EP0028403A1

EP0028403A1 - Stub for matching microstrip circuits

Info

Publication number: EP0028403A1
Application number: EP80106677A
Authority: EP
Inventors: Antonio Scudellari
Original assignee: CSELT Centro Studi e Laboratori Telecomunicazioni SpA
Current assignee: Telecom Italia SpA
Priority date: 1979-11-05
Filing date: 1980-10-31
Publication date: 1981-05-13
Also published as: IT7969148A0; IT1119942B

Abstract

Introduction of a minimum redundancy into a microstrip stub for microwave circuits allowing reversibility of adjustment, always operating in the same manner, that is by only removing or by only adding conductive material.

Description

The present invention relates to microwave circuits implemented by the so called microstrip technology and more particularly it concerns a stub for adjusting microstrip circuits.
One of the fields in which microstrip stubs can be used is the measurements of characteristics of the active components.
In fact to obtain the complete description of an active microwave device often it is not enough to effect on it just the measurement of the known diffusion parameters S, whose determination requires no adjustment.
In a number of cases it is necessary to connect at the input and at the output of the active device apparatuses able to introduce arbitrary variations in the impedances they present at the active device.
This is valid for instance in the case of noise-figure measurement at the variation in the input impedance or in case of optimum input-output impedances measurement to check the operation of a power device of non-linear type.
It would be of course preferable to effect such variations directly on the microstrip circuits containing the active device; but this is not convenient due to the typical irreversibility of microstrip matching devices.
In fact once the adjustment circuit has been modified, values prior to the adjustment cannot be recovered.
This difficulty is generally overcome by means of coaxial cable matching devices, provided that the transition from microstrip to coaxial-cable has taken place.
The inconvenient of the measurement method effected by means of coaxial matching devices resides in that a dismembering must be effected of component holder to measure the unknown impedance.
A further field in which microstrip stubs are generally used is the implementation of amplifiers, oscillators, mixers and others. For these devices direct adjustment is generally required of the matching networks, stubs enclosed, to compensate for the unavoidable parameter losses characterizing the active components (as commercially available) and to compensate for possible parassitic parameters of passive circuity.
Such adjustment is critical when the active components must be pushed beyond their threshold of power dissipation; certain load impedances,in fact where imprudently varied can give rise to a breaking of the active components to which they are connected. These dangers are often present in microstrip circuit embodiments as the known stubs, not being of reversible type, must be precalibrated with values in excess so as to allow the progressive approximation for reduction to required operation conditions.
In fact operating on a microstrip in the microwave range (bands X, K_u, etc.....), the usually method used for the circuit adjustment consists in removing the conductive material forming the microstrip; said removement is generally obtained through a laser beam, as the required precision in the adjustment is very high.
Otherwise many active power components require for conjugated matching a very low impedance, but they do not tolerate short circuit conditions.
Operating by laser removing technique, the main inconvenient is that of acting only by removing the material without any recovery, if the removement was too severe.
Obviuosly, the determination of the optimal condition can be obtained only by verifying worse performances beyond that condition.
That is why it is necessary to dispose of microstrip stubs of the reversible type, that is such as to allow easy scanning about the optimal condition.
Otherwise, in the superior microwave range, this reversibility cannot simply be obtained by adding other material after the removement of conductive material, since: above all the precision that could be achieved by adding further material would be poor, besides a much varied structure would result with respect to the original one with subsequent increased losses; finally such corrective material addition, necessarily requiring manual intervention, would not allow the automatic calibration process.
Operating always in microstrip in the lower microwave range (bands L, C) an adjustment method alternative to the one previously described of laser removement, is the one of providing while planning a certain number of areolae of conductive material at the edges of metalized strips of microstrip and duly connecting a certain number to the main strip to the attainment of the required conditions.
In this case due to the rather low frequencies these connections do not present serious problems.
From the literature it is not clear haw the reversibility can be obtained by utilizing said areolae method.
These and other problems of the present invention will be solved by a method devised to obtain the reversibility in stub microstrip adjustment modalities, that by introducing minimum redundancy in the stub topology allows this reversibility and optimal operation conditions to be obtained, operating always in the same direction, that is by the only removement of conductive material, for instance by means of laser, or by the only addition of conductive material by means of areolae method.
Another characteristic of the present invention resides in the fact that said slight added redundancy can be positevely utilized by interpreting the stubs realized in that way as elements of abroad band matching network or by sending a polarization through a choke connected to a short-cricuit section of the stub.
It is a particular object of the present invention a stub for the adjustement of microstrip circuits consisting of two cascaded lines, the second of which having a length comprised between 1/4 and 1/2 of the used wavelength.
These and other characteristics of the present invention will become clearer from the following description thereof of a particular embodiment of the same, taken by way of example and not in a limiting sense in connection with the annexed drawings in which.

- Fig. 1 is the most general version of a symmetrical reversible stub of the the type with metalization addition (method of areolae );
- Fig. 2 is the equivalent scheme of the stub depictedin Fig. 1;
- Fig. 3 is a particular asymmetrical case of the reversible symmetrical stub depicted in Fig. 1 in which there are just two series of aereolae, one for carrying out the lengthening and the other one the short enin^g;
- Fig. 4 is the complementary version of the stub of Fig. 3 as the short- enning and the lengthening operations are effected not by addition but by removement of conductive material;
- Fig. 5 is, analogously to Fig. 4, the complementary version of the stub of Fig. 1;
- Fig. 6 representes an intermediate adjusting method between the one depicted in Fig. 1 and that of Fig. 5.

Fig. 1 shows the typical form of a symmetrical reversible stub; it consists of two cascaded lines, the fist, connected to main line L, has'high characteristic impedance, and the second, left in open circuit, has low characteristic impedance.
References w₁, 1₁ denote respectively the width and the length of the first line prior to adjustment; references w₂, 1₂ denote respectively the analogous values of the second line prior to adjustement.
Reference 1"₃ denotes the lengthening of the second line at the open end; reference 1'₃ denotes the lengtheing of the second line at the end of the connection with the first line, that then corresponds also to a shortening of the first line.
Reference w'₂ denotes the widening of the microstrip relating to the second line.
References a, b, c denote some areolae placed in adwance while planning the microstrip in suitable number and size, to carry out the required adjustments.
In the equivalent circuit of Fig. 2 reference Y₀ denotes the characteristic admittance of the main line L of Fig. 1.
References Y₁, 1₁ denote the characteristic admithance and the length of the first line wi, 1₁ prior to adjustement; references Y₂, 1₂ denote the analogous values of the second line w₂, 1₂ always before adjustement.
In Fig. 3 references L, 1₁, 1₂, 1'₃, 1"₃, w₁, w₂ denote the same values as in Fig. 1.
Reference a denotes the same anolae as in Fig. 1; reference c' denotes the areolae that correspond, in the asymmetric version, to those denoted by a in the symmetric version of Fig. 1.
In Fig. 4 references L, 1₁, 1₂, wi, w₂ denote the same values as in Fig. 1.
References 1'₄, 1"₄ denote areas of conductive material that can be partially or totally removed to realize the wonted adjustments.
In Fig. 5 the originary configuration of the microstrip is the same as in Fig. 1; the dotted lines denoted by d, e, f, represent areas of conductive material that can be removed while adjusting.
References 1₁, 1₂, w₁, w₂ denote the same values as in in Fig. 1.
In Fig. 6 references a, b, c denote the same areolae as in Fig. 1, but this time during the planning they have not been plotted outside of (as they have in Fig.1), but they have been plotted inside the original configuration of the microstrip and have been subsequently covered with conductive material (dotted zone) easy to remove, for instance by a sharpened stylus. References 1₁, 1₂, w₁ w₂ denote the same values as in Fig. 1.
With reference to the annexed drawings, the function will be now described of the adjusting method of the stub, object of the invention.
With reference to the generalized version with metalizing addition in Fig. 1, the initial equivalent circuit is the one of Fig. 2.
By adding to a second line (1₂, w₂) areolae of type a, length 1₂ is increased by an entity that, for instance, in case the two first areolae of type a were incorporated is equal to 1"₃, i. e. passing from length 1₂, to 1₂ + 1"₃.
By adding to second line (1₂, w2) some areolae of type b, width w₂ increases; in case for instance both areolae b represented in Fig. 1 were incorporated one would pass from width w₂ to w'₂: characteristic admittance Y₂ of Fig. 2 increases accordingly.
By adding to the second line (1₂, w₂) areolae of type c, length 1₂ increases, for instance in the case depicted in Fig. 1, by1'₃ and 1₁ diminishes by the same value.
Let us consider now the known expression of the input admittance B, normalized with respect to Y₀
in which is the phase constant, considered as equal, in the two microstrip lines. The expression (1) is for instance reported in "Foundation for Microwave Engineering" by R. Collin, Chapter 5. New York Mc Graw - Hill, 1966.
From (1) it derives that when 1₂ increases (for instance by 1''₃), B increses; but if tang β1₂ < 0 (that is if
< 12 <
being λ the microstrip wavelength), B decreases when Y_Z increases or when 1₂ increases, for instance by 1'₃ (and correspondingly 1₁ decreases by the same value).
Consequently, while connecting areolae of type a there is an increase in electric length (as it happens in case of a single line stub), incorporating areolae of type a and b everything works as if the stub electric length decreased.
The same reversibility property is present if the removement operation of conductive material was taken into consideration instead of area conglobation.
In fact, by considering (1) again it can be seen from Fig. 5 that when the conductive material is removed from side d (that is when 1₂ decreases) B decreases, but if tang β1₂<0 B increases when Y₂ decreases (removement of material from side e) or 1₂ decreases to the advantag of 1₁ (removement of material from side f).
As a consequence a single transmission line in open circuit used in shunt (that is a stub) can be converted into a reversible line (that is into a line that can be lengthened or shortened by the same method of adjustment no matter whether it consists of the addition or of the removement of conductive material) if said line is replaced by the cascaded connection of two different transmission lines, the second of which has a length 1₂ comprised between λ/4 and λ/2 (λ/4< 1₂ < λ/2); the length 1₁ of the first line is free and depends on the initial value positive or negative of B that is to be realized.
The operating modalities of the stub of Fig. 6 are strictly analogous to those of Fig. 5; however in this case while designing the microstrip initial areolae a, b, c are to be prearranged (as in Fig. 1) that must be covered with conductive material of the soft type that is easy to remove.
It has to be specified that the diagrams with areolae (Fig. 1, 3 and 6) are particularly suitable for not too high frequencies (<3GHz); while diagrams of Figures 1, 4 and 5 are necessary in case of high frequencies (>3GHz).
The methodology described here is based on the replacement of a single line by a cascade of two lines and then by the introduction of a redundancy.
Such redundancy can be usefully employed also, to send the polarization to active devices or to match loads on wide band.
It is enough to note that the second line (1₂, w₂) is a line with low characteristic impedance and has a length comprised between λ/4 and λ/2. Therefore an intermediate position exists (corresponding to λ/4) in which a short circuit at radio frequency is present with broad band due to the low impedance characteristic of the line.
This short circuit section can be utilized to insert a choke to send polarization voltage.

Claims

1, Stub for matching microstrip circuits, characterized in that it consists of two cascaded lines, the second (1₂, w₂) of which has a length comprised between 1/4 and 1/2 of the used wavelength.

2. Stub according to claim 1 characterized in that the only addition of conductive material at suitable points of said sub causes both the increase and the decrease of the admittance (B) characteristic of the line; said increase or said decrease in the admittance exclusively depending on the point of the stub to which the conductive material is added.

3. Stub as in claim 1, characterized in that the only removement of conductive material at suitable points of said stub causes both the increase and the decrease in the characteristic admittance (B) of the line; said increase or said decrease in the admittance depending only on the point of the stub from which the conductive material is removed.

4. Transmission stub for matching microstrip circuits, the whole as described in the text and depicted in the annexed drawings.