GB2129623A - Improvements in or relating to microwave circulators - Google Patents

Abstract

A microwave circulator for use in MIC's comprises an insulating substrate (1) backed by a ground- plane (2) and having a circular disc (3) of metallising on its opposite surface from whose periphery extend strip transmission lines (7). A ferrite disc (5) of larger diameter than disc (3) is secured concentrically over the latter disc (3) and has a metallising layer (6) on its free surface. The line-portions (7) below the ferrite disc (5) are narrower than the line-portions (4) extending beyond its periphery to maintain constant impedance. The shoulders between these two line-portions aid location of the ferrite disc (5) during manufacture, and this circulator reduces tolerance problems generally. <IMAGE>

Description

SPECIFICATION Improvements in or relating to microwave circulators This invention relates to microwave circulators and has a principal application in microwave integrated circuits (MIC's).

MIC's are commonly produced by forming conducting-layer transmission lines on an insulating substrate, using either thick film or thin-film techniques, backed by a continuous metal-layer ground-plane, an arrangement commonly known as microstrip. Circulators are components comprising a plurality of circumferentially spaced ports (usually three, 120 apart) such that substantially all the input energy to any port will travel in one direction only around the circulator and leave by the port next to the input port. Circulators have one application, for example, in interconnecting a transmitter and receiver to a common antenna.

A familiar form of MIC circulator comprises a ferrite disc, commonly termed a "puck", let into the substrate so that its surface is flush therewith. The upper surface of the puck is metallised, and metallised transmission lines extending from its periphery are formed simultaneously to constitute the ports. A permanent magnet mounted under the substrate in register with the puck provides a magnetic field normal thereto. However, drilling the substrate (usually of alumina) to insert the puck is expensive, as is the alternative of using an allferrite substrate.

In an alterative form of circulator, eg as described in UK Patent Application No 2,005,924A, the puck is mounted on the under-surface of the substrate and has its exposed surfaces metallised and electrically continuous with the ground-plane. A disadvantage of this form is the need for good precision in the diameter and roundness of the ferrite puck, and for good alignment between the puck and the disc of metallising on the upper surface of the substrate. Such alignment is difficult to achieve since they are on opposite sides of the substrate.

The present invention provides a form of circulator which enables these disadvantages to be avoided.

According to the present invention a microwave circulator comprises: an insulating substrate having a conducting ground-plane on one surface thereof; a conducting area on the opposite surface of said substrate and a plurality of strip transmission lines extending on said opposite surface from the periphery of said area; a ferrite element secured on said opposite surface of said substrate with one face of the element in contact with said conducting area; and a conducting layer on the opposite face of said ferrite element; whereby said conducting area in contact with said ferrite element is in triplate configuration between said ground-plane and said conducting layer on said opposite face of the ferrite element.

Preferably the conducting area is a circular disc and the ferrite element substantially overlaps the periphery of the conducting area, being preferably a circular disc of larger diameter than the conducting area. In this case preferably the resulting triplate transmissionline portions extending between the peripheries of the conducting area and of the ferrite element are of reduced width relative to the microstrip transmission-line portions extending beyond the ferrite element in order to match the impedance of these two portions of each line. Preferably the two discs are substantially concentric with one another.

To enable the nature of the present invention to be more readily understood, attention is directed, by way of example, to the accompanying drawings wherein; Figure 1 is a sectional elevation of a threeport circulator embodying the present invention.

Figure 2 is a plan view of the embodiment of Fig. 1.

Figures 3 and 4 are graphs showing respectively the return loss and the insertion loss for the embodiment of Figs. 1 and 2.

Fig. 1 shows a ceramic substrate 1, suitably of alumina, having a metallised ground plane 2 on one surface. On its opposite surface are printed, by conventional thick-or thin-film techniques, a conducting disc 3 and three strip transmission lines 4 which extend symmetrically at 120 spacing from the periphery of disc 3 in the manner of a conventional MIC circulator.

A ferrite disc (or puck) 5 is adhesively secured to the opposite surface of substrate 1 over and concentric with the disc 3. The upper surface of disc 5 is coated with a layer of metallising 6. The disc 3 and the portions 7 of the lines 4 extending between the peripheries of discs 3 and 5 are thus effectively in a mixed-dielectric triplate configuration between conducting layers 2 and 6. The lineportions 7 are of reduced width relative to the microstriop portions extending beyond disc 5, so that the characteristic impedancies of these two portions of each line are substantially the same. The shoulder thus formed between the two line portions facilitates accurate location of disc 5 over disc 3 during construction of the circulator.Good concentricity is important in this embodiment for good impedance matching of the prior-printed line-portions, not for the circulator action as such; circulator action as such is obtained whether the disc 5 diameter is less than, equals, or exceeds (as described) the diameter of disc 3.

A permanent magnet or magnets (not shown) provides a magnetic field, indicated by arrow 8, normal to the plane of the substrate, in a conventional manner. A single magnet can be mounted either above or below the substrate, or two magnets (one above and one below) can be used to give a higher field strength and a more uniform field. The performance of the circular is optimised by adjusting the spacing of the magnet or magnets to vary the field in the disc 5, in the conventional manner.

The ferrite disc dimensions, strip transmission-line dimensions, magnetic strengths, etc are determined in a similar manner as for the aforesaid known forms of circulator, eg the diameter of disc 3 (which alone defines the resonant frequency of operation when, as described, the disc 5 overlaps disc 3) is made approximately 1,84A/2n where À is the wavelength in the transmission-line portions 7.

Figs. 3 and 4 show typical results obtained with a circulator as shown in Figs. 1 and 2 having the following characteristics: Substrate 1: alumina, 0.635 mm thick Ferrite disc 5: Marconi M2A, 0.635 mm thick, 10.25 mm diameter.

Disc 3: 4.05 mm diameter.

Transmission line widths: portion 7 (in triplate) 0.2 mm; remainder (in microstrip) 0.62 mm.

Magnetic field strength: 1 k oe.

Metallisation: copper.

Fig. 3 shows the return loss, ie the proportion of the input to any port which is reflected therefrom; Fig. 4 shows the isolation loss, ie the proportion of the input to any port which travels beyond the next adjacent port and appears at the third port.

In its principal application, the present circulator constitutes a localised triplate zone in an otherwise microstrip MIC. One advantage of the present circulator is that by making the diameter of the ferrite disc 5 greater than that of the conducting disc 3, as in the described embodiment, the resonant frequency of operation is then dependent only on the diameter of disc 3, which can be defined accurately by thin-or thick-film techniques. Moreover when, as described, the metallisation pattern on the upper surface of substrate 1 includes shoulders between the two portions of each transmission lines, these shoulders facilitate the subsequent accurate location of the disc 5 on the upper surface. Thus the present invention can reduce tolerance problems and simplify the manufacture of high-frequency circulators in microstrip MIC's.

Claims (6)

1. A microwave circulator comprising: an insulating substrate having a conducting ground-plane on one surface thereof; a conducting area on the opposite surface of said substrate and a plurality of strip transmission lines extending on said opposite surface from the periphery of said area; a ferrite element secured on said opposite surface of said substrate with one face of the element in contact with said conducting area; and a conducting layer on the opposite face of said ferrite element; whereby said conducting area in contact with said ferrite element is in triplate configuration between said ground-plane and said conducting layer on said opposite face of the ferrite element.

2. A circulator as claimed in claim 1 wherein the conducting area is a circular disc and the ferrite element substantially overlaps the periphery of the conducting area.

3. A circulator as claimed in claim 2 wherein the ferrite element is a circular disc of larger diameter than the conducting area.

4. A circulator as claimed in claim 2 or claim 3 wherein the resulting triplate transmission-line portions extending between the peripheries of the conducting area and of the ferrite element are of reduced width relative to the microstrip transmission-line portions extending beyond the ferrite element in order to match the impedances of these two portions of each line.

5. A circulator as claimed in any one of claims 2, 3 or 4 wherein the two discs are substantially concentric with one another.

6. A microwave circulator substantially as hereinbefore described with reference to the accomDanvina drawinas.