GB2031658A - Microwave circuit arrangements - Google Patents

Abstract

A tunable microstrip band-stop filter comprising two quarter-wave short circuit resonators 3 (one shown) which are each tuned by a titania disc 16 eccentrically mounted near the short circuited ends of the resonators such that pivotal motion of the discs about their mountings varies their coupling to the conductors of the respective resonators. The resonator line 3 is short-circuited to the earth plane at its outer end by a metallization on the inside of an aperture 10 in the dielectric substrate 1. The mechanism 12, 13, 15 for adjusting the disc 16 extends through the aperture 10. <IMAGE>

Description

SPECIFICATION Microwave circuit arrangements The present invention relates to microwave circuit arrangements.

The requirement often arises in radio engineering to produce a notch, or a narrow suppression band, in an otherwise flat transmission characteristic, in order to reduce the effect of a particular source of interference or noise. In addition it is frequently necessary that the centre frequency ofthe suppression band be tunable. At the same time it is important that, because of the narrow bandwidth, the centre frequency, once set, should remain stable against changes of temperature.

Band-stop filters have been realised using resonant lengths of waveguide or coaxial transmission line, with the tuning accomplished by mechanically varying the resonant length or volume. For economic and other reasons the form of microwave circuit arrangement known as microstrip has become important at frequencies of the order of a few gigahertz, but it has until the present suffered from the disadvantage that means for varying component values are less well-developed than for waveguide or coaxial line.

According to one aspect of the present invention in a microwave circuit arrangement including at least one microstrip section, there is provided a dielectric element which is mounted adjacent said microstrip section for movement with respect to said microstrip section such as to vary the coupling between said dielectric element and said microstrip section.

Preferably some at least of the permittivity, temperature coefficient of permittivity, dimensions and the range of positions of said dielectric element with respect to said microstrip section are chosen to compensate at least in part for a variation with temperature of a parameter of the microstrip section.

According to another aspect of the present invention in a microwave circuit arrangement including at least one microstrip section, there is provided a dielectric element which is mounted on pivotal means to lie adjacent said microstrip section, pivotal motion of said dielectric element or of the mounting for said dielectric element being arranged to vary the effective position of said dielectric element with respect to said microstrip section and thereby the coupling between said dielectric element and said microstrip section.

According to another aspect of the present invention in a microwave filter arrangementformed at least in part in microstrip there is provided an element of said filter arrangement comprising a section of microstrip line and a dielectric element which is pivotally mounted adjacent said section of microstrip line, pivotal motion of said dielectric element or of the mounting for said dielectric element being arranged to vary the effective position of said dielectric element with respect to said section of microstrip line and thereby the coupling between said dielectric element and said section of microstrip line.

The dielectric element may comprise a substantially flat plate of dielectric material mounted for pivotal motion generally in the plane of the plate about an axis offset from the centre of area of the plate.

According to another aspect of the present invention in a microwave filter arrangement formed at least in part in microstrip there is provided a tunable microstrip resonator comprising a section of microstrip line, a substantially flat plate of dielectric material, and means to mount said plate with its plane generally parallel to the plane of the substrate of the microstrip line for pivotal motion about an axis which is perpendicular to said planes and which is offset from the centre of area of the plate, pivotal motion of said plate about said axis being arranged to vary the effective position of said plate with respect to the microstrip line, and thereby the coupling between said plate and the microstrip line, to vary the tuning of said resonator.

The plate may be generally circular in outline and may be pivoted at a point offset from its centre about a pivot axis which is fixed relative to said substrate of the microstrip line. Alternatively the plate may be mounted on a pin which is eccentrically disposed on a member which itself is pivotable about an axis perpendicular to said planes, the plate being annular in form and pivotable about said pin.

A microwave circuit arrangement in accordance with the present invention will now be described by way of example with reference to the accompanying drawing, of which: Figure 1 shows in plan view a part of a microstrip circuit arrangement, and Figure 2 shows a sectional view along the line ll-il in Figure 1.

Referring first to Figure 1 the microwave circuit arrangement comprises a band-stop filter for use at frequencies of the order of 6G Hz. The circuit arrangement is formed in microstrip on a planar dielectric substrate 1 of alumina some 0.635 miliimetres (mm) thick, bearing on one major surface a generally continuous ground-plane conductor (not shown) approximately 0.005 mm thick and on the other major surface a pattern of conductive strips 2,3 and 4 some 0.635 mm wide and 0.005 mm thick.

The band-stop filter comprises the two strips 3 and 4 each of a length of approximately one quarter of a wave-length at the frequency of the intended stopband, which are separated by approximately one quarter of a wavelength along the strip-line path 2.

The strips 3 and 4 are capacitively coupled to the path 2, across gaps 5 and 6 respectively, and are terminated at their outer ends by short circuits 7 and 8 respectively to the ground plane through holes 9 and 10 in the substrate 1. The short circuits 7 and 8 may be formed as a substantially complete metallic platings of the bores of the respective holes 9 and 10.

Spaced around the holes 9 and 10 are additional areas 11 of the same general form and thickness as the strips 3 and 4.

Referring also to Figure 2 a circular-section metallic pin 12 with a slotted head 13 extends through a metallic base 14 and through the hole 10 in the substrate 1. An extension 15, also of circular section but with its axis offset from that of the pin 12, passes through the hole in the centre of an annular plate 16 of a dielectric material, such astitania, which is of similar thickness to that of the substrate 1 and which rests on the conductive strip 3 and areas 11. The plate 16 is held in place by a screw 17 into the extension 15, a washer 18 and spring washer 19.

The screw 17 and the washer 18 may be of a plastics material such as polytetrafluorethylene, while the spring washer 19 may be of synthetic rubber. The extra conductive areas 11 serve to support the plate 16 with its major faces parallel to the substrate 1 and the strip 3.

That part of the strip 3 which extends beneath the plate 16 has a lower characteristic impedance than the open stripline, has a positive temperature coefficient of impedance in contrast to the negative coeffi cientofthe open stripline, and a negative temperature coefficient of permittivity as opposed to the positive coefficient of the open stripline. The composte stubs made up by the titania plates 16 and the striplines 3 and 4 can therefore be designed with compromise characteristics.

The two-resonator band-stop filter shown can be tuned over a bandwidth of up to ten per cent of the centre frequency by rotation of the pin 12 such that, due to the eccentricity of the extension 15, the dielectric plate 16 extends over more or less of the strip 3, whereby the effective electrical length of the strip 3 is varied. By correctly choosing the permittivity, temperature coefficient of permittivity, dimensions and the range of overlay of the plates 16, the temperature dependence of the resonant frequency of the strips 3 and 4 can be kept to an acceptably low value.

For example titania plates 16 with a permittivity of about 100 and a temperature coefficient of minus 1000 parts per million per degree Centigrade were used to tune 6GHz resonators on an alumina substrate to give a frequency coefficient with temperature of plus or minus 25 parts per million per degree Centigrade over a five per cent tuning range.

The dielectric plates 16 are placed at regions of low electric field strengths, that is near the shortcircuited ends of the strips 3 and 4, to give finer control and more stable results. The plates 16 could, however, be mounted at any point along the strips 3 and 4 if there were no objection to the coarse tuning achieved.

The strips 3 and 4 and their separation may be otherthan a quarter wavelength, and the resonators may be open-circuit at their outer ends.

In the example described above the strips are some 12.5 mm long, while the plates 16 are 6.35 mm in diameter.

Claims (9)

1. A microwave circuit arrangement including at least one microstrip section wherein there is provided a dielectric element which is mounted adjacent said microstrip section for movement with respect to said microstrip section such as to vary the coupling between said dielectric element and said microstrip section.

2. A microwave circuit arrangement in accor dance with Claim 1 wherein some at least of the permittivity, temperature coefficient of permittivity, dimensions and the range of positions of said dielectric element with respect to said microstrip section are chosen to compensate at least in part for a variation with temperature of a parameter of the microstrip section.

3. A microwave circuit arrangement including at least one microstrip section wherein there is provided a dielectric element which is mounted on pivotal means to lie adjacent said microstrip section, pivotal motion of said dielectric element or of the mounting for said dielectric element being arranged to vary the effective position of said dielectric element with respect to said microstrip section and thereby the coupling between said dielectric element and said microstrip section.

4. A microwave filter arrangement formed at least in part in microstrip wherein there is provided an element of said filter arrangement comprising a section of microstrip line and a dielectric element which is pivotally mounted adjacent said section of microstrip line, pivotal motion of said dielectric elementor of the mounting for said dielectric element being arranged to vary the effective position of said dielectric element with respect to said section of microstrip line and thereby the coupling between said dielectric element and said section of microstrip line.

5. A microwave filter arrangement formed at least in part in microstrip wherein there is provided a flat plate of dielectric material mounted for pivotal motion generally in the plane of the plate about an axis offset from the centre of area of the plate.

6. A microwave filter arrangement formed at least in part in microstrip wherein there is provided a tunable microstrip resonator comprising a section of microstrip line, a substantially flat plate of dielectric material, and means to mount said plate with its plane generally parallel to the plane of the substrate of the microstrip line for pivotal motion about an axis which is perpendicular to said planes and which is offset from the centre of area of the plate, pivotal motion of said plate about said axis being arranged to vary the effective position of said plate with respect to the microstrip line, and thereby the coupling between said plate and the microstrip line, to vary the tuning of said resonator.

7. A microwave filter arrangement in accordance with Claim 6 wherein the plate is generally circular in outline and is pivoted at a point offset from its centre about a pivot axis which is fixed relative to the substrate of the microstrip line.

8. A microwavefilter ariangement in accordance with Claim 6 wherein the plate is mounted on a pin which is eccentrically disposed on a member which itself is pivotable about an axis perpendicularto said planes, the plate being annular in form and pivotable about said pin.

9. A microwave circuit arrangement substantially as hereinbefore described with reference to the accompanying drawing.