GB2192494A - Strip transmission line impedance transformation - Google Patents

Abstract

To provide a desired impedance transformation between two impedances, for example to match them, a strip transmission line circuit comprises two strip conductors (8,9) having coextensive parallel portions which are closely spaced so as to be mutually coupled along their lengths, and two bridge pieces (10,11) providing local RF short-circuit interconnections between the coupled portions of the strip conductors (8,9); the impedance transformation between the input (4) and the output (5) of the circuit depends on the positions of the bridge pieces (10,11). By using, if necessary, an additional length of line (7), any desired transformation of a passive impedance may be obtained. The bridge pieces (10,11) may be pieces of metal foil or wire which are removably bonded to the strip conductors (8,9) or which, in the case of stripline, are clamped between two dielectric substrates. Alternatively, the bridge pieces may be carried by members slidable along a slot in the substrate and releasably securable in adjusted positions. <IMAGE>

Description

SPECIFICATION "Strip transmission line impedance transformation" The invention relates to a strip transmission line circuit for providing an impedance transformation between two impedances. The term "transformation" is of course to be understood to comprise unity transformation.

The invention relates particularly but not exclusively to a said circuit wherein the impedance transformation can be adjusted. The invention further relates to a method of providing a substantially optimised impedance transformation between two impedances using such a circuit.

A circuit wherein the strip transmission line is microstrip and which includes impedance-adjusting elements is known from EP 0 154496 A. The impedance-adjusting elements each comprise a length of wire. One end of the wire is connected to the ground plane of the microstrip; the remainder of the wire has a non-conductive sheath, and the free end of the sheathed wire is arranged in close proximity to a strip conductor of the circuit.

The specification discloses such wire elements in two positions, one at the open-circuit end of a shunt stub and the other over a signal transmission line between two shunt stubs. By bending the wire so that its distance from the strip conductor or the angular position of its intersection with the strip conductor is altered, the input impedance is adjusted. Such impedance-adjusting elements can provide only a small range of variation in the input impedance, and the number of adjustments which can be made before a wire breaks is limited. Their use is therefore essentially limited to "once-only" fine tuning.

According to a first aspect of the invention, a strip transmission line circuit for providing an impedance transformation between two impedances comprises two strip conductors each having a respective portion which is elongate, substantially rectilinear and of substantially uniform width, the two portions being substantially coplanar, substantially parallel, substantially coextensive and closely spaced so as to be mutually coupled along their lengths in the operating frequency range of the circuit, and further comprising ground plane means spaced from the plane of said portions and means providing two local interconnections between opposed respective points on said portions, the interconnections being substantial short-circuits in said frequency range. For one or both strip conductors, the respective portion may be the whole length of the conductor.

If the impedance match is to be adjustable, the interconnections should not be integral with said strip conductors. However, if for example a plurality of similar circuits are to be made and the circuits do not require individual tuning, then once a sample circuit with adjustable interconnections has been made and optimum positions found for the interconnections, then further circuits can be made wherein interconnections in those positions form for example integral parts of the same conductive layer as the strip conductors.

In one form of adjustable circuit, the interconnections, which may be short lengths of metal wire or foil, are held in position by thermo-compression or ultrasonic bonding to said strip conductors.

As an alternative, the circuit may comprise two dielectric substrates each having a pair of opposed major surfaces, wherein two major surfaces, one of each substrate, are substantially contiguous, wherein a ground plane is disposed on the major surface remote from said contiguous surfaces of at least one of the substrates, and wherein the interconnections comprise respective metal foils which are clamped in position by and between the substrates.

As a further alternative, the positions along said portions of the interconnections may be continuously adjustable in operation.

In order to widen the range of impedances between which a suitable transformation can be provided by a circuit embodying the invention and having an input coupled to one of the strip conductors at one end of the two portions and an output coupled to one of the strip conductors at the other end of the two portions, the circuit may further comprise means for introducing between the input and the output a phase delay additional to that produced by the mutually coupled and interconnected strip conductor portions. Such means may for example comprise an open loop of line in the circuit pattern that may be coupled between one end of the strips and the input or output by further interconnections.

To obtain a wide range of impedances between which a suitable impedance transformation can be provided without the tuning being unduly sensitive to the exact positions of the interconnections, the length of each said portion suitably is substantially three quarters of a wavelength in said frequency range.

The widths of said strip conductor portions may be substantially equal.

According to a second aspect of the invention, a method of providing a substantially optimised impedance transformation between two impedances each having a normalised reflection coefficient less than or equal to unity comprises: coupling in cascade between the two impedances a strip transmission line circuit embodying the first aspect of the invention, supplying microwave energy from one impedance to the other via the strip transmission line circuit, measuring a quantity which constitutes or is representative of a parameter which it is desired to optimise, the quantity being dependent on the impedance transformation, and adjusting the positions along said portions of said interconnections so as substantially to optimise said parameter.

The parameter which it is desired to match may be (but is not necessarily) the power transfer from the one impedance to the other, in which case the impedances will be substantially matched. In a typical case in which a suitable impedance transformation for a mixer circuit is to be found, the parameter to be optimised may for example be the noise figure or the intermodulation distortion.

A circuit embodying the invention may constitute a strip transmission line analogue of the well-known coaxial tuner which is used mainly in the field of microwave measurements, for example load pull measurements on large signal amplifying devices and noise parameter measurements on low noise FETs.

When used for measurements on a strip transmission line circuit, a circuit embodying the invention can obviate the need for a change in the form of transmission line. Furthermore, whereas coaxial tuners present a short-circuit to DC and therefore when used in conjunction with a circuit including an active device that requires DC bias necessitate the use of a DC break, the latter is not required with a circuit embodying the invention.

A circuit embodying the invention may form part of a microwave integrated circuit. Such a circuit may for example be a narrow-band circuit wherein a large spread in device characteristics results in a need for individual circuit tuning or wherein the use of a non-linear device has resulted in some uncertainty in the design of the circuit; a typical example is a frequency multiplier.

Embodiments of the invention will now be described, by way of example, with reference to the diagrammatic drawings, in which: Figure 1 is a plan view of the strip conductor pattern of a strip transmission line tuning circuit embodying the invention; Figure 2 is a side view of a strip transmission line circuit embodying the invention; Figure 3 depicts lengths of coupled strip conductors; Figures 4(a) and 4(c) are equivalent circuits; Figures 5(a) and 5(b) are respectively a Smith chart and complex impedance plane diagram depicting the effect of continuously varying the position of one interconnection between the strip conductors; Figures 6(a) and 6(b) analogously depict the effect of continuously varying the position of the other interconnection, and Figure 7 is a perspective schematic view of a microstrip circuit embodying the invention.

Figure 1 is a plan view of a planar substrate 1 consisting of a dielectric sheet carrying on its upper major surface a circuit pattern 2 of strip conductors and on its lower major surface a ground plane 3 (see Figure 2).

The circuit has an input 4 and an output 5, and comprises an input strip conductor 6, a U-shaped strip conductor 7, and two parallel rectilinear strip conductors 8 and 9; the conductor 8 extends to the output 5.

Conductor 8 includes a portion of reduced constant width that is co-extensive with the whole of conductor 9; conductor 9 is of uniform width. The co-extensive portions of conductors 8 and 9 are closely spaced so that they are mutually coupled along their lengths in the operating RF range of the circuit. Two bridge pieces 10 and 11, in this case pieces of metal foil, that constitute substantial short-circuits at RF provide local interconnections between two points on conductor 9 and opposite respective points on conductor 8.

The circuit provides an impedance transformation between its input and output that depends on the positions of the bridge pieces 10 and 11, and the circuit may therefore be used for providing a desired impedance transmission, for example an impedance match, between two impedances respectively coupled to the input and the output by adjusting the positions of the bridge pieces. A further sheet 12 of dielectric may be brought up to the substrate 1 as shown in Figure 2, the substrates being secured together (by means not shown) so that the bridge pieces are clamped in position by and between the dielectric sheets.This is particularly suitable for a strip line circuit wherein the circuit pattern 2 is sandwiched between two ground planes, the sheet 12 in that case carrying a further ground plane 13 as shown in Figure 2; alternatively, the bridge pieces may for example be wires removably held in position by ultrasonic bonding to the strip conductors 8 and 9. In the case of a microstrip circuit, the bridge pieces may for example be pieces of metal foil removably held in position bythermo-compression bonding to the strip conductors 8 and 9.

In use, the strip conductor 8 is usually connected to the input strip conductor 6 by a further bridge piece 14, but as will be further explained below, the conductors 6 and 8 may alternatively be interconnected via the U-shaped conductor 7 by bridge pieces 15 and 16 (shown in dashed lines).

The input and output of the tuning circuit may be coupled, at opposite ends of the pair of coupled strip conductor portions, to the same one of the pair of coupled strip portions, as in the embodiment described above with reference to Figure 1, or to different ones of the pair of coupled strip conductor portions. Suitably, each of the remaining two of the four ends of the pair of coupled strip conductor portions is open-circuit, as in the embodiment described above with reference to Figure 1.

The tuning portion of the circuit, comprising the coupled strips 8 and 9 and the bridge pieces 10 and 11, may be analysed as follows. As shown in Figure 3, the overall phase length (at a given operating frequency) of the coextensive portions of the strips designated 0; this may be divided into three sections, the phase distance from the left-hand (input) end to the bridge piece 10 being designated 1, the phase distance between the bridge pieces 10 and 11 being designated 2, and the phase distance between the bridge piece 11 and the right-hand (output) end being designated 03, SO that 0=01+62+03.

Figures 4(a), (b) and (c) shows respectively the equivalent two-wire transmission line circuits of the three sections from left to right, of the coupled strips, with the lengths and characteristic impedances. The two end sections each comprise a transmission line of characteristic impedance Z0 and a shunt stub of the same electrical length (01 or 63 respectively) and of characteristic impedance Za, while the central section comprises a transmission line of characteristic impedance Zrn. These impedances may be expressed as:: Zo=a(Cb+Cab)/[CaCb+Cab(Ca+Cb)] Z,'a(Cb+C,b)/Cb2 Zm=al(ca+cb) where Ca, Cb and Cab are the distributed capacitances per unit length between the strip 8 and ground, between the strip 9 and ground, and between the strips 8 and 9 respectively, and where a=377/ < r is the effective dielectric constant of the dielectric medium (including air in the case of microstrip).

The effect of varying the positions of the bridge pieces 10 and 11 will now be considered for the case where the strips 8 and 9 are of equal width so that Ca=Cb, and where 0=3A14 (as discussed above). Figures 5(a) and (b) are respectively a Smith chart (the plane of the reflection coefficient p) and a complex impedance plane diagram showing how the normalised impedance presented at the input (left-hand) end of the coupled strips varies, the output (right-hand) end being terminated in a matched load RL, as the length 63 is varied while 1 is maintained constant at each of a plurality of values.For each value of Or, the input impedance follows a circular locus as 63 is varied: on the Smith chart, each circle is tangential to the boundary circle of the chart (representing Ipl=l; values of pare indicated around the boundary circle) and to a circle which is centred on the axis of zero reactance, which passes through the infinite resistance point, and which represents'the region of the Smith chart that is inaccessible however Oi and 63 are varied. This latter circle is shown shaded in Figure 5(a): its diameter may be designated R,,,, i.e. the maximum value ofthe resistive component of the normalised input impedance. It can be shown that Rmax=(Zm/RL)2.

In the complex impedance (Z) plane, the loci for constant 01 are circles of diameter Rmax whose centres are at Rmax/2, (ZmZo tan 01)/(ZsR). In each plane, the circle for 0a =0 is centred on the zero reactance axis. As Oi increases, the circular locus in the p plane moves clockwise around the Smith chart, corresponding to vertically-upward movement of the circular locus in the Z plane (with a discontinuity at the change at infinity from positive to negative values of the imaginary (reactive) part of the impedance) as denoted by the arrows at the centres of the circles. The arrows adjacent the circles in each plane indicate the direction of movement along around the respective circle as 63 increases.

Figures 6(a) and (b) are respectively a Smith chart and Z-plane diagram showing how the normalised input impedance varies as 01 is varied while 63 is maintained constant at each of a plurality of values. In the Smith chart, the input impedance follows circles of constant resistance which transform to vertical rectilinear lines in the Z-plane; these loci for the input impedance apply when Ca=Cb (which results from the strips 8 and 9 being of equal width). The inaccessible circular region of the Smith chart is again shaded. Thus, varying the position of the bridge piece 10 affects only the input reactance. When 63 is zero, the locus in the p plane is the boundary of the inaccessible region, corresponding to the line through Rmax in the Z plane.As 63 increases, the diameter of the circular locus in the p plane increases, corresponding to leftward movement (towards zero) of the rectilinear locus in the Z plane. The arrows adjacent the circles in Figure 6(a) indicate the direction of movement around the respective circle as 01 increases.

If the shunt stubs are to be able to provide the full range of susceptances from - oc to + oo, the phase length 6 should be at least"', i.e. half the wavelength at the operating frequency. 6 should not be equal to n (or an integral multiple thereof) since the circles of constant 01 and 63 respectively in the Smith charts would then be coincident. The tuning sensitivity is a minimum when 6 is an integral multiple of /2; if this is a desired criterion, a suitable phase length is therefore 3"r/2, corresponding to 3X/4.Reducing the length towards )ç/2 would increase Rmax and hence increase the range of resistive impedances that could be matched by only adjusting the bridge pieces 10 and 11 but would considerably increase the tuning sensitivity.

If the range of resistive impedances that can be matched by only adjusting the bridge pieces 10 and 11 is to be as large as possible, Rmax should be correspondingly large, and hence (Zm/RL) should be as high as possible. For a given value of RL, this is achieved by making the separation and the widths of the coupled portions of the strip conductors as small as possible. A suitably high value for Zm that can readily be realised is 75 ohms, and with the usual value for Rv of 50 ohms, this leads (when û=3A/4) to a value for Rmax of 2.25.

Corresponding values of Z0 and Za (with Er=2.2) are respectively 114 ohms and 221 ohms.

The real and imaginary parts of the input impedance, Re(Zjn) and Im(Zjn) respectively, when the circuit is terminated in a matched load RL and when 8=32 may be expressed as: Re(Zjn)=Zm2R/[RL2+(ZOZmYs tan 03)2] lm(Zin)=ZcZmYa [tan B1-(Re(Zi,) tan 03/RL)] where Ys=1/Zs.

In order to be able to match an impedance that falls within the shaded circular region on the Smith chart, the impedance may be transferred out of this region by using the U-shaped strip conductor 7 to introduce a phase shift between the impedance to the matched and the coupled strip conductors. With the above-mentioned specific values, the minimum additional phase length which will ensure this is approximately 26.4 degrees.

It will be apparent from Figures 5(a) and 6(a) that in using a circuit embodying the invention to match an unknown impedance lying outside the shaded circle to a standard impedance in general be possible to transform the unknown impedance to a value closer to the centre of the Smith chart by adjusting either O or 63. Which adjustment will have the more pronounced effect in improving the reflection coefficient will depend on the location of the unknown impedance in the Smith chart: for impedances near p=+1, adjusting Oi will tend to be most effective, while for impedances near p= -1, adjusting 63 will tend to be most effective.

Iterative adjustment of Oi and 63 in alternation to minimise the reflection coefficient should in general enable a substantial match to be obtained. A suitable starting point is with the interconnecting bridges at opposite ends so that û1 =Û3=0. If a substantial match cannot be obtained, it may be assumed that the unknown impedance lies within the shaded circle, and the U-shaped conductor 7 may be used to introduce an additional phase shift between the unknown impedance and the coupled pair of strip conductors.

Figure 7 is a perspective schematic view of a further embodiment of the invention. In this instance, the circuit is formed in microstrip, and the positions of the interconnections between the coupled strip conductors are continuously adjustable inuse. The substrate 21 is mounted on a rigid conductive baseplate 23, which may form the ground plane of the circuit. The circuit pattern on the upper, free surface of the substrate comprises a strip conductor 28, opposite ends of which are respectively coupled to the input and output (not shown) of the circuit, and which comprises a portion of reduced width that it coextensive with a closely adjacent parallel strip conductor 29. A slot aperture 31 extends alongside the strip conductors through the substrate 21 and the base plate 23.Two rigid L-shaped members 32 and 33 respectively of insulating material extend through the aperture 31 and across the air of strip conductors. The lower ends (not shown) of the vertical legs of the members 32 and 33 project from the lower surface of the base plate 23, are threaded, and carry nuts (not shown). The lower sides of the horizontal legs of the members 32 and 33, adjacent the upper surface of the substrate, are formed as knife edges and are metallised. By tightening the nuts on the vertical legs of the members, the members can be secured in position with their metallised knife edges engaging the upper surface of the substrate and providing local interconnections between the strip conductors 28 and 29. The nut on either member may be loosened to allow the member to be slid along the aperture 31, and hence the position of each interconnection may be adjusted as desired in operation.

The circuit pattern may include, as in the embodiment of Figure 1, include an additional length of line (not shown) which may be introduced between the input and the coupled strip conductors if necessary.

Claims (13)

1. A strip transmission line circuit for providing an impedance transformation between two impedances, comprising two strip conductors each having a respective portion which is elongate, substantially rectilinear and of substantially uniform width, the two portions being substantially coplanar, substantially parallel, substantially coextensive and closely spaced so as to be mutually coupled along their lengths in the operating frequency range of the circuit, and further comprising ground plane means spaced from the plane of said portions and means providing two local interconnections between opposed respective points on said portions, the interconnections being substantial short-circuits in said frequency range.

2. A circuit as claimed in Claim 1 wherein the interconnections are not integral with the strip conductors.

3. A circuit in any preceding claim wherein the interconnections are held in position bythermo- compression or ultrasonic bonding to said strip conductors.

4. A circuit as claimed in Claim 2 wherein the circuit comprises two dielectric substrates each having a pair of opposed major surfaces, wherein two major surfaces, one of each substrate, are substantially contiguous, wherein a ground plane is disposed on the major surface remote from said contiguous surfaces of at least one of the substrates, and wherein the interconnections comprise respective metal foils which are clamped in position by and between the substrates.

5. A circuit as claimed in Claim 2 wherein the positions along said portions of the interconnections are continuously adjustable in operation.

6. A circuit as claimed in any preceding claim wherein the circuit has an input coupled to one of the strip conductors at one end of the two portions and an output coupled to one of the strip conductors at the other end of the two portions, and wherein the circuit further comprises means for introducing between the input and the output a phase delay additional to that produced by the mutually coupled and interconnected strip conductor portions.

7. A circuit as claimed in any preceding claim wherein the length of both said portions is between half a wavelength and one wavelength in said frequency range.

8. A circuit as claimed in Claim 7 wherein the length of both said portions is substantially three quarters of a wavelength in said frequency range.

9. A circuit as claimed in any preceding claim wherein the widths of said strip conductor portions are substantially equal.

10. A circuit as claimed in Claim 9 for matching to an impedance of 50 ohms, wherein the section of the coupled strip conductors between the two interconnections has an effective characteristic impedance of substantially 75 ohms.

11. A method of providing a substantially optimised impedance transformation between the two impedances each having a normalised reflection coefficient less than or equal to unity, the method comprising.

coupling in cascade between the two impedances a strip transmission line circuit as claimed in any preceding claim, supplying microwave energy from one impedance to the other via the strip transmission line circuit, measuring a quantity which constitutes or is representative of a parameter which it is desired to optimise, the quantity being dependent on the impedance transformation, and adjusting the positions along said portions of said interconnections so as substantially to optimise said parameter.

12. A strip transmission line circuit for providing an impedance transformation between two impedances, substantially as herein described with reference to the drawings.

13. A method of providing a substantially optimised impedance transformation between two dissimilar impedances with a strip transmission line circuit, substantially as herein described with reference to the drawings.