GB2104750A

GB2104750A - Microwave single-balanced mixer

Info

Publication number: GB2104750A
Application number: GB08220037A
Authority: GB
Inventors: Robert Neil Bates
Original assignee: Philips Electronic and Associated Industries Ltd
Current assignee: Philips Electronics UK Ltd
Priority date: 1979-08-23
Filing date: 1982-07-09
Publication date: 1983-03-09
Also published as: GB2104750B

Abstract

A planar microwave single- balanced mixer includes a series-T junction (3) comprising a slot line (41) formed in a ground plane on one major surface of a dielectric substrate (11) and a strip conductor (51) on the other major surface crossing the slot a quarter-wavelength from a short- circuit at one end (60) thereof. A pair of further strip conductors (141, 151) are connected at one end to the strip conductor (51) on opposite sides of the slot and extend closely adjacent to one another so as to be coupled together. Two diodes (33, 34) are connected in opposite senses between the other ends of the pair of strip conductors (141, 151) and a common R.F. input terminal (25). A local oscillator signal supplied along the slot line (41), e.g. from a microstrip line (71), is coupled to the diodes (33, 34) along the pair of conductors (141, 151) in the odd mode, and an intermediate frequency signal is derived between the strip conductor (51) to which the I.F. signal propagates along the pair of conductors (141, 151) in the even mode, and the diodes (33, 34) via a low-pass filter (35-38) and a short- circuited stub (27) a quarter- wavelength long at the signal frequency. The circuit provides inherent isolation between the R.F. and L.O. ports, and simplifies filtering to reject undesired mixer products, e.g. at the image frequency. <IMAGE>

Description

SPECIFICATION Microwave single-balanced mixer The present application is divided out of our application 7929438 (Serial No. 2 057 196 A), as is our copending application 8220036.

The invention relates to a microwave singlebalanced mixer comprising a series-T junction.

Embodiments of the invention are particularly suited to manufacture on a single dielectric substrate.

Microwave mixers of generally planar form are known from, for example, U.S. Patents 3,652,941 and 4,125,810. These mixers use baluns which have the disadvantage that they inherently cannot completely decouple an unbalanced mode of propagation from a balanced mode of propagation.

According to the invention, a microwave single-balanced mixer comprises a series-T junction, said junction 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 short-circuit termination spaced from the region where the first strip conductor crosses the slot by a quarter of a wavelength in the operating frequency range of the mixer so as to present a high impedance at said region, 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 mixer further comprising second and third strip conductors which are of substantially equal electrical lengths, which are each connected at one end to the first strip conductor at the second and third arms of the junction respectively, and which over substantially their whole lengths extend closely adjacent to one another so as to be coupled together, two diodes connected in opposite senses between a common input terminal and the other ends of the second and third strip conductors respectively, 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, and 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.

Each of said other ends of the strip conductors may be electrically spaced m A/2, wherein m=0, 1, 2..., from an open-circuited stub having an electrical length (2n+ 1 ) :1/4, where n=O, 1, 2..., for presenting said effective substantial shortcircuit, and wherein said output means comprises a stub short-circuited at one end to the conductive plane, having an electrical length (2p+ 1 ) :1/2, where p=O, 1, 2 . . ., and connected at its other end to the common terminal, A being a wavelength in the operating frequency range of the first signal.

The mixer may comprise an input transmission line for the first signal connected to the common terminal, wherein the output means comprises a further stub short-circuited at one end to the conductive plane, having an electrical length (2q+ 1) :1i4, where q=O, 1, 2 and connected at its other end to the input line substantially :1/4 from the common terminal.

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.

Embodiments of the invention will now be described, by way of example, with reference to the diagrammatic drawing, which is a plan view 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 being a ground plane. A series-T junction 31 comprises a slot line 41, formed in the ground plane (the edges of the slot being denoted by dashed lines), and a strip conductor 51 crossing the slot. The right-hand end (as drawn) of the slot is terminated at 60 by a short-circuit, and the lefthand 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 narrow-band operation is required: 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 quarterwavelength beyond the slot. The open-circuit termination 91 at the left-hand end of the slot line could be 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. (Reference may be made to our above-mentioned application 7929438 for a more detailed description of an analogous series-T junction.) 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 quarter-wavelength 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 short-circuited 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, 1 51. 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 single-balanced mixer comprising a four-part 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 Q-factor since it must reject the image frequency but pass both the local oscillator and R.F. signal frequencies. However, in a singlebalanced 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 0factor and consequently be easier to design.

(Similar considerations may apply to the rejection of other undesired mixer products.) In order to iiicrnase 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 shortcircuit 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 the drawing has been constructed for operation at 10 GHz with gold coatings on a 3 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 170 .ym. The diodes (33, 34) where 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 dB.

It may be noted that whereas in the described embodiment, strip conductor 71 crosses slot 41 substantially at short-circuit 91, i.e. the centre line of the strip conductors is substantially aligned with the edge of the open-circuit aperture extending away from the slot, the strip conductor may cross the slot immediately adjacent the open-circuit. Both arrangements appear to operate satisfactorily.

Claims

1. A microwave single-balanced mixer comprising a series-T junction, said junction 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 short circuit termination spaced from the region where the first strip conductor crosses the slot by a quarter of a wavelength in the operating frequency range of the mixer so as to present a high impedance at said region, 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 mixer further comprising second and third strip conductors which are of substantially equal electrical length, which are each connected at one end to the first strip conductor at the second and third arms of the junction respectively, and which over substantially their whole lengths extend closely adjacent to one another so as to be coupled together, two diodes connected in opposite senses between a common input terminal and the other ends of the second and third strip conductors respectively, 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, and 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.

2. A mixer as claimed in Claim 1 wherein each of said other ends of the strip conductors is electrically spaced m A/2, wherein m=0, 1, 2 ....

from an open-circuited stub having an electrical length (2n+ 1 ) :1/4, where n=O, 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 ) :1/4, where p=0, 1,2 2..., and connected at its other end to the common terminal, A being a wavelength in the operating frequency range of the first signal.

3. A mixer as claimed in Claim 1 or 2 comprising an input transmission line for the first signal connected to the common terminal, wherein the output means comprises a further stub short-circuited at one end to the conductive plane, having an electrical length (2q+ 1 ) :1/4, where q=0, 1, 2..., and connected at its other end to the input substantially :1/4 from the common terminal.

4. A mixer as claimed in any preceding Claim 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.

5. A microwave single-balanced mixer substantially as herein described with reference to the drawing.