GB2059171A

GB2059171A - Improvements in or relating to microwave circulators

Info

Publication number: GB2059171A
Application number: GB8026145A
Authority: GB
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1979-08-22
Filing date: 1980-08-11
Publication date: 1981-04-15

Abstract

A microwave circulator comprises a ferrite element 3 having one face secured to that surface of a microstrip substrate 1 which carries the ground plane 2. The opposite face of the element 3 is coated with a conducting layer 4 which extends over the whole side surface of the element between its two faces and contacts the ground plane 2 so that the latter is effectively continuous. A conducting area 6 in register with the element 3 is located on the opposite side of the substrate, and strip transmission lines extend from the periphery of this area. A permanent magnet 9 is adjustably located beyond the ground plane, in register with the element 3. <IMAGE>

Description

SPECIFICATION Improvements in or relating to microwave circulators This invention relates to microwave circulators and has a principal application in thick-film microwave integrated circuits.

Thick-film microwave integrated circuits (MIC's) are inherently cheap to manufacture provided some difficult types of components, such as circulators, are avoided. Commonly, such circuits are produced by screen-printing conducting-layer transmission lines on an insulating substrate 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) 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.

The usual form of MIC circulator comprises a ferrite disc let into the substrate so that its upper surface is flush therewith. The upper surface of the disc is metallised by the screen-printing process, which simultaneously prints transmission lines which extend from its periphery and form the ports. A magnet is mounted under the substrate in register with the disc, adjustably spaced to vary the magnetic field therein.

Unfortunately drilling the substrate (usually of alumina) to insert the ferrite disc is expensive, as is the alternative of using an all-ferrite substrate. For this reason some cheap microwave systems are designed without circulators, but the performance of such systems is severely restricted.

The present invention provides a cheaper form of circulator for use in MlC's, which does not require drilling of the substrate.

According to the present invention a microwave circulator comprises a ferrite element having one face secured to that surface of an insulating microstrip substrate which elsewhere carries the ground plane, and having a conducting layer on its opposite face which extends over substantially the whole side surface of the element between said two faces to contact the ground plane so that the ground plane is substantially continuous, the opposite surface of the substrate comprising a conducting area thereon substantially in register with said element and a plurality of strip transmission lines extending from the periphery of said area.

The ferrite element may be a substantially circular disc, but alternative shapes, such as ferrite triangles or rings, used in known circulators, can likewise be used in the present invention.

The usual magnet is located beyond the ground plane substantially in register with the element and may be adjustably spaced therefrom in a known manner. More readily understood, attention is directed by way of example, to the accompanying drawings wherein: Figure 1 is a sectional elevation of a three-port circulator embodying the present invention.

Figure 2 is a plan view of the embodiment of Figure 1 to a reduced scale.

Figures 3, 4 and 5 are graphs showing respectively the insertion loss, return loss and isolation for the embodiment of Figures 1 and 2.

Figure 1 shows a ceramic substrate 1,suitably of alumina, having a metallised ground plane 2 on one surface. To this surface is adhesively secured one face of a ferrite disc 3 whose opposite face is metallised at 4. The latter metallising extends down the side of disc 4 at 5 to contact the ground plane around the periphery of the disc. The metallising 5 covers the whole side of the disc so that the ground plane is effectively continuous. On the opposite surface of the substrate are printed a conducting disc 6 in register with disc 3 and three strip transmission lines 7 (Figure 2) which extend symmetrically from the periphery of disc 6, in the manner of a conventional printed circulator.

The substrate is mounted on a block 8 having a threaded hole in which a permanent magnet 9 coaxial with disc 3 is adjustably located at the end of a screw 10. The spacing of magnet 9 from disc 3 is adjusted in the conventional way to optimise the performance of the circulator by varying the magnetic field in the disc.

Where the substrate is supplied with the ground plane already applied thereto, a hole is made in the ground plane to receive the ferrite disc. The disc can be metallised, eg by vapour deposition, either before or after sticking in position, and is connected to the surrounding ground plane, eg by soldering, all round its circumference. Alternatively the ferrite disc can be secured to the substrate before the ground plane is applied, in which case the metallising of the disc and the deposition of the ground plane may be performed together.

The ferrite disc dimensions, strip transmission line dimensions, magnetic strengths, etc are determined in a similar manner as for known circulators in which the disc is inserted in the substrate, eg the disc diameter is made approximately 1 .8k where k is the wavelength in the transmission lines.

Figures 3-5 show typical results obtained with a circulator as shown in Figures 1 and 2 having the following characteristics: Substrate: alumina, 0.625 mm thick Ferrite disc: Marconi G5A, 0.625 mm thick, 3.81 mm diameter.

Disc 6: diameter 3.81 mm Strips 7: width 0.619 mm MagnetS: 1.5 x 103 gauss Metallising: copper Figure 3 shows the variation of insertion loss with frequency, eg in Figure 2 the loss between an input to port A and the output from port B; Figure 4 similarly shows the return loss, ie the proportion of the input reflected at port A; and Figure 5 shows the isolation, ie the proportion of the input at port A which appears at port C.

If a ferrite ring is substituted for the described disc, the metallising must also extend down its inner periphery to contact the ground plane within the ring, in order to maintain continuity.

Claims

1. A microwave circulator comprising a ferrite element having one face secured to that surface of an insulating microstrip substrate which elsewhere carries the ground plane, and having a conducting layer on its opposite face which extends over substantially the whole side surface of the element between said two faces to contact the ground plane so that the ground plane is substantially continuous, the opposite surface of the substrate comprising a conducting area thereon substantially in register with said element and a plurality of strip transmission lines extending from the periphery of said area

2. A circulator as claimed in claim 1 wherein the ferrite element is a substantially circular disc.

3. A circulator as claimed in claim 1 or claim 2 wherein a permanent magnet is located beyond said ground plane, substantially in register with the ferrite element.

4. A microwave circulator substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.