GB2269704A

GB2269704A - Microwave filter

Info

Publication number: GB2269704A
Application number: GB9217470A
Authority: GB
Inventors: Philip David Sleigh
Original assignee: Filtronic Components Ltd
Current assignee: Teledyne Defence Ltd
Priority date: 1992-08-15
Filing date: 1992-08-15
Publication date: 1994-02-16
Also published as: GB9217470D0; WO1994005056A1

Abstract

A microwave filter having a narrow bandstop within its passband, comprises a first set of resonant cavities (21 to 28) coupled together in series between an input (IN) and an output (OUT), to form a bandpass filter, and a second set of resonant cavities (31 to 38) coupled in shunt manner to respective cavities of the first set, to define the bandstop. <IMAGE>

Description

Microwave Filter This invention relates to a microwave filter, and more particularly to a microwave bandpass filter having a relatively narrowbandstop within its passband.

Hitherto in order to provide a microwave filter having a broad passband which includes a narrow bandstop, a bandpass filter and a bandstop filter have been connected in series.

A conventional microwave bandpass filter comprises a plurality of resonant cavities coupled together in series, typically with a small amount of coupling between non-adjacent pairs of cavities. A conventional microwave bandstop filter comprises a transmission line to which a plurality of resonant cavities are coupled at intervals: these cavities attenuate, over a frequency range defining the bandstop, the signal passing along the transmission line.

We have now devised a microwave filter having a relatively narrow bandstop within its passband, this filter being more compact and having improved characteristics relative to the above-described series combination of bandpass and bandstop filters.

In accordance with this invention, there is provided a microwave filter which comprises a first set of resonant cavities coupled together in series, and a second set of resonant cavities, the cavities of said second set being coupled in a shunt manner to respective cavities of the first set.

It will be appreciated that the first set of cavities, being coupled together in series, form a bandpass filter, and the second set of cavities in combination define the bandstop.

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 diagram of a conventional bandpass microwave filter; FIGURE 2 is a diagram of a conventional bandstop microwave filter; FIGURE 3 is a diagram showing the conventional series arrangement of a bandpass filter of Figure 1 and a bandstop filter of Figure 2; FIGURE 4 is a diagram showing an integrated bandpass/bandstop filter in accordance with this invention; FIGURE 5 is a plan view of a portion of a filter in accordance with this invention, shown with its lid removed; and FIGURE 6 is a section on the line VI - VI of Figure 5, with the lid fitted in place.

Referring to Figure 1, a conventional microwave bandpass filter comprises a plurality of resonant cavities 1 to 8 coupled together in series between an input IN and an output OUT. In the example shown, the cavities 1 to 4 and 5 to 8 are arranged in two groups to enable a small amount of coupling between cavities 1 and 4 and between cavities 5 and 8 (these two pairs of cavities being non-adjacent on the series path 1 to 8). Figure 2 shows a conventional microwave bandstop filter, comprising a transmission line 10 and a plurality of resonant cavities 11 to 18 which are coupled to the transmission line 10 at intervals along the latter. Figure 3 shows the conventional series connection of the bandpass filter BPF of Figure 1 and the bandstop filter BSF of Figure 2, to form a filter having a broad passband with a relatively narrow bandstop.

Figure 4 shows a microwave filter in accordance with this invention, comprising a first set of resonant cavities 21 to 28 coupled together in series between an input IN and an output OUT. Cavities 21 to 24 and cavities 25 to 28 are arranged in two closely-packed groups and a small amount of coupling is provided between cavities 21 and 24 and between cavities 25 and 28. The filter further comprises a second set of resonant cavities 31 to 38, the cavities 31 to 38 being coupled to respective cavities 21 to 28 of the first set: it will be appreciated that each of the cavities 31 to 38 is coupled in a shunt manner with respect to the main signal path from the input IN to the output OUT.

The filter in accordance with this invention may be constructed in a compact form, but also it gives improved performance. Preferably the filter comprises a single block 40 of metal (Figures 5 and 6) having all of the cylindrical cavities formed in it, the cavities being open to the top surface of the body and a flat metal lid 42 being fitted over the top of the block 40 to close the cavities. Figure 5 shows cavities 21,22 and 32 and Figure 6 is a section through cavity 32. Each cavity has a coaxial, cylindrical post 44 upstanding from its base, to form a resonator for the cavity: the resonator stops short of the lid and is formed with a blind bore 46: a plurality of tuning screws 47, one for each cavity, are threaded through the lid and project into the respective blind bores 46. Each adjacent pair of cavities are physically separated by a thin wall W, but are inductively coupled together through an appropriately dimensioned aperture 48. The wall W stops short of the lid, as shown at 49. The input and output comprise connectors to which coaxial cables are connected, each connector including a central probe e.g. 50 extending into the cavity to the resonator 44.

Claims

1) A microwave filter which comprises a first set of resonant cavities coupled together in series, and a second set of resonant cavities, the cavities of said second set being coupled in a shunt manner to respective cavities of the first set.

2) A microwave filter as claimed in claim 1, comprising a block of metal having said first and second sets of cavities formed therein.

3) A microwave filter as claimed in claim 2, in which adjacent cavities of said first set are coupled together, and the cavities of said second set are coupled to said respective cavities of the first set, by apertures formed in said block between the respective pairs of cavities.

4) A microwave filter as claimed in claim 2 or 3, in which a relatively small degree of coupling is provided between at least one non-adjacent pair of cavities in said first set.

5) A microwave filter substantially as herein described with reference to Figures 4 to 6 of the accompanying drawings.