ASSEMBLY AND METHOD FOR COUPLING A MICROSTRIP CIRCUIT TO A CAVITY RESONATOR

Abstract: The present invention concerns an assembly and a method for coupling a microstrip circuit (3) to a cavity resonator (4), said assembly comprising a substrate plate (1), a microstrip circuit (3) fabricated on one side of said substrate plate (1), a ground plane (2) fabricated on the other side of said substrate plate (1), and a cavity resonator (4). According to the invention, the microstrip circuit (3) is coupled to the cavity resonator (4) by means of a slot (6) fabricated to the ground plane (2) and a planar radiator (7) disposed between the ground plane (2) and the cavity resonator (4). By virtue of the invention the frequency range 1...100 GHz is readily covered.

Description: Assembly and method for coupling a microstrip circuit to a cavity resonator * The present invention relates to an assembly in accordance 5 with the preamble of claim 1 for coupling a microstrip * circuit to a cavity resonator. The invention also concerns a method for coupling a microstrip circuit to a cavity resonator. 10 A cavity resonator has a structure which can be mathemati¬ cally modelled as an LC resonant circuit. The dimensions of the cavity determine its resonant frequencies, several of which are possible depending on the principal dimensions of 15 the cavity. The cavity resonator is excited by a transistor and a microstrip circuit connected to the transistor device. According to conventional technology, microstrip circuits are used in conjunction with dielectric resonators up to 20 30 GHz frequency. Above this frequency the size of the resonator at high frequencies becomes so small that its Q (quality factor) deteriorates significantly. In addition, the size of the dielectric resonator becomes so small that the reliable placement of the resonator onto the microstrip 25 circuit in mass production becomes extremely difficult. Waveguide systems operating at millimeter wavelengths typi¬ cally employ diode oscillators. These combinations are, however, clumsy and expensive. 30 Combinations of microstrip circuits with cavity resonators have been in use up to frequencies of several GHz, but in the millimeter wavelength range the typical coupling method based on a small probe antenna reaches its limits in terms t 35 of manufacturing possibilities. It is an object of the present invention to overcome the drawbacks of the above described techniques and to achieve a novel type of assembly and method for coupling a microstrip circuit to a cavity resonator. The invention is based on forming the coupling from the microstrip to the cavity resonator by means of slot made in the ground plane and a planar radiator disposed on the surface of a coupling piece made of a suitable dielectric material. More specifically, the assembly according to the invention is characterized by what is stated in the characterizing part of claim 1. Furthermore, the method according to the invention is char- acterized by what is stated in the characterizing part of claim 4. The invention provides outstanding benefits. The resonator according to the invention can be readily manufactured for frequencies in the range 1 ... 100 GHz. The upper ground plane can be omitted from the design, because the planar radiator directs the radiating field toward the cavity resonator. Selection and/or attenuation of different resonant modes is easy to attain by altering the position and dimensions of the planar radiator in respect to the cavity resonator. Further, temperature compensation of the operating frequency can be readily implemented by suitable material choice of the planar radiator substrate with a compensating temperature coefficient of the dielectric constant ε p . The invention is next examined with the help of exemplifying embodiments illustrated in the attached drawings, in which Figure 1 shows an expanded view in perspective of the coupling circuit according to the invention between a microstrip circuit and a cavity resonator. Figure 2a shows a first alternative coupling coefficient of the circuit according to the invention in a microstrip line. Figure 2b shows another alternative coupling coefficient of the circuit according to the invention in a microstrip line. Figure 3 shows in a top view the entire coupling configura¬ tion according to the invention. For the sake of clarity, the components which in reality are closely connected are in Fig. 1 drawn detached from each other. In practice the substrate plate 1 and the ground plane 2 form are bonded together into a single element using, e.g., .an adhesive. Onto the upper surface of the substrate plate 1 is formed a matching circuit 11 of the microstrip circuit 3 that matches the circuit 3 to a resonator 4. The microstrip circuit 3 is fabricated onto the substrate plate 1 using, e.g., thin-film techniques. The thickness of the microstrip is advantageously in the range 10...15 μm and strip width is typically 0.2 mm. The resonator 4 itself is located below the ground plane 2, while the ground plane 2 and the resonator 4 are separated from each other by a dielectric plate 5 which is located at a slot 6 fabricated to the ground plane 2. In this context, the dielectric plate 5 is also called the radiator substrate. The dielectric plate 5 is fixed in its place by adhesive bonding. The conductive planar radiator 7 proper is located to the that side of the dielectric plate 5 which faces the resonator 4. Thus, the dielectric plate 5 performs galvanic isolation of the planar radiator 7 from the ground plane 2. The planar radiator 7 itself has a square form, whose side length conventionally is half wavelength at the operating frequency. Therefore, the wavelength-related dimensions are determined by the operating frequency of the resonator. The vertical position of the planar radiator 7, orthogonally to the substrate plate 1, is not particularly critical. In the exemplifying embodiment, the planar radiator 7 is spaced by the thickness of the dielectric plate 5 from the ground plane 2 so as to bring it flush with the upper sur ace 10 of the cavity resonator 4. In regards to its function, the planar radiator 7 acts as a Yagi antenna which directs the energy from the microstrip circuit 3 toward the cavity resonator 4. A suitable exemplif ing dimensioning for a 39 GHz resonator could be such as given below: Thickness of substrate plate 1 0.254 mm Material of substrate plate 1 Aluminium oxide (A1 2 0 3 ) Dielectric constant ε r of substrate plate 1 9.9 Thickness of substrate plate 1 0.254 mm Cavity diameter (d) of resonator 4 6 mm Cavity height (h) of resonator 4 3 mm Material of resonator 4 Conductive, e.g. a metal such as gold or nickel alloy Length 1 of slot 6, approx. half wavelength 2.0 mm Width w of slot 6 0.3 mm Material of radiator substrate 5 PTFE Dielectric const. ε r of radiator substrate 5 2.2 Thickness of radiator substrate 5 0.5 mm Dimensions of planar radiator 7, a = b = λ/2 2.5 mm Material of planar radiator 7 Gold or copper Thickness of planar radiator 7 10...15 μm The circuit illustrated in Fig. 1 was measured with the results shown in Fig. 2a after the position of the cavity resonator 4 is offset with respect to the other elements.. The offset is made in the upper plane 10 of the cavity resonator 4. The coordinate system employed can be reely chosen; thus, the cavity resonator 4 is offset in the x- direction by 5 mm in reference to the other elements, while no offset in the y-direction was made. The frequencies of the resonance peaks were at approx. 35.8 GHz and 37.8 GHz. The same circuit illustrated in Fig. 1 was measured with the results shown in Fig. 2b when the position of the cavity resonator 4 was offset from its initial position by 1.2 mm in the y-direction, while no offset in the x-direction was made. The frequency of the resonance peak was at approx. 31.5 GHz. Fig. 3 illustrates a practical microstrip circuit for 39 GHz frequency. The diagram is drawn to scale, and a 1 mm reference line is placed to the lower left corner of the diagram. According to Fig. 3, a MESFET device 20 is configured in the microstrip circuit so that its drain is connected to a DC supply 21 via leads 22 and bonding (not shown) . Its source is correspondingly connected via a biasing resistor 23 to ground. The ground potential is provided by a plate 24, which further is connected to the ground plane behind the substrate 1. To the left of the MESFET 20 is its gate which is further bonded to a microstrip 25. The other end of the microstrip 25 is connected to ground via a 50 ohm resistor. At the cavity resonator 4, the microstrip 25 has a matching circuit 26 that matches the microstrip 25 to the cavity resonator 4. Under the matching circuit 26, a slot 6 is fabricated to the ground plane that further is covered underneath by a planar radiator (not shown) . The drain of the MESFET is connected to an output strip line 28 by way of a thin-film capacitor 27. The function of the capacitor 27 is to block the DC component. A larger-diameter resonator 4' illustrates an alternative resonator design.

Claims: WHAT IS CLAIMED IS: 1. An assembly for coupling a microstrip circuit (3) to a cavity resonator (4) , said assembly comprising - a substrate plate (1) , - a microstrip circuit (3) fabricated on one side of said substrate plate (1) , - a ground plane (2) fabricated on the other side of said substrate plate (1) , and - a cavity resonator (4) , c h a r a c t e r i z e d in that - the microstrip circuit (3) is coupled to said cavity resonator (4) by means of a slot (6) fabricated to said ground plane (2) and a planar radiator (7) disposed between said ground plane (2) and said cavity resonator (4) . 2. An assembly as defined in claim 1, c h a r a c - t e r i z e d in that said planar radiator (7) has a planar and square shape, in which the square is dimensioned as λ/2 x λ/2, where λ is the wavelength at the operating frequency of the resonator. 3. An assembly as defined in claim 1, c h a r a c ¬ t e r i z e d in that the planar radiator (7) is fabri¬ cated onto a substrate (5) of polytetrafluorethene, PTFE. 4. A method for coupling a microstrip circuit to a cavity resonator when said microstrip circuit comprises - a substrate plate (1) , - a microstrip circuit (3) fabricated on one side of said substrate plate (1) , - a ground plane (2) fabricated on the other side of said substrate plate (1) , and - a cavity resonator (4) , c h a r a c t e r i z e d in that - said microstrip circuit (3) is coupled to said cavity resonator (4) by means of a slot (6) fabricated to said ground plane (2) and a planar radiator (7) disposed between said ground plane (2) and said cavity resonator (4) .

Inventors: SCHECK HANS-OTTO, SCHECK, HANS-OTTO
Applicants: VALTION TEKNILLINEN, VALTION TEKNILLINEN TUTKIMUSKESKUS
Application year: 1992
Main classification: H01P7
IPC codes: H01P5, H01P7