GB2067848A - Cavity Filters - Google Patents

Abstract

A cavity resonator (1), which may form part of an array of such coupled by slots (5), is of cylindrical section with a central tuning post (3) projecting into a hollow post (2) to form a coaxial capacitance. Each cavity has a height and a diameter much less than g/4 where g is the wavelength of the microwaves to be filtered. The cavity thus acts as a hybrid of a coaxial resonator and a radial line resonator. The cavity is easily formed by conventional milling and drilling techniques on a solid block using a numerically controlled machine tool. <IMAGE>

Description

SPECIFICATION Cavity Filters The present invention relates to cavity filters.

An exemplarly application of a cavity filter of the invention is in an array of filters.

An array or bank of filters is essential in many situations, for example in a receiver designed to monitor simultaneously many signals occupying different frequencies over a iarge bandwidth.

There has been proposed, in the literature, the user of surface acoustic wave (SAW) filters in preference to cavity filters because cavity filters are considered to be too bulky and too expensive to manufacture. A filter bank based on existing commercial quarter wave coaxial resonators would be very bulky and expensive. However, a cavity filter has a lower insertion loss, and less propagation delay than an SAW filter and furthermore a cavity filter may be realised at microwave frequencies.

It is an object of the present invention to provide a cavity filter suitable for use in an array or bank of filters.

According to the invention, there is provided a cavity filter comprising a cavity containing a central post and a tuning member moveable relative to, and coaxial with the post, one of the post and member entering into the other the cavity having dimensions much less than one quarter of a wavelength of microwave energy fed to the cavity.

Preferably the height of the cavity (measured parallel to the post) approximately equals the transverse dimension (measured perpendicular to the post). Preferably the central post is hollow, and the tuning member is a rod which enters the post.

In a preferred embodiment of the invention, the cavity is cylindrical, the height of the cavity equalling its diameter. Preferably, in the preferred embodiment, the central post and tuning member are cylindrical.

For a better understanding of the invention, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a plan view of a portion of the body of a bank of cavity filters, Figure 2A is a section on line A-A of Figure 1, Figure 2B is a sectional view of a portion of a lid of the bank, and Figure 3 is a perspective view of a complete body.

Referring to Figures 1 and 2, an exemplary cavity filter comprises a body A in which a cylindrical cavity 1 contains a hollow central post 2, and a lid B in which a rod 3 is mounted, to locate coaxially within the hollow post 2. The lid B is screwed to the body A by screws 4 to complete the cavity. The rod 3 is mounted in screw thread manner in the lid B so as to provide adjustment of the amount of rod 3 within the post 2, for tuning purposes.

The dimensions of the cavity are much less than a quarter wavelength of a microwave energy fed to it, i.e. the diameter d of the cavity and the height / of the cavity are both much less than A/4, where A is the wavelength. Furthermore the exemplary cavity filter in accordance with the invention has dimensions such that d is approximately equal to /. The filter can be regarded as a lumped L-C resonator, in which the cavity is the inductance, and the hollow central post 2 and the tuning rod 3 form a coaxial capacitance within the hollow post. The coaxial capacitance thus formed has the relatively high magnitude needed to tune a cylindrical cavity resonator in which all the dimensions are less than quarter wave.

If the cavity filter had dimensions d and / such that the diameter d is much less than the height / (both land d being much less than A/4) it would be regarded as foreshortened coaxial resonators or if the dimensions were such that (I d, both v1/4) it would be regarded as a foreshortened radial line resonator. With d roughly equal to I (both çl/4), then in the exemplary filter the cavity is a hybrid of a coaxial resonator and of a radiai line resonator.

The cavity is cylindrical for ease of manufacture, and to provide a good volume to surface area ratio to keep the insert;on loss low.

The post 2 is short and therefore not microphonic.

Furthermore, the tuning rod 3 need not be accurately centralised relative to the post.

Although the filter has been described above as having a hollow post 2 within which a solid rod 3 projects, the post 2 could be solid and project within a hollow rod 3.

Figures 1 and 2 show a filter comprising two cavities coupled by a slot. In practice, more than two cavities may be coupled.

For example Figure 3 shows a bank of four filters 31-34 each comprising a row of four cavities 1 (only a representative sample being numbered) coupled by slots 5. For ease of manufacture the cavities are cylindrical and the cavities in each row are identical, although they could be different. The slots in each row controls the type of frequency response of the filter bank.

As shown in Figure 3 the slots in each row are identical giving a particular response. However, if, say a Chebyshev or Butterworth response is required then the slots in a row must differ. The slots also affect bandwidth. The input 36 and output 37 couplings of each row use conventional capacitive probes and coaxial connectors.

The filter bank of Figure 3 is easily formed from a solid metal block by milling and drilling operations. Using a numerically controlled machine large banks can be repeatedly formed quickly and accurately.

One method of manufacturing the filter bank shown in Figure 3 comprises the following steps: a) drill an array of holes in a metal block to define the hollows in the posts 2; b) form the cylindrical cavities 1 by using an end cutter having a central bore which is placed concentrically with the holes to form the posts and cavities simultaneously, and c) form the slots using a miller.

Claims (11)

Claims

1. A cavity filter comprising a cavity containing a central post and a tuning member moveable relative to, and coaxial with the post, one of the post and member entering into the other the cavity having dimensions much less than one quarter of a wavelength of microwave energy fed to the cavity.

2. A filter according to claim 1 ,wherein the height of the cavity (measured parallel to the post) approximately equals the transverse dimension (measured perpendicular to the post).

3. A filter according to claim 1 or 2, wherein the post is hollow, and the member enters the hollow post.

4. A filter according to claim 1 or 2, wherein the member is hollow and the post enters the hollow member,

5. A filter according to claim 1, 2, 3 or 4, wherein the cavity is circularly cylindrical.

6. A filter according to any preceding claim, wherein the central post and tuning member are each circularly cylindrical.

7. A filter circuit comprising a plurality of cavity filters according to any preceding claim, the cavities of the filters being coupled.

8. A filter circuit according to claim 7, wherein the cavities at least one pair of the filters are coupled by a slot.

9. A filter circuit according to claim 7 or 8, comprising a first metal member defining a plurality of cavities each having one open end and a plurality of central posts or tuning members in the cavities, and a second metal member supporting a plurality of members or posts complementary to the posts or members in the cavities, the second member being fixed to the first member and closing said open ends, the first and second members together defining the said plurality of cavity filters.

10. A filter circuit according to claim 9, wherein the first member is formed by machining a solid metal block.

11. A filter substantally as hereinbefore described with reference to Figures 1 to 3 of the drawings.