GB2153155A - Improvements on or relating to microwave filters - Google Patents

Abstract

A microwave spur-line band-pass filter is constituted by a strip transmission line including one or more longitudinal portions 2 separated by gaps 7 from the adjacent portions 3,4 at either end, the portions having finger-portions 6 extending from their ends to effect coupling between their spaced- apart parallel edges 5. The portions 6 are preferably approximately a quarter-wavelength long at the pass-band centre frequency and, unless a known band-stop filter (Fig. 4, not shown) is incorporated therein, the length of the longitudinal portion between the inner ends of the portions 6 is preferably likewise approximately a quarter-wavelength. <IMAGE>

Description

SPECIFICATION Improvements in or relating to microwave filters This invention relates to microwave filters and in particular to band-pass filters, and provides forms of filters particularly, though not exclusively, suitable for use in microwave integrated circuits (MICs), eg for intergration with antennas deposited on the same substrate of dielectric material backed by a conducting ground-plane. In MICs the interconnections are normally made by strip transmission lines, usually by microstrip lines consisting of a strip of conduting material deposited on the substrate, but sometimes by triplate lines in which the strip is sandwiched between two layers of dielectric material each backed by a ground-plane.

The filters of the present invention are of the kind know as spur-line filters, in which there is coupled to such a transmission line a short length of similar line, the coupling being effected along their spaced-apart longitudinal edges. Such filters have the advantages of compactness, because they can be formed within the width of the line, and, when using microstrip, of lower radiation losses than with other kinds of filters. Band-stop filters of this kind, formed in microstrip, have been described by R N Bates et al in "Microwaves, Optics and Acoustics", vol 1, no 6, pp 209-214, November 1977 and in "Electronic Engineering", April 1978, pp 39-41.

The present invention provides band-pass filters of this kind.

According to the present invention, a microwave spur-line band-pass filter is constituted by a strip tramsmission line including at least one longitudinal portion separated by gaps from the adjacent portions either end thereof, said portions each including at least one finger-portion extending from the ends thereof to effect coupling between spacedapart parallel edges of the respective fingerportions; the lengths of said finger-portions being approximately an odd multiple of one quarterwavelength in the strip at the centre frequency of the require pass-band and the length of the longitudinal portion between the inner ends of its projecting finger-portions also being an odd multiple of said one quarterfinger portions also being approximately an odd muliple of said one quarter-wavelength.

Preferably the length of each finger-portion is approximately one quarter-wavelength and, unless a spur-line band-stop filter (as hereinbefore defined) is incorporated therein, preferably the length of the longitudinal portion between the inner ends of its projecting finger-portions is also approximately one quarterwavelength. Said portions may each include one or more respective finger-portions arranged interdigitally.

The filter may comprise a plurality of said longitudinal portions coupled to each other in series by projecting finger-portions as aforesaid.

One or more of the longitudinal portions may incorporate a spur-line band-stop filter (as hereinbefore defined), or such a filter or filters may be included elsewhere in the transmission line.

To enable the nature of the present invention to be more readily understood, attention is directed, by way of example, to the accompanying drawings, wherein: Figure 1 is a plan view of a microstrip transmission line embodying the present band-pass filter.

Figures 2 and 3 are plan views of modified forms of the filter of Fig. 1.

Figure 4 is a plan view of the filter of Fig. 1 incorporating also a band-stop filter.

Figure 5 is a graph of the frequency response of examples of the present filter.

Fig 1 shows a microstrip transmission line comprising a metallic conducting strip 1. As in the other Figures, the dielectric substrate on which the strip is formed is omitted for simplicity, as is the conducting ground-plane.

In accordance with the invention, the line 1 includes a longitudinal portion 2 separated by insulating gaps from the adjacent portions 3 and 4 at either end. Coupling of the portion 2 to portions 3 and 4 takes place between the spaced parallel edges 5 of the respective projecting finger-portions 6, the arrangement thereby forming a band-pass filter. A plurality of portions 2 can be connected in series, as shown by the second such portion 2' coupled to portion 2.

The lengths of the finger-portions 6, p, and of the portion 2 between the bases of the finger-portions 6, q, is made approximately At/4, where Ag is the wavelength in the line 1 at the centre frequency of the required passband; in other embodiments these lengths can be an odd multiple of Ag/4.

The design of embodiments of the present filter having specified characteristics can follow conventional procedures such as that described in Chapter 8.4 of "Foundations for microstrip circuit design" by T C Edwards (John Wiley and Sons Ltd, 1981) with particular reference to Fig. 8.1 5 Although the latter shows a different kind of band-pass filter comprising a staggered array of Ag/4 elements connected in series, the same design procedure can be followed. This procedure yields a set of even- and odd-mode characteristic impedances Zoe and Zoo respectively, for all the series-connected portions 2 2', etc and hence the spacings s between the parallel edges 5 (the latter corresponding to the values s" S2, etc in Fig. 8.15).This procedure also gives the total number of series-connected portions required. The length t of the gaps 7 at the ends of the finger-portions 6 can be determined as described by Bates et al in the aforesaid papers for a spur-line bandstop filter, which includes one or more similar gaps.

(It may be noted that the Bates et al bandstop filter can have some structural similarity to the present band-pass filter, but with the essential difference that in Bates et al the corresponding "finger-portions 6" projecting from "portion 2" are both joined to the adjacent line portions, instead of there being a "gap 7" therebetween; see eg Fig 7 (a) of the aforesaid paper in "Electronic Engineering".

However, there is a complete change of behaviour when the DC connection of Bates et al becomes a gap as in the present invention.) The widths of the finger-portions are fixed once the separations of the edges 5 is known.

If the filter is to be the full width w of the line 1, the finger-portion widths are > (w-s). It is not essential, however, that the filter be the same width as the remainder of the line, and the finger-width can be varied accordingly. In that case the filter will have a different overall impedance Z0 (= Z,,.Z,,), since the latter impedances depend upon both the line widths w and the separations s.

Matching must be provided between the feeder lines either side of the filter and the filter itself. This can frequently be arranged automatically by the initial design of the first and last filter portions, using the procedure described in the aforesaid Chapter 8.4. If the resulting first and last separations s are too small to manufacture, then quarter-wavelength matching line-portions can be provided in series therewith, as described for example by D Rubin et al in a paper, "Millimeter-wave coupled line filters", in "Microwave Journal", October 1980, pp67-7l, 78 and by J A Mosko in a report "Design techniques for UHF stripline filters", NAVWEPS Report 8677, April 1965.

Fig 2 shows a modification of Fig. 1 in which the finger-portions 1 6 projecting from each end of portion 1 2 are on opposite sides of portion 1 6 instead of the same side. The behaviour of the two structures is identical.

Instead of single finger-portions arranged side-by-side to effect coupling along only one edge of each, as in Figs 1 and 2, coupling can be effected between fingers arranged interdigitally, ie at least a single finger-portion at the end of one line-portion disposed between two finger-portions at the end of the adjacent line-portion to effect coupling along both edges of the former. In the example of Fig. 3, two finger portions 26 at the end of portion 22 interdigitate with three similar finger-portions on the adjacent line portions.

Clearly either or both ends of portion 22 may alternatively have three finger-portions interdigitating with two on the adjacent line-portions. Such interdigital couplings are of particular use with wide transmission lines. Design calculations similar to those for Figs 1 and 2 are applicable, with such appropriate adaptations as will be apparent to those skilled in the art.

In Fig. 4 a band-pass filter of the present kind comprising portion 32 coupled to portions 33 and 34 by finger-portions 36 as in Fig. 1, incorporates within portion 2 a bandstop filter of the kind shown by Bates et al in Fig. 7(b) of the aforesaid paper in "Electronic Engineering". Portion 32 here comprises a finger-portion 38 projecting between two finger-portions 39, all of length r approximately Ag/4, portion 38 being joined to the line at the base of portions 39 as described earlier.

In order to accommodate the band-stop filter, portion 32 is made approximately 3Ag/4 long, the lengths u each being approximately Ag/4.

Fig. 5 shows the responses of two examples of the invention of the form shown in Fig. 1 having a single portion 2 and intended to have a pass-band centred on 1 6GHz. The relevant dimensions of these example were: Substrate dielectric permittivity 9.8 Substrate thickness: 0.635mm Strip material and thickness: copper, 0.005mm Strip width (w): 0.635mm Finger length (p): 1.6mm Length between finger bases (q): 1.5mm Finger spacing (s): 0.05mm Gap length (t): 0.05mm The unwanted pass-band centred on 9.5 GHz can be removed by incorporating a suitable spur-line band-stop filter as shown in Fig.

4 or locating such a filter elsewhere along the line, or by other known means.

Although in the described embodiments the transmission lines are all of microstrip type, the invention can also be incorporated in triplate lines, whereby structures of the kind shown would have a further dielectric layer and ground-plane superimposed thereon.

Claims (7)

1. A microwave spur-line band-pass filter constituted by a strip transmission line including at least one longitudinal portion separated by gaps from the adjacent portions either end thereof, said portions each including at least one finger-portion extending from the ends thereof to effect coupling between spacedapart parallel edges of the respective fingerportions; the lengths of said finger portions being approximately an odd multiple of one quarterwavelength in the strip at the centre frequency of the required pass-band and the length of the longitudinal portion between the inner ends of its projecting finger-portions also being approximately an odd multiple of said one quarter-wavelength.

2. A filter as claimed in claim 1 wherein the length of each finger-portion is approximately one quarter-wavelength.

3. A filter as claimed in claim 2 wherein the length of the longitudinal portion between the inner ends of its projecting finger-portions is approximately one quarter-wavelength.

4. A filter as claimed in any of claims 1-3 wherein the portions each include one or more respective finger-portions arranged interdigitally.

5. A filter as claimed in any preceding claim comprising a plurality of said longitudinal finger-portions coupled to each other in series by projecting finger-portions.

6. A filter as claimed in claim 1 or claim 2, or in claim 4 or claim 5 other than as dependent on claim 3, wherein one or more of said longitudinal portions incorporates a spur-line band-stop filter (as herein before defined).

7. A microwave spur-line band-pass filter substantially as hereinbefore described with reference to the accompanying drawings.