwavelength portion of slot line 41 by virtue of their proximity and of the currents in the ground plane in the vicinity of the short-circuit.

Two diodes 33,34 are respectively connected, in opposite senses, between the right-hand ends of conductors 141,151 and a common terminal 25, to which are also connected a strip conductor 26 of a further microstrip line, forming an R.F. input signal port, and a quarter-wavelength stub 27 shortcircuited at its other end to the ground conductor.

A low-pass filter arrangement, comprising two quarter-wavelength portions of high-impedance microstrip conductors 35,36 connected in series and two quarter-wavelength open-circuited microstrip stubs 37,38, is connected to strip conductor 51 and to an output conductor 39. Stub 37 has the effect of presenting to a signal applied to common terminal 25 a short-circuit to the ground conductor at the right-hand ends of strip conductors 141,151, i.e. at the connections of the diodes 33,34 to those conductors, since the connections are spaced an integral number (in this case, one) of half-wavelengths from the stub. (Stub 37 could of course have a length of any other odd number of quarter-wavelengths.) The mixer operates as follows.An R.F. input signal is applied to common terminal 25 via strip conductor 26, and a local oscillator signal is applied to the slot line of the series - T junction 31 via strip conductor 71. For the R.F. input signal, the diodes 33 and 34 appear to be connected in parallel (in opposite senses) between common terminal 25 and earth; the R.F. input signal does not propagate along conductors 141,151. The series - T junction 31 couples the local oscillator signal to coupled strip conductors 141,151; the local oscillator signal propagates along the transmission line formed by the coupled conductors and the ground plane (from left to right as drawn) in the odd mode. Diodes 33 and 34 appear to the local oscillator signal to be connected in series (in the same sense) across the end of the odd-mode line.The intermediate frequency signal produced in the diodes by the mixing process propagates along the transmission line formed by the coupled conductors 141,151 and the ground plane (from right to left as drawn) in the even mode and is taken from output conductor 39 via the low-pass filter. The quarterwave short-circuited stub 27, which at the signal frequency appears as an open-circuit across the R.F.

signal input line at 25, provides a return path for the intermediate frequency signal.

Owing to the different manners in which the local oscillator and R.F. input signals are fed into the diodes, the local oscillator and R.F. input ports of the mixer are inherently isolated from one another, assuming the two diodes to be substantially the same. It was indeed found in a constructed embodiment that satisfactory isolation in excess of 20 dB was obtained with two randomly selected diodes of the same type; careful matching of the diodes was therefore unnecessary (although matching might have provided even better isolation). It will be seen that the only part of the mixer circuit through which the local oscillator and R.F. input signals both pass is the pair of diodes and their interconnection via terminal 25.This is related to a particular advantage of a single-balanced mixer embodying the invention compared with, for example, a conventional singlebalanced mixer comprising a four-port hybrid ring wherein the local oscillator and R.F. input signals are applied to two respective ports and the two diodes are respectively connected to the other two ports, and wherein the two signals thus pass from the hybrid ring to the diodes along the same two lines. If in this latter type of mixer, image-rejection filtering is required to reflect image-frequency signals back to the diodes for remixing (as in a low-noise mixer), each of the two requisite filters must have a high Q4actorsince it must reject the image frequency but pass both the local oscillator and R.F. signal frequencies.However, in a single-balanced mixer embodying the invention, it is merely necessary to use a single filter in the R.F. signal input line to reject the image frequency and pass the R.F. signal frequency; this filter may have a high insertion loss at the local oscillator frequency, and may therefore have a lower Q-factor and consequently be easier to design.

(Similar considerations may apply to the rejection of other undesired mixer products.) In order to increase the R.F. input signal bandwidth, a second quarter-wave short-circuited stub may be connected to the signal input line 26 a quarter-wavelength from the common terminal 25, as indicated by dash-dot lines at 28. The stub(s) 27(28) may each be short-circuited by a pin extending through the substrate or, if the stub terminates adjacent an edge of the substrate, by a conductive foil wrapped around the edge of the substrate.

The coupled conductors 141,151 need not be a quarter-wavelength long, and other means may be used to present an effective substantial short-circuit to the ground plane at the connections of the diodes to the coupled conductors. For example, a respective quarter-wavelength open-circuited stub may be connected to each of the conductors 141,151 at the connection of the respective diode thereto. Furthermore, the local oscillator and R.F. input signal ports could be interchanged, but in that case, a high-pass filter would be required in the signal input line, and it should also be noted that the transitions from the microstrip conductor 71 to the coupled conductors 141,151 may have an insertion loss of about 2 dB, which may be of some importance for the R.F. input signal but of no great significance for the local oscillator signal.

An embodiment of the form shown in Figure 3 has been constructed for operation at 10 GHz with gold coatings on a 1/2 mm thick alumina substrate. The R.F. input and local oscillator signal input lines (71,26) had characteristic impedances of 50 ohms, as did the quarter-wavelength stubs (27, 28, 37, 38) and the slot line (41). The coupled parallel strip conductors (141,151), the gap between them, and the strip conductor (51) joining them across the slot all had a width of 170cm. The diodes (33,34) were beam-lead devices. The measured noise figure was 6.8 dB including a 30 MHz l.F. amplifier with a 1 dB noise figure, and the isolation between the R.F. input and local oscillator signal ports was greater than 20 do.

Figure 4 is a plan view, analogous to Figures 1 and 3, of a broadband double-balanced ring mixer/ modulator embodying the invention. The circuit is formed on a dielectric substrate 12 with conductive coatings on both major surfaces, the coating on the lower surface of the substrate again being the ground plane. The majority of the circuit consists of left-hand and right-hand portions which are mirror images of each other, and in which the reference numerals of corresponding parts are suffixed A and B respectively. One of these portions will now be described without suffixes. It comprises a slot line 42 terminated at each end by an open-circuit, 62 and 92 respectively. An input strip conductor 72 crosses the slot at open-circuit 92 and is connected through the substrate to the ground plane by a conductive pin 82.

Another strip conductor 52 crosses the slot at open-circuit 62 to form a series - T junction 32.

Connected to strip conductor 52 on opposite sides of the slot are two further strip conductors 142,152 respectively, which at their ends remote from strip conductor 52 terminate close to one another near the centre of the substrate, these ends being electrically equidistant from the second and third arms of the series - T junction 32.

Another strip conductor 63 extends towards the series - T junction and is connected to a point on strip conductor 52 substantially mid-way above slot 42 by a bond wire 64. This arrangement acts like a centre-tapped transformer, the second and third arms of the series - T junction constituting the ends of the secondary winding of the transformer and said point on strip conductor 52 to which bond wire 64 is connected constituting the "centre-tap".

Disposed on the upper surface of the substrate at the centre thereof is a packaged diode quad 65, the electrical circuit of which is shown in two equivalent forms in Figures 5A and 5B respectively. It consists of four similar diodes 43,44,45,46, connected in series in a ring or bridge having terminals 53, 54, 55, 56. Each arm may comprise a plurality of diodes in series rather than a single diode, provided of course that there are the same number of diodes in each arm.

In the mixer circuit, the ends of strip conductors 142A and 152A are connected to one pair of opposed terminals, 53 and 55 respectively, of the diode quad, and the ends of strip conductors 142B and 152B are connected to the other pair of opposed terminals, 54 and 56 respectively.

For operation as a mixer, an R.F. input signal is applied to one of the series - T junctions (e.g. 32A) via its associated microstrip conductor (72A), and a local oscillator signal is applied to the other series - T junction (32B) via its associated microstrip conductor (72B). The l.F. output signal produced by the diodes is derived between the "centre-taps" of the series - T junctions via conductors 64A and 63A, 64B and 63B; this output signal may be taken as a balanced signal, or one of the two conductors 63 may be connected to the ground plane to give an unbalanced signal between the other conductor 63 and the ground plane.As an example of a further alternative, the l.F. signal may be derived from the "centre-tap" of each series - T junction via a respective balun; each balun may comprise bifilar wire or a twisted pair of wires looped through a longitudinally-apertured cylindrical ferrite core (for example, as shown in Figures 6 and 7 of the above-mentioned U.S. Patent 4,063,176), the pair of wires on one side of each balun being connected respectively to the "centre-tap" of a respective series - T junction and to the round plane (for example through an aperture in the substrate) at a point adjacent that junction, and the pairs of wires on the other sides of the two baluns being combined. The ferrite core of each balun may have a single, central, longitudinal aperture (as shown in Figure 6 of U.S.

Patent 4,063,176) or two longitudinal apertures sym metrically disposed about the longitudinal axis of the core. A suitable choice of bifilar wire may be used to obtain a transmission line with a characteris tic impedance approximating that desired: for exam ple, 36 s.w.g. bifilar wire can provide a characteristic impedance of approximately 50 ohms.

A double-balanced mixer embodying the inven tion has the advantages compared with, for exam ple, the mixer described in U.S. Patent 3,652,941 that no large or complicated holes are required to be made in the substrate (the holes for the connections to the ground plane can readily be made in an alumina substrate with an ultrasonic drill or a laser), that the baluns used for the R.F. input and local oscillator signals may be compact and have a large bandwidth, and that the bandwidth of the circuit is not compromised by the necessity of including series capacitors which must have a low impedance at the R.F. and local oscillator frequencies and a high impedance at the intermediate frequency. Furthermore, compared with for example the mixer described in U.S.Patent 4,125,810, it has the advantage that the passive R.F. components of the mixer circuit can be formed with only two conductive layers respectively on opposite sides of a single substrate.

A mixer of the form shown in Figure 4 has been constructed with gold coatings on a 1/4 mm thick alumina substrate. The input and output lines comprising the conductors 72 and 63 respectively were formed as microstrip lines having a characteristic impedance of 50 ohms, as had the slot lines 42; the microstrip lines comprising conductors 52, 142 and 152 had a characteristic impedance of 25 ohms. The I.F. output lines 63 were connected to the "centretaps" of the series - T junctions with thin gold wires 64 (typically 20item diameter) secured to the strip conductors with a ball bonder. With one of the conductors 63 earthed and an I.F. of 30 MHztaken from the other conductor 63, a conversion loss of less than 6 dB was obtained over the range of 1.5-9.0 GHz.

It has been found that the points at which bond wires 64 are connected to strip conductors 52 are not critical for the isolation of the l.F.from the R.F. input and local oscillator ports; satisfactory isolation was obtained provided the points were approximately aligned with slots 42.

A double-balanced mixer embodying the invention can of course also be used as a modulator. For example, an R.F. signal to be modualated can be fed into one of the series - T junctions, the modulation applied to the "centre-taps" of the junctions, and the modulated R.F. signal taken from the other series -T junction.

It may be noted that whereas in the embodiment of Figure 1, strip conductors 5 and 7 cross slot 4 immediately adjacent open-circuits 6 and 9 respectively, in the embodiments of Figures 3 and 4, strip conductor 71 crosses slot 41 substantially at short circuit 91 and strip conductors 52 and 72 cross slot 42 substantially at short-circuits 62 and 92 respectively, i.e. the centre lines of the strip conductors are substantially aligned with the edges of the open circuit apertures extending away from the slots. Both arrangements appear to operate satisfactorily.

Claims (31)

1. A microwave series - Tjunction comprising a first transmission line formed by a slot in a conductive plane and a strip second transmission line, wherein the conductive plane is a ground plane of the second line, wherein a first strip conductor which is a strip conductor of the second line crosses the slot so as to be coupled therewith, the first line having at one end a termination which presents in the operating frequency range of the junction a high impedance at the region where the strip conductor crosses the slot, wherein a portion of the first transmission line extending from said region and away from said end constitutes a first arm of the junction and wherein two portions of the second transmission line extending from the slot on opposite sides thereof constitute second and third arms respectively of the junction, whereby microwave energy fed into the junction on the first arm produces signals leaving the junction on the second and third arms, the signals produced in the second and third arms being in antiphase with one another at points electrically equidistant from the slot.

2. A junction as claimed in Claim 1 wherein the second line is substantially symmetrical about the first line.

3. A junction as claimed in Claim 1 or 2 wherein said termination is an open-circuit at or immediately adjacent said region.

4. A junction as claimed in Claim 3 wherein the open-circuit is formed by an aperture in the conductive plane, the edges of the aperture adjacent the slot extending away therefrom substantially perpendicular thereto.

5. A junction as claimed in Claim 1 or 2 wherein said termination is a short-circuit spaced from said region by a quarter of a wavelength in the operating frequency range of the junction.

6. A junction as claimed in any preceding claim wherein the conductive plane and the strip conductor are on opposite major surfaces of a dielectric substrate.

7. A series - Tjunction as claimed in any preceding claim in combination with a balanced third transmission line comprising said conductive plane and further comprising second and third strip conductors which are each connected at one end to the first strip conductor at the second and third arms of the junction respectively and at least parts of which extend closely adjacent to one another so as to be coupled together.

8. A microwave balun comprising a series - T junction as claimed in Claim 2 or in any of Claims 3 to 6 when appendant to Claim 2, or comprising the combination of a series - T junction and a balanced third transmission line as claimed in Claim 7 when appendant to Claim 2, the balun further comprising an unbalanced fourth transmission line coupled to the first line, portions of the second line on opposite sides of the slot forming a balanced system.

9. A balun as claimed in Claim 8 wherein the fourth transmission line is a microstrip line comprising the conductive plane and a fourth strip conductor, wherein the other end of the first line is terminated by an open-circuit, and wherein the fourth strip conductor crosses the slot at or immediately adjacent the open-circuit termination at said other end.

10. A balun as claimed in Claim 8 wherein the fourth transmission line is a microstrip line comprising the conductive plane and a fourth strip conductor and wherein the fourth line is coupled to the first line by a transducer as claimed in any claim of U.K.

Patent Specification (U.K. Patent Application 42929176).

11. A balun as claimed in any of Claims 8 to 10 when appendant to Claim 7 wherein the only parts of the second and third strip conductors which extend closely adjacent to one another are remote from the slot.

12. A balun as claimed in any Claims 8 to 11 when appendant to Claim 7 and to Claim 3 or 4 wherein the first, second and third strip conductors are substantially spaced from the open-circuit except where the first strip conductor crosses the slot.

13. A balun as claimed in any of Claims 8 to 10 when appendant to Claim 7 and to Claim 5 wherein the second and third strip conductors extend closely adjacent to one another from the second and third arms of the junction.

14. A balun as claimed in Claim 11,12 or 13 wherein the coupled strip conductors are juxtaposed to a portion of the conductive plane which has a termination so that in operationtheelectricfields between each of the coupled conductors and the conductive plane are mainly or wholly transformed into an electric field between the coupled conductors.

15. A balun as claimed in Claim 14 wherein said portion of the conductive plane is tapered towards the termination thereof with a progressively decreasing width transverse to the longitudinal direction of the balanced third line so as progressively to restrict the extent of the electric fields between each one of said coupled conductors and the conductive plane in a direction transversely away from the other of said coupled conductors.

16. A balun as claimed in Claim 14wherein the termination of the conductive plane is an edge generally transverse to the longitudinal direction of the balanced third line, said edge having a plurality of slots each of which extends into the conductive plane to form a quarter-wave choke.

17. A balun as claimed in any of Claims 14to 16 wherein a portion of a dieletric substrate supporting the coupled conductors extends longitudinally beyond the termination of the conductive plane and is disposed between and parallel to second and third conductive planes so that the coupled conductors and the second and third conductive planes form a suspended odd-mode microstrip line, as described in Netherlands Patent Application 7810942.

18. A balun as claimed in any of Claims 14to 16 wherein each of the coupled conductors is of substantially increased width beyond the termination of the conductive plane so that the closely adjacent edge regions of the widened conductors form a balanced slot transmission line.

19. A balun as claimed in Claim 18 wherein the increase in width of each of the coupled conductors is progressive from the region of the termination of the conductive plane.

20. A balun as claimed in Claim 18 when appendant to Claim 14 or 16 wherein the width of each of the coupled conductors increases at an edge which is generally transverse to the slot line and which has a plurality of slots each extending into the respective conductor to form a quarter-wave choke.

21. A microwave single-balanced mixer comprising a combination of a series - T junction and a balanced third tmnsmission line as claimed in Claim 7 when appendant to Claim 5 wherein the second and third strip conductors are of substantially equal electrical lengths and are closely adjacent over substantially their whole lengths, the mixer further comprising two diodes connected in opposite senses between a common input terminal and the other ends of the second and third strip conductors respectively, and wherein an effective substantial short-circuit to the conductive plane is presented to a first signal applied to the common terminal at each of said other ends of the second and third strip conductors, the mixer further comprising output means for deriving from the diodes a difference frequency signal produced when a second signal of a different frequency from the first signal is simultaneously applied to the first arm of the series - T junction.

22. A mixer as claimed in Claim 21 wherein each of said other ends of the strip conductors is electrically spaced m ~J2, where m = 0, 1, 2..., from an open-circuited stub having an electrical length (2n + 1) XI4, where n = 0, 1, 2..., for presenting said effective substantial short-circuit, and wherein said output means comprises a stub short-circuited at one end to the conductive plane, having an electrical length (2p + 1 ) it4, where p = 0, 1, 2..., and connected at its other end to the common terminal, ;; being a wavelength in the operating frequency range of the first signal.

23. A mixer as claimed in Claim 21 or 22 comprising an input transmission line for the first signal connected to the common terminal, wherein the output means comprises a further stub shortcircuited at one end to the conductive plane, having an electrical length (2q 1 1) i.,'4, where q = 0, 1,2..., and connected at its other end to the input line substantially 1.4 4 from the common terminal.

24. A mixer as claimed in any of Claims 21 to 23 wherein the electrical length of each of the second and third strip conductors is substantially a quarter wavelength in the operating frequency range of the first signal.

25. A microwave double-balanced ring mixer or modulator having three ports and comprising two series - T junctions each as claimed in Claim 2 or in any of Claims 3 to 6 when appendant to Claim 2 and a diode quad having two pairs of opposed terminals, wherein the two terminals of each pair of opposed terminals are respectively connected to the second and third arms of a respective one of said two junctions by two transmission lines of equal electrical lengths, wherein the first arms of said two junctions respectively constitute first and second of the three ports, and wherein coupling means to two points each electrically substantially mid-way between the second and third arms of a respective one of the two junctions constitute the third of the three ports.

26. A mixer or modulator as claimed in Claim 25 wherein the coupling means comprise two wires respectively conductively connected to the first strip conductors of the two junctions at the regions where those strip conductors cross the respective slots of the two junctions.

27. A mixer or modulator as claimed in Claim 26 wherein the coupling means comprise two further wires conductively connected to the conductive plane adjacent a respective one of the two junctions, and wherein the two wires thereby associated with each junction are looped through a respective apertured ferrite core to form a respective balun.

28. A microwave series - T junction substantially as herein described with reference to Figure 1,3 or 4 of the accompanying drawings.

29. A microwave balun substantially as herein described with reference to Figure 1,to Figures 1 and 2, to Figure 3 orto Figure 4 of the accompanying drawings.

30. A microwave single-balanced mixer substantially as herein described with reference to Figure 3 of the accompanying drawings.

31. A microwave double-balanced mixer or modulator substantially as herein described with referpence to Figures 4, 5A and 5B of the accompanying drawings.