Microwave Quadrature Coupler Having Lumped-element Capacitors

Abstract

A microwave quadrature coupler constructed of a plurality of thin conductive strips on a substrate, preferably formed by photolithographic means. Each of these strips comprises a section of transmission line having a physical length less than one-quarter wavelength at the midband frequency. The ends of these lines are coupled to other circuit elements through lumped capacitors also formed of thin layers coplanar with the conductive strips on the same substrate.

Description

GOVERNMENT CONTRACT

The invention herein claimed was made in the course of, or under contract with The Department of the Army.

BACKGROUND OF THE INVENTION

This invention relates to microwave transmission devices and more particularly to quadrature couplers.

Quadrature couplers of the prior art have been constructed using several sections of transmission line each a quarter wavelength long. This has resulted in structures having larger physical dimensions than are desired for many modern applications and a solution of the problem of reducing the size seems to have been impeded by the requirement that each of the line sections must be a quarter wavelength long. An example of a prior art device with quarter wavelength dimensions is disclosed in U.S. Pat. 3,063,026 granted Nov. 6, 1962 to J. E. McFarland. From an economy standpoint it would also be desirable that the entire circuit in the deice is constructed in thin film using photolithographic methods. Applicant has discovered that the desirable objectives of reduces size and economy can be readily met by employing a plurality of transmission line sections each less than a quarter wavelength long and having distributed parameters. These transmission line sections are combined with lumped impedance elements connected at their ends to provide the required phase shifts and impedance transformations.

Summary of the Invention

This invention comprises a four-port microwave quadrature coupler having its entire circuit structure composed of thin conductive film on a substrate of dielectric material. A portion of the circuit structure comprises a plurality of conductive strips forming sections of transmission line with distributed parameters, each section having a physical length substantially less than a quarter wavelength at the midband frequency. Other portions of the thin film circuit structure comprises lumped capacitors coplanar with and connected at the ends of the sections of transmission line. The sections of transmission line cooperate with the lumped capacitors to provide the necessary phase shifts and impedance transformations between the sections to act as a conventional four-port quadrature coupler. Signals may be applied to any one port and two outputs in phase quadrature can be obtained with a 3 db. power split at the two opposite ports while the adjacent conjugate port is isolated from the input port.

BRIEF DESCRIPTION OF THE DRAWINGS.

The invention may be better understood by reference to the accompanying drawings in which:

FIG. 1 discloses the invention embodied in a coupler of the branch line type;

FIG. 2 is a schematic representation of the circuits of FIG. 1;

FIG. 3 discloses the invention embodied in a coupler of the proximity type;

FIG. 4 is a schematic representation of the circuits of FIG. 3; and

FIG. 5 shows a schematic of a proximity coupler of the same type shown in FIG. 3 but with three lumped capacitors periodically placed along the coupled transmission lines.

The quadrature coupler of FIG. 1 is shown with four ports 1, 2, 3 and 4, any one of which may be used as the input port. A signal applied to port 1, for example, will emerge from output ports 3 and 4 while port 2 will deliver no power. Port 2 is, therefore, conjugate with port 1 and is generally referred to as the isolated port. Any power division may be obtained at the output ports with their voltages in quadrature and, with proper design, a 3 db. split is readily obtained. The conventional branch line coupler does not include the lumped capacitors 13, 14, 15, 16, 17 and 18 but generally comprises two or more series-connected transmission line sections such as sections 6 and 7 between ports 1 and 4 and sections and 9 between ports 2 and 3. Branch line coupling is provided by the transmission line sections 10, 11 and 12. In the conventional branch line coupler, these transmission line sections are all one-quarter wavelength long at the midband frequency.

In order to reduce the size of the coupler, it is obvious that the lengths of these transmission line sections must be substantially reduced. By supporting thin film transmission line sections on a substrate 20 and connecting each of their junctions to a ground plane through lumped capacitors, these transmission line sections can be made substantially shorter than in the conventional structure. For example, in one specific embodiment of this invention, lines 6, 7, 8 and 9 were made approximately 18 percent of a quarter wavelength, thereby materially reducing their length. In this same embodiment, the outputs were 3 db. coupled throughout a band extending from about 1 to 1.3 gHz. The isolation and return loss were both better than 15 db. over this same band. Throughout the slightly narrower band from 1 to 1.25 gHz, both the isolation and return loss were better than 20 db.

It is to be understood that all of the circuit structure in FIG. 1 is formed of a thin film supported by substrate 20. This includes the ground conductors 5A and 5B, the transmission lines 6 through 12 and the lumped capacitors 13 through 18. It is also to be understood that this substrate 20 is supported between a pair of conventional ground planes, not shown, with the ground conductors 5A and 5B connected thereto. Since the entire circuit structure can be fabricated in thin film, it is possible to manufacture these devices in quantity by photolithographic means with great economy.

The presence of substrate 20 in an otherwise air dielectric environment produces an inhomogeneous dielectric medium between the ground planes and the circuit structure. It has been discovered that the transmission line sections can be shortened provided that lumped capacitors, such as capacitor 13, are also connected at the junctions formed by the transmission line sections and the branch lines so as to correct the phase and provide the impedance transformation necessary to effect proper quadrature coupler action. Thus a lumped capacitor 13 is connected between ground and the junction of branch line 10 and transmission line section 6; capacitor 14 at the junction of transmission lines 6 and 7 and branch line 11; and capacitor 15 at the junction of transmission line section 7 and branch line 12. Lumped capacitors 16, 17 and 18 are similarly connected between ports 2 and 3.

The circuit structure of FIG. 1 is disclosed in FIG. 2 with the reference numerals in this figure corresponding to their similar parts in FIG. 1. Here the lumped capacitors are shown as conventional capacitors 13 through 18 connected between ground and the junctions of the transmission line sections as previously described for FIG. 1.

FIG. 3 discloses a proximity quadrature coupler also embodying the principles of this invention. As in FIG. 1, any one of the ports 1 through 4 may be used as the input port. Assuming again that port 1 is used as the input port, port 2 will be the isolated port while power can be derived from output ports 3 and 4 with a 3 db. power split, if desired, their voltages being in quadrature. As with the circuits of FIG. 1, the entire circuit structure of FIG. 3 is formed of a thin conductive layer on substrate 30 and it is also to be understood that this structure will be supported between ground planes, not shown, in accordance with conventional practice. While transmission line sections 31 and 32 are physically short, they may be regarded as electrically long transmission lines closely coupled by reason of their proximity with each other. The classical proximity coupler will not work in thin film construction because of the large velocity difference between their even and odd mode propagations. However, when these lines are arranged in an inhomogeneous dielectric medium as shown in FIG. 3 with lumped capacitors periodically placed between the lines to provide a match in both their even and odd modes, the combination of the lumped capacitors and the distributed coupling between the lines cooperate to produce the coupling and phase shift required to provide the desired outputs at ports 3 and 4 and isolation at port 2.

Transmission line section 31 is coupled to port 1 through a thin film lead 35 while its other end is connected to port 4 through a thin film lead 37. Similarly, thin film leads 36 and 38 couple transmission line section 32 to ports 2 and 3, respectively. The ends of transmission line sections 31 and 32 are intercoupled through lumped capacitors 33 and 34. This thin film construction which combines transmission line sections with lumped capacitors results in the same advantages for the proximity coupler of FIG. 3 as were previously described for the branch line coupler of FIG. 1.

FIG. 4 is a schematic representation of the circuits of FIG. 3, the corresponding parts bearing the same reference numerals. As in the case of FIG. 2, the lumped capacitors 33 and 34 are here given a conventional representation.

FIG. 5 is a schematic diagram of a proximity coupler of the same type shown in FIG. 3 but with three lumped capacitors 33, 34 and 39 periodically spaced along the coupled transmission lines 31 and 32. The operation is essentially the same, however, as previously described for FIG. 3.

It will be noted from the above description that both the branch line type coupler of FIG. 1 and the proximity type coupler of FIG. 3 have the same convenient topological feature of having the input and isolated ports adjacent on their substrates. While these couplers operate on slightly different principles, each is the full functional equivalent of the other.

Claims

What I claim is:

1. A microwave quadrature coupler having an input port, an isolated port and two output ports, said coupler being particularly characterized in that it comprises a plurality of thin conductive strips supported coplanar on an insulating substrate, said conductive strips comprising sections of transmission line, each of said sections having a physical length less than one-quarter wavelength at a midband frequency, said sections of transmission line comprising a first series circuit of two of said sections of line connected between said input port and one of said output ports, a second series circuit of two of said sections of line connected between said isolated port and the other of said output ports, and first, second and third branch lines, said first branch line connected between the ends of said first and second series circuits at the input and isolated ports, respectively, said second branch line connected between the junctions formed by the two line lengths in said first and second series circuits, said third branch line connected between the ends of said first and second series circuits at the two output ports, respectively, and said conductive strips further comprising lumped capacitors coupling the ends of said line sections to ground.

2. The combination of claim 1 wherein said lumped capacitors comprise six fixed capacitors each having a grounded end and an ungrounded end, said ungrounded ends being connected, respectively, to the ends of said three branch lines.

3. A microwave quadrature coupler having an input port, an isolated port and two output ports, said coupler comprising an insulating substrate, a plurality of thin conductive strips supported coplanar on said substrate, said strips comprising sections of transmission line having distributed parameters, each of said sections of said transmission line having a length less than a quarter wavelength at a midband frequency, and said strips further comprising lumped capacitors coupled to the ends of said line sections, said sections of transmission line comprising a first series circuit comprising two of said sections of said transmission line connected between said input port and one of said output ports, a second series circuit comprising two of said sections of said transmission line connected between said isolated port and the other of said output ports, and first, second and third branch lines, said first branch line connected between the ends of said first and second series circuits at the input and isolated ports, respectively, said second branch line connected between the junctions formed by the two line lengths in said first and second series circuits, said third branch line connected between the ends of said first and second series circuits at the two output ports, respectively.

4. The combination of claim 3 wherein said lumped capacitors comprise six fixed capacitors each having a grounded end and an ungrounded end, said ungrounded ends being connected, respectively, to the ends of said three branch lines.

5. A microwave quadrature coupler comprising:

an input port, an isolated port, and two output ports;

an insulating substrate;

first and second series circuits each comprising at least one section of transmission line having a length less than a quarter wavelength at the midband frequency, each said section comprising a thin conductive strip supported coplanar on said substrate, said first series circuit connecting said input port to one of said output ports, said second series circuit connecting said isolated port to the other of said output ports, said first and second series circuits extending parallel to each other; and

a plurality of lumped-element capacitors connected between said first series circuit and said second series circuit at the ends of said sections of transmission line, each of said capacitors comprising thin conductive strips supported coplanar on said substrate.