Four port hybrid microstrip circuit of Lange type

A four port hybrid microstrip circuit of modified Lange type, comprising a microstrip pattern including a first strip conductor between an input port (P1) and a direct port (P2) and a second strip conductor between an isolated port (P3) and a coupled port (P4). These two conductors are divided into parallel sections being mutually interdigitated and divided ibnto two parts (10,20) located side by side to each other.

FIELD OF THE INVENTION

The present invention relates to a four port hybrid microstrip circuit of modified Lange type, with a microstrip pattern having first and second strip conductors extending between an input port and a direct port and between an isolated port and a coupled port, respectively. More particularly, the microstrip circuit is of the kind defined in the preamble of claim 1.

BACKGROUNTD OF THE INVENTION AND RELATED ART

Lange couplers are generally used to couple electromagnetic energy between transmission lines. In a four port hybrid, there is an input port and a direct port, these two ports being directly and conductively connected to each other, as well as a coupled port, the latter being connected to transmission lines coupled electromagneticly (inductively and capacitively) to the conductors extending between the input and direct ports. Such hybrid couplers are used extensively as essential components in balanced circuits, such as balanced amplifiers.

In a Lange type coupler, each strip conductor is divided into mutually parallel sections, and the conductor sections from the two different strip conductors are interdigitated, so that each strip section is located between two sections from the other conductor. In a planar arrangement, it is necessary to have cross-over connectors in order to establish a direct conductive connection between the various sections extending in parallel.

A four port hybrid microstrip circuit of this kind is disclosed in U.S. Pat. No. 4,937,541 (Pacific Monolithics). The device has a reduced size and improved performance being obtained by capacitors added between the input and coupled ports and between the direct and isolated ports. Moreover, the known device is designed for R frequencies in the order of 10 GHz.

The present invention also aims at obtaining a reduced size o: the circuit, in particular for much lower frequencies in the range 0.5 to 5.0 GHz, in particular in the frequency range used for wireless communication systems.

However, when trying to obtain a satisfactory coupling, normally 3 dB, the strip conductors and the gaps between them become very narrow. With associated degradation of microstrip line Q factor and a high insertion loss. Therefore, it is difficult to use standard methods, especially production methods based on PCB technology.

SUMMARY OF THE INVENTION

Accordingly, the main object of the invention is to reduce the problems indicated above and to provide a circuit structure which enables the use of standard technology for producing a microstrip circuit which is operative even in relatively low frequency bands.

This object is obtained for a microstrip circuit of the kind referred to in the coning paragraph, where the strip conductor sections of the first and second strip conductors are divided into first and second parts extending longitudinally in opposite directions side by side, the parallel conductor sections of each strip conduct or in the first part being joined to a first and a second junction strip section, respectively, leading sideways to the associated parallel conductor sections in the second part. Preferably, the cross-over connectors are constituted by standard microstrip technology components, such as zero ohm resistors.

By arranging the circuit in two parts side by side, the overall dimensions of the device can he reduced, and it is also possible to use relatively wide conductor strips with relatively wide gaps therebetween. As a consequence, the microstrip line Q factor will be high and the insertion loss will be low. Moreover, standard PCB technology for microstrip circuits can be used, and the cross-over connectors may be constituted by commercially available zero ohm resistors.

BRIEF DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1illustrates the basic arrangement of the strip conductors included in the four port hybrid microstrip circuit according to the invention. The four ports are denoted P1to P4, where P1is an input port and P2is a direct port which is directly and conductively connected to the input port P1. The port P3is an isolated port, whereas P4is a coupled port, these two ports being directly and conductively connected to each other. A RF signal applied to the input port P1will be conductively transmitted to the direct port P2and, simultaneously, a part of the electromagnetic energy will be transferred, by way of electromagnetic coupling, to the coupled port P4.

According to the invention, the microstrip circuit includes two parts, viz. a first part generally denoted10, and a second part, generally denoted20. The two parts10and20are physically located side by side, but they are electrically connected in serious to one another.

Thus, the input port P1is connected to two parallel conductor sections11,13in the first circuit part10, these two sections being jointly connected to a terminal30. The terminal30is connected to a first junction strip section31leading sideways to another terminal32. The terminal32is connected to two parallel strip conductor sections21,23in second circuit part20, these conductor sections21and23being jointly connected to the direct port P2. So, there is a continuos conductive path from the input port P1to the direct port P2, having the general shape of the letter U and extending generally along the longitudinal direction L.

In a similar manner, the isolated port P3is connected to two parallel conductor sections12and14, which are jointly connected to a terminal40. The terminal40is connected by a second junction strip section41leading sideways to a terminal42, which in turn is connected to two parallel conductor sections22and24in the second circuit part20. These conductor sections22and24are jointly connected to the coupled port P4. So, the ports P3and P4are connected conductively to each other by way of a conductive path which is also configured like the letter U.

By way of this general configuration, the overall dimensions of the device can be kept relatively small.

A practical embodiment implementing the general structure shown inFIG. 1is shown inFIGS. 2,3and4.

The hybrid microstrip circuit is arranged on a planar, generally rectangular substrate1of a dielectric material of the kind DICLAD 527, a commercially available product obtainable from Arlon. This material has a permittivity of 2.55, and the thickness of the dielectric substrate is 0.76 mm in the preferred embodiment.

On a first surface of the substrate1(the lower side in FIG.4), there is a ground plane layer2constituted by a thin metal layer, in the preferred embodiment of Cu, haling a thickness of 0.035 mm. On a second surface opposite to the first surface (the upper side in FIG.4), there is a microstrip pattern3implementing the general structure shown in FIG.1.

However, in the practical embodiment, as illustrated most clearly inFIG. 3, the strip conductors are arranged in a slightly modified manner in order to minimise the number of crossing conductor sections.

The pattern3is obtained e.g. by printing or etching a thin metal layer, e.g. likewise of Cu with the same thickness as the ground plane layer2, i.e. 0.035 mm.

The four ports P1, P2, P3and P4are constituted by terminal pads arranged in the four corners of the device.

As appears fromFIG. 3, the first strip conductor connected to the input port P1comprises a conductor section with two portions11A and11B (corresponding to section11inFIG. 1) and a parallel conductor section with two portions13A and13B (corresponding to the conductor section13in FIG.1). The conductor section portion11A is connected to the conductor section portion11B by means of a diagonally extending crossover connector15in the form of a 0 ohm resistor of ordinary type.

The input port P1is connected to the conductor section portion13A by means of a transverse cross-over connector16, and the conductor section portion13A is connected to the connector section portion133by means of a diagonally extending cross-over connector17. All these conductor section portions11A,11B,13A,13B belong to the first strip conductor in the first part10of the device.

The conductor section portions11B and13B are jointly connected to a terminal or point30, which in turn is connected to the first junction strip section31leading sideways to the second part20.

From the terminal or point32, the first strip conductor has two parallel branches or conductor section portions21B and23B being connected respectively, by means of diagonally extending connecting sections21C and23C, to conductor section portions21A and23A, both being connected to the direct port P2. The conductor section portions21A and213correspond to the conductor section21inFIG. 1, and the conductor section portion23A and23B correspond to the conductor23in FIG.1. The conductor section portion21A is connected to the direct port P2by means of a transverse cross-over connector27.

In a similar manner, the isolated port P3and the coupled port P4are connected by a second strip conductor having conductor section portions12A,12B and14A,14B in the first part10, a second junction strip section41between the terminals40and42of the first and second circuit parts10,20, respectively, and mutually parallel conductor section portions22A,22B and24A,24B.

The conductor section portions12A,12B are connected by a diagonally extending conductor section12C, and the conductor section portions14A and14B are connected by a diagonally extending conductor section14C. The conductor sections22A and22B are connected by a diagonally extending cross-over connector25, and the conductor section portions24A,24B are connected by a diagonally extending cross-over connector26. The isolated port P3is connected to the conductor section portion145by a transverse connector18, and the coupled port P4is connected to the connector section portion223by a transverse connector28.

The connectors16-18and25-28are all of the same kind as the connector15.

As known per se, there will be a strong electromagnetic coupling between the parallel conductor sections belonging to the two different strip conductors, e.g. between the conductor sections11A,11B,13A,13B, on one hand, and the conductor sections12A,12B,14A,14B, on the other hand (in the first part10of the circuit device). Accordingly, an input signal applied to the input port P1will be divided into a first signal component appearing at the direct port P2, and a second signal component appearing at the coupled port P4. These two signal components have generally the same energy content and amplitude, provided that the coupling is effectively 3 dB. In order to achieve such an effective coupling, the length of the conductor sections11A,11B, etc, and thus of the longer side of the rectangular configuration of the whole circuit, should be a quarter wavelength or, generally, N/4 of a wavelength, N being an odd integer.

The signal components appearing at the direct and coupled ports P2and P4are mutually phase shifted 90°.

With the structure of the microstrip pattern shown inFIG. 3, it has turned out that such a coupling is achieved in the RF frequency range normally used in mobile telephone communication systems, in spite of the fact that the strip conductor sections are relatively wide, 0.3-0.7 mm, preferably about 0.5 mm, and have corresponding gaps between them, likewise 0.3-0.7 mm, preferably about 0.5 mm, whereby the circuit can be produced by normal methods used in ordinary PCB technology, including the cross-over connectors or jumpers15-18and25-28.

The illustrated embodiment may be modified within the scope of the claims. For example, each strip conductor may comprise three or more parallel sections in each part10,20. Also, if desired, it is possible to use very narrow strip conductors and gaps therebetween (as narrow as in conventional Lange couplers) and thereby achieve an even tighter coupling factor, such as 1 dB instead of 3 dB.