MICROWAVE FILTER

Abstract: A microwave filter comprises a body (10) having a plurality of resonant cavities (e.g. 11 to 16) formed therein, the cavities being open to one surface of the body (10), and a lid (44) disposed over that surface of the body to close the cavities. The cavities (e.g. 11 t0 16) are arranged in a series with each pair of adjacent cavities physically separated from one another by a wall (40) and coupled together by an aperture (41) in that wall; the wall (40) terminates short of the lid (44), with the result that the formation of passive intermodulation products is substantially reduced.

Description: This invention relates to a microwave filter fo filtering input signals to within a predetermined frequenc band. One known type of microwave filter is the so-calle comb-line filter which comprises a plurality of resonan cavities coupled together in a series, each resonant cavit having a central element or resonator. Such a filter i normally considered to be a passive linear device. However, at high power levels, non-linear effects become apparent. A a result, signals having frequencies in the filter's frequenc passband may mix to form inter odulation products (passiv intermodulation products) . Two comb-line filters may be forme together in a single structure, to form a diplexer: one filte acts as a transmission filter and the other as a receiv filter. Depending upon the particular wavebands of the tw filters, the frequency of the intermodulation products of th transmission filter may fall within the waveband of the receiv filter. According to a first aspect of this invention, there i provided a microwave filter comprising a body having plurality of resonant cavities formed therein, the cavitie being open to one surface of said body, and a lid disposed ove said surface of the body to close said cavities, the cavitie being arranged in a series with each adjacent pair of cavitie physically separated from one another by a wall and couple together by an aperture in that wall, the wall terminatin short of said lid. We have found that by terminating the separating wall short of the lid, the formation of passive intermodulatio products is substantially reduced. This is due to avoidin contact between two components, the body and the lid, at region of high current density. Preferably the body of the microwave filter is forme of a single piece of material including an upstanding elemen or resonator projecting from the base of each resonant cavity. If it is not possible to form the resonators integrally wit the body (e.g. if it is necessary to use different metals) , then each resonator is fixed in intimate contact with the bas of its resonant cavity. By forming the resonators integral with or in intimate contact with the body component, then imperfections at the junction between different components are avoided or minimised at regions of high current density: accordingly the passive intermodulation product is further reduced. According to a second aspect of this invention, there is provided a microwave filter comprising a body having at least one resonant cavity formed therein, with an element or resonator projecting into the cavity and formed integrally with or in intimate contact with said body. Preferably at least the internal surfaces of the cavities are formed with a layer of silver which has a thickness of at least three times the maximum skin depth for the frequencies passed by the filter: typically the silver layer is at least 8 microns thick. With such a relatively thick layer of silver, it is ensured that all current is carried within the silver layer. Preferably the non-adjacent cavities, at one end of the series of cavities, are coupled directly so as to form an attenuation pole at one edge of the filter's passband. As a result, a required level of signal rejection adjacent that edge is achieved with a minimum number of resonant cavities in the series, for the required width of the passband: this minimises the bandwidth of the highest Q resonator and therefore the current density and hence the passive intermodulation products. A diplexer may comprise two filters in accordance with the invention, one a transmission filter and the second a receive filter. The diplexer may be used for example in a cellular radio base station, typically with the transmission filter having a passband of 935 to 960 MHz and the receive filter a passband of 890 to 915 MHz. In general however, a multiplexer may be provided, comprising any desired number of filters each in accordance with this invention. An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings in which: FIGURE 1 is a block diagram of a diplexer in accordance with the invention; FIGURE 2 is a plan view of the diplexer shown with it lid removed; FIGURE 3 is a cross-section through the diplexer on th line III-III of Figure 2; and FIGURE 4 is a cross-section through the diplexer on th line IV-IV of Figure 2. Referring to Figure 1, there is shown a diplexer whic may be used in cellular radio system, for example. Th diplexer has a common terminal connected by a coaxial cable t a transmit/receive antenna 1, and a receive filter Fl and transmission filter F2 both connected to the common terminal and having respective terminals connected by coaxial cables t receiver and transmitter systems. Receive filter Fl has waveband of 890 to 915 MHz and transmission filter F2 has waveband of 935 to 960 MHz, in the example shown. Referring to Figure 2, the diplexer comprises a singl body 10 in which the two filters Fl and F2 are formed. Bod 10 is rectangular in shape in the example shown. Each filte comprises a series of n cylindrical cavities 11 to 16 and 2 to 26 having their axes parallel to each other, the cavitie being open to a top surface of the body 10: in the exampl shown n=6. Each cavity has a coaxial, cylindrical elemen 30 upstanding from its base to form a resonator for th respective resonant cavity. The common coaxial terminal 31 ha its centre pin connected via conductors 32,33 to the resonator 30 of the first resonant cavities 11,21 of the respectiv filters F1,F2. The resonators of the last resonant cavit 16,26 of the respective filters are connected by conductor 34,35 to the centre pin of coaxial terminals 36,37. In each filter, adjacent cylindrical resonant cavitie e.g. 12,13 are physically separated by a thin wall 40, but ar inductively coupled together through a rectangular aperture 4 formed in the centre of this wall and extending towards th base of the cavities from the top surface of the body 10. However, the remaining portions of the wall 40, either side o the coupling aperture or iris 41, terminate short of the to surface of the body 10, as shown in Figures 3 and 4. It wil be noted that a portion 42 of the wall 40 remains at the bas of the cavities. The portions of the wall with upper surfaces short of the top surface of the body 10 are indicated by W: each of these lies between the adjacent edge of the iris 41 and a line e.g. L which is tangential to the two adjacent cavities. A flat lid 44 is fixed against the top surface of the body 10 to close the resonant cavities. The upper surfaces W of the various walls 40 accordingly stop short of the lid 44. The resonators 30 also stop short of the lid 44, and are formed with a coaxial blind bore 45: tuning screws (not shown)are threaded through the lid and project into the respective bores 45. It will be noted that the resonators 30 are formed integrally with the body 10. The body 10 may be is cast in one-piece from e.g. aluminium. At least the internal surfaces of the resonant cavities are given a relatively thick layer of silver, to a thickness at least 3 times the maximum skin depth for frequencies within the respective waveband (and preferably at least 8 microns) . In the receive filter Fl, the fourth to sixth cavities 14 to 16 are arranged in a triangle and cavities 14 and 16 are inductively coupled together by a rectangular aperture or iris 46 formed in the wall which separates them. This coupling provides an attenuation pole at the higher-frequency side of the receive waveband. In the transmission filter F2, the fourth to sixth cavities 24 to 26 are also arranged in a triangle: a notch 48 is formed in the top of the wall between cavities 24 and 26, and a member (not shown) fits into this notch, the member having a pin projecting into each cavity to provide capacitive coupling between them. This provides an attenuation pole at the lower-frequency side of the transmission waveband. The diplexer which is shown in the drawings exhibits minimal passive intermodulation products, and hence minimal intermodulation distortion, for the reasons explained above. Whilst the drawings show a diplexer, in general a multiplexer may be provided, having any desired number of filters each in accordance with the inventions and having a common terminal: this is represented in Figure 1 by dotted lines to indicate the additional filters Fn which would then be included.

Claims: CLAIMS (1) A microwave filter comprising a body (10) having plurality of resonant cavities (e.g. 11 to 16)_ formed therein, the cavities being open to one surface of said body (10) , an a lid (44) disposed over said surface of the body to close sai cavities, the cavities being arranged in a series with eac pair of adjacent cavities physically separated from one anothe by a wall (40) and coupled together by an aperture (41) in tha wall, the wall (40) terminating short of the lid. (2) A microwave filter as claimed in claim 1, in which two non-adjacent cavities (e.g. 14,16), at one end of the series of cavities, are coupled directly to form an attenuation pole at one edge of the filter's passband. (3) A microwave filter as claimed in claim 2, in which three cavities (e.g. 14,15,16), at one end of the series of cavities, are arranged in a triangle. (4) A microwave filter as claimed in claim 3, in which the end and end-but-one cavities (16,14) are inductively coupled by an aperture formed in a wall which separates them. (5) A microwave filter as claimed in claim 3, in which the end and end-but-one cavities (26,24) are capacitively coupled by a probe device which projects into both of them. (6) A microwave filter as claimed in any preceding claim, having an upstanding resonator element (30) projecting from the base of each cavity. (7) A microwave filter as claimed in any preceding claim, in which the internal surfaces of the cavities are formed with a layer of silver or other precious metal. (8) A microwave filter comprising a body (10) having at least one resonant cavity formed therein, with a resonator element (30) projecting into the cavity and formed integrally with or in intimate contact with said body.

Inventors: SLEIGH PHILIP DAVID, SLEIGH, PHILIP, DAVID
Applicants: FILTRONICS COMPONENTS, FILTRONIC COMPONENTS LIMITED
Application year: 1992
Main classification: H01P1
IPC codes: H01P1