TRIPLE MODE MICROWAVE FILTER

Abstract: A triple mode microwave filter comprises first and second resonant cavities (10, 20) coupled by an iris arrangement (6, 7) which couples only a single resonant mode of the first cavity to only a single resonant mode of the second cavity. The attenuation poles of the filter can be selected independently of each other: the bandpass has six poles and the filter is capable of an elliptic-function response.

Description: This invention relates to a triple mode microwave filter, the attenuation poles of which can be selected independently of each other. Microwave bandpass filters are known which comprise two resonant cavities, each supporting three resonant modes and the two cavities being coupled together. More particularly, the coupling between the two cavities has been via two or three pairs of modes and by selecting the parameters of the cavities (e.g. size and arrangement of coupling irises) to predetermine one of the attenuation poles, the other attenuation poles of the bandpass are predetermined in consequence. We have now devised a triple mode microwave filter which enables its attenuation poles to be selected or determined independently of each other. In accordance with this invention there is provided a triple mode microwave filter which comprises first and second resonant cavities coupled by an iris arrangement which couples a single resonant mode of the first cavity to a single resonant mode of the second cavity. An embodiment of this invention will now be described by way of example only and with reference to the accompanying drawings, in which: FIGURE 1 is a longitudinal section through a microwave filter in accordance with the invention; FIGURE 2 is a view of one end of the filter; FIGURE 3 is a view of the opposite end of the filter; and FIGURE 4 is a plan view of a diaphragm with the coupling iris arrangement of the filter. Referring to the drawings, there is shown a triple mode microwave filter having first and second cavities each of which supports three resonant modes, with only a single mode of one cavity coupled to a single mode of the other cavity: as a result we have found that the attenuation poles of the filter can be selected independently of each other. The bandpass has six poles and the filter is capable of an elliptic-function response. The filter comprises two cylindrical members l, 2 each with a closed end, so that each member is cup-shaped. The two members are fixed together with their open ends facing each other, but with a circular diaphragm 3 interposed, by means of bolts passed through co-operating annular flanges 4, 5 of the members 1, 2. The two members l, 2 thus define two cylindrical resonant cavities 10, 20. The diaphragm 3 is formed with two slots 6, 7 on a common diameter and equidistant from the centre (i.e. axis of the two cavities). These slots provide an iris arrangement for coupling a single mode of one cavity to a single mode of the other, as will be described below. The first cavity 10 has an input probe 11 connectable to a coaxial cable carrying the input signal. This couples into a first TE U1 resonant mode in cavity 10, which in turn couples to a TM 010 mode within the same cavity, which in turn couples to a second TE IH mode (orthogonal to the first) within that cavity. The latter mode, with its electric vector parallel to the diameter on which the slots 6, 7 are positioned, couples into the second cavity to support a first TE U1 mode, which in turn couples to a TM 010 mode within that cavity, which in turn couples to a second TE, U mode in that cavity (orthogonal to the first such mode in that cavity. Finally, the last-mentioned mode couples to an output probe 21 which is connectable to a coaxial cable to carry the output signal. The input and output probes 11, 21 are aligned longitudinally on one side of the structure, at one end of the diameter on which slots 6, 7 are positioned, and each at the mid-height of its respective cavity. Each cavity has three tuning screws to control the resonant frequency of its respective resonant modes: each such screw is positioned in a direction parallel to the maximum electrical field strength of the mode which it controls. Thus, cavity 10 has a first tuning screw 12 in its side wall diametrically opposite the input probe 11 and at the mid-height of the cavity, a second tuning screw 13 in the middle of its end wall, and a third turning screw 14 in its side wall at its mid height and at 90° to the first screw 12. Similarly, cavity 20 has a first tuning screw 22 in its side wall at its mid- height and at 90° to the diameter on which the slots 6, 7 are positioned, a second tuning screw 23 in the middle of its end wall, and a third tuning screw 24 in its side wall at its mid- height and diametrically opposite the output probe 21. Each cavity has three additional screws to control the coupling. between its resonant modes. These screws are shown at 15, 16, 17 for cavity 10 and at 25, 26, 27 for cavity 20, and are positioned at the junction between the side wall and end wall of each cavity in a direction 45° to the axis of the cavity. For cavity 10, screw 15 lies in the same plane as the slots 6, 7 and controls the coupling between the first and second modes of that cavity , screw 16 lies at 90° to screw 15 and controls the coupling between the second and third modes, and screw 17 lies at 45° to screw 16 and controls coupling between the first and third modes. For cavity 20, screw 25 lies at 90° to the plane containing slots 6, 7 and controls the coupling between the first and second modes of that cavity, screw 26 lies in the plane of slots 6, 7 and controls the coupling between the second and third modes of the cavity, and screw 27 lies at 45° to screw 25 and controls the coupling between the first and third modes of the cavity. Each of the tuning and coupling screws 12 to 17 and 22 to 27 is threaded through the wall of the structure to project a short distance into the cavity (e.g. 2-3mm) , the precise distance being adjusted to achieve the required tuning or coupling.

Claims: CLAIMS (1) A triple mode microwave filter which comprises first and second resonant cavities (10,20) coupled by an iris arrangement (6,7) which couples a single resonant mode of the first cavity to a single resonant mode of the second cavity. (2) A triple mode microwave filter as claimed in claim 1, in which the first and second resonant cavities comprise cylindrical cavities (10,20) aligned on a common axis and separated by a diaphragm (3) in which said iris arrangement (6,7) is formed. (3) A triple mode microwave filter as claimed in claim 2, in which the first resonant cavity (10) includes an input probe (11) arranged to couple into a first TE 1M resonant mode in the first resonant cavity, said first TE,,, resonant mode coupling into a TM 010 mode within the same cavity, and said TM 0I0 mode coupling into a second TE ιn mode within the same cavity. (4) A triple mode microwave filter as claimed in claim 3, in which the iris arrangement (6,7) is arranged to couple said second TE n , mode in the first resonant cavity (10) with a first TE mode in the second cavity (20) , which TE U1 mode couples to a TM 010 mode within the same cavity (20) , which TM 010 mode couples to a second TE 1U mode in the same cavity (20) . (5) A triple mode microwave filter as claimed in claim 3 or 4, in which the second resonant cavity (20) has an output probe (21) , said input and output probes (11,21) being longitudinally aligned and positioned substantially mid-way between opposite ends of the respective cavities (10,20). (6) A triple mode microwave filter as claimed in any preceding claim, in which each resonant cavity is provided with a set of tuning screws (12 to 14, 22 to 24) to control the resonant frequency of its respective resonant modes. (7) A triple mode microwave filter as claimed in any preceding claim, in which each resonant cavity is provided wit a further set of screws (15 to 17, 25 to 27) to control th coupling between respective pairs of its resonant modes.

Inventors: POPPLETON DAVID MARTIN, RHODES JOHN DAVID, POPPLETON, DAVID, MARTIN, RHODES, JOHN, DAVID
Applicants: FILTRONICS COMPONENTS, FILTRONIC COMPONENTS LIMITED
Application year: 1992
Main classification: H01P1
IPC codes: H01P1