Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to electrical resona-
tors 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 be readily and rapidly
adjusted so as to operate at a variable resonant fre-
quency.
According to this invention a frequency
selective resonator includes 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 haviny 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 utilized to
vary said inductance so as to adjust and control the
resonant frequency of said resonator.
Typically, a frequency selective resonator
which is Eormed out of a body of dielectric material
is mounted upon one surface of an electrically in-
sulating plate, the opposite surface of which is in
contact with a ground plane. Conveniently an electri-
cal signal is communicated to said dielectric resonator
by means of a microstrip line formed upon the upper
surface of said dielectric plàte. I'he resonant fre-
quency of said dielectric resonator is dependent on
the characteristics of its external electromagnetic
field. In practice the electric field is almost
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-- 2 --
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 vary-
ing the bias applied to a diode which is connected to
said conductor. Preferably the conductor and one or
more diodes form a closed electrical path around which
current can circulate under the influence of the
external electromagnetic field. The electrical con-
ductor 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 ar~ 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. ~he electrical conductor can be provided
~0 in several sections which are lin]ced by respective diodes
to form the closed electrical path~
The invention is further described by way of
example with reference to the accompanying drawing in
which:-
Figure 1 shows a sectional perspective view
of a frequency selective resonator, and
Figure 2 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 groundplane
3 on its lower surface. An elongate electrically con- ;
ductive 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
-- 3 -
field which couples into the body 1. Dielectric
materials for this purpose are now well known, and
a range of suitable materials is available. Fre-
quently 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 Techniques, page 754, Volurne 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 frequency 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 stron~ly affected by the characteristics of any
conductive housing within which it is situated. In this
example, the microstrip circuit is mounted on the base-
plate 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 sa that
access is available to the interior of the chamber.
Electrical tuning of the dielectric resonator
is achieved by the provision of two semicircular
annular 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 di-
J3~35
electric material by means of a quartz spacer 12. Anadditional 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 yold tracks are bonded
directly on to the quartz. Small gaps are left between
the ends of the tracks 9 and 10 and these are electric-
ally 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 ~ 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 com~onent 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.
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-- 5 --
The presence of the varactor diodes intro-
duces an inductive component into the circuit so
formed, and this inductance modifies the electro-
magnetic field associated with the dielectric
resona-tor, thereby altering its resonance frequency.
It has been found that the effect can be quite
appreciable, and Figure 2 shows the e~Efect 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 fl,
where the difference between these two frequencies
is of the order of about 50 MEIz.
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.
