Filter apparatus

Abstract: A filter apparatus in accordance with the present invention comprises, for example, a band-pass filter (16) and a plurality of band-stop filters (18,20,22) connected as the post-stage to the band-pass filter. Then, the band-pass filter transmits a band containing a desired frequency band, and the band-stop filters attenuate undesirable frequency bands. Another filter apparatus in accordance with the present invention comprises, for example, a first band-pass filter (16), a circulator (19) connected to the first band-pass filter, and second band-pass filters (21,23,25) connected to the circulator. Then, the first band-pass filter transmits a band containing a desired frequency band to the circulator, and the second band-pass filters transmit undesirable frequency bands and reflect the desire frequency band to the cerculator. The desired frequency band is outputted from this circulator. Still another filter apparatus in accordance with the present invention comprises, for example, a circulator and a band-pass filter connected to the circulator. Then, the band-pass filter transmits an undesirable frequency band and reflects a desired frequency band to the circulator. The desired frequency band is outputted from this circulator at (19).

Description: The present invention relates to a filter apparatus, and specifically to a filter apparatus used for a car telephone of a cellular system and other radio equipment. In recent years, in the United States, attending on a rapid propagation and expansion of car telephones of cellular system and a rapid increase in cellular base station, extention of the width of frequency bands to extention of the width of frequency bands to be used has been studied to increase the number of channels to be used, and assignments of frequency bands to be used have been announced. In this case, the frequency to be used in a service system sometimes approaches the frequency to be used in another service system, and therefore to prevent mutual interference between those service system, for example, a filter apparatus having a sharp characteristic as shown in Fig. 14 and a filter apparatus having a sharp characteristic as shown in Fig. 15 are requested. To obtain such a sharp characteristic, it is considered to configurate a filter apparatus using a cavity resonator, but this apparatus becomes large in shape, going against the request for miniaturization. Also, conventionally, in a filter apparatus using resonators, to obtain such a sharp characteristic, the value of Q is required to be increased by coupling each active circuit to each resonator respectively, resulting in a complicated structure. Furthermore, in the conventional filter using resonators, a load is connected to the output end thereof, and therefore the value of Q 0 of the resonator has to be made considerably large taking the effect of the load into account. In such a case, an active circuit coupled to the resonator has to be operated in unstable state, and thereby the apparatus lacks practical usability. The United States Patent No. 4,538,123 discloses a millimeter wavelength band-pass filter arrangement comprising an input line, a circulator, a band-stop filter, and an output line. In this filter arrangement all frequencies with the exception of a narrow band of frequencies pass through a band-stop filter to a load. In the said narrow band of frequencies, the band-stop filter reflects the desired band-pass frequency(s) back to the circulator where it is delivered to the output line. The present invention is based on the object of providing a filter apparatus which is small in size and has a sharp characteristic. This object is achieved by a filter apparatus in accordance with claim 1. A filter apparatus for solving the principal object of the present invention is a filter apparatus wherein a desired frequency band is located in a pass band thereof and undesirable frequency bands are located in attenuation bands thereof, and comprises an input terminal, a first band-pass filter an input end of which is connected to the input terminal and which is for transmitting a band containing the above-mentioned pass band, a non-reversible circuit element a first terminal of which is connected to an output end of the first band-pass filter, a plurality of second band-pass filters input ends of which are connected in common to a second terminal of the non-reversible circuit element which becomes the output side when the first terminal is set as the input side and pass bands of which are located in the above-mentioned attenuation bands, and an output terminal which is connected to a third terminal of the non-reversible circuit element which becomes the output side when the above-mentioned second terminal is set as the input side, and at least one of the plurality of second band-pass filters comprises a resonator and an active circuit which is coupled to the resonator and becomes a negative resistance in an operating band of the at least one of the second band-pass filters. In this filter apparatus, the first band-pass filter transmits a signal of the band containing the desired frequency band, and the non-reversible circuit element gives the signal to a plurality of the second band-pass filters. The second band-pass filters transmit signals of the undesirable frequency bands, and reflect the signal of the desired frequency band to the non-reversible circuit element. In this case, the active circuit coupled to the resonator in the second band-pass filter becomes a negative resistance, and therefore the value of Q of the second band-pass filter becomes large. For this reason, a plurality of the second band-pass filters sharply transmit the undesirable frequency bands, and sharply reflect the desired frequency band. Then, the non-reversible circuit element gives the reflected desired frequency band to the output terminal. In accordance with the present invesntion, a filter apparatus having a sharp characteristic can be obtained. Moreover, this filter apparatus uses the resonator in place of the cavity resonator, therefore being small in size. Also, in accordance with the present invention, a filter apparatus is containable which can obtain a sharp characteristic by a simple structure that the active circuit is coupled to any one of the resonators of the band-pass filter. Furthermore, in this filter apparatus, no load is required to be connect to the output end of the band-pass filter, and therefore the value of Q 0 of the resonator in the band-pass filter is not required to be taken so large, and the active circuit coupled to the resonator of the band-pass filter can be operated in stable state, and thereby the apparatus excels in practical usability. The above and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the embodiments of the present invention when taken in conjunction with accompanying drawings. Fig. 1 is illustration view showing an embodiment of the present invention. Fig. 2A is an illustration view showing a major part of a second band-pass filter used for the embodiment of Fig. 1, and Fig. 2B is an equivalent circuit diagram of the major part of the same. Fig. 3 is an equivalent circuit diagram of the second band-pass filter used for the embodiment of Fig. 1. Fig. 4 is a graph showing a frequency characteristic of reflection at the input end side of the second band-pass filter as shown in Fig. 3. Fig. 5 is a graph showing a frequency characteristic of the embodiment of Fig. 1. Fig. 6 is an illustration view showing a major part of another example of the second band-pass filter used for the embodiment of Fig. 1. Fig. 7 is an equivalent circuit diagram of the second band-pass filter as shown in Fig. 6, Fig. 8 and Fig. 9 are graphs showing frequency characteristics which are the background of the present invention and are required for it. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 is an illustration view showing an embodiment of the present invention. This filter apparatus 10 comprises, for example, a box-shaped case 12 composed of metal. For example, a coaxial connector 14 is fixed to one side wall of this case 12 as an input terminal in a manner of penetrating therethrough. Also, a first band-pass filter 16 comprising, for example, a dielectric coaxial resonators is housed in this case 12. This band-pass filter 16 has a pass band in a band (about 820 - 853 MHz in this embodiment) containing desired frequency bands (a band of 835 - 845 MHz and a band of 846.5 - 849 MHz in this embodiment). Accordingly, this band-pass filter 16 transmits a signal of the band containing the desired frequency bands. An input end 16a of this band-pass filter 16 is connected to the coaxial connector 14 as an input terminal, for example, by a coaxial cable. An output end 16b of the first band-pass filter 16 is connected to a first terminal 19a of a circulator 19 as a non-reversible circuit element. Furthermore, input ends 21a, 23a and 25a of three second band-pass filters 21, 23 and 25 are connected in common to a second terminal 19b which becomes the output side when the terminal 19a of this circulator 19 is set as the input side. These second band-pass filters 21, 23 and 25 have pass bands in bands wanted to be attenuated among bands transmitted by the band-pass filter 16, for example, a band of 845 - 846.5 MHz, a band of 849 MHz and higher and a band of 835 MHz and lower, respectively. These second band-pass filters 21, 23 and 25 differ in the number of stages of the dielectric coaxial resonator to be used or the like and the pass band, but have structures similar to one another, and therefore detailed description is made particularly on one of the second band-pass filters 21. This second band-pass filter 21 comprises eight sets of a dielectric coaxial resonator 31 of λ/4 whose outer conductor 31a is grounded and a substrate 33 having an amplifier circuit 45 as an active circuit which is coupled to the dielectric coaxial resonator 31. This means that this second band-pass filter 21 comprises, for example, a cuboid-shaped case 27 composed of metal, and space in this case 27 is divided into eight housing parts by seven partition plates 29 composed of metal. Then, in each of these housing parts, for example, the dielectric coaxial resonator 31 of λ/4 and the substrate 33 composed of an insulating material such as epoxy resin are housed. In this case, the outer conductor 31a as one end of each dielectric coaxial resonator 31 is grounded to the case 27. Also, each substrate 33 is disposed at the open end side of the dielectric coaxial resonator 31. As shown in Fig. 2A, on this substrate 33, for example, an L-shaped conductor pattern 35 is formed, and to one end of this conductor pattern 35, an inner conductor 31b as the other end of the dielectric coaxial resonator 31 is connected, for example, by a conductor ribbon 37. Furthermore, on the substrate 33, two electrodes 39a and 39b are formed in the vicinity of one end of the conductor pattern 35, and these electrodes 39a and 39b are coupled to the conductor pattern 35 respectively through gap capacities produced by gaps 41a and 41b. Also, to these electrodes 39a and 39b, the amplifier circuit 45 as an active circuit which is configurated, for example, with a Si bipolar transistor is connected by lines 43a and 43b for phase adjustment. Accordingly, this amplifier circuit 45 is coupled to the dielectric coaxial resonator 31. Also, this amplifier circuit 45 is configurated so as to become a negative resistance in the operating band of the second band-pass filter 21. For this reason, the apparent value of Q of the second band-pass filter 21 becomes large in the operation band of the second band-pass filter 21. Furthermore, on the substrate 33, a conductor pattern 47 for supplying power to this amplifier circuit 45 is formed. Furthermore, the other end of the conductor pattern 35 on each substrates 33 is connected to each electrode 51 as a transmission line by each conductor ribbon 49. Also, these electrodes 51 are disposed in a line with intervals kept so that gap capacities are produced by gaps 52 between them. Accordingly, an equivalent circuit of this dielectric coaxial resonator 31 and surrounding thereof is formed as shown in Fig. 2B. Furthermore, the electrodes 51 at the both ends are coupled to the input end 21a and the output 21b, for example, through gap capacities produces by gaps 54. Accordingly, this second band-pass filter 21 is expressed by an equivalent circuit as shown in Fig. 3. The output end 21b of this second band-pass filter 21 is put in no-load state in this embodiment. In addition, to the output end 21b of this second band-pass filter 21, a load, for example, a resistance may be connected. Also, the dielectric coaxial resonator 31 and the gap capacities such as the one between the electrodes 51 in the second band-pass filter 21 are selected so that the resonance frequencies thereof are located in the band of 845 - 846.5 MHz to be attenuated. Accordingly, for this second band-pass filter 21, the frequency characteristic of reflection at the input end 21a side becomes as shown in Fig. 4. In addition, though not illustrated, the conductor pattern 47 of each substrate 33 is connected to a power source end 21c penetrating through the side wall of the case 27. Then, to this power source end 21c, the DC power source 48 in the case 12 is connected, and power is supplied from the DC power source 48. Another second band-pass filter 23 has a structure similar to the above-described band-pass filter 21, but differs therefrom in that this has a 10-stage dielectric coaxial resonator and the like and the pass band is the band of 849 MHz and higher. Still another second band-pass filter 25 differs from the above-described band-pass filter 21 in that this has a 18-stage dielectric coaxial resonator and the like, and the pass band is the band of 835 MHz and lower. Then, for these second band-pass filters 23 and 25, the output ends thereof are also put in no-load state, and power is supplied from the DC power source 48 also to power source ends 23c and 25c of these second band-pass filters 23 and 25. In addition, loads, for example, resistances may be connected also to the output ends of these second band-pass filters 23 and 25. In addition, the amplifier circuit in each second band-pass filter is configurated so as to become a negative resistance in the operating band of the second band-pass filter. On the other hand, a third terminal 19c becoming the output side when the second terminal 19b of the circulator 19 is set as the input side is connected to the coaxial connector 50 as the output terminal penetrating through the side wall of the case 12. In this filter apparatus 10, the first band-pass filter 16 transmits a signal of the band containing the desired frequency bands to the first end 19a of the circulator 19. Then, this circulator 19 gives the signal to the input ends 21a, 23a and 25a of three second band-pass filters 21, 23 and 25. One second band-pass filter 21 has the pass band in the frequency band of 845 - 846.5 MHz to be attenuated, and therefore sharply reflects signals of bands other than this band, giving them to the second terminal 19b of the circulator 19. Similarly, the other second band-pass filter 23 sharply reflects signals of bands other than the band of 849 MHz and higher, and gives them to the second terminal 19b of the circulator 19, and the still other second band-pass filter 25 sharply reflects signals of bands other than the band of 835 MHz and lower, giving them to the second terminal 19b of the circulator 19. Accordingly, the signal of the desired frequency bands is given to the second terminal 19b of the circulator 19. For this reason, the signal of the desired frequency bands is outputted from the third terminal 19c of the circulator 19, and the signal is given to the coaxial connector 50 as the output terminal. Accordingly, in this filter apparatus 10, a frequency characteristic as shown in Fig. 5 is obtained. In addition, in the embodiment as shown in Fig. 1, the first band-pass filter 16 is installed in the pre-stage of the circulator 19, but in the present invention, it is also possible that no filter is installed in the pre-stage of the circulator 19. In this case, the first terminal 19a of the circulator 19 is connected directly to the coaxial connector 14 as the input terminal. Also, in the embodiment as shown in Fig. 1, the dielectric coaxial resonator of λ/4 is used in the second band-pass filter, but the dielectric coaxial resonator of λ/2 may be used in place of λ/4. In this case, as shown in Fig. 6 and Fig. 7, one end side of the inner conductor 31b of the dielectric coaxial resonator 31 of λ/2 is coupled to the amplifier circuit 45 as an active circuit, and the other end side thereof is connected to the electrode 50 as a transmission line. Or, for the resonator used in the second band-pass filter, a resonator having another structure such as the strip-line resonator may be used without being limited to the dielectric coaxial resonator, and the resonance mode thereof may be a mode such as the TE mode or the TM mode. Furthermore, the embodiment as shown in Fig. 1 has a characteristic of attenuating a plurality of bands, but the apparatus of the present invention is applicable also to a filter apparatus having a characteristic of attenuating one band. In this case, for example, in the embodiment as shown in Fig. 1, only the input end of one second band-pass filter having the pass band in the band to be attenuated has only to be connected to the second terminal 19b of the circulator 19. Also, it is also possible that an active circuit is coupled to only any one of the resonators. In this case, the active circuit is desirably coupled only to the pre-stage or post-stage resonator, and in view of noise generated on the circulator side from the second band-pass filter, it is most desirable to couple the active circuit to the post-stage resonator. The embodiment as shown in Fig. 1 has a characteristic of transmitting two different bands, but in the present invention, a filter apparatus having a characteristic of transitting three or more different bands may be configurated. In this case in the embodiment as shown in Fig. 1, three or more second band-pass filters having pass bands differing from one another have only to be connected to the circulator. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being limited only by the terms of the appended claims.

Claims: A filter apparatus (10) wherein a desired frequency band is located in a pass band thereof and undesirable frequency bands are located in attenuation bands thereof, comprising an input terminal (14), and a circulator (19), characterised in that said circulator (19) has a first terminal (19a) which is connected to an output end (16b) of a first band-pass filter (16), said first band-pass filter (16) having an input end (16a) which is connected to said input terminal (14), said first band-pass filter being for transmitting a band containing said pass band, further characterised in that the filter apparatus comprises: a plurality of second band-pass filters (21,23,25), input ends (21a,23a,25a) of which are connected in common to a second terminal (19b) of said circulator (19) which is the output side when said first terminal (19a) is set as the input side, and pass bands of which are located in said attenuation bands, and an output terminal (50) which is connected to a third terminal (19c) of said circulator (19) which is the output side when said second terminal (19b) is set as the input side, wherein at least one of said plurality of second band-pass filters (21,23,25) comprises a resonator (31), and an active circuit (45) which is coupled to said resonator (31), for establishing a negative resistance in an operating band of said at least one of the plurality of second band-pass filters, and wherein said at least one of said plurality of second band-pass filters (21,23,25) transmits signals of undesirable frequency bands and reflects signals of desired frequency bands, and wherein an output end of said at least one of said plurality of second band-pass filter (21,23,25) is in non-load state. A filter apparatus in accordance with claim 1, wherein said resonator (31) includes a dielectric coaxial resonator. A filter apparatus in accordance with claim 1 wherein said at least one of said plurality of second band-pass filters (21,23,25) includes a plurality of said resonators (31), and said active circuit (45) is coupled to a last-stage resonator of said at least one of said plurality of second band-pass filters. A filter apparatus in accordance with claim 3, which further comprises a case (12), wherein said input terminal (14) is fixed to one side wall of said case (12); said circulator (19) is located in said case (12); said at least one of said plurality of second band-pass filters (21,23,25) are located in said case (12); and said output terminal (50) is fixed to said one side wall of said case (12).

Inventors: HATTORI JUN, ISHIKAWA YOUHEI, NISHIKAWA TOSHIO, HATTORI, JUN, ISHIKAWA, YOUHEI, NISHIKAWA, TOSHIO
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1990
Main classification: H03H11
IPC codes: H03H11, H01P1