Active-type band-pass filter

Abstract: A minimum noise design is achieved by setting the coupling a factor (Q e1 ) on the input side of an amplifier 18, the coupling Q factor (Q e2 ) on the output side of the amplifier 18, and the gain G of the amplifier 18 so that they satisfy the condition that Q e1 < Q e2 and 1/Q e1 = G/Q e2 . By increasing the term with the negative symbol in the following Equation, the total no-load Q factor (Q₀) is set in the negative region. With the above-mentioned arrangement, the NF can be reduced and the pertinent apparatus can be compacted to allow a practical active-type BPF to be obtained.

Description: BACKGROUND OF THE INVENTION The present invention relates to an active-type band-pass filter (referred to as "active-type BPF" hereinafter), and more particularly to an active-type BPF for use in a mobile communication system such as a portable telephone, mobile phone, and the like. An active-type BPF has been conventionally proposed. According to a conventional design, a no-load Q (Quality) factor of a practical resonator is around 1,000 in a transverse electromagnetic mode (referred to as "TEM mode" hereinafter), 6,000 in a transverse magnetic mode (referred to as "TM mode" hereinafter"), and 20,000 in a transverse electric mode (referred to as "TE mode" hereinafter), where the no-load Q factors mentioned above are increased in the negative regions. When a no-load Q factor of a resonator is designed to be, for example, increased by five times, even if such a resonator is used as an active-type BPF, the resonator is accompanied by an excessively great noise figure (referred to as "NF" hereinafter). Therefore, it has been impossible to provide any practical active-type BPF. The NF can be reduced when the no-load Q factor of the resonator is increased, while the no-load Q factor of the resonator is approximately proportional to the size of the resonator. Therefore, the resonator is required to have a big size when the no-load Q factor of the resonator is increased, which means that the resulting resonator has an impractical large size for use as an active-type BPF. IEEE MTT-S International Microwave Symposium Digest; 30. April - 2. May 1979; Orlando Florida; H. Matsumura et. al. : "An active microwave filter with dielectric resonator", pages 323 - 325, relates to an active microwave filter comprising a dielectric resonator and an active element. The active element is a bipolar transistor or a FET. The active microwave filter is configured as band pass filter and has an attenuation which is negative at the center frequency of the filter. SUMMARY OF THE INVENTION Accordingly, an essential objective of the present invention is to provide a compact and practical active-type BPF with its NF suppressed. In order to achieve the objective mentioned above, the present invention is contrived to provide an active-type BPF which comprises an active feedback resonator including an amplifier, where a no-load Q factor of the active feedback resonator is set in a negative region, wherein a ratio between a coupling intensity Q e1 on the input side of the amplifier and an coupling intensity Q e2 on the output side of the amplifier is made equal to a gain G of the amplifier thereby to achieve a minimum noise temperature design. Furthermore, the total no-load Q factor of the active feedback resonator is set in the negative region by adjusting the values of the intensity Q e1 , Q e2 and gain G, whereby the NF reduction is achieved. According to the present invention, since the NF can be improved without increasing the no-load Q factor of the resonator, there can be obtained a compact and practical active-type BPF with its NF suppressed. BRIEF DESCRIPTION OF THE DRAWINGS These and other objectives, features and advantages of the present invention will become apparent from the following description taken in conjunction with the preferred embodiment thereof with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of an embodiment of an active-type BPF in accordance with the present invention with a part thereof removed; Fig. 2 is an equivalent circuit of the embodiment shown in Fig. 1; Fig. 3 is a graph showing a minimum noise point in designing an active-type BPF; Fig. 4 is a graph showing a comparison between a NF characteristic of an active-type BPF and a NF characteristic of a conventional passive-type BPF when the BPFs have the same gain S 21 ; Fig. 5 is a graph showing a comparison between the NF characteristic of the active-type BPF and the NF characteristic of the passive-type BPF; Fig. 6 is a graph showing a comparison between the no-load Q factor of the active-type BPF and the no-load Q factor of the conventional passive-type BPF when the BPFs have the same NF value; and Fig. 7 is an equivalent circuit of the conventional passive-type BPF. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following describes a preferred embodiment of the present invention with reference to the attached drawings. Referring to Figs. 1 and 2, an active-type BPF 10 of the present embodiment includes an active feedback resonator 12 having a housing 14 for forming a cavity. In the cavity of the housing 14, there is provided a resonator 16 such as a TM single-mode resonator. To an external surface of the housing 14, there is mounted an amplifier 18 having its input and output loops 20 and 22 disposed in parallel within the housing 14. The amplifier 18 is magnetically coupled to the resonator 16 in magnetic field by way of the amplifier input and output loops 20 and 22 located within the housing 14. With the above-mentioned arrangement of the active feedback resonator 12, a part of the resonance electromagnetic field power of the resonator 16 is taken out through the input loop 20, the taken out power being amplified by means of the amplifier 18. Then the resultant amplified power is fed back to the resonator 16 through the output loop 22. The active feedback resonator 12 is further provided with an input port 24 and an output port 26 which are linearly mounted to respective external surfaces of the housing 14 opposing to each other. In more detail, the input port 24 and the output port 26 are inwardly passed through the housing 14 in such a manner that the input and output ports 24 and 26 are magnetically coupled to the resonator 16 in magnetic field by way of its magnetic field coupling loops 28 and 30 which are located within the housing 14. As described above, it is to be noted that the band-pass filter constructed by providing such input and output ports 24 and 26 in the active feedback resonator 12 is referred to as the active-type BPF 10. First, an electric design of the active-type BPF 10 is determined as expressed by the following Equations 1 through 4, and a noise design is determined as expressed by the following Equation 5. It is noted that a relation as expressed by the following Equation 6 holds between the noise power N a , NF of the amplifier 18, and the gain G of the amplifier 18. Equation 1: S 21 2 = ( 2 Q L Q e ) 2 Equation 2: 1 Q L = 1 Q 0 + 2 Q e Equation 3: 1 Q 0 = 1 Q 00 + 1 Q e1 + 1 Q e2 - 2 G Q e1 Q e2 Equation 4: 1 Q e1 = G Q e2 Equation 5: N out = 4 Q L 2 Q e Q 00 N 0 + 4 Q L 2 Q e Q e2 Na + 4 Q L 2 Q e 2 N in Equation 6: NF = 10 log(1 + N a GN 0 ) According to Equation 6, the noise power N a can be expressed by the following Equation 7. Equation 7: N a = GN 0 (10 NF 10 - 1) The total NF of the active-type BPF 10 can be calculated according to the following Equation 8. Equation 8: NF = 10 log N out S 21 2 N in In Equations 1 through 8, S 21 denotes the gain at the center frequency after filtering, Q 0 the no-load Q factor of the active feedback resonator 12, Q 00 the original no-load Q factor of the resonator 16, Q e the coupling Q factor of the input/output port, Q e1 the coupling Q factor on the input side of the amplifier external to the feedback loop, Q e2 the coupling Q factor on the output side of the amplifier external to the feedback loop, Q L the load Q factor of the total circuit of the active feedback resonator,- G the gain (power ratio) of the amplifier, N in the noise power at the input port, N out the noise power at the output port, N a the noise power on the output side of the amplifier, and N 0 the white-noise power. As to Equations 2 and 3, according to a conventionally known passiv-typ BPF as shown in Fig. 7, the active feedback resonator is designed to have the no-load Q factor Q 0 in a range of Q 0 > Q 00 . While in the embodiment of the present invention, the no-load Q factor Q 0 of the active feedback resonator is set in the negative region with the load Q factor Q L set within the positive range. For example, assuming that the active feedback resonator is designed to have Q e = 400 and Q 0 = - 300, the load Q factor Q L is determined to be 600 according to Equation 2. The solution for satisfying both Equations 3 and 4 at the same time is given as following: 1/Q e1 = (1/Q 00 - 1/Q 0 ) · G/(G-1) > G/(G-1) · 1/Q 00 1/Q e2 = (1/Q 00 - 1/Q 0 ) · 1/(G-1) > 1/(G-1) · 1/Q 00 when the no-load Q factor Q 0 is negative (Q 0 < 0). Accordingly, the following inequality is obtained: Q e1 < Q 00 (G-1)/G Q e2 < Q 00 (G-1) For example, assuming that G=16dB, i.e., 39.8 and Q 00 =1000, the following range is obtained: Q e1 < 975 Q e2 < 38800 As to the value of G, the design is principally possible so long as G exceeds 0dB (i.e., power ratio greater than 1). As to the white-noise power N 0 in Equation 5, the position of occurrence of the white-noise power N 0 in the equivalent circuit of Fig. 2 corresponds to the resistance portion of the no-load Q factor Q 00 . The no-load Q factor Q 00 is calculated using circuit constants as following: Q 00 = ω 0 L/R where ω 0 denotes a resonance frequency, L denotes a sum of inductances of the equivalent circuit of the resonator, and R denotes an internal resistance of the resonator which corresponds to Q 00 . In this equation, the noise power generated by the resistance R is approximately represented by the following equation when the resistance R is substantially smaller than 50Ω: N = (4R/Z 0 )N 0 Accordingly, Equation 5 includes the term of N 0 . As expressed by Equation 4 in connection with the design of the active-type BPF 10, the minimum noise temperature design is achieved when the coupling Q factor (Q e1 ) on the input side of the amplifier 18 and the coupling Q factor (Q e2 ) on the output side of the amplifier 18 have such a relation as Q e1 < Q e2 under the condition that the ratio between Q e1 and Q e2 coincides with the gain G of the amplifier 18. The above fact can be also apparent from Fig. 3. Fig. 3 shows a characteristic of a standardized noise temperature  with respect to Q e2 /GQ e1 , where the amplifier 18 has a gain G of 16 dB. The standardized noise temperature  is expressed by the following Equation 9. Equation 9:  = 1 T a {( Q 00 Q 0 - Q 00 ) T n - ( Q 0 Q 0 - Q 00 ) T 0 } = G Q e2 ( 2 G Q e1 Q e2 - 1 Q e1 - 1 Q e2 ) -1 where T n denotes the noise temperature and T 0 denotes the white-noise temperature. It is to be noted here that the noise temperature is defined under such a condition that a constant white-noise power N 0 is generated from a standard resistor at a temperature of 290°K. In Equation 9, one of Q e1 and Q e2 can be obtained according to the electrical design of the active-type BPF, and the other can be obtained according to the design of the minimum noise temperature as expressed by Equation 4. By making the value of the term with the negative symbol in the right member of Equation 3 greater than (1/Q 00 + 1/Q e1 + 1/Q e2 ), the no-load Q factor of the active feedback resonator 12 can be set in the negative region. According to the design conforming to Equations 3 and 4, the active-type BPF 10 has a gain S 21 which exceeds 0 dB level in the vicinity of the center frequency as indicated by the curve A in Fig. 4. That is, the power is amplified without attenuation in the pass band region to have the power ratio greater than 1. Fig. 4 shows a comparison between a NF characteristic of an active-type BPF and a NF characteristic of a conventional passive-type BPF when the BPFs have the same gain S 21 , where the curve A in Fig. 4 indicates the frequency characteristic of the gain S 21 of the active-type BPF 10 and the curve B in Fig. 4 indicates the frequency characteristic of the gain S 21 of the passive-type BPF. The active-type BPF has not been put into practical use yet, and therefore a conventional passive-type BPF 1 as shown in Fig. 7 is used as a comparison example. Referring to Fig. 7, a passive-type BPF 1 comprises a resonator 2 and an amplifier 3 which is coupled to the output side of the resonator 2 in magnetic field. Similarly to Fig. 2, Q 00 denotes a no-load Q factor of the resonator 2, Q e a coupling Q factor of the input/output port, G the gain (power ratio) of the amplifier 3, N in a noise power at the input port, N out a noise power at the output port. A comparison between the present embodiment and a conventional passive-type BPF 1 is then provided by setting the following points in designing. An amplifier 18 having a gain G of 16 dB and a noise figure NF AMP of 2.5 dB is used on the condition that the no-load Q factor (Q 00 ) of the resonator 16 is 1,000, the total load Q factor (Q L ) of the circuit is 500, and the gain S 21 at the center frequency (f 0 = 900 MHz) obtained after filtering is +10 dB. It is noted that the gain S 21 of +10 dB is obtained by increasing the gain S 21 from -6 dB by the gain G of 16 dB of the amplifier 18, and the no-load Q factor (Q 0 ) of the active feedback resonator is set at 231 in the present embodiment. With the above-mentioned arrangement, a total NF of 4.3 dB in the present embodiment (indicated by the line C in Fig. 4) can be achieved in the active-type BPF 10 as compared with the total NF of 7.9 dB (indicated by the curve B in Fig. 4) at the center frequency in the comparison example of the passive-type BPF 1. Therefore, in the present embodiment, the total NF can be improved by 3.6 dB. Furthermore, it can be found that the NF of the active-type BPF 10 has almost no frequency dependence. Fig. 5 shows the NF characteristics with respect to Q 00 /Q L , where the NF of the active-type BPF 10 of the present embodiment is indicated by the curve D. As apparent from the graph, the total NF of the present embodiment is remarkably improved as compared with the NF of the passive-type BPF 1 indicated by the curve E. As shown in Fig. 6, with a noise design having the NF of the active-type BPF 10 made coincident with that of the passive-type BPF 1, the active-type BPF 10 is allowed to have a smaller Q 00 /Q L value as indicated by the curve F. Therefore, a resonator 16 having a small no-load Q factor can be utilized. Assuming that the active-type BPF 10 is allowed to have a total NF of 7.86 dB, the resonator 16 is only required to have a no-load Q factor (Q 00 ) of 141. Therefore, if the active-type BPF 10 is allowed to have an NF on the current level, a resonator 16 having a small no-load Q factor, i.e., a compacted resonator 16 can be used. A further compacting of the resonator can be achieved in the order of TE01δ mode resonator → TM mode resonator → TEM mode resonator → strip line filter, with which the pertinent apparatus can be reduced in size. Practically, the no-load Q factor of the resonator 16 can be reduced to approximately 1/7 in the above example of design setting, and therefore the single unit of the resonator 16 can be reduced in volume to approximately 1/25. It is to be noted here that Fig. 5 indicates the fact that the NF can be improved in the region where the Q 00 /Q L value is smaller than the value at the intersection of the active-type BPF 10 and the passive-type BPF 1 (i.e., intersection of the curve D and the curve E). Further, Fig. 6 indicates the fact that the active-type BPF 10 can be compacted in the region where the Q 00 /Q L value is smaller than the value at the intersection of the curve F and a reference line G. However, there is no problem since the practical Q 00 value is a small value. As described above, in the present embodiment, the gain S 21 at the center frequency of the active-type BPF is increased from -6 dB to 10 dB by using the amplifier 18 having a gain G of 16 dB. However, in the active-type BPF 10, the gain S 21 at the center frequency can be increased from -6 dB to +10 dB by adjusting the coupling Q (Q e ) at the input/output port even when an amplifier 18 having a gain G different from 16 dB is used so long as the gain is greater than a specified value, and then the same characteristics as described hereinbefore can be obtained. Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be noted here that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention as defined by the appended claims, they should be construed as included therein.

Claims: An active-type band-pass filter comprising an active feedback resonator (12) having an amplifier (18) to be magnetically coupled thereto, wherein the no-load quality factor (Q 0 ) of said active feedback resonator is set in the negative region; characterized in that said active-type band-pass filter is so set to as to satisfy the condition that Q e1 < Q e2 and 1/Q e1 = G/Q e2 assuming that the intensity of the coupling on the input side of the amplifier (18) to be coupled to the active feedback resonator (12) is Q e1 , the intensity of coupling on the output side of the amplifier (18) to be coupled to the active feedback resonator (12) is Q e2 , and the gain of the amplifier is G. The active-type band-pass filter as claimed in Claim 1, wherein said active feedback resonator (12) has a housing (14) in which there is provided a resonator (16) and said amplifier is mounted to an external surface of the housing. The active-type band-pass filter as claimed in Claim 2, wherein said amplifier (18) is coupled in magnetic field to said resonator (16) by way of an amplifier input loop (20) and an amplifier output loop (22) disposed in parallel within the housing (14), whereby said active feedback resonator (12) takes out a part of the resonation electromagnetic field power of the resonator (16) through the input loop (20), and the obtained power is amplified by means of said amplifier (18) and fed back to the resonator (16) through the output loop (22). The active-type band-pass filter as claimed in Claim 2, wherein said active feedback resonator (12) is further provided with an input port (24) having its magnetic field coupling loop (28) and an output port (26) having its magnetic field coupling loop (30) linearly disposed opposing to each other, wherein said input port (24) and output port (26) are inwardly passed through the housing (14) as coupled in magnetic field to the resonator (16) by way of the magnetic field coupling loops (28) and (30) disposed within the housing (14). The active-type band-pass filter as claimed in Claim 1 having an electric design determined as expressed by the following Equations 1 through 4, and having a noise design determined as expressed by the following Equation 5: Equation 1: S 21 2 = ( 2 Q L Q e ) 2 Equation 2: 1 Q L = 1 Q 0 + 2 Q e Equation 3: 1 Q 0 = 1 Q 00 + 1 Q e1 + 1 Q e2 - 2 G Q e1 Q e2 Equation 4: 1 Q e1 = G Q e2 Equation 5: N out = 4 Q L 2 Q e Q 00 N 0 + 4 Q L 2 Q e Q e2 Na + 4 Q L 2 Q e 2 N in |S 21 | 2 = gain at the center frequency, Q 0 = no-load Q factor of the active feedback resonator (12), Q L = load Q factor of the total circuit of the active resonator (10), Q e = coupling Q factor of the input/output port, Q 00 = no-load Q factor of resonator (16), Q e1 = coupling Q factor on the input side of the amplifier (18) external to the feedback loop, Q e2 = coupling Q factor on the output side of the amplifier (18) external to the feedback loop, N in = noise power at the input port, N out = noise power at the output port, N a = noise power at the output port of the amplifier (18), N 0 = white-noise power. The active-type band-pass filter as claimed in Claim 5, wherein the no-load quality factor (Q 0 ) of said active feedback resonator is set in the negative region by increasing the term with the negative symbol in the Equation 3.

Inventors: HIDAKA SEIJI, ISHIKAWA YOUHEI, HIDAKA, SEIJI, ISHIKAWA, YOUHEI, HIDAKA, SEIJI, MURATA MANUFACTURING CO.,LTD., ISHIKAWA, YOUHEI, MURATA MANUFACTURING CO.,LTD.
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1993
Main classification: H01P7
IPC codes: H01P7, H01P1