GB2289167A - Electrical filters - Google Patents

Abstract

An electrical filter 10 comprises a block 12A, 12B of dielectric material. An electrode structure 14 is embedded within the block. Further electrodes 16 are associated with the electrode structure 14 and maintained at a reference potential (usually earth) during use. The electrode structure 14 and the further electrodes 16 provide an electrically resonant arrangement operable to provide electrical filtering. The filter characteristics can be widely varied according to the pattern of the electrode structure 14. Examples are described which can be used as bandpass filters in the UHF to low microwave frequency bands. <IMAGE>

Description

Electrical Filters The present invention relates to the construction of electrical filters.

Many types of electronic communications systems require miniaturised electrical frequency filters.

Examples include mobile telephones operating in the 900 MHz and 1.8 (;Hz bands and wireless local area networks (LANs) operating in the 2.5 CHz bands. Such systems require bandpass filters for the purposes of defining the spectral range of transmitted signals and also for isolating transmitted signals from received signals.

These filters may have to satisfy stringent requirements such as small size, surface mountability, narrow passband bandwidth, low passband loss, high frequency selectivity, broad stopband bandwidth and good electrical shielding.

The present invention seeks to provide an improved filter construction, particularly but not exclusively for use in the UHF to low microwave frequency bands.

The present invention provides an electrical filter construction comprising a block of dielectric material, an electrode structure embedded within the block, and a further electrode associated with the electrode structure and maintained at a reference potential during use, the electrode structure and further electrode providing an electrically resonant arrangement operable to provide electrical filtering.

The dielectric material may be chosen to have a relative permittivity in the range 30-40. The dielectric material may comprise a ceramic material.

For example, ceramic materials based on titania (TiO2) and alumina (A1203) have been shown to exhibit appropriate dielectric properties. Such materials may be solid solutions including any one or more of the following oxides: Magnesia (MgO), Strontium Oxide (SrO), Calcium Oxide (CaO), Barium Oxide (BaO), Lanthanum Oxide (La203), Cerium Oxide (Ce203), Tin Oxide (SnO), Zirconia (ZrO2).

The electrode structure may be formed at the interface of two pieces of high relative permittivity material which together form the block. The electrode structure is preferably generally planar. The electrode structure may comprise a stripline structure of electrodes. The stripline structure may comprise an interdigital stripline structure and/or may comprise a parallel-coupled transmission line structure. The construction may further comprise means for coupling to and from the electrode structure. The coupling means may provide capacitive coupling or a direct connection, such as by elongate members protruding into the dielectric material, or extensions of the electrode structure extending outside the dielectric material.

The further electrode is preferably grounded during use. The or each further electrode may be formed at the outer surface of the dielectric material. The or each further electrode may be provided by an electrically conductive layer. The electrode structure may be generally planar, there being two generally planar and generally parallel further electrodes between which the electrode structure is located.

The construction may provide a bandpass filter, such as a bandpass filter with a centre frequency between 500 MHz, and 10 GHz.

Embodiments of the present invention will now be described in more detail, by way of example only, and with reference to the accompanying drawings, in which: Fig. 1 is an exploded perspective view of a first electrical filter construction according to the invention; Fig. 2 is a partial schematic section along the line 2-2 of the structure of Fig. 1 (not exploded); Figs. 3 and 4 show examples of electrode patterns for use in the construction of Figs. 1 and 2; Fig. 5 shows a computer plot of the expected performance of a filter using the pattern of Fig. 4; Fig. 6 shows an alternative electrode pattern; and Fig. 7 shows the expected performance of a filter using the pattern of Fig. 6.

Turning to Fig. 1, there is shown an electrical filter construction 10 comprising a block 12A, 128 of dielectric material. An electrode structure indicated generally at 14 is embedded within the block. Further electrodes 16 are associated with the electrode structure 14 and maintained at a reference potential (usually earth) during use. The electrode structure 14 and the further electrodes 16 provide an electrically resonant arrangement operable to provide electrical filtering.

In more detail, the block 12 is formed of two slabs 12A, 128 of high relative permittivity dielectric material such as a ceramic material. Zirconia titania stannic oxide is used in the preferred example and has a relative permittivity in the region of 38. Other materials could be chosen but are preferred to have a relative permittivity in the region of 40, and preferably greater than 35. For instance, ceramic materials based on titania and alumina have been shown to exhibit appropriate dielectric properties. Such materials may be solid solutions including any one or more of the following oxides: Magnesia (MgO), Strontium Oxide (SrO), Calcium Oxide (CaO), Barium Oxide (BaO), Lanthanum Oxide (La2O3), Cerium Oxide (cue203), Tin Oxide (SnO), Zirconia (ZrO2).

The two slabs 12A, 12B are generally parallelepipedal. They are brought together to meet at the two major faces 18 which are parallel to the further electrodes 16. The slabs 12A, 12B are bonded together to form a unitary block. The further electrodes 16 are then formed as a conformal layer on the outer surface of the block 12, for instance in a thick silver film. This layer may be formed by plating, or otherwise conformally coating to form a uniform, highly conductive layer which closely follows the contours of the block. The whole outer surface of the block 12 could be coated. This layer forms a continuous electrode which can be earthed (or maintained at another reference potential) to form ground planes at 16, for reasons to be described.

The electrode structure 14 is formed on the surface 18 of one of the slabs 12B, before the slabs are bonded together to embed the electrode structure in the dielectric material. The electrode structure 14 may be formed by metal electrode patterns such as by stripline or printed circuit techniques applied to the surface 18.

Various electrode patterns can be used to determine the final filter characteristics and physical dimensions of the construction. Three examples of electrode structures are shown in Figs. 3, 4 and 6.

Other components of the construction are omitted from these figures, in the interests of clarity.

In Fig. 3, the electrode pattern 20 consists of a series of five parallel electrodes 22, each of which forms a transmission line against the ground planes 16.

Each electrode 22 projects one quarter wavelength beyond the extremity of its neighbour to tune the filter to the corresponding frequency. The bandwidth of the filter is determined by proximity coupling between electrodes, and thus by their spacing. The five electrodes form a 5th degree filter.

This pattern is well suited for use in the present invention because no connections are required to intermediate electrodes 22. It is only necessary to couple to the first and last electrode 22 to provide an input and output, such as by extending these electrodes to the boundary of the dielectric block or by capacitive coupling. However, the construction is larger than can be achieved using the alternative pattern of Fig. 4. Furthermore, in comparison with the pattern of Fig. 4, the pattern of Fig. 3 has poorer upper stopband performance (due to its larger width and consequent lower cut-off frequency for the TEO1 waveguide mode).

The electrode pattern 30 of Fig. 4, which is preferred to the pattern of Fig. 3, uses five interdigitated electrodes 32, 34 to form a fifth degree filter. The electrodes 32, 34 are parallel and each forms a transmission line against the ground planes 16.

The length of each transmission line is one quarter wavelength. The electrodes 32 are interconnected by an electrode 36 running transverse to their length. The electrodes 34 are similarly connected by an electrode 38 running transverse to their length. The electrodes 32, 34 alternate. The electrodes 36, 38 are earthed. Thus, the electrodes 32, 34, 36, 38 provide an array of parallel, one quarter wavelength transmission lines earthed alternately at each end.

Experimental work has indicated that the interdigitated electrode structure of Fig. 4 will allow the construction of a fifth degree bandpass filter with a 1.4 CHz centre frequency and 68 MHz bandwidth (at -1 dB), using a composition within the zirconium titanium tin oxide system as the dielectric material.

The external dimensions are expected to be approximately 30 mm x 15 mm x 6 mm.

Fig. 5 shows the performance expected from this construction on the basis of computer simulation of a 1.4 CHz centre frequency, 5th degree, 70 MHz bandwidth filter built on the basis of the pattern of Fig. 4.

Fig. 5 shows the expected MS21 transmission characteristic 40 and MS11 reflection characteristic 42 of the filter.

Input and output coupling to the filter is, in this example, achieved using series capacitive coupling from the first and last electrodes 32, 34. Coupling would be at the end remote from the transverse electrodes 36, 38. The use of capacitive coupling in this manner simplifies the mechanical construction and avoids the need for the construction to increase in size to allow coupling. Furthermore, the use of capacitive coupling in this manner allows the device readily to be surface mounted. However, it may sometimes be desirable to provide direct connections, for instance by extending the electrodes 32, 34 to protrude from the ceramic block 12, or by inserting connector pins into the block, perhaps in pre-drilled holes.

It can be readily understood from Figs. 3 and 4 that the filter characteristics can be widely modified by varying the pattern of the electrode structure embedded in the block 12, and by varying other dimensions of the construction. This is facilitated by the generally planar nature of the electrode structures which allows them to be formed by conventional printed circuit techniques. Complex patterns can be formed to high tolerances, allowing complex filter characteristics to be achieved.

By way of background, it is appropriate to explain some generalised calculations. If each ceramic block 12 is assumed to be 2.5 mm thick, giving a 5 mm spacing between the ground planes 16, and a single 3 mm wide copper transmission line is printed on the surface 16, the transmission line will act in the TEM prorogation mode. The characteristic impedence Zo can be calculated from the static capacitance between the copper line and the ground planes. Assuming a relative permittivity in the region of 38, the characteristic impedence Zo becomes about 14.5 ohms and it can be shown that the theoretical Q factor would be in the region of 540 at 1GHz increasing by the square root of frequency to about 760 at 2GHz. (Dielectric losses etc. may be expected to reduce the Q factor by about 5 or 10 in practice).

The quarter wavelength dimension at 1.4 GHz would be approximately 8.7 mm. A fifth degree interdigital filter (as shown in Fig. 4) made from lines with these dimensions and with a centre frequency of 1.4 CHz and 100 MHz bandwidth would have a midband dissipation loss of about 1.5 dB.

The centre frequency of the filter would of course be affected by printing or etching tolerances in forming the copper transmission line, and in variations in permittivity throughout the block 12. Assuming printing tolerances of +/- 0.025 mm, and a relative permittivity variation of 38 +/- 0.25, the centre frequency would not be expected to vary by more than +/- 9 MHz. Thus, the structures described are expected to be particularly useful for applications in the low microwave frequency band.

It will also be apparent that various technologies are readily available for producing the electrode structures on the surface 16. Various printing and etching techniques could be used such as those conventional in the production of stripline, printed circuit boards etc.

The basis on which the electrode patterns are designed may need to be varied according to the filter characteristics and dimensions required. For instance, if the structure of Fig. 4 is designed for a centre frequency of about 2.66 GHz, it is found that for the same ground plane spacing the passband insertion loss is 40, higher. The filter becomes longer because coupling admittances are inversely proportional to the bandwidth percentage. The combined effects of etching and dielectric constant tolerances produce a centre frequency tolerance of +/- 23 MHz. Thus, the design must aim for a larger bandwidth which implies more resonators and a longer filter. Lower loss would be achieved by a larger ground plane spacing but the filter would then become even larger.

An alternative approach is thus preferred at these frequencies. Generalised Chebychev filters are preferred, because they would allow the designer to arbitrarily choose the frequency of the poles of infinite attenuation.

One example of an electrode pattern for implementing a generalised Chebychev filter is shown in Fig. 6.

The pattern 44 in Fig. 6 uses five finger electrodes 46, each one quarter wavelength long, and reducing in width at a discontinuity 48 part way along their length. The electrodes 46 are bordered by a grounded electrode 50. The discontinuities 48 represent impedance steps in the resonators, which produce resonant coupling between the resonators, and attenuation poles on either side of the filter passband. This is shown in Fig. 7 which is a plot similar to Fig. 5, showing expected performance for the structure of Fig. 6. It can be seen that although the MS11 reflection characteristic 52 is broadly similar to the characteristic 42 of Fig. 5 the MS21 transmission characteristic 54 is markedly different to the characterstic 40 of Fig. 5, having poles 56 of substantially infinite attenuation spaced to either side of the passband, to sharply define the passband.

Input and ouput coupling to the filter can be achieved in the ways described above in relation to Fig. 4.

It will be apparent that many variations and modifications can be made to the constructions described above, without departing from the spirit of the present invention. In particular, the choice of dielectric material, dimensions and electrode patterns will all affect the resultant filter characteristics. The resonant electrodes have been described as being formed between a sandwich of dielectric but it may be possible to provide an embedded electrode structure in other ways.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims (21)

1. An electrical filter construction comprising a block of dielectric material, an electrode structure embedded within the block, and a further electrode associated with the electrode structure and maintained at a reference potential during use, the electrode structure and further electrode providing an electrically resonant arrangement operable to provide electrical filtering.

2. A filter construction according to claim 1, wherein the dielectric material is chosen to have a relative permittivity in the range 30-40.

3. A filter construction according to claim 1 or 2, wherein the dielectric material comprises a ceramic material.

4. A filter construction according to claim 3, wherein the ceramic material is based on titania (TiO2) or alumina (Al203).

5. A filter construction according to claim 4, wherein the ceramic material comprises a solid solution including any one or more of the following oxides: Magnesia (MgO), Strontium Oxide (SrO), Calcium Oxide (CaO), Barium Oxide (BaO), Lanthanum Oxide (La203), Cerium Oxide (Ce203), Tin Oxide (SnO), Zirconia (ZrO2).

6. A filter construction according to any preceding claim, wherein the electrode structure is formed at the interface of two pieces of high relative permittivity material which together form the block.

7. A filter construction according to any preceding claim, wherein the electrode structure is generally planar.

8. A filter construction according to any preceding claim, wherein the electrode structure comprises a stripline structure of electrodes.

9. A filter construction according to claim 8, wherein the stripline structure comprises an interdigital stripline structure.

10. A filter construction according to claim 8 or 9, wherein the stripline structure comprises a parallel-coupled transmission line structure.

11. A filter construction according to any preceding claim, further comprising means for coupling to and from the electrode structure.

12. A filter construction according to claim 11, therein the coupling means provide capacitive coupling or a direct connection.

13. A filter construction according to claim 12, wherein the coupling means comprise elongate members protruding into the dielectric material or extensions of the electrode structure extending outside the dielectric material.

14. A filter construction according to any preceding claim, wherein the further electrode is maintained at ground potential during use.

15. Afilter construction according to any preceding claim, wherein the or each further electrode is formed at the outer surface of the dielectric material.

16. A filter construction according to any preceding claim, wherein the or each further electrode is provided by an electrically conductive layer.

17. A filter construction according to any preceding claim, wherein the electrode structure is generally planar, there being two generally planar and generally parallel further electrodes between which the electrode structure is located.

18. A filter construction according to any preceding claim, so arranged as to provide a bandpass filter.

19. A filter construction according to claim 18, wherein the bandpass filter has a centre frequency between 500 MHz, and 10 GHz.

20. A filter contruction substantially as described above with referenc eto any of the accompanying drawings.

21. Any novel subject matter or combination including novel subject matter disclosed, whether or not within the scope of or relating to the same invention as any of the preceding Claims.