GB2302453A

GB2302453A - Dielectric filter

Info

Publication number: GB2302453A
Application number: GB9612765A
Authority: GB
Inventors: Masayuki Atokawa; Yasuo Yamada; Haruo Matsumoto
Original assignee: Murata Manufacturing Co Ltd
Current assignee: Murata Manufacturing Co Ltd
Priority date: 1995-06-20
Filing date: 1996-06-19
Publication date: 1997-01-15
Anticipated expiration: 2016-06-19
Also published as: JP3050090B2; GB9612765D0; DE19624691C2; DE19624691A1; GB2302453B; JPH098505A

Description

2302453 1 DIELECTRIC FILTER The present invention relates generally to a

dielectric filter and, more particularly, to a dielectric filter for use in a high-frequency circuit for a mobile communication unit or the like.

As the use of cellular mobile communication systems is becoming widespread, there is an increased demand for a filter for use in a communication unit that is applicable to two types of mobile communication systems employing different frequency bands, or a filter applicable for the common use of communication units for individual communication systems. For example, in a high-frequency circuit applicable to both a mobile communication system using the 800 MHz band and a system using the 1.5 GHz band, the following type of filter for selectively passing or attenuating the two types of waves in the different frequency bands is conventionally provided. According to the conventional technique, as illustrated in Fig. 14, a filter F1 for passing or attenuating a wave in the 800 MHz band and a filter F2 for passing or attenuating a wave in the 1.S GHz band are connected in parallel to each other, whereby a filter circuit can be configured. In this manner, two band-pass filters having respective pass bands with different center frequencies may be combined, thereby implementing 2 bandpass characteristics for allowing waves with frequencies fl and f2 to pass through the respective filters, as illustrated in Fig. 15. on the other hand, two filters having respective attenuation bands with different center frequencies may be combined, whereby band attenuation characteristics for attenuating waves with frequencies fl and f2, as shown in Fig. 16, can be obtained.

However, the conventional filter of the above type present the following problem. Simply connecting a pair of filters in parallel to each other, as described above, causes interference between the filters. Accordingly, desired characteristics cannot be obtained. Matching circuits are thus required for the filters. Figs. 17 and 18 illustrate examples of the matching circuits. Referring to Fig. 17, BPF1 and BPF2 indicate band-pass filters each having two stages of dielectric resonators, while M1 and M2 represent phasing circuits. The two filters are thus matched by the phasing on the other hand, referring to Fig. 18, BEF1 band-pass circuits. and BEF2 designate band attenuation filters each having the three stages of dielectric resonators, while Mi and M2 indicate phasing circuits. The two band attenuation filters are thus matched by the phasing circuits. In this manner, according to the conventional technique, a pair 3 of filters are required, which also necessitates matching circuits for connecting the two filters. This enlarges the overall filter and also increases costs.

Accordingly, it is an object of the present invention to provide a downsized and inexpensive dielectric filter that can selectively pass or attenuate waves having two respective frequencies.

In order to pass or attenuate two frequencies with a pair of filter circuits, according to one aspect of the present invention, there is provided a dielectric filter comprising a TEM-mode dielectric resonator, the TEM-mode dielectric resonator including one end, which is shortcircuited, and the other end, which is opened, wherein the frequencies of a fundamental-wave resonant mode and/or a third-order resonant mode are determined in such a manner that a first frequency is passed or attenuated according to the fundamental-wave resonant mode and that a second frequency is passed or attenuated according to the third-order resonant mode.

For setting the first and second frequencies to predetermined values, the impedance ratio of the dielectric filter of the present invention between the short-circuit end and the opened end may be varied to set the frequencies of the fundamental-wave resonant mode and the third-order resonant mode to the predetermined values.

4 In order to generate an attenuation pole in a higher-frequency band or a lower-frequency band of the first frequency or the second frequency, a coupling circuit for coupling the adjacent dielectric resonators or coupling the dielectric resonator and an external circuit may be provided, and a reactance device may be disposed between the coupling circuit and the dielectric resonator, thereby determining the frequency of an attenuation pole. in one type of dielectric filter of the present invention, one end of the TEM-mode dielectric resonator is shortcircuited, while the other end thereof is opened, whereby at least two modes, i.e., the fundamental-wave resonant mode and the third-order resonant mode, can be generated. The frequencies of both the resonant modes are determined so that a first frequency can be passed or attenuated according to the fundamental-wave resonant mode and that a second frequency can be passed or attenuated according to the third-order resonant mode. It is thus possible to pass or attenuate the two frequencies by use of a pair of dielectric filters.

In another type of dielectric filter of the present invention, the impedance ratio between the short-circuit end and the opened end of the filter is varied, whereby the frequency of the fundamental-wave resonant mode is set to be, for example, 800 MHz, while the frequency of the third-order resonant mode is set to be, for example, 1.5 GHz. With this arrangement, the two frequencies in the 800 MHz band and in the 1.5 GHz band, respectively, can be passed or attenuated.

In still another type of dielectric filter of the present invention, a coupling circuit for coupling the adjacent dielectric resonators or coupling the dielectric resonator and an external circuit is provided, and a reactance device is further disposed between the coupling circuit and the dielectric resonator. The provision of the above-mentioned coupling circuit and the reactance device causes the generation of an attenuation pole in a higher-frequency band or a lower-frequency band of the first frequency or the second frequency. Accordingly, unnecessary frequency signals in a higher-frequency band or a lower-frequency band of the first frequency or the second frequency can be attenuated efficiently and significantly.

Exemplary embodiments of the present invention will now be described with reference to the accompanying drawings of which:- Fig. 1 is a perspective view of a band-pass filter according to an embodiment of the present invention; Fig. 2 schematically illustrates the configuration of the dielectric plate shown in Fig. 1; Fig. 3 is an band-pass filter Fig. 4 is a filter shown in Figs. 5A to in the impedance 6 equivalent circuit diagram of the shown in Fig. 1; characteristic diagram of the band-pass Fig. 1; 5D, illustrate examples of the variations ratio between the short-circuit end and the opened end of the filter; Fig. 6, which is comprised of Figs. 6A and 6B, illustrates examples of a change in the frequency of the third-order resonant mode relative to the frequency of the fundamental-wave resonant mode achieved by the variations shown in Fig. 5; Fig. 7 is an elevation of a band-pass filter according to a second embodiment of the present invention; Fig. 8 is an equivalent circuit diagram of the band-pass filter shown in Fig. 7; Fig. 9 is a characteristic diagram of the band-pass filter shown in Fig. 7; Figs. IOA and 10B are perspective views of a band-pass filter according to a third embodiment of the present invention; Fig. 11 is an equivalent circuit diagram of the band-pass filter shown in Fig. 10; 7 Fig. 12 is an elevation of a band attenuation filter according to a fourth embodiment of the present invention; Fig. 13 is an equivalent circuit diagram of the band attenuation filter shown in Fig. 12; Fig. 14 is a schematic diagram of a conventional filter circuit; Fig. 15 illustrates an example of the characteristics of the filter shown in Fig. 14; Fig. 16 illustrates another example of the characteristics of the filter shown in Fig. 14; Fig. 17 illustrates an example of matching circuits for use in a conventional band-pass filter; and Fig. 18 illustrates another example of matching circuits for use in a conventional band-pass filter.

A description will now be given of the configuration of a band-pass filter according to a first embodiment of the present invention with reference to Figs. 1 to 6.

Fig. 1 illustrates a band-pass filter without having a shield cover attached thereto. A pair of /4-type transverse electromagnetic (TEM)-mode dielectric resonators Ra and Rb are configured in the following manner. A through-hole is formed in each of dielectric blocks la and lb along its central axis. Also, an internal conductor is formed within the inner peripheral indicated by la and 1b desianated bv S 8 surface of each block, while external conductors 2a and 2b are formed on the surfaces other than the surfaces. As a result, the surface I can be an opened end, while the surface represented by S2 can be a short-circuit end. Terminals 3a and 3b are inserted into the respective through-holes. The two dielectric resonators Ra and Rb constructed as described above and a dielectric plate 4 are mounted on the top surface of a substrate 7. Two electrodes deposited on the obverse surface of the dielectric plate 4 are each connected to a respective one of the terminals 3a and 3b, while other electrodes are deposited on the reverse surface of the dielectric plate 4 and are electrically connected to input/output electrodes 8 provided for the surface of the substrate 7. Referring to Fig. 2, electrodes 5a and 5b are mounted on the obverse surface 100 of the dielectric plate 4, while electrodes 6a and 6b are formed on the reverse surface 200 of the plate 4. A capacitance C1 is generated between the electrodes 5a and 6a; a capacitance C3 is produced between the electrodes 5b and 6b; and a capacitance C2 is generated between the electrodes Sa and 5b.

Fig. 3 illustrates an equivalent circuit of the band-pass filter shown in Fig. 1. As is seen from Fig.

9 3, the filter includes a band-pass filter circuit formed of two dielectric resonators. Fig. 4 is a characteristic diagram of the bandpass filter shown in Fig. 1. In this diagram, the horizontal axis indicates the frequency, while the vertical axis designates attenuation magnitude (dB). S21 indicates bandpass characteristics between input and output ends; Sil designates reflection characteristics of the input end; and S22 represents reflection characteristics of the output end. In this manner, the frequency of the fundamental-wave resonant mode of the resonators Ra and Rb is set to be 800 MHz, while the frequency of the third-order resonant mode is set to be 1.9 GHz, whereby band-pass filter characteristics can be obtained in which two types of waves in the 800 MHz band and in the 1.9 GHz band are allowed to pass therethrough.

In order to set the frequencies of the fundamental-wave resonant mode and the third-order resonant mode to predetermined values, adjustments are made of the impedance ratio between the short-circuit end and the opened end of the dielectric resonators. Figs. 5 and 6 illustrate examples of the variations in the impedance ratio. For example, as illustrated in Fig. 5A, the diameter of the internal conductor at the opened end is set larger than that at the short-circuit end, thereby increasing the capacitance' component in a region of the fundamental-wave resonant mode having a great magnitude of electric-field energy. This decreases the resonant frequency of the fundamental-wave resonant mode. On the other hand, in the third-order resonant mode, both the peak and trough of the electric-field energy are distributed in a region L1 in which the internal conductor is enlarged, whereby the resonant frequency of the third-order resonant mode cannot be significantly changed. As a result, as shown in Fig. 6A, the frequency f2 of the third-order resonant mode relative to the frequency fl of the fundamental-wave resonant mode becomes a higher frequency f21. In other words, a difference between the frequencies fl and f2 becomes larger.

Conversely, as shown in Fig. 5B, the diameter of the internal conductor at the short-circuit end is made larger than that at the opened end, thereby increasing the capacitance component in a region of the third- order resonant mode on which electric-field energy comparatively concentrates. This decreases the resonant frequency of the third-order resonant mode. In the fundamental-wave resonant mode, on the other hand, electricfield energy is relatively small in a region L2 in which the internal conductor is enlarged, whereby the

11 resonant frequency of the fundamental-wave resonant mode is not significantly changed. As a consequence, the frequency f2 of the thirdorder resonant mode relative to the frequency fl of the fundamental-wave resonant mode becomes a lower frequency f21, as shown in Fig. 6B. Namely, a difference between the frequencies fl and f2 becomes smaller.

In place of varying the diameter of the internal conductor, the size of the external conductor may be changed, as illustrated in Figs. SC and 5D. For example, as illustrated in Fig. SC, the external conductor at the short-circuit end is made smaller than that at the opened end, thereby offering advantages similar to those of the example shown in Fig. 5B. A difference between the frequencies fl and f2 can thus be reduced. On the other hand, as shown in Fig. SI), the external conductor at the opened end is made smaller than that at the short-circuit end, thereby obtaining advantages similar to those of the example shown in Fig. SA. A difference between the frequencies fl and f2 can thus be increased.

As has been discussed above, the dimensions Ll and L2 along the axis of the resonator and the diameter of the internal conductor or the size of the external conductor are varied, whereby the frequencies of the fundamental-wave resonant mode and the third-order 12 resonant mode can be set to the respective predetermined values.

The configuration of the band-pass filter according to a second embodiment of the present invention will now be explained with reference to Figs. 7 to 9.

Fig. 7 is an elevation of a band-pass filter of this embodiment. The filter includes two TEM-mode dielectric resonators Ra and Rb and a dielectric plate 4 mounted on the top surface of the substrate 7. The second embodiment differs from the first embodiment in that electrodes 10a and lob are disposed on the dielectric plate 4, and chip inductors Ila and lib used as reactance devices are mounted between the two electrodes loa and lob and the other two electrodes 5a and 5b, respectively, the terminals 3a and 3b of the dielectric resonators being further connected to the electrodes 10a and lob, respectively.

Fig. 8 is an equivalent circuit of the band-pass filter shown in Fig. 7, and Fig. 9 is a characteristic diagram of the equivalent circuit shown in Fig. S. Referring to Fig. 8, inductors La and Lb correspond to the abovedescribed chip inductors Ila and llb. In this fashion, the inductors La and Lb are connected between the resonators Ra and Rb and a coupling circuit, thereby generating attenuation poles between the first and second 13 frequencies fl and f2 and in a frequency band above the second frequency f2, as shown in Fig. 9. In this case, increased inductance of the inductors La and Lb decreases the frequency fdl of the first attenuation pole and increases the frequency fd2 of the second attenuation pole. Conversely, decreased inductance of the inductors La and Lb increases the frequency fdl of the first attenuation pole and decreases the frequency fd2 of the second attenuation pole.

Although chip inductors are employed in the embodiment shown in Fig. 7, coils may instead be used. Further, the lengths of the terminals 3a and 3b pulled out of the resonators (the distance between the resonators and the dielectric plate 4) may be changed to determine the inductance of the inductors La and Lb.

An explanation will now be given of the construction of a band-pass filter according to a third embodiment of the present invention with reference to Figs. 10A, 10B and 11. In this embodiment, the band pass filter is formed of a single dielectric block.

Fig. 10A is a perspective view of the overall dielectric resonator, and Fig. 10B is a partially cutaway perspective view illustrating the resonator shown in Fig. 10A turned upside down. Two through-holes 12a and 12b formed with internal steps are provided for the dielectric block 1, and an internal conductor 15 is formed on the inner peripheral surface of each of the through holes 12a and 12b. The internal conductor 15 is partially provided with a gap 16 where a capacitance is generated. An external conductor 2, an input/output conductor 14 and so on are formed on the outer surface of the dielectric block 1. Fig. 11 is an equivalent circuit of the filter shown in Fig. 10A. In Fig. 11, Ra and Rb indicate dielectric resonators constituted by the internal conductors 15 formed on the inner peripheral surfaces of the through-holes 12a and 12b, the dielectric block 1 and the external conductor 2, while Ca and Cb designate external coupling capacitances generated between the internal conductors 15 and the input-output conductors 14.

As illustrated in Fig. 10B, the internal diameter of each of the throughholes 12a and 12b at the short-circuit end is differentiated from that at the opened end, thereby varying the impedance ratio between the shortcircuit end and the opened end. At the same time, the axial length of the resonators, the pitch between the through-holes 12a and 12b, and the size of the gap provided for part of the internal conductor 15 are determined. Accordingly, band-pass filter characteristics for allowing waves in the two different band ranges to pass the filter can be obtained.

is The construction of a band-attenuation filter according to a fourth embodiment of the present invention will now be described with reference to Figs. 12 and 13. Fig. 12 is an elevation of a band-attenuation filter. Mounted on the substrate 7 are electrodes connected to the reverse surfaces of chip capacitors 17a, 17b and 17c, input electrodes 8 and 9, and A/4 transmission lines 18a and 18b. The three TEM-mode dielectric resonators Ra, Rb and Rc and chip capacitors 17a, 17b and 17c are further mounted on the top surface of the substrate 7. Also, the terminals 3a, 3b and 3c of the resonators are connected to the surface electrodes of the chip capacitors 17a, 17b and 17c, respectively.

Fig. 13 is an equivalent circuit of the band-attenuation filter shown in Fig. 12. In Fig. 13, Ca, Cb and Cc correspond to the chip capacitors 17a, 17b and 17c, respectively, shown in Fig. 12. It will now be assumed that the electrical length of the /4 transmission lines 18a and 18b is equal to 1/4 of the wavelength at the frequency of the fundamental -wave resonant mode. Then, the electrical length is also substantially equal to 3/4 of the wavelength at the frequency of the third-order resonant mode, and the adjacent resonators are out of phase by substantially 90". Accordingly, the filter shown in Fig. 12 exhibits band-attenuation characteristics in the third-order resonant mode, as well as in the fundamental-wave resonant mode.

The band-attenuation filter according to the fourth embodiment exhibits characteristics similar to those of the filter shown in Fig. 16. In Fig. 16, it is determined that A indicates an attenuation band ranging from 810 to 830 MHz; B designates a pass band ranging from 940 to 960 MHz; C represents a pass band ranging from 1429 to 1453 MHz; and D denotes an attenuation band ranging from 1477 to 1501 MHz. Thus, the above-described filter for use in, for example, a transmission line, can be applied for the common use of a mobile communication system using the 800 MHz band and another system using the 1.5 GHz band.

Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art.

17

Claims

CLAIMS:

1. A dielectric filter comprising a TEM-mode dielectric resonator, said TEM-mode dielectric resonator including two ends, one of which is shortcircuited, and the other of which is opened; said resonator having a fundamental-wave resonant mode and a third-order resonant mode corresponding to respective frequencies, at least one of said modes being determined in such a manner that said filter passes or attenuates a first frequency according to the fundamental-wave resonant mode and passes or attenuates a second frequency according to the third- order resonant mode.

2. A dielectric filter according to claim 1, wherein the frequencies of the fundamental-wave resonant mode and the third-order resonant mode are set to predetermined values by adjusting an impedance ratio between the short-circuit end and the opened end of said dielectric resonator.

3. A dielectric filter according to claim 1, wherein a coupling circuit is disposed to couple a pair of said dielectric resonators or to couple said dielectric resonator and an external circuit, and a reactance device is also disposed between said coupling circuit and said dielectric resonator, thereby obtaining bandpass 4 h 1_) ( 4 643k characteristics for passing said first and second frequencies and generating an attenuation pole in a band other than said first and second frequencies.

4. A method of adjusting a dielectric filter comprising a TEM-mode dielectric resonator, said TEM-mode dielectric resonator including two ends, one of which is shortcircuited, and the other of which is opened, in such a manner that said filter passes or attenuates a first frequency and a second frequency; said method comprising the step of adjusting at least one of a fundamental-wave resonant mode and a third- order resonant mode of said dielectric resonator, said modes corresponding to respective frequencies, such that said filter passes or attenuates said first frequency according to the fundamental-wave resonant mode and passes or attenuates said second frequency according to the third-order resonant mode.

5. A method according to claim 4, comprising the steps of adjusting both of said fundamental-wave and thirdorder resonant modes.

6. A method according to claim 4, comprising the steps of setting the frequencies of the fundamental-wave resonant mode and the third-order resonant mode to predetermined values by adjusting an impedance ratio between the short-circuit end and the opened end of said dielectric resonator.

a-l- W

7. A method according to claim of providing a coupling circuit to dielectric resonators or to couple resonator and an external circuit, and providing a reactance device between said coupling circuit and said dielectric resonator, thereby obtaining bandpass characteristics for passing said first and second frequencies and generating an attenuation pole in a band other than said first and second frequencies.

8. A dielectric filter substantially as hereinbefore described with reference to the accompanying drawings.

4, comprising the steps couple a pair of said said dielectric

3. A dielectric filter according to claim 1, wherein a coupling circuit is disposed to couple a pair of said dielectric resonators or to couple said dielectric resonator and an external circuit, and is also disposed between said coupling dielectric resonator, thereby obtaining a reactance device circuit and said bandpass characteristics for passing said first and second frequencies and generating an attenuation pole in a band other than said first and second frequencies. 4. A method of adjusting a dielectric filter comprising a TEM-mode dielectric resonator, said TEM-mode dielectric resonator including two ends, one of which is shortcircuited, and the other of which is opened, in such a manner that said filter passes or attenuates a first frequency and a second frequency; said method comprising the step of adjusting at least one of a fundamental-wave resonant mode and a third- order resonant mode of said dielectric resonator, said modes corresponding to respective frequencies, such that said filter passes or attenuates said first frequency according to the fundamental-wave resonant mode and passes or attenuates said second frequency according to the third-order resonant mode. 5. A method according to claim 4, comprising the steps of adjusting both of said fundamental-wave and thirdorder resonant modes. 6. A method according to claim 4, comprising the steps of setting the frequencies of the fundamental-wave resonant mode and the third-order resonant mode to predetermined values by adjusting an impedance ratio between the short-circuit end and the opened end of said dielectric resonator.

Icj 7. A method according to claim 4, comprising the steps of providing a coupling circuit to couple a pair of said dielectric resonators or to couple said dielectric resonator and an external circuit, and providing a reactance device between said coupling circuit and said dielectric resonator, thereby obtaining bandpass characteristics for passing said first and second frequencies and generating an attenuation pole in a band other than said first and second frequencies. 8. A dielectric filter substantially as hereinbefore described with reference to the Figures 1 to 13 of the accompanying drawings.

A 2-o Amendments to the claims have been filed as follows CLAIMS:

1. A dielectric filter comprising a TEM-mode dielectric resonator, said TEM-mode dielectric resonator including two ends, one of which is shortcircuited, and the other of which is opened; said resonator having a fundamental-wave resonant mode and a thirdorder resonant mode corresponding to respective frequencies, at least one of said modes being determined in such a manner that said filter passes or attenuates a first frequency according to the fundamental-wave resonant mode and passes or attenuates a second frequency according to the third- order resonant mode.