Active filter

Abstract: An active filter includes a resonator surface acoustic wave (SAW) filter with at least two ports, an amplifier, and a phase shifter. The SAW filter has either one resonant mode or two or more resonant modes which couple mutually to show characteristics of a multi-stage filter. The amplifier and the phase shifter are connected so as to compensate loss caused by the SAW filter.

Description: The present invention relates to surface acoustic wave (SAW) filters, and in particular to active SAW filters used in radio frequency units and intermediate frequency units of communication equipment. One example of conventional active filter which compensates loss caused by a resonator is shown in Fig. 5. Resonators each including an active resonator 31, a coupling device 32 and a l/4-strip line 33 are connected in multi-stages to form the active filter 30. An internal block diagram of the active resonator 31 is shown in Fig. 6. The active resonator 31 includes a dielectric resonator 31a, an amplifier 31b, phase shift lines 31c and coupling devices 31d. The dielectric resonator 31a and the amplifier 31b are connected in the form of a loop via the phase shift lines 31c and the coupling devices 31d to form a positive feedback circuit. The amplifier 31b compensates energy loss in the dielectric resonator 31a. Thereby, no-load Q on the active resonator 31 increases, and performance of the active filter 30 is consequently improved. Since production of a filter using a plurality of resonators needs an amplifier, a phase shifter and a coupling device for each stage of resonator, the size of the filter increases as the number of stages increases. In addition, adjustment of characteristics of each resonator, and adjustment among the resonators is also needed, which thus causes a problem in which adjustment of the whole filter is very complicated. The dielectric constant of a dielectric which can be used for a resonator is approximately 100 at its highest. In a sub-microwave band (several hundred MHz to 2 GHz) the size of a dielectric resonator is large. For example, let the resonance frequency be 1 GHz. Wavelength I in a vacuum is approximately 300 mm. Let the dielectric constant be 100. The wavelength shortening rate is 0.1, and wavelength I is approximately 30 mm. Thus, the length of one l/4-resonator in this case is 7.5 mm. Accordingly, it is impossible to reduce the size of a filter using resonators connected in multi-stages. Accordingly, it is an object of the present invention to provide a small active filter which can be easily adjusted. To this end, the foregoing object has been achieved through the provision of an active filter including a resonator surface acoustic wave (SAW) filter with at least two ports, an amplifier, and a phase shifter, in which the SAW filter has either one resonant mode or two or more resonant modes coupling mutually to show characteristics of a multi-stage filter, and the amplifier and the phase shifter are connected so as to compensate loss caused by the SAW filter. A balanced input/output SAW filter is used as the SAW filter, a differential amplifier is used as the amplifier, and two sets of input/output ports for the balanced input/output SAW filter are connected to two sets of input/output ports for the differential amplifier. The active filter has a multi-chip module structure formed by the SAW filter, the amplifier and the phase shifter. According to the present invention, only one amplifier and one phase shifter are needed for a filter equivalently having a plurality of resonant stages. Thus, an active filter in which adjustment-needed portions are reduced and adjustment is easily performed is realized. In addition, by using a differential amplifier, control of an amplifier unit can be omitted, so that a filter is easily constituted in the form of an integrated circuit. Moreover, an active filter which is extremely small-sized and does not need adjustment is realized. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a circuit diagram showing an active filter according to an embodiment of the present invention. Fig. 2 is a graph showing a comparison between a transmission characteristic of an active filter according to the present invention and that of a single SAW filter. Fig. 3 is a circuit diagram showing an active filter according to another embodiment of the present invention. Fig. 4 is a perspective view showing an active filter according to a further embodiment of the present invention. Fig. 5 is a circuit diagram showing a conventional active filter. Fig. 6 is a circuit diagram showing an active resonator in the active filter shown in Fig. 5. DESCRIPTION OF THE PREFERRED EMBODIMENTS In Fig. 1, an active filter according to an embodiment of the present invention is shown. The active filter 1 includes a resonator SAW filter 2, an amplifier 3, a phase shifter 4, and coupling devices 5. The active filter 1 is provided with a first terminal 1a and a second terminal 1b through which a signal is inputted or outputted. The SAW filter 2 is provided with a first port 2a and a second port 2b for input and output. The first port 2a of the SAW filter 2 is connected to the input of the amplifier 3 via the coupling device 5. The output of the amplifier 3 is connected to the second port 2b of the SAW filter 2 via the phase shifter 4 and the coupling device 5. The first and second ports 2a and 2b of the SAW filter 2 are connected to the first and second terminals 1a and 1b, respectively. The active filter 1 operates as follows: The resonator SAW filter 2 resonates in one, two or more modes. In particular, when the SAW filter 2 resonates in a plurality of modes, the resonant modes inside the SAW filter 2 couple mutually to provide characteristics equivalent to those of a multi-stage filter. In addition, the amplifier 3 has a frequency range covering the pass band of the SAW filter 2. The phase shifter 4 is designed so that positive feedback occurs in the SAW filter 2 and the amplifier 3. In the above-described arrangement, negative resistance energy held by the amplifier 3 couples with the resonant modes of the SAW filter 2 to provide energy. As a result, feedback occurs in all the resonant modes, which is equivalent to a condition in which loss caused by each resonant mode is improved. Thus, a signal is amplified in the entire pass band of the SAW filter 2. A filter characteristic of this circuit arrangement, and a characteristic of the single SAW filter 2 are shown in Fig. 2. Fig. 2 shows a graph of the transmission characteristics of the filters. The horizontal axis represents frequency, and the vertical axis represents insertion loss. The characteristic of the active filter 1 is denoted by a, and the characteristic of the single SAW filter 2 is denoted by b. The characteristic b of the single SAW filter 2 in Fig. 2 shows that insertion loss is approximately -5 dB in the pass band, and is approximately -15 to -20 dB outside the band. To the contrary, the characteristic a of the active filter 1 shows that insertion loss is similar to that in the case of the single SAW filter 2 but is approximately +1 dB in the pass band. The insertion loss in the pass band significantly decreases. An active filter according to another embodiment of the present invention is shown in Fig. 3. The active filter 10 includes a resonator SAW filter 11 with balanced input and output, a differential amplifier 12, phase shifters 13, and coupling devices 14. The active filter 10 is provided with a first terminal 10a, a second terminal 10b, a third terminal 10c and a fourth terminal 10d through which a signal is inputted or outputted. The SAW filter 11 is provided with a first port 11a, a second port 11b, a third port 11c and a fourth port 11d. The first and second ports 11a and 11b of the SAW filter 11 are respectively connected to the two input ports of the differential amplifier 12 via the coupling devices 14. The two outputs of the differential amplifier 12 are respectively connected to the third and fourth ports 11c and 11d of the SAW filter 11 via the phase shifters 13 and the coupling devices 14. The first, second, third and fourth ports 11a, 11b, 11c and 11d of the SAW filter 11 are connected to the first, second, third and fourth terminals 10a, 10b, 10c and 10d of the active filter 10, respectively. The active filter 10 operates as follows: When two signals whose phases differ mutually at 180 degrees are inputted to the two terminals 10a and 10b of the active filter 10, the active filter 10 functions as a balanced active filter. Loss compensation by the differential amplifier 12 is performed in the same manner as in the circuit shown in Fig. 1. Accordingly, a description of the operation will be omitted. Next, a case where the above-described active filter is constituted in the form of a multi-chip module (MCM) will be described. Fig. 4 shows a perspective view of an active filter according to the present invention. The active filter 20 includes an SAW filter chip 21 obtained by forming an SAW filter in the form of a chip, an MMIC chip 22 including an amplifier and a phase shift circuit, and a mother board 23. The active filter 20 has an MCM structure in which the SAW filter chip 21 and the MMIC chip 22 are mounted on the mother board 23. Operation of the active filter 20 is similar to the cases shown in Figs. 1 and 3. Accordingly, a description of the operation will be omitted.

Claims: An active filter including a resonator surface acoustic wave SAW filter (2) with at least two ports (1a, 1b), an amplifier (3), and a phase shifter (4), characterized in that the SAW filter has either one resonant mode or two or more resonant modes coupling mutually to show characteristics of a multi-stage filter, and the amplifier (3) and the phase shifter (4) are connected so as to compensate loss caused by the SAW filter. An active filter according to Claim 1, characterized in that a balanced input/output SAW filter (2) is used as said SAW filter, a differential amplifier is used as said amplifier (3), and two sets of input/output ports for said balanced input/output SAW filter are connected to two sets of input/output ports for said differential amplifier. An active filter according to Claim 1 or 2, characterized in that the active filter has a multi-chip module structure formed by said SAW filter (2), said amplifier (3) and said phase shifter (4).

Inventors: TANAKA HIROAKI, TANAKA, HIROAKI
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1997
Main classification: H03H9
IPC codes: H03H9, H03H11, H03H7