GB2141880A - Electrical resonators - Google Patents

Abstract

A dielectric resonator consists of a body of dielectric material (1) mounted in close proximity to a microstrip line (4) carrying a high frequency electrical signal. The resonance frequency of the sonator is adjusted by varying a d.c. bias voltage applied to two varactor diodes (14, 15) which form part of an inductive closed loop (9, 10) mounted adjacent to the end of the body (1) which is remote from the micostrip line. By varying the bias level, a change in inductance is obtained by virtue of the fact that the loop lies within the magnetic field of the resonator. <IMAGE>

Description

SPECIFICATION Electrical resonators This invention relates to electrical resonators and is specifically concerned with resonators which comprise one or more bodies of a shaped dielectric material. Resonators of this kind are usually mounted adjacent to a ground plane and are arranged to operate at very high frequencies, typically of the order of a Gigahertz or more. It is, at present, customary to operate a resonator at a single predetermined centre frequency which is determined by the shape and size of the dielectric body and the housing in which it is mounted. The present invention seeks to provide an electrical resonator which can oe readily and rapidly adjusted so as to operate at a variable resonant frequency.

According to this invention a frequency selective resonator includes a body of dielectric material which is mounted on an electrical insulator which serves to solace it apart from a ground plane; an electrical conductor having a variable inductance associated therewith located adjacent to said body but spaced aparttherefrom so as to influence the external electromagnetic field of said body; and means for receiving an electrical signal which is utilised to vary said inductance so as to adjust and control the resonant frequency of said resonator.

Typically, a frequency selective resonator which is formed out of a body of dielectric material is mounted upon one surface of an electrically insulating plate, the opposite surface of which is in contact with a ground plane. Conveniently an electrical signal is communicated to said dielectric resonator by means of a microstrip line formed upon the upper surface of said dielectric plate. The resonant frequency of said dielectric resonator is dependent on the characteristics of its external electromagnetic field. In practice the electric field is almost entirely confined to the physical surface of the resonator and the external field is largely due to the magnetic contribution.

Tuning is achieved by placing an electrical conductor adjacent to one surface of the resonator, and altering its effective inductance, preferably by varying the bias applied to a diode which is connected to said conductor. Preferably the conductor and one or more diodes from a closed electrical path around which current can circulate under the influence of the external electromagnetic field. The electrical conductor is preferably positioned adjacent to an end face of the dielectric resonator which is remote from the ground plane. Because of the presence of the ground plane the magnetic field lines tend to extend through those end faces which are parallel with the ground plane and the magnetic field can be greatly influenced by positioning the tuning means so as to intercept these field lines.The electrical conductor can be provided in several sections wnich are linked by respective diodes to form the closed electrical path.

The invention is further described by way of example with reference to the accompanying draw ing in which: Figure 1 shows a sectional perspective view of a frequency selective resonator, and Figure2 is an explanatory diagram.

Referring to Figure 1 a frequency selective resonator comprises a solid cylindrical body 1 of dielectric material which is mounted upon a microstrip circuit assembly comprising an electrically insulating alumina substrate 2, having an electrical ground plane 3 on its lower surface. An elongate electrically conductive track 4 is formed upon its upper surface and in operation a signal is applied from an external source to the track 4 to generate an electromagnetic field which couples into the body 1. Dielectric materials for this purpose are now well known, and a range of suitable materials is available. Frequently the materials are composed of ceramic mixtures containing titanium dioxide, various titanates or zirconates. The resonance frequency of the resonator is determined primarily by the physical size and shape of the body 1. These devices are further described in a Paper by J. K.Plourde, IEEE Transactions on Microwave Theory and Techiques, page 754, Volume MTT/29, No. 8 August 1981.

Although the frequency characteristics of a dielectric resonator are generally fixed by its physical shape and size, the invention permits its resonant requency to be varied over an appreciable range.

Typically, in this example, the centre frequency of resonance of the dielectric resonator is of the order of 7 GHz and the invention permits that value to be readily altered over a range of typically 50 MHz. The resonance properties of the dielectric resonator are also strongly affected by the characteristics of any conductive housing within which it is situated. In this example, the microstrip circuit is mounted on the baseplate 5 of a container having walls 6 and 7 and a top plate 8. In practice all four walls will be provided to constitute a sealed chamber having a removable lid, and in some applications more than just a single dielectric resonator may be provided within the same housing. Conveniently, the top plate 8 is detachable so that access is available to the interior of the chamber.

Electrical tuning of the dielectric resonator is achieved by the provision of two semicircularannu- lar conductive tracks 9 and 10, formed of gold, which have a diameter conforming with that of the cylindrical dielectric body 1. However the tracks are spaced apart from the top surface 11 of the dielectric material by means of a quartz spacer 12. An additional very thin electrically insulating spacer disc 13 is positioned above the quartz spacer 12 so as to support and secure the conductive tracks 9 and 10. In practice, the spacer disc 13 may not be necessary, in which case the gold tracks are bonded directly on to the quartz. Small gaps are left between the ends of the tracks 9 and 10 and these are electrically bridged by a pair of varactor diodes 14 and 15.Small recesses can be formed in the supporting spacer 13 so as to accommodate the electrical connections which are made to these varactor diodes. If the spacer 13 is not provided, the varactor diodes are mounted directly on the gold tracks, so the recesses may not be necessary. It is necessary to provide the quartz spacer so that the presence of the electrically conductive tracks and the varactor diodes do not adversely affect the resonance mode of oscillation within the dielectric body itself. A variable d.c.

voltage bias is applied to the varactor diodes via leads 16, 17 which are attached to the gold tracks 9, 10.

In operation, a high frequency electrical signal is applied to the microstrip conductor 4 from an external source, not shown, and the electromagnetic field which it produces couples into the dielectric body 1 to induce resonance. When in resonance, the body 1 produces an external electromagnetic field, but in practice the electric component of the field is almost entirely confined to the interior of the body, whilst the external magnetic component is significant. This magnetic component is believed to couple with the conductive tracks 9 and 10, to cause induced currents to flow around the closed loop which is formed with the varactor diodes 14 and 15. The conductive tracks are very thin, but are formed of gold so as to exhibit an extremely low electrical resistance.

The presence of the varactor diodes introduces an inductive component into the circuit so formed, and this inductance modifies the electromagnetic field associated with the dielectric resonator, thereby altering its resonance frequency. It has been found that the effect can be quite appreciable, and Figure 2 shows the effect on the resonance frequency by altering the d.c. bias which is applied to the two varactor diodes. It will be seen that a small reverse bias of -1 volt produces the lowest resonance frequency and that increasing the bias progressively to about 15 volts produces a corresponding increase in resonance frequency. The variation of resonance frequency with voltage bias is not linear but is steepest in the area in which the applied bias is near to zero. Typically the frequency of resonance can be altered from f0 to f1,where the difference between these two frequencies is of the order of about 50 MHz.

Although two varactor diodes 14 and 15 are illustrated in combination with two semicircular conductive tracks 9 and 10, the other configurations are possible. For example a circular annular conductor can be provided having only a single break which is bridged by one varactor diode but an extra capacitive break is needed to avoid shorting out the varactor diode. On the other hand, additional breaks can be provided as desired, each of which is bridged by an appropriate diode or other device by means of which the inductance associated with the loop can be altered and controlled.

Claims (9)

1. A frequency selective resonator including a body of dielectric material which is mounted on an electrical insulator which serves to space it apart from a ground plane; an electrical conductor having a variable inductance associated therewith located adjacent to said body but spaced apart therefrom so as to influence the external electromagnetic field of said body; and means for receiving an electrical signal which is utilised to vary said inductance so as to adjust and control the resonant frequency of said resonator.

2. A resonator as claimed in claim 1 and wherein said body of dielectric material is mounted upon one surface of an electrically insulating plate, the opposite surface of which is in contact with a ground plane.

3. A resonator as claimed in claim 2 and wherein said plate and ground plane form part of a microstrip circuit, by means of which an electrical signal is coupled to said dielectric body.

4. A resonator as claimed in claim 3 and wherein the microstrip circuit includes a conductive track formed upon that surface of said plate which supports said body, with the track being spaced apart from the body itself.

5. A resonator as claimed in claim 1 or 2 wherein said conductor is positioned adjacent to an end face of the dielectric resonator which is remote from said ground plane.

6. A resonator as claimed in claim 5 and wherein said electrical conductor is associated with a diode, the bias of which is variable so as to vary its effective inductance.

7. A resonator as claimed in claim 6 and wherein the conductor is spaced apart from the body of dielectric material by a quartz spacer.

8. A resonator as claimed in claim 5,6 or 7 and wherein said electrical conductor consists of two semicircular portions, with two varactor diodes respectively positioned to bridge the two gaps in the conductor.

9. A frequency selective resonator substantially as illustrated in and described with reference to Figure 1 of the accompanying drawing.