GB2153598A - Microwave resonator device - Google Patents

Abstract

A microwave device includes a dielectric e.g. ceramic resonator 10 coupled to a signal path 11 and to a control path 12 which contains a varactor diode 15 to vary the frequency at which resonance occurs. The control path preferably includes a return port 13, and a low pass filter 14 to prevent microwave signals reaching the control means. The device is implemented as microstrips on a dielectric substrate. Resonator control is achieved entirely by a control voltage applied to input port 16. The device is described for use in a waveguide filter and a frequency discriminator. <IMAGE>

Description

SPECIFICATION Microwave resonator The invention relates to a microwave resonator which can be tuned electronically, e.g. by means of a control voltage.

Microwave devices are usually implemented as wave guides together with active elements and the device operates at a frequency dependent on its dimensions. Since the dimensions change with temperature the frequency of the device is affected by temperature. An important exception is a dielectric resonator, e.g. a ceramic resonator, and therefore, such resonators are sometimes used in microwave devices. Thus dielectric resonators are utilised as the frequency standard in a frequency disciminator such as a frequency demodulator and also as the standard in a feed-back loop to control a voltage controlled oscillator.

There is a disadvantage in that dielectric resonators are difficult to control and this invention relates to electrical control of a dielectric resonator.

A microwave device, e.g. a microstrip device, according to the invention includes a ceramic resonator coupled to a signal path to provide resonance therein and also coupled to a control path which contains a varactor diode to vary the frequency at which resonance occurs. The control path, which conveniently includes a return port, is adapted to receive a control voltage from a controller and a low pass filter is conveniently included in the control path to prevent high frequency signals interference with the controller. The controller may be a simple manual device such as a potentiometer or, preferably, it may be an automatic control circuit, e.g. an analogue or digital computer.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a diagram showing the obverse side of a microstrip device according to the invention.

Figure 2 is a diagram showing the device of Figure 1 as the control element of a dielectric filter; and Figure 3 is a diagram showing the device of Figure 1 as the control element of a dielectric frequency discriminator.

The microstrip device illustrated in Figure 1 comprises a dielectric substrate, e.g. alumina or glass filled polytetrafluoroethylene, having conductive layers, e.g. copper layers or gold layers, on both surfaces. As is well known in the art the dimensions are related to the frequency band for which the device is intended. Figure 1 illustrates only that part of the device embodying the inventive feature (i.e. it omits connections to neighbouring devices) and in this region the reverse is a complete conductive layer so it is sufficient to show the obverse.

On the obverse the substrate supports a ceramic resonator 10 which is located between conductive strips 11 and 12. Strip 11 provides a path for a microwave signal whereas strip 12 constitutes a coupling element for a control voltage. The distances are such that there is interaction between the resonator 10 and the fields originating from the vicinity of the strips 11 and 12.

In addition to the coupling element 12, the control path of Figure 1 is constituted by low pass filter 14 which is connected to the coupling element 12 by varactor diode 15; the coupling element 12 is short-circuited at one end by area 13 and the filter 14 provides an input port for a control voltage.

The varactor diode 15 and the resonator are fixed to and supported by the substrate, e.g. by a low-loss adhesive. The device is made by conventional methods. For example the starting point comprises the substrate coated on both sides with metal and the unwanted metal is etched away to leave the desired pattern. The resonator 10 is attached e.g. by bonding and the varactor diode 15 is connected to filter 15 and coupling element 12.

The connection is by soldering, heat bonding or a conductive epoxy resin.

In the use of the device a microwave signal passes principally in the substrate between the strip 11 on the obverse and the (continuous) conductive area on the reverse. The magnetic field associated with this signal extends beyond the strip 11 and resonator 10 is subjected to the variations of the field whereby conventional resonance effects occur. In addition the resonator 10 is also subjected to a magnetic field from the coupling element 12. The resonance is affected by the electrical state of the coupling element 12 and therefore, altering the state of the coupling element 12 alters the resonance. Thus changing the electrical state of the coupling element 12 provides a means of controlling the resonance frequency.

The electrical state of the coupling element 12 can be altered by a voltage signal applied to varactor diode 15 via low pass filter 14 and port 16. The low pass filter 14 prevents high frequency signals derived from the microwave reaching port 16 and the control unit.

The arrangement shown in Figure 1 is applicable in most environments where dielectric resonators are used. Instead of a device which is fixed in frequency the arrangement provides simple and convenient adjustment of the frequency. Two examples of this will now be described.

Figure 2 illustrates a microstrip device suitable for insertion into a wave guide to act as a filter.

The device comprises the arrangement shown in Figure 1 together with input and output paths adapted to transfer the wave from and to the wave guide. In Figure 2 the edge of the reverse layer is shown by a dotted line whereas the obverse layer is shown by continuous lines and the parts shown in Figure 1 are repeated with the same reference numerals. It should be noted, as stated with reference to Figure 1, that the reverse layer is continuous behind these parts.

To form the input path the strip 11 extends to an edge region 20 having an operative border 21. The reverse layer has an operative border 22 and these two borders cross at 23 leaving a region 24 where both obverse and reverse layers are missing. The output path is similar to the input path being formed by an edge region 25 with an operative border 26. The reverse layer has an operative border 27. The two borders cross at 28 ieaving a region 29 where both obverse and reverse layers are missing. This arrangement forms a transition suitable for passing microwaves between a waveguide and the strip 11 and will be known to those skilled in the art.

When mounted for use the device is clamped inside a chamber (the wall of which is indicated as 32) which can be regarded as an augmentation to a wave guide. The input wave guide 30 meets the microstrip at the border 21 and 22; the borders 26 and 27 meet the output wave guide 31. The lead 33 passes through an insulating plug 34 to provide an external connection for a control voltage. (Area 13 is clamped into the wall 32 to provide a return path for the control voltage).

During the use of the device microwave energy travelling principally in wave guide 30 arrives at the edge of the device where it converts to a wave travelling in the substrate between borders 21 and 22. This mode continues until the wave reaches crossing 23 where it continues principally in the substrate between the obverse and reverse layers so that the path is defined by strip 11. Thus the wave passes the resonator 10 where it is modified as described with reference to Figure 1. The filtered wave therefore continues to crossing 28 where it changes its mode to propagation between borders 26 and 27 until it is released into wave guide 31.

The device is a conventional filter but the filter frequency is easily controlled by varying the voltage on lead 33.

Figure 3 shows the control system according to the invention applied to a microwave discriminator as published in our corresponding European Patent Application 91201A1. As described in the publication the disciminator has a mixer 41 with two par allel input paths 12 and 40, one 12 of which includes a dielectric resonator 10. After filtering out a microwave component in filter 42, the output of the mixer 41 is a low frequency signal which measures the discrepancy between the frequency of the input and a standard frequency set by the resonator. This output is available at port 43. The discriminator can be used to stabilise an oscillator using the low frequency output as negative feedback. It can also be used as a frequency demodulator.

Figure 3 shows the published discriminator modified by the addition of a control as shown in Figure 1. The components are numbered as in Figure 1 and 2 and the function is the same. The standard frequency of the discriminator is therefore adjustable by a voltage control applied to port 33.

The device according to the invention is simple to produce requiring only the etching of paths and the mounting of two components in the control path. The control of the device is entirely electrical.

The resonator (shown as 10 in all Figures) is preferably a ceramic based on barium oxide and titanium oxide e.g. with mole ratio (BaO) : (TiO2) = 2 : 9. Ceramics based on titanium oxide, zinconium oxide, tin oxide and other substances are also suitable.

Claims (5)

1. A microwave device implemented in microstrip form which device includes a dielectric resonator coupled to a signal path to provide resonance therein and also to a control path which contains a varactor diode to vary the frequencies at which resonance occurs wherein all of the said paths are defined by conductive regions on both sides of a dielectric substrate.

2. A microwave device according to claim 1, wherein the control path includes a low pass filter.

3. A microwave device according to claim 2, wherein the control path comprises a coupling element adjacent the resonator said coupling element being connected (a) to the low pass filter via the varactor diode and (b) to a return path.

4. A microwave device according to claim 3 in which the low pass filter has a port for receiving a control voltage.

5. A microwave device according to any one of the preceding claims in which the dielectric resonator is a ceramic resonator based on a titanium oxide.