Resonator having a variable resonance frequency

Abstract: A resonator having a variable resonance frequency includes a cavity enclosed by a conducting wall (25). A resonating element (21) and a conductive plate (31) are located within the cavity. A photo-conductive element (22) is connected between two points on the conductive plate (31). The resonator (21) also includes a light source (29) for irradiating the photo-conductive element (22) with light of the predetermined wavelength. In the preferred embodiment, the conductive plate (31) is circular and includes a gap, the photo-conductive element (22) connecting two points on the gap and the resonating element (21) is a cylindrical dielectric resonator element having a TE 01δ mode electromagnetic field distribution. The circular conductive plate (31) is preferably placed parallel to the top surface of the cylindrical dielectric resonator (21) substantially midway between the top surface and the inner surface of the conducting wall (25). The diameter of the circular plate is preferably greater than that of the cylindrical dielectric resonator. In one embodiment, the photo-conductive element (22) includes first and second photo-conductive regions, the first photo-conductive region connecting first and second points on the conductive plate (25) and the second photo-conductive region connecting third and fourth points on the conductive plate (25). In this embodiment, the light source includes first and second light emitting elements, for respectively illuminating said first and second photo-conductive regions. The magnitude of the change in resonance frequency induced by illuminating the photo-conductive region can be altered by adjusting the relative position of the photo-conductive element and the light source.

Description: The present invention relates to variable frequency resonator, preferably a resonator whose frequencies can be broadly and rapidly varied. Dielectric resonator elements are used in oscillators and filters as the frequency adjustment elements for narrow-band frequencies in the microwave and millimetre wave bands. Dielectric resonator elements are small in size, and have high Q values. In addition, the resonant frequency of dielectric resonators is insensitive to temperature fluctuations. Accordingly, such resonators are being used on a rapidly-increasing scale in satellite communication devices, mobile radio devices, variable-frequency oscillators, and other applications. While a dielectric resonator having a high Q value provides a stable resonance frequency, its resonance frequency cannot be widely varied electronically. Instead, its resonance frequency is varied mechanically, such as by mechanically varying the cavity size. However, mechanical adjustments change the resonance frequency slowly, and are bulky and expensive. In one form of communication referred to as Minimum Shift Keying (MSK), data is sent by using two different frequencies to represent the binary data values 0 and 1, respectively. This system requires an oscillator whose frequency can be rapidly changed. The rate at which data can be transmitted in this system depends on the difference in frequency used to transmit the two data states. Systems that allow the oscillator frequency to be shifted in response to a signal are known to the art. For example, unexamined Japanese Patent Publication No. HEI 9-205324 discloses a system in which the resonance frequency of an oscillator is altered by applying a signal to a variable capacitance element in an auxiliary transmission line. The magnetic field in the resonating element is coupled to the magnetic field of the auxiliary transmission line in this system. By altering the magnetic field, this device alters the resonant frequency. Generally, the variation in the resonance frequency is limited to about 0.1 % of the resonance frequency. Hence, when a resonator of this type is used to construct a voltage-controlled oscillator that oscillates in the 5 GHz band for MSK communications, the oscillation frequency can only be varied by about 5 MHz. This limits the data transmission rate to about 10 Mbps. However, speeds exceeding 20 Mbps are sought for 5 GHz band radio communications. Furthermore, published paper XP-002 12 60 88 presents a new method of electronic tuning, i.e. frequency modulation of dielectric resonator oscillators. The present invention seeks to provide an improved resonator, preferably a resonator whose resonance frequency can be shifted in response to an external electrical signal. According to an aspect of the present invention, there is provided a resonator as specified in claim 1. The preferred embodiment provides a resonator having a variable resonance frequency. The resonator includes a cavity enclosed by a conductive wall. A resonating element and a conductive plate are located within the cavity. A photo-conductive element is connected between two points on the conductive plate. The photo-conductive element has a first impedance when illuminated with light and a second impedance when not so illuminated, the second impedance being greater than the first impedance. The resonator also includes a light source for irradiating the photo-conductive element. In the preferred embodiment of the present invention, the conductive plate is circular and includes a gap, the photo-conductive element connecting two points on the gap. The preferred resonating element is a cylindrical dielectric resonator element having a TE 01δ mode electromagnetic field distribution, the cylindrical dielectric resonator having a cylindrical shape characterised by top and bottom surfaces and a diameter. The circular conductive plate is preferably placed parallel to the top surface of the cylindrical dielectric resonator substantially midway between the top surface and the inner surface of the top of the conducting wall. The diameter of the circular plate is preferably greater than that of the cylindrical dielectric resonator. In one embodiment of the present invention, the photo-conductive element includes first and second photo-conductive regions, the first photo-conductive region connecting first and second points on the conductive plate and the second photo-conductive region connecting third and fourth points on the conductive plate. In this embodiment, the light source includes first and second light emitting elements, for respectively illuminating said first and second photo-conductive regions. The magnitude of the change in resonance frequency induced by illuminating the photo-conductive region is altered by adjusting the relative position of the photo-conductive element and the light source. An embodiment of the present invention is described below, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a cut-away perspective view of one embodiment of resonator; Figure 2 is a longitudinal sectional view of the resonator through line AA' shown in Figure 1; Figure 3 is a perspective view of a circular conductive plate and photo-conductive element; and Figure 4 is a cross-sectional view of a second embodiment of resonator. Referring to Figures 1 and 2, resonator 20 includes a cylindrical conducting cavity defined by a conductive wall 25 which has an opening in its bottom, and a printed circuit board 23, which seals the opening and forms the bottom surface of the cylindrical cavity. The axis of the cavity is perpendicular to the surface of printed circuit board 23. Conductive wall 25 may be copper, aluminium, or brass. Microstrip line 28 on printed circuit board 23 is used for input and output of high frequency signals to the cavity. Conductive wall 25 confines the electromagnetic field, thus preventing leaks of the electromagnetic field, which, in turn, increases the Q of the resonator. Conductive wall 25 also supports the entire structure. The inner surface of the conductive wall 25 is smooth. Any irregularities in the finish are preferably small compared to the wavelength of the electromagnetic energy at the resonance frequency. The inner surface of conductive wall 25 may be electroplated to improve the smoothness of the finish. Printed circuit board 23 is a conventional printed circuit board. Printed circuit board 23 has conductors on both sides of an insulating base of ceramic or polytetrafluoroethylene. A ground plane 33 is formed on the bottom surface of printed circuit board 23. Ground plane 33 of printed circuit board 23 is electrically connected to conductive wall 25 by metallic screw 34 or by a conductive bonding agent. The interior dimensions of the cavity formed by conductive wall 25 and printed circuit board 23 are related to the height and diameter of the cylindrical dielectric resonator element 21. The diameter and height of the cavity are about three times the diameter and height of the cylindrical dielectric resonator element 21, respectively. The resonant frequency of resonator 20 is determined primarily by cylindrical dielectric resonator element 21 having a TE 01δ mode electromagnetic field distribution in the cavity. Cylindrical dielectric resonator element 21 has its axis approximately aligned to the axis of the cavity and is preferably fixedly bonded by an epoxy to the printed circuit board 23. Cylindrical dielectric resonator element 21 is constructed from a material having a high dielectric constant such as barium titanate, titanium oxide, or another ceramic oxide with a high dielectric constant. In one preferred embodiment, Ba(MgTa)O 3 is utilised. The dimensions of the cylinder are determined by the material used to construct dielectric resonator element 21 and by the resonance frequency. For a resonance frequency in the 5 GHz band, the diameter is about 10 cm, and the height is half the diameter. This embodiment utilises a circular conductive plate 31 to fine tune the resonant frequency and to switch the resonant frequency between two closely spaced values. The conductive plate is formed on a dielectric substrate 24. Dielectric substrate 24 is placed midway between the top of the dielectric resonator element 21 and the inner surface of conductive wall 25 such that the substrate surface is parallel to the top of the dielectric resonator element 21 and parallel to the inner surface of conducting wall 25 that is above dielectric substrate 24. Dielectric substrate 24 is bonded to conductive wall 25. Dielectric substrate 24 is preferably a single-layer printed circuit board of a thin ceramic insulator whose thickness is between 100 to 300 µm. A circular conductive plate 31 having substantially the same diameter as dielectric resonator element 21 is formed on the top surface dielectric substrate 24 by patterning the conductive layer of the substrate. The centre of circular conductive plate 31 lies on the axis of the cavity. Circular conductive plate 31 has a radial slit 30 directed from the centre towards the edge. The thickness of circular conductive plate 31 is 20 µm for a resonance frequency of 5 GHz, but a plate with a thickness from 15 µm to 30 µm will also function adequately. The thickness of conductive circular plate 31 is proportional to the wavelength of an electromagnetic signal at the resonant frequency. At a resonance frequency of 60 GHz, a plate having a thickness of 1 to 3 µm is utilised. A photo-conductive element 22 extending across the slit is mounted on the outer edge of the circular conductive plate 31. A more detailed view of circular conductive plate 31 is provided in Figure 3 which is a prospective view of circular conductive plate 31 and photo-conductive element 22. For a resonance frequency in the 5 GHz band, the width of the slit is about 1 mm and can be expanded or reduced in proportion to the resonance wavelength. Photo-conductive element 22 is preferably a p-i-n diode. Since any electrical resistance introduced by photo-conductive element 22 when it is in the conducting state will decrease the Q of the cavity, the preferred conductive element has a resistance in the conducting state of around 1 ohm or less. The terminals 22A and 22B of the p-i-n diode are electrically connected to the circular conductive plate 31 on opposite sides of slit 30. The photo-conductive element 22 functions to short-circuit the slit 30 when the photo-conductive element is in the conducting state. Returning now to Figures 1 and 2, photo-conductive element 22 is activated by a light source 27 that preferably includes a semiconductor laser 29 and a lens 26. The semiconductor laser should preferably be a surface-emitting semiconductor laser. Laser light emitted from the light source 27 is focused and adjusted by the lens 26, and is input into the photo-conductive element 22. At a higher frequency such as 60 GHz, the distance between the semiconductor laser 26 and the photo-conductive element 22 is reduced because the dimensions of the cavity are smaller. In such cases, the lens can be omitted. The light source 27 is energised by a signal applied to leads 45 from a transmission apparatus. To simplify the drawing, the transmission apparatus is not shown. In the preferred embodiment of the present invention, light source 27 extends through a hole in conductive wall 25. The diameter of circular conductive plate 31 is preferably the same or slightly larger than the diameter of cylindrical dielectric resonator element 21. The slit 30 extends from approximately the centre of circular conductive plate 31 to the edge of the plate. On the outside of the circular conductive plate 31, the terminals 22A and 22B of photo-conductive element 22 are connected to the ends 31A and 31B of the vertical wall of the circular conductive plate 31 on both sides of the slit 30. The position of light source 27 is adjusted such that the irradiation spot generated by light source 27 hits light receptor 22C of photo-conductive element 22. The irradiation pattern must be sufficiently broad to assure that the photo-conductive element shorts the edges of the slit together when irradiated. The precise radial position at which the short occurs can be adjusted by altering the position of light source 27 relative to photo-conductive element 22. The position of the short determines the shift in resonance frequency of resonator 20 when light source 27 is irradiating photo-conductive element 22. The manner in which the resonance frequency of resonator 20 is modulated by the signal to light source 27 will now be explained in more detail The resonating electrical field is confined to a cross-sectional surface inside dielectric resonator element 21, and has a concentrated doughnut-shaped distribution. When conductive plate 31 is placed parallel and close to the top of dielectric resonator element 21 on the centre axis of dielectric resonator element 21 and the photo-conductive element 22 is not irradiated, a concentrated portion of an electrical field also forms in the slit 30. Intermittent emission of laser light can turn the photo-conductive element 22 on and off and bring about wide variations in the distribution of the electrical field concentrated in slit 30. The variations in the electrical field distribution change the effective electrical length of the resonator element thereby causing the resonance frequency to change. When light source 27 is off, slit 30 is substantially open at the circumferential edge of circular conductive plate 31, and the electrical field is primarily concentrated on the outside of the cylindrical conductive plate surface around the circumference. In this configuration, the electrical field at slit 30 is parallel to, and is oriented in the opposite direction from, the electrical field of dielectric resonator element 21. This is equivalent to reducing the portion of the electrical field distribution in resonator element 21, which has the same effect as reducing the size of resonator element 21 which, in turn, increases the resonance frequency. When light source 27 is on, slit 30 can be regarded as having a short-circuit along the circumference of circular conductive plate 31. In this case, the electrical field in the centre part of slit 30 is parallel to, and is oriented in the same direction as, the electrical field in the dielectric resonator element 21. This is equivalent to increasing the portion of the electrical field distribution in the resonator element 21, which has the same effect as increasing the size of resonator element 21 which, in turn, decreases the resonance frequency. It should be noted that the coupling between the electric field in slit 30 and resonating element 21 is much higher than the magnetic coupling between an auxiliary transmission line and a resonating element used in the prior art device discussed above. This can provide a much greater variation in the resonance frequency than such prior art devices. The resonator has two calibrations, one to set the resonant frequency when light source 27 is turned off and one to set the shift in resonant frequency induced when light source 27 is turned on. The first calibration is performed by varying the position of printed circuit board 23 relative to conductive wall 25 while the resonance state is monitored from the microstrip line 28. When the desired resonance frequency is obtained, the frequency variation is optimised by turning on the light source and varying the irradiation position while monitoring the altered resonance frequency. Once the correct position is determined, light source 27 is fixed in position by a suitable bonding agent or mechanical mechanism such as a set-screw. The adjustment of printed circuit board 24 can be facilitated by separating conductive wall 25 into two parts as shown in Figure 4. Figure 4 is a cross-sectional view of a second embodiment of resonator. To simplify the following discussion, elements that perform the same function in Figure 4 as elements shown in Figures 1-3 have been given the same numeric designations. Resonator 30 shown in Figure 4 differs from resonator 20 shown in Figures 1-3 in that the conductive wall of resonator 20 has been replaced by a two piece wall 25'. The two parts are screwed together with the aid of the mating threaded regions shown at 35 at the position where the dielectric substrate 24' is to be located. Dielectric substrate 24' is held between the two parts. In addition, dielectric substrate 24' differs from dielectric substrate 24 shown in Figures 1-3 in that a portion of the dielectric substrate 24' projects into conducting wall 25'. In addition, the slot in wall 25' that engages dielectric substrate 24' includes enough space 26 to allow dielectric substrate 24' to move relative to dielectric resonator 21. Hence, during calibration, dielectric substrate 24' can be more easily positioned with respect to conductive wall 25'. The above-described embodiments utilise a single photo-conductive element and light source. This arrangement provides a means for modulating the resonance frequency between two values. However, embodiments in which the resonance frequency is modulated between more than two values can also be constructed. In such embodiments, a plurality of photo-conductive elements are installed along the slit, and the resonance frequency is modulated with multiple values by switching the appropriate photo-conductive elements on and off with the aid of a plurality of corresponding light sources. It should be noted that the plurality of photo-conductive elements described above can be replaced by a single continuous photo-conductive element covering the appropriate portion of the slit. The above-described embodiments utilise a p-i-n diode as the photo-conductive element. However, it will be evident to those skilled in the art from the preceding discussion that any photo-conductive element that can be switched between a conducting and insulating state can be utilised.

Claims: A resonator (20) having a variable resonance frequency, said resonator comprising: a cavity enclosed by a conductive wall (25); a resonator element (21) and a conductive plate (31) located in said cavity, said conductive plate comprising a gap (30); the resonator (20) being characterized in that the conductive plate further comprises a photo-conductive element (22) connected between two points on said gap, said photo-conductive element having a first impedance when illuminated with light and a second impedance when not so illuminated, said second impedance being greater than said first impedance; and a light source (20) for irradiating said photo-conductive element with light. A resonator as in claim 1, wherein the relative position of said photo-conductive element and said light source is adjustable. A resonator as in claim 1 or 2, wherein said resonator element comprises a cylindrical dielectric resonator element having a TE 01δ mode electromagnetic field distribution, said cylindrical dielectric resonator having a cylindrical shape. A resonator as in any preceding claim, wherein said conductive plate is disposed parallel to said top surface of said dielectric resonator element substantially midway between said top surface and an inner surface of said conducting wall. A resonator as in claim 4, wherein said conductive plate is a substantially circular metal plate having a diameter larger than said diameter of said dielectric resonator element. A resonator as in any preceding claim, wherein said photo-conductive element comprises first and second photo-conductive regions, said first photo-conductive region connecting first and second points on said conductive plate and said second photo-conductive region connecting third and fourth points on said conductive plate. A resonator as in any preceding claim, wherein said light source comprises a plurality of light emitting elements, each light emitting element illuminating a different location of said conductive plate. A resonator as in claim 1, wherein said conducting wall comprises first and second separable sections that can be secured to one another, wherein said first and second separable sections form a region for engaging a dielectric sheet on which said conductive plate is mounted, wherein said region is sufficiently large to allow said dielectric sheet to move relative to said resonating element when said separable sections are not secured to one another, and wherein said dielectric sheet is fixed relative to said resonating element when said sections are secured to one another.

Inventors: TAKENAKA TSUTOMU, TAKENAKA, TSUTOMU
Applicants: AGILENT TECHNOLOGIES INC, AGILENT TECHNOLOGIES, INC. (A DELAWARE CORPORATION)
Application year: 1999
Main classification: H01P7
IPC codes: H03C7, H03B5, H01P7, H01P1