EP0398419A2

EP0398419A2 - One-octave 90 degrees 3dB directional coupler embodied in microstrip and slotline

Info

Publication number: EP0398419A2
Application number: EP90201164A
Authority: EP
Inventors: Carlo Buoli; Nicolangelo Palermo
Original assignee: Siemens Telecomunicazioni SpA; Societa Italiana Telecomunicazioni Siemens SpA
Current assignee: Italtel SpA; Siemens Telecomunicazioni SpA
Priority date: 1989-05-15
Filing date: 1990-05-08
Publication date: 1990-11-22
Also published as: IT1229726B; IT8920508A0; ZA903701B; NO902108D0; NO902108L; AU5498690A; EP0398419A3

Abstract

A 3dB 90 degrees directional coupler having a one-octave band extension for microwave circuits is described. It consists of two coupled lines (2,3) embodied in microstrip form and a slotline loop (4,5) made in the ground plane (6) of the microstrip lines.

Description

The present invention relates to the field of directional couplers for microwave circuits and more specifically to a one-octave 90 degrees 3dB directional coupler embodied in microstrip and slotline form.
The theory of directional couplers operating in the microwave range by means of coupled lines has been known for some time. An explanatory description of the theory of microstrip and slotline couplers is given for example in the volume entitled "Microstrip Lines and Slotlines" by K. C. Gupta published by Artech House in 1979, pages 234-244.
In accordance with this theory it is possible to analyze a directional coupler by dividing the structure in two simpler structures termed 'even mode' and 'odd mode', each having its own phase velocity and characteristic impedance. Said impedances can be taken when it is known that it is a 'normalization' impedance which is the load impedance of the coupler gates.
In the majority of practical cases the normalization impedance is 50 Ohm. For a 3dB coupler this means a value of approximately 20.7 Ohm for the characteristic odd mode impedance and approximately 120.7 Ohm for the characteristic even mode impedance.
Directional couplers operating in the frequency range of 2 GHz to 30 GHz are in general obtained by the thin film technique using alumina substrates having standard thicknesses of 25 or 10 mils depending on the frequency range.
A first example of embodiment of said 90 degrees 3dB directional couplers consists of two parallel lines of microstrip having predetermined spacing. Despite their simplicity these couplers have the drawback that in practice they cannot be embodied on alumina substrates having the above-mentioned standard thicknesses. Indeed, by using a 25 mil substrate the above characteristic even and odd mode impedances could be made with a distance between lines of approximately 8µm, which is too small for the present techniques of thin film deposit.
Another example of a known directional coupler is the one called 'Lange coupler' the description of which is given in the article 'The Design of Interdigitated Couplers for MIC Applications' by Vittorio Rizzoli and Alessandro Lipparini published in IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-26, No. 1, pages 7-15, Jan. 1978.
This coupler is embodied in microstrip form and, compared with the previous one, has the advantage of allowing greater distance between the coupled lines, i.e. 50µm with a 25 mil substrate. It is the interdigitated type and includes conductors which appropriately connect together the interdigitated lines.
On the other hand embodiment of the Lange couplers requires complex technology, principally for connection of said conductors. In addition they have the drawback of not possessing high directivity, a parameter which will be defined below. This is due mainly to the fact that the even and odd mode phase velocities differ slightly.
A third example of a known type of directional coupler embodied in microstrip and slotline form is the one termed 'de Ronde' from the name of its inventor. A detailed description thereof is given in Gupta's book mentioned above on pages 244-249.
Said coupler includes microstrip lines coupled together and deposited on one face of a dielectric substrate and some slotlines made in the metallization deposited on the other face of the substrate.
The main drawback of this coupler is the fact that it radiates electromagnetic power from the breaks, i.e. open circuits, in the slotline.
Therefore the object of the present invention is to overcome the above drawbacks and indicate a 3dB 90 degrees direction coupler for microwave circuits having a one-octave band extension and consisting of two coupled lines embodied in microstrip form and a slotline loop made in the ground plane of the microstrip lines.
The slotline loop is rectangular and placed at the outer edge of the microstrip. Its purpose is to increase the characteristic even mode impedance and thus provide the required characteristic even and odd mode impedances with more space between the microstrips than without the loop.
To achieve this the object of the present invention is a 90 degrees 3dB directional coupler for microwave circuits as described in claim 1.
Another object of the present invention is a variant of the subject coupler as described in claims 6 to 10.
In comparison with previous couplers, the directional coupler in accordance with the invention possesses the basic advantage of being simple to embody even on alumina substrates with thicknesses of 10 or 25 mils. In addition it does not radiate electromagnetic power because the slotline loop has no breaks capable of acting as antennas.
Another advantage of the coupler which is the object of this invention is that it maintains good operating performance in a one-octave band, which should be considered wide in comparison with the one obtained with known couplers. A particular advantage is the high directivity of the coupler due to the fact that it holds the even and odd mode phase velocities equal to one another.
Other objects and advantages of the present invention are made clear in the detailed description given below of an example of embodiment thereof and of a variant thereof and by the annexed drawings given only for nonlimiting explanatory purposes wherein:

Fig. 1 shows a plan view of the 3dB 90 degrees directional coupler which is the object of the invention,
Fig. 2 shows a cross section along plane of cut A-A of a partial isometric drawing of the directional coupler shown in FIG. 1 of said figure, and
FIG. 3 shows a plan view of a variant of the directional coupler of FIG. 1.

With reference to FIG. 1 reference number 1 indicates an alumina dielectric medium on which are deposited two microstrip lines indicated by 2 and 3 respectively. Each end belonging to the lines 2 and 3 is connected to a radiofrequency signal gate. Said gates are indicated by P1, P2, P3 and P4 respectively.
The microstrip lines 2 and 3 are parallel and separated by an appropriately calculated distance D. Two broken lines 4 and 5 delimit the contour of a slotline SL having a closed rectangular shape and made by engraving a metallization plane (indicated by 6 in FIG. 2.) placed on the rear face of the dielectric medium 1.
In FIG. 1 the distance between the broken line 5 and the contour of the microstrips 2 and 3 is exaggerated to make the figure easier to understand while in reality the line 5 is superimposed on the outer edge of the microstrips 2 and 3.
FIG. 2 makes clear the exact position of the slotline SL in relation to the microstrips 2 and 3. With reference to the figure there are seen the microstrips 2 and 3 deposited on the top face of the dielectric medium 1, on the bottom face of which is deposited a metallization plane 6 which acts as a ground plane for said microstrips and at the same time includes the slotline loop SL.
The directional coupler is placed over a metallic medium 7 having a base and four side walls which delimit an empty rectangular interior space 7′. The side walls are placed in contact with the metallization plane 6 and enclose the space 7′ to prevent escape of any electromagnetic radiation emitted by the slotline SL.
In operation, due to the electromagnetic coupling being formed between the microstrips 2 and 3 and the slotline SL, a sinusoidal radiofrequency signal at the input to the gate P1 issues from the gates P2 and P3 with a mutual 90 degrees phase shift and an electrical power one-half the input power, said gates P2 and P3 being coupled to the gate P1. There is no signal at the gate P4 and therefore it is termed an isolated gate.
The directivity mentioned above is a valuable parameter of the directional coupler. It is given by the ratio expressed in dB of the power of the signal present at either of the gates P2 or P3, which are coupled to the gate P1, to the power of the residual signal at the isolated gate P4.
In a practical case of embodiment of the coupler which is the object of the invention, on a 25 mil substrate and with a normalization impedance of 50 Ohm it is appropriate to choose for example a value of 50µm for the distance D between the microstrips 2 and 3. Then the width of said microstrips is calculated in such a manner as to obtain a value of 20 Ohm for the odd mode characteristic impedance. The value of 50µm for the distance D is easily obtained with present thin-film technology. The electrical length of the microstrips 2 and 3 is 90 electrical degrees at the central frequency of use.
At this point of the fabbrication process, there being no slotline, the even mode characteristic impedance is only 45 Ohm, thus quite far from the desired value of 120 Ohm. To increase this value while leaving unchanged the previous value of 20 Ohm it is necessary to calculate the width of the slotline SL so that it will have a characteristic impedance of 75 Ohm. This value is given by the difference between the theoretical value of the even mode characteristic impedance (120 Ohm) of the microstrips 2 and 3 and the value (45 Ohm) which said even mode characteristic impedance would have without the slotline SL.
A value of 75 Ohm for the characteristic impedance of the slotline SL can be obtained with a width of SL (distance bewtween the broken lines 4 and 5) of approximately 300µm.
It is to be understood that the above-described embodiment is simply illustrative of the principles of the invention. Various other modifications and changes may be made by those skilled in the art which will embody the principles of the invention and fall within the spirit and scope thereof.
FIG. 3 shows a variant of the coupler of FIG. 1 which allows obtaining the isolated gate in a position different from that given in the above example.
With reference to FIG. 3, where the same elements of FIG. 1 are indicated by the same reference numbers, there are seen four microstrip lines of the same length indicated by numbers 8, 9, 11 and 12. The microstrips 8 and 12 are positioned along a first horizontal direction common to both. Similarly the microstrips 9 and 11 are positioned along a second horizontal direction parallel to the one mentioned above.
The ends of the microstrips 8 and 9 located in the centre of the coupler are connected together by a thin section of microstrip line 10 positioned obliquely. Similarly the ends of the microstrips 11 and 12 located in the centre of the coupler are connected together by a metal wire 13 which is welded to said ends and which bypasses the section of microstrip 10 while remaining isolated therefrom.
The other ends of the microstrip 8, 12, 11 and 9 are connected in that order to gates indicated by P1′, P2′, P3′ and P4′ positioned in the same manner as the coupler shown in FIG. 1.
The dielectric medium 1 and the slotline loop SL are indentical to those of the coupler of FIG. 1.
In operation, due to the electromagnetic coupling being formed between the microstrips 2 and 3 and the slotline SL a sinusoidal radiofrequency signal at the input to the gate P1′ issues from the gates P3′ and P4′, said gates being coupled to the gate P1′ with a mutual 90 degrees phase shift and an electrical power one-half the input power. There is no signal at the gate P2′, said gate being isolated in the variant example.
A coupler with the above characteristics could be obtained from that of FIG. 1 by crossing together the microstrips and the slotline. But analysis of the structure shows that crossing of the slotline SL has no effect on the operation of the coupler and therefore is not necessary as shown in FIG. 3.
Consequently the coupler shown in FIG. 3 presents an operation similar to that of the coupler shown in FIG. 1. The only difference lies in the position of the isolated gate.
Accordingly the distance D separating the microstrips 8, 11 and 9, 12 and the width thereof is calculated as in FIG. 1. The electrical length of the microstrips 8, 9 and 10, like that of the microstrips 11, 12 and the wire 13, is 90 electrical degrees to the central frequency of use.
In a first embodiment of directional couplers like those shown in the examples of FIGS. 1 and 3 operating in the frequency band from 6GHz to 12GHz there is adopted an alumina substrate 25 mils thick. In a second embodiment of couplers operating in the frequency band from 12GHz to 24GHz there is adopted a substrate 10 mils thick.
The couplers of both embodiments have the following functions measured over the entire one-octave operating band:
- Adaptation to the four gates > 25dB.
- Directivity > 25dB.
- Unbalance between the coupled gates ≦ 0.5dB.
- Phase shift between coupled gates 90° ±1°.
These values show that the coupler which is the object of the invention and its variant have excellent performance over the entire band of operation.

Claims

1. 3dB 90 degrees directional coupler comprising a first microstrip line (2) whose ends are connected to a first gate (P1) and a second gate (P2) respectively, a second microstrip line (3) parallel to the first and coupled electromagnetically thereto and whose ends are connected to a third gate (P3) and a fourth gate (P4) respectively, said microstrip lines being deposited on a first face of a dielectric substrate (1) of appropriate thickness on whose second face is deposited a broad metallization plane (6), characterized in that on said metallization plane there is made a slotline loop (SL) at the outer contour (4, 5) of the package consisting of said microstrip lines (2, 3) which are parallel to each other.

2. 3dB 90 degrees directional coupler in accordance with claim 1 characterized in that said slotline loop (SL) forms a closed rectangle.

3. 3dB 90 degrees directional coupler in accordance with any of the above claims characterized in that the distance (D) between said first (2) and said second (3) microstrip has a freely selected value such as to allow ready deposit thereof on said dielectric substrate (1).

4. 3dB 90 degrees directional coupler in accordance with any one of the above claims characterized in that the width of said microstrips (2, 3) defines a characteristic impedance thereof with a value equal to the predetermined value for the characteristic odd mode impedance of said directional coupler.

5. 3dB 90 degrees directional coupler in accordance with any one of the above claims characterized in that the width of said slotline (SL) defines a characteristic impedance thereof whose value corresponds to the difference between the predetermined value for the even mode characteristic impedance of said directional coupler and the value of the even mode characteristic impedance which it would have without said slotline loop (SL).

6. 3dB 90 degrees directional coupler comprising a first (8), second (9), third (11) and fourth (12) microstrip lines deposited on a first face of a dielectric substrate (1) of appropriate thickness on whose second face is deposited a broad metallization plane (6) characterized in that said first (8) and fourth (12) microstrip lines are positioned along a first horizontal direction and said second (9) and third (11) microstrip lines are positioned along a second horizontal direction parallel to said first direction; in that first nonadjacent ends of the first (8) and fourth (12) microstrip lines connected to a first (P1′) and second gate (P2′) respectively, and first nonadjacent ends of the second (9) and third (11) microstrip lines are connected to a fourth (P4′) and a third (P3′) gate respectively; in that a first conductor (10) connects together a second end of said first (8) and second (9) microstrip lines and a second conductor (13), isolated from the first, connects together a second end of said third (11) and fourth (12) microstrip lines; and in that on said metallization plane (6) is made a slotline loop (SL) at the outer contour (4, 5) of the package consisting of all of said microstrip lines (8, 9, 11, 12).

7. 3dB 90 degrees directional coupler in accordance with claim 6 characterized in that said slotline loop (SL) forms a closed rectangle.

8. 3dB 90 degrees directional coupler in accordance with claims 6 or 7 characterized in that the distance (D) between said first (8) and third (11) and between said second (9) and fourth (12) microstrip lines has a freely chosen value such that it allows ready deposit thereof on said dielectric substrate (1).

9. 3dB 90 degrees directional coupler in accordance with any one of claims 6 to 8 characterized in that the width of said microstrips (8, 9, 11, 12) defines a characteristic impedance thereof equal to the predetermined value for the odd mode characteristic impedance of said directional coupler.

10. 3dB 90 degrees directional coupler in accordance with any one of claims 6 to 9 characterized in that the width of said slotline (SL) defines a characteristic impedance thereof with a value equal to the difference between the predetermined value of the even mode characteristic impedance of said directional coupler and the value of the even mode characteristic impedance it would have without said slotline loop (SL).