GB2355597A

GB2355597A - Dielectric resonator filter with mirror surface

Info

Publication number: GB2355597A
Application number: GB9917838A
Authority: GB
Inventors: Farhat Abbas
Original assignee: Nokia Telecommunications Oy
Current assignee: Nokia Oyj
Priority date: 1999-07-29
Filing date: 1999-07-29
Publication date: 2001-04-25
Also published as: GB9917838D0

Abstract

A filter 100 comprises a dielectric resonator 102 with at least one mirror layer 106 on the resonator which acts to reflect signals from the resonator back into the resonator. Conducting or superconducting layers 110 may be positioned on the opposite face of the mirror layers.

Description

2355597 FILTERS The present invention relates to filters and in

particular, but not exclusively, to dielectric f ilters for use in a wireless telecommunications system.

Figure 1 illustrates a known wireless telecommunication network 2. The area covered by the network 2 is divided into a number of cells 4. Each cell has associated therewith a base transceiver station 6. Each base transceiver station 6 is arranged to communicate with terminals 8 located in the cell associated with that base transceiver station 6. The terminals may be mobile stations which are able to move between the cells 4. In practice, the network will comprise more than three cells which are shown for illustrative purposes only in Figure 1.

Base transceiver stations incorporate a number of components including a number of filters. These filters are typically bandpass filters and are in the form of cavity filters. Cavity f ilters are relatively bulky. The base station has to be large enough to accommodate these bulky filters. This is disadvantageous, particularly for certain types of base station. For example, base stations for office buildings or for use in certain urban environments are easier to accommodate if they are 3'0 p com act. A further disadvantage of cavity filters is that the performance provided by cavity filters is not particularly close to that of the hypothetical ideal filter. with an ideal filter there is a relatively sharp cut off point between the frequencies which are passed through the filter and the frequencies which are removed by the filter.

It is an aim of embodiments of the present invention to provide a filter which is capable of being smaller than a cavity filter and/or which provides improved performance compared to a cavity filter.

According to one aspect of the present invention, there is provided a filter comprising a dielectric resonator arranged to receive signals to be filtered; and at least one mirror layer arranged in use to reflect signals from the dielectric resonator back into said dielectric resonator.

The presence of at least one mirror layer means that at least part of the electric field can be confined within the dielectric resonator which improves the performance of the filter. The signals are at least partially reflected by the mirror layer.

The at least one mirror layer is preferably arranged between the resonator and at least one conducting layer. That conducting layer may be in the form of a film. The conducting layer may be normally conducting or superconducting. The presence of the additional conducting layer may further improve the performance of the filter by further confining the electric field within the f ilter.

The at least one mirror layer may be of a dielectric material.

The dielectric constant of the resonator layer may be greater than that of the at least one mirror layer. The dielectric resonator is preferably thicker than the at least one mirror layer. The dielectric resonator is preferably thicker than the at least one mirror layer. The dissipation factor of the resonator and/or the at least one mirror layer is preferably negligible. Preferably, the dissipation factor of both of the resonator and the at least one mirror layer is negligible.

The filter may be substantially cuboid. Alternatively, the filter may be substantially cylindrical.

Two mirror layers are provided in embodiments of the present invention, the mirror layers being provided on opposite sides of the filter.

According to a second aspect of the present invention, there is provided a base station incorporating a f ilter as described hereinbefore.

For a better understanding of the present invention and as to how the same may be carried into effect, re-Eeren-ce will now be made by way of example to the accompanying drawings in which:

Figure 1 shows a typical wireless cellular telecommunications network; Figure 2 shows a filter embodying the present invention; Figure 3 shows a graph of quality factor against various thicknesses of the mirrors of Figure 2, having different dielectric constants; Figure 4 shows a graph of quality factor against various different thicknesses of dielectric resonator, having different dielectric constants; Figure 5 shows fourth order Butterworth bandpass amplitude for various quality values; Figure 6 shows fourth order Chebyshev bandpass amplitudes for various quality values; and Figure 7 shows part of a base transceiver station including the filter of Figure 2.

Reference will now be made to Figure 2 which shows a filter 100 embodying the present invention.

The filter 100 has a dielectric resonator 102. Attached to opposite sides 104 of the dielectric resonator 102 are dielectric mirrors 106. Two dielectric mirrors 106 are provided. on the side 108 of each dielectric mirror 106, opposite to the side which is attached to the dielectric resonator 102, is a normal or superconducting layer 110. The normal or superconducting layers 110 may be in the form of thin films.

The dielectric resonator 102 has a thickness T, a depth D and a width W. The width W is the distance between the sides 104 of the resonator 102 to which the respective mirrors 106 are attached.

4 The signals which are to be filtered are input to one 112 of the sides of the dielectric resonator 102 which are not attached to the mirrors 106. The filtered signals are output from the side 114 of the dielectric resonator 102 which is opposite to the input side 112 on which the unfiltered signals are received. The distance between the input and output sides 112 and 114 is the thickness T. The input and output sides 112 and 114 each have the width W and the depth D.

The thickness T of the filter 100 may be generally equal to half the wavelength of the signals to be filtered by the filter. The thickness T should be selected so as to ensure that resonance occurs in the filter 100. The thickness T may of course have values other than half the wavelength of the signals to be filtered provided that resonance occurs.

The dielectric constant of the dielectric mirrors 106 is selected to be less than the dielectric constant of the dielectric resonator 102. This is so that the dielectric mirrors 106 are able to confine the electric field in the dielectric resonator

102 due to reflection of the signals by the dielectric mirrors 106. The reflection will be caused the boundary between the dielectric resonator 102 and the dielectric mirrors 106. The confinement of the field within the dielectric resonator 102, caused by the dielectric mirrors 106 improves the quality factors and hence the performance of the filter 100. For the purposes of this document, improved performance means that the filter 100 acts more like a perfect filter. In other words, the cut off between the frequencies which are passed through the filter and those which are filtered out by the filter is sharp.

The dielectric constant of the dielectric resonator 102 should be as large as possible whilst tan 6 (dissipation factor) for the material of the dielectric resonator 102 should be as small as possible. The dissipation factor is equal to the tangent of the loss angle 6 which is the phase angle between the external electric field vector and the resulting displacement factor. The dissipation factor should also be as small as possible for the mirrors 106. The dissipation factor can be the same or different for the dielectric resonator 102 and the mirrors 106.

The mirrors 106 have a thickness which ensures that there is good internal reflection in the dielectric resonator 102. Generally, the thicker the mirrors 106, the better the confinement of the field in the dielectric resonator 102. However, the thicker the mirrors 106, the more expensive and thicker the filter 100. The balance of performance versus cost and size of the filter can be selected based on the intended application of the filter. The mirrors 106 are arranged to have a thickness which is generally much less than that of the dielectric resonator 102.

The mirrors 106 are arranged to have a dielectric constant which is less than that of the dielectric resonator 102. A relatively large difference between the dielectric constants of the mirrors 106 and the dielectric resonator 102 is preferred.

The normal or superconducting layers 110 are preferably thin films which give fewer losses than a thicker layer and thus result in improved performance. A typical thin film may be of the order of 700nm thick. It should be noted that thin films are more expensive to achieve than thicker layers. Accordingly, in some embodiments, cheaper relatively thick layers may be used to reduce the cost of the filter. Again, the thickness of the normal superconducting layers 110 and can be selected on the basis of the intended use of the filter.

The f ilter 100 shown in Figure 2 has a cubic shape. The sides of the f ilter 102 may be of the same size or of dif f erent sizes - The width w of the input and output sides of the filter 100 is, in preferred embodiments of the invention ' smaller than the depth D or thickness T. It is of course possible for the filter to have any other suitable shape. For example, the filter may be cylindrical or the like. If the filter were to be cylindrical, the longitudinal axis of the filter would preferably extend 6 between the input and output sides of the filter. The dielectric mirrors may completely surround the dielectric resonator except on the input and output sides.

In some embodiments of the present invention, the normal or superconducting layer 110 may be omitted. The normal or superconducting layer 110 may be omitted in the case where the mirror 106 is particularly good at ensuring that the field is confined within the dielectric resonator.

In embodiments of the present invention, any one or more of the dielectric resonator 102, the mirrors 106 and the normal or superconducting layers 110 may be replaced by a structure made up of more than one layer. For example, a plurality of mirror layers could be attached to each side of the dielectric resonator 102.

The f ilter shown in Figure 2 can be classed as a planar microwave narrow bandpass filter. The filter shown in Figure 2 can be modified to filter a wide range of frequencies. For example, embodiments of the present invention are suitable for use with frequencies between 1GHz to 30GHz. However, it is possible to also use embodiments of the invention outside this range. In particular embodiments of the invention may be used at frequencies above and below this range.

One example of a filter 100 has thin superconducting layers of YBaCuO (Yttrium Barum Copper Oxide). The film 110 has a thickness or depth of 700nm. The penetration depth for Y3aCuO is 140nm. Thus, the conducting layers may have a thickness equal to around 5 penetration depths. The conductivity of the films 110 are around 1.7 x 106(ohms.m)-'. The operating frequency is assumed to be IOMHz whilst the reduced temperatures T/Tc=0.5 and 0.05. The operating frequency is the frequency at which the penetration depth is given. The super conducting layer may be Yttrium Boron Cobalt.

7 The dielectric resonator 102 is of sapphire and the mirrors 1OG are of magnesium oxide MgO. Magnesium oxide has a dielectric constant of 2, 4, 6, 8 or 9 depending on its structure. Magnesium oxide has a negligible dissipation factor which can therefore be assumed to be zero. Sapphire has a dielectric constant of 10 and a negligible dissipation factor which again can be assumed to be zero.

Reference is made to Figure 3 which shows a graph of the conduction quality factor Qx of the filter against thickness of the dielectric mirrors 106. It is assumed in this model that the dielectric resonator 102 has a width of 5001.4m. Curve A represents the results when the magnesium oxide has a dielectric constant of 2, curve B the results when the dielectric constant is 4, curve C the results when the dielectric constant is 6, curve D the results when the dielectric constant is 8 and curve E the results when the dielectric constant is 9. The dielectric constant of the dielectric resonator 102 is 10 in this example.

As can be seen from Figure 3, the quality factor due to conduction increases as the thickness of the mirror 106 increases. It can also be seen that the quality factor due to conduction increases as the dielectric constant of the mirror 102 is decreased. This is because the difference in the dielectric constant of the mirror 106 and that of the dielectric resonator 102 is increased. This increases the internal reflection of the signals back into the dielectric resonator 102. As the dielectric constant of the dielectric mirrors 106 decreases, the internal reflection at the interface between the dielectric resonator 102 and the mirrors 106 increases and the field at the surface of the normal or superconducting layer 110 decreases. This leads to an increase in the quality value due to conduction. This also reduces the radiation lost by the conducting layers which gives rise to an increased radiation quality factor QR.

Reference is now made to Figure 4 which shows a graph of the conduction quality factor Qx of the filter against the thickness 8 of the dielectric resonator. The dielectric mirrors 106 have a constant thickness and a constant dielectric constant for all of the curves. Curve A represents the results when the dielectric resonator 102 has a first dielectric constant, curve 3 the results when a second dielectric constant is used, curve C the results when a third dielectric constant is used, curve D the results when a fourth dielectric constant is used and curve E is the results when a fifth dielectric constant is used. The first dielectric is greater than the seccnd dielectric constant which is greater than the third which is greater than the fourth which is greater than the fifth.

As can be seen from Figure 4, the quality factor increases with the width W of the dielectric resonator 102. Additionally, as the dielectric constant increases, the quality factor also increases.

Again, this is a function of the difference between the dielectric constant of the mirrors 106 and that of the dielectric resonator 102. The larger the difference, the better the internal reflection and accordingly the better the quality value due to conduction.

Reference is now made to Figure 5 which shows a fourth order Butterworth bandpass amplitude for various values of Q. The graph plots amplitude against frequency with the central frequency being one rad/second per second. Curve A is the result when the quality value is 2, curve B is the result when the quality value is 5 and curve C represents the results when the quality value is 10. As can be seen, larger values of the quality value provide narrower passbands and a cleaner cut off between those frequencies which are filtered and those which are allowed to pass through the filter. In other words, with higher values of the quality value, the filter behaves more like a perfect filter than with lower quality values. As can be seen from Figure 5, the value of Q increases and that embodiments of the invention are able to perform closer to the ideal filter model.

Reference is also made to Figure 6, which is similar to Figure 9 5, but which shows the fourth order Chebyshev bandpass amplitude against frequency for various quality factor values. Curve A represents the results when the quality value is 2, curve B represents the results when the quality value is 5 and curve C represents the results when the quality value is 10. Again, the larger values of the quality value provide narrower passbands. Like Figure 5, this figure illustrates that the value of Q increases with embodiments of the invention and that a performance closer to the ideal filter model can be achieved.

To assist in the understanding of embodiments of the present invention, the following analysis is presented. The regions adjacent the normal or superconducting layers 110, on the side opposite to the mirrors 106 are considered to be very thick so that the f ields in these regions can be assumed to exponentionally decay away from the conductive layers 110.

The following is a "field analysis". Consider the propagation of an electromagnetic wave in the direction from the input side 112 of the f ilter to the output side. It is assumed that the thickness of the mirrors 106, the width W of the dielectric resonator 102 and the penetration depth of the normal or superconducting layers 110 are small compared to the depth D of the filter, which in turn is very small compared to the thickness T of the filter 100. With these assumptions, the edge effects can be ignored and there is no dependence in the width direction W of the filter of the fields and the currents.

It is known to use a two fluid model for superconductors. In this model, the total current is the sum of the supercurrent and the normal current. These are effectively two mechanisms which cause current to be generated. The classical skin effect model is used to calculate the normal current whilst the London theory is used for calculation of the supercurrent.

If a transmission wave is considered, the following equations apply:

to 2 2 HY k) E,, E-=-l d E (2) dx d' E, k 2 E. 0, (3) dx' where Hy is the magnetic field in the width W direction, E. is the electric field in the direction of the thickness T, E,, is the electric field in the depth D direction, u is the propagation constant, u, is the permeability constant, w is the angular 2 frequency and the k, for dielectrics is equal to K 2 r = U2 _ Cj 2 ErAo 1 (4) and for the superconductors is equal I + U2 _ C02E"'Uo + 1WA" G-' (5) x2 and for normal conductors k 2 is 2- 2E.Ij a W.0 + lWg. 6, (6) where Er. is the dielectric constant of the dielectrics, E. is the permitivility of a vacuum, X is the penetration depth of the superconductors, and c is the conductivity of the superconductor.

Equation 3 is the second order of dif f erential equation which has two independent solutions of the form e" and e-" where k is the square root of K 2 with a positive real part. In the region next to the superconducting layers (ie on the opposite side of the super conducting layers to the mirrors) in the positive direction only the solution e -k 3x and in the negative direction only the solution e"3x is taken. In other words e' 3.'< and e-'3x is discarded for the positive and negative directions respectively. However, in the superconducting layers 110, the dielectric mirrors 106 and in the dielectric resonator 102, both solutions are retained in order to satisfy the boundary conditions.

With these solutions for the various media, there are 12 arbitrary constants for the amplitudes of the fields. The field is within the resonator and all the dielectrics around it. There are 12 boundary conditions that must be satisfied, namely the continuity of the tangential fields Ez and Hy at the six boundaries shown in Figure 1 (including the boundaries with the regions next to the superconducting layers 110). If any non linearity in the system is ignored, the characteristics of the filter are independent of the amplitude of the wave and eleven of the constants can be determined in terms of the twelfth by using eleven of the twelve boundary conditions. The twelfth boundary condition gives an equation for the propagation constant a which must be satisfied in order for the solution to exist.

That condition is a transcendental equation for which an exact solution cannot be readily obtained. Approximations which are made are Kjx thickness m of the mirror 106 is very much less than 1 and K2 x thickness of the resonator is very much less than 1.

K is the wave number. These approximations mean that higher order modes are ignored. With W and the thickness of the mirrors being small, higher order modes will not be excited. With these assumptions, the transcendental equation becomes:

2 = 2 j. 'E (Y Cji2 2Xcoth( 2m+ (6) 2mE2+wE,) X) E, is the dielectric constant of the mirrors and E2 is the dielectric constant of the resonator.

It should be appreciated that the factor 2 in equation 6 indicates that there are two dielectric mirrors 106 and two normal or superconducting layers 110. This is because a symmetric case has been considered.

12 The wave velocity Vr relative to that in a vacuum can be derived from equation 7 as follows:

I Vr r (2ME2+WEl) f Z Ell-2E2Xcoth (1/X) +2m+W This equation indicates that the wave is dispersionless even though there is a component of the electric field in the direction of propagation that is through the filter. This means that the group velocity and the phase velocity are equal and independent of frequency. Attenuation of the wave due to losses in each medium and the wave velocity have been obtained by replacing the dielectric constants E,, E2 and the penetration depth X by their complex form.

The loaded quality factor Q1 of a transmission resonator is evaluated from a measured resonant curve by dividing the resonant frequency f. by the 3-dB width of the resonant curve. The unloaded quality factor Q0 can be calculated from the insertion loss of the resonator at the resonated frequency:

Q0 01- (8) P,/Pi) 41 where P. is the power transmitted by the dielectric resonator and Pi is the power incident on the dielectric resonator.

When the transmission line discussed in the analysis hereinbefore resonates as a half wave resonator, four kinds of quality factor are obtained due to losses in the dielectric resonator (Qd2), the dielectric mirrors (Qdl), the normal or superconducting layers (Qc) and radiation from the sides of the resonator 102 not in contact with the mirrors (Qr). This gives rise to the following equation:

13 1 (9) Q. Qc Qdl Qd2 Qr The Q,, defined in this equation is the same as Q. defined in the preceding equation. The various Q factors can be written as f ollows:

Qc= 2 a,V9 Qdl= () 2 adIV9 Qd2 2 ad2Vg where Vg is the group velocity and is defined as follows:

V9= V22 VR and v2= FT(C 2 A.] -112 where R represents the "real part". The uc refers to the propagation constant of the normal or superconducting layer 110, ad, refers to the propagation constant of the mirror layer 106 and ad2 refers to the propagation constant of the resonator f ilter.

Examples of other possible dielectric materials which can be used for the dielectric mirror or resonator are as follows:- COMPOSITION DIELECTRIC CONSTANT I/TAN6 Barium Titanate 35.0-36.5 >28,000 BaLnTi Oxide 80.0 3,000 BaZnTaTi Oxide 27.6-30.6 10,000 14 BaTitanium Oxide 36.6-30.6 >6,000 Zr, Sn titanate 36.5 Mg.Ca titanate 19.5 Ba,Nd titanate 88.0 Da, Zn tantalate 29.5 Steatite 6 tan6=5xi 04 The dielectric constants of the resonator may be at least 2. Likewise the dielectric constant of the mirror layer may be at least 2.

In one preferred embodiment of the present invention: the dielectric resonator has a dielectric constant 10, and a width 10'm, the dielectric mirror has a dielectric constant of 2 and a thickness 500tm, and the normal or superconducting layer has a thickness of 700nm.

It should be noted that the values selected will be dependent on the application of the filter, the required cost and performance as well as the materials selected. The wavelength with which the filter is to be used will also have an effect.

Reference is made to Figure 7 which shows part of a base transceiver station 9 which is arranged to receive N frequency channels at the same time. For clarity, only the receiving part of the base transceiver station 9 is shown. The base transceiver station 9 has an antenna 10 which is arranged to receive signals from mobile stations in the cell served by the base transceiver station 9. The base transceiver station 9 comprises N receivers R1, R2... RN. Thus one receiver is provided for each frequency channel which is to be received by the base transceiver station 9. All of the receivers Rl-RN are the same. Accordingly the components of the first receiver I only are shown.

The first receiver R1 comprises a first bandpass filter 12 which is arranged to filter out signals which fall outside for example the 25MHz bandwidth (in the global system for mobile communications GSM standard) in which the available channels are located. The filtered output is input to a first low noise amplifier 14 which amplifies the received signals. The signal is then passed through a second bandpass filter 16 which filters out 10 any noise, such as harmonics or the like introduced by the first amplifier 14. The output of the second bandpass filter 16 is connected to a mixer 18 which receives a second input from a local oscillator 20. The frequency of the output of the local oscillator 20 will depend on the frequency of the channel 15 allocated to the particular receiver. The output of the second bandpass filter 16 is mixed with the output of the local oscillator 20 to provide a signal at an intermediate frequency IF, which is less than the radio frequency at which the signals are received. The intermediate frequency IF output by the mixer 20 18 of each receiver will be the same for all receivers and may, for example, be 180MHz. For example, if the channel allocated to a given receiver has the frequency of 880MHz, then the local oscillator 20 of that receiver will be tuned to 70OMHz. On the other hand, if the channel allocated to a given receiver has a 25 frequency of 90OMHz, then the local oscillator will be tuned to a frequency of 720MHz. The output of the mixer 18 is input to a third bandpass filter 22 which filters out any noise introduced by the mixer 18. The 30 output of the third bandpass filter 22 is amplified by a second amplifier 24 and output to a surface acoustic wave (SAW) filter 26 or another filter of an appropriate type such as a bandpass filter. The surface acoustic wave filter 26 filters out all signals except that of the channel allocated to that particular 35 receiver. In other words, all the channels received by the antenna 10 with the exception of the channel allocated to the receiver will be filtered out by the surface acoustic wave filter 26. The output of the surface acoustic wave filter 26 is connected to an automatic gain control unit 28 which alters the 40 gain of the signal so that it falls within the dynamic range of an analogue to digital converter 30.

16 Any one or more of the f ilters of the receivers shown in Figure 8 may be a f ilter such as shown in Figure 2.

Whilst embodiments of the present invention have been described generally in the context of a time division multiple access system, embodiments of the present invention can be used in base stations using any other suitable access system such as spread spectrum access systems including code division multiple access etc, frequency division multiple access, or hybrids of any one or more of these access systems.

Embodiments of the present invention can also be incorporated in mobile terminals. Furthermore, embodiments of the present invention are not limited to use in base stations or mobile stations. Embodiments of the present invention have a wide application and can be used in any other telecommunications or non telecommunications application.

17

Claims

CLAIMS:

1. A filter comprising: a dielectric resonator arranged to receive signals to be filtered; and at least one mirror layer arranged in use to reflect signals from the dielectric resonator back into said dielectric resonator.

2. A f ilter as claimed in claim 1, wherein the at least one mirror layer is arranged between said resonator and at least one conducting layer.

3. A f ilter as claimed in claim 2, wherein the conducting layer is in the form of a film.

4. A filter as claimed in claim 2 or 3, wherein the conducting layer is a superconducting layer.

5. A filter as claimed in claim 2, 3 or 4, wherein the at least one superconducting layer is of one of the following materials: YBCO; YBaCuO.

6. A filter as claimed in any one of the preceding claims wherein said at least one mirror layer is of a dielectric material.

7. A filter as claimed in claim 6, wherein the dielectric constant of the resonator layer is greater than that of the at least one mirror layer.

8. A filter as claimed in any one of the preceding claims, wherein the dielectric resonator is thicker than the at least one mirror layer.

9. A filter as claimed in any one of the preceding claims, 18 wherein the dissipation -factor of the resonator and/or the at least one mirror layer is negligible.

10. A filter as claimed in any one of the preceding claims, wherein the dielectric resonator is of one of the following dielectric materials:

sapphire; MgO; Barium Titanate; BaLnTi Oxide.- steatite; Da, Zn tantalate; Ba Nd titanate; Mg, Ca titanate; Zr, Sn titanate; Ba Zn Ta Ti Oxide; BaTitanium Oxide.

11. A f ilter as claimed in any one of the preceding claims wherein the at least one mirror layer is one of the following dielectric materials:

MgO;sapphire; Barium Titanate; BaLnTi Oxide; steatite; Ba, Zn tantalate; Ba Nd titanate; Mg, Ca titanate; Zr, Sn titanate; Ba Zn Ta Ti Oxide; BaTitanium Oxide.

12. A filter as claimed in any one of the preceding claims wherein said filter is substantially cuboid.

13. A filter as claimed in any preceding claim wherein two mirror layers are provided.

14. A filter as claimed in claim 13, wherein a conducting layer is formed on each mirror layer.

15. A base station including a filter as claimed in any one of the preceding claims.