WO2005088832A1 - Filter circuit - Google Patents

Abstract

A variable frequency filter in which a variation in cut-off characteristics is suppressed when the frequency is varied. The filter comprises two parallel resonance circuits, a coupling inductor coupling them, and a multipath circuit. The parallel resonance circuit comprises, connected in parallel, a resonance inductor and a series circuit of a variable capacitance element and a resonance capacitor. The multipath circuit comprises a coupling capacitor, and a variable capacitance element inserted between the coupling capacitor and each parallel resonance circuit. Since a variation in cut-off characteristics is reversed between the parallel resonance circuit and the multipath circuit when the frequency is varied, variations in cut-off characteristics are canceled when frequencies are varied simultaneously at the two and thereby variations in cut-off characteristics can be reduced when frequencies are varied.

Description

 Specification

 Filter circuit

 Technical field

 The present invention relates to a filter circuit, and more particularly, to a variable frequency filter circuit capable of changing a pass frequency characteristic.

 Background art

 [0002] In a recent portable wireless system represented by a mobile phone, a multi-band operation using a plurality of carrier frequencies is being demanded. In the future, the demand is expected to increase. For example, dual-band support equipment is becoming popular in wireless systems for consumers. For example, domestic mobile phones include dual-band terminals such as 800 MHz band and 1.5 GHz band PDC (Personal Digital Cellular) terminals, or 2 GHz band W-CDMA (Wideband Code Division Multiple Access) and 800 MHz band PDC. In wireless LAN, there are three mode terminals, 2.4GHz band IEEE802.11lbZg and 5.2GHz band IEEE802.11a. In order to support more frequencies (bands) in the future, it is necessary to continuously change the frequency characteristics of a filter circuit or filter element (hereinafter referred to as a filter) used in a wireless terminal. Filters are used in wireless terminals to prevent the emission of radio waves outside of a specified range during transmission, and to suppress poor communication quality due to input other than the desired signal during reception. Parts or circuits.

[0003] A circuit for realizing such a filter capable of changing the frequency characteristics (hereinafter, referred to as a frequency variable filter) is described in Japanese Patent Application Laid-Open No. 2002-9573 (first prior art). Figure 1 shows the circuit of this filter. 1 is an input terminal, 2 is an output terminal, 3 is a signal line, 4 is a parallel resonance circuit, 5 is a band control circuit, 6 and 13 are capacitors for cutting DC current, 7 is a resonance inductor, and 8 and 9 are resonance capacitors. , 10 is a variable capacitance element using a varactor diode, 11 is a frequency control terminal for the parallel resonance circuit 4, 12 is a bias resistor for inputting the frequency control voltage of the parallel resonance circuit 4, and 14 is a band control circuit using a varactor diode. 15 is a frequency control terminal for the band control circuit 5, and 16 is a frequency control terminal for the band control circuit 5. This is a bias resistor for inputting several control voltages.

 [0004] The first conventional technique is a circuit in which a pass frequency band is defined by a parallel resonance circuit 4, and the spread of the pass frequency bandwidth is suppressed by a band control circuit 5. The parallel resonance circuit 4 and the band control circuit 5 have variable capacitance elements 10 and 14 (varactor diodes) whose capacitances can be changed by control voltages input from frequency control terminals 11 and 15. The parallel resonance circuit 4 and the band control circuit 5 change the capacitance value, so that the pass frequency bandwidth is kept almost constant while changing the resonance frequency. However, as shown in Fig. 2 (horizontal axis; frequency, vertical axis; gain) showing the frequency characteristics of this conventional circuit, the width of the pass frequency band characteristic becomes wider, Is hard to say.

 [0005] Circuits that can solve this problem are described in Keisuke Kageyama, Kohki Saito, Hisanori Murase, Hideki Utaki, and Toshishige Yamamoto, "Tunable Active Filters Having Multilayer Structure Using LTCCJ, IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL.49, NO.12, December 2001, P.2421-2424, this second conventional circuit is shown in Figure 3. 21 is an input terminal, 22 is an output terminal, and 23 is a parallel resonant circuit 24,25 26 and 27 are variable capacitance elements for each parallel resonance circuit 24 and 25, and 28 and 29 are for each parallel resonance circuit 24 and 25 connected in series with each variable capacitance element 26 and 27. 30 and 31 are resonance inductors for the parallel resonance circuits 24 and 25. The variable capacitance elements 26 and 27, the resonance capacitors 28 and 29, and the resonance inductors 30 and 31 are respectively parallel resonance inductors. It forms circuits 24 and 25. Resonant inductors 30 and 31 are electromagnetically coupled and mutual inductors And a coupling capacitor 37. A direct current cut capacitor 35 and a coupling capacitor 37. The direct current cut capacitors 35 and 36 and the coupling capacitor 37 form a band control circuit ( Multipath circuit) 38 is formed.33 and 34 are bias resistors for inputting the frequency control voltage of each parallel resonance circuit 24,25.

The second conventional example couples two parallel resonance circuits 24 and 25 with a transformer 32 and also has a signal path passing through a coupling capacitor 37 separately from a signal path passing through the transformer 32. Te ru. Such a circuit having two signal paths is called a multipath circuit. The According to the multipath circuit, when the phase difference between the signals passing through the two paths is 180 degrees, the signals cancel each other, so that a dip occurs in the frequency characteristic of the filter and the cutoff characteristic can be sharpened. Figure 4 shows the frequency characteristics of this filter (horizontal axis; frequency, vertical axis; gain). It is clearly seen that a sharper cutoff characteristic is obtained as compared with FIG. 2 showing the characteristic of the first conventional example.

 Disclosure of the invention

 [0007] However, a problem of the conventional technique is that when the resonance frequency is changed, the pass frequency band characteristic changes. For example, as shown in Fig. 4 showing the frequency characteristics of the second conventional example, as the pass frequency band is changed to the high frequency side by increasing the resonance frequency, the cutoff characteristics (the slope of the frequency characteristics) on the low frequency side are increased. Become gentle. This is because the frequency characteristics of the multipath circuit do not change.

 [0008] It is an object of the present invention to solve the above-mentioned problems of the prior art, and to provide a frequency variable filter circuit in which a change in cutoff characteristics at the time of frequency change is small. It is to realize.

 [0009] To achieve the above object, a filter circuit according to the present invention comprises two variable capacitance elements for parallel resonance circuits arranged between a signal path between input and output and ground, and capable of changing the resonance frequency. A filter circuit comprising: a parallel resonance circuit; and a first reactance coupling between the two parallel resonance circuits. The filter circuit connects a second reactance connected between the input and output and a connection point between the first reactance and the two parallel resonance circuits to the input and the output, respectively. A variable capacitance element for a band control circuit that can be varied so as to generate a dip near a pass frequency band of the resonance frequency.

[0010] Another filter circuit of the present invention is a filter circuit including a first signal transmission path and a second signal transmission path for transmitting a signal from an input to an output. In this filter circuit, the first signal transmission path connects two parallel resonance circuits, each including a variable capacitance element for a parallel resonance circuit, in series with a first reactance. In this configuration, the input and the output are connected to each other via a capacitive element. Further, the second signal transmission path transmits a second reactance between the input and the output to the first signal. This is a configuration connected in parallel with the signal transmission path. Further, the parallel resonance circuit can change the resonance frequency by the variable capacitance element for the parallel resonance circuit. The variable capacitance element for a band control circuit can be changed so that a dip occurs in a pass frequency band near the resonance frequency.

 Further, the above-described filter circuit of the present invention includes means for controlling the variable capacitance element for the parallel resonance circuit and the variable capacitance element for the band control circuit so as to change the capacitance while keeping the capacitance ratio constant. You may have more.

 [0012] In the above-described filter circuit of the present invention, the variable capacitance element for the parallel resonance circuit and the variable capacitance element for the band control circuit may be changed in capacitance with the same control voltage.

FIG. 1 is a circuit diagram showing a first conventional example

 FIG. 2 is a graph showing frequency characteristics of a first conventional example.

 [FIG. 3] A circuit diagram showing a second conventional example

 FIG. 4 is a graph showing frequency characteristics of a second conventional example.

 FIG. 5 is a circuit diagram for explaining the principle of the present invention.

 FIG. 6 is a circuit diagram showing a first embodiment of the present invention.

 FIG. 7 is a circuit diagram showing a second embodiment of the present invention.

 FIG. 8 is a circuit diagram showing a third embodiment of the present invention.

 FIG. 9 is a circuit diagram showing a fourth embodiment of the present invention.

 FIG. 10 is a circuit diagram showing a fifth embodiment of the present invention.

 FIG. 11 is a circuit diagram showing a sixth embodiment of the present invention.

 FIG. 12 is a circuit diagram showing a seventh embodiment of the present invention.

 FIG. 13 is a graph showing frequency characteristics of the present invention.

 FIG. 14 is a graph showing frequency characteristics for explaining the operation of the present invention.

 BEST MODE FOR CARRYING OUT THE INVENTION

According to the present invention, in addition to changing the resonance frequency and changing the pass frequency band in a conventional parallel resonance circuit using a variable capacitance element, the variable capacitance element is also used in a multipath circuit. By introducing it, the frequency characteristic of the multipath circuit is also changed, thereby suppressing the change in the cutoff characteristic of the filter circuit due to the frequency change.

 [0015] The principle of the present invention utilizes the fact that the slope of the cutoff characteristic changes in the opposite direction when each variable capacitance element is changed. First, as shown in FIG. 4 in the description of the background art, the slope of the cutoff characteristic of the parallel resonance circuit becomes gentler when the pass frequency band is changed to a higher frequency side. On the other hand, when the capacitance of only the variable capacitance element in the multipath circuit is changed, as shown in the analysis result of the inventor shown in FIG. 14, as the pass frequency band is changed to the higher frequency side, the cutoff characteristic becomes higher. Becomes steep. According to the present invention, the change of the cutoff characteristic accompanying the change of the frequency can be suppressed by these two canceling effects.

 [0016] The conditions for changing the cutoff characteristics in the principle of the present invention will be described in more detail. Referring to the filter circuit of FIG. 5, the center frequency fc of the pass band of the filter is determined by the inductor L and the capacitance C constituting the resonance circuit, and is given by the following equation (1).

 [0017] [number 1]

"One 2π ∑0 ()) The pass bandwidth Δ ί is calculated by the following equation (2) using the multipath circuit capacitance Cm, input / output capacitance Cc, coupling inductance Lm, and resonance circuit capacitance C. Approximately given.

[0018] [number 2]

From this equation (2), it is found that if Lm and Cm are fixed and the ratio between the input / output capacitance Cc and the resonance circuit capacitance C is kept constant and these capacitances are changed, the pass bandwidth can be changed. It can be seen that the center frequency can be changed without any change. An example of a frequency variable characteristic according to the present invention Figure 14 shows. The center frequency is changed while keeping the slope of the cutoff characteristic and the passband width constant, and it can be said that keeping the passband constant corresponds to keeping the slope of the cutoff characteristic constant.

 The method for keeping the ratio between the input / output capacitance Cc and the resonance circuit capacitance C constant involves not only controlling the control voltage to each variable capacitance element so that this ratio is constant, but also using two variable capacitances. Can be controlled by the same voltage. At this time, the ratio between the input / output capacitance Cc and the resonance circuit capacitance C is given by the number of parallel variable elements or the area of the elements.

 Hereinafter, a specific description will be given according to an embodiment.

 (First Embodiment)

 Embodiments of the present invention will be described in detail with reference to the drawings. FIG. 6 is a circuit diagram showing the first embodiment of the present invention.

 Description of configuration>

 Referring to FIG. 6, in the embodiment of the present invention, two parallel resonance circuits 44 and 45, a coupling inductor 53 connecting them, and a multipath circuit 46 are also configured. Each of the two parallel resonance circuits 44 and 45 is composed of a series circuit of variable capacitance elements 51 and 52 and resonance capacitors 49 and 50, and resonance inductors 47 and 48 connected in parallel to the series circuit. The multipath circuit 46 includes a coupling capacitor 56 and variable capacitance elements 54 and 55 inserted between the coupling capacitor 56 and the parallel resonance circuits 44 and 45.

Further, capacitors 61 and 62 are connected in series between the input / output terminals 41 and 42 and the variable capacitance elements 54 and 55, respectively. The terminals of the varactors 51, 52, 54, and 55 that are not directly connected to the ground 63 (the anode side of the varactor diode that is the varactors 51, 52, 54, and 55 is connected via the resonant inductors 47 and 48). Are connected to the frequency control terminal 43 via bias resistors 57-60 for frequency control voltage input, respectively. This terminal is controlled by a single control voltage! RU

 Explanation of operation>

This circuit operates as follows. The two parallel resonant circuits 44 and 45 have variable capacitance elements 51 and 52 (such as varactor diodes) whose capacitance can be changed by a control voltage input to a frequency control terminal 43, respectively. With high impedance It looks open. Therefore, in the input signal from the input terminal 41, the component flowing from the signal path to the ground 63 becomes small, and almost 100% of the input signal passes through the path of the coupling inductor 53 between the parallel resonance circuits 44 and 45 or the coupling capacitor of the multipath circuit 46. The signal is transmitted to the output terminal 42 through the! / Of 56 paths. Therefore, this resonance frequency determines the pass frequency band. This resonance frequency can be changed by changing the control voltage from the frequency control terminal 43 to change the capacitance of the variable capacitance elements 51, 52 in the parallel resonance circuits 44, 45. On the other hand, as the frequency departs from the resonance frequency, the impedance of the parallel resonance circuits 44 and 45 decreases, the ratio flowing to the ground 63 increases, and the signal input to the input terminal 41 is not transmitted to the output terminal 42.

Here, if the phase difference between the signal passing through coupling capacitor 56 and the signal passing through coupling inductor 53 is designed to be 180 degrees at a frequency near the pass frequency band, the frequency Then, the signals of these two paths cancel each other out and are cancelled, and a dip can be created in the frequency characteristics. With this dip, the cutoff characteristics can be made steep. Here, the frequency at which the dip occurs can be changed by changing the control voltage input from the frequency control terminal 43 to change the capacitance of the variable capacitance elements 54 and 55 in the multipath circuit 46.

 When changing these variable capacitance elements 51, 52, 54, 55, the capacitance ratio is changed by changing the variable capacitance elements 51, 52 of the resonance circuit and the variable capacitance elements 54, 55 of the multipath circuit with the same control voltage. By keeping the constant, the change of the cutoff characteristic is suppressed. When the pass frequency band is changed to the higher frequency side, the slope becomes steep in the multi-nos circuit 46 as shown in FIG. 14, and becomes gentle in the parallel resonance circuits 44 and 45 as shown in FIG. By canceling out the changes in the slopes of the two cutoff characteristics, in the filter circuit of the present invention, the cutoff characteristics are maintained in a substantially constant shape even when the pass frequency band changes, as in the frequency characteristics shown in FIG. .

 (Second embodiment)

 Next, a second embodiment will be described as another embodiment of the present invention.

FIG. 7 is a circuit diagram showing a second embodiment of the present invention. In FIG. 7, the same reference numerals as those in FIG. 6 denote the same components. In this embodiment, in the parallel resonance circuits 64 and 65, the variable capacitance elements 51 and 52 are connected to the ground 63 and the resonance capacitors 49 and 50 are connected to the signal. The difference from the first embodiment is that the connection is made on the route side. In order to make the capacitance change of the variable capacitance elements 51 and 52 due to the bias voltage the same as in the first embodiment, the variable capacitance elements 51 and 52 are oriented such that the anode side of the nodal diode is DC-connected to the ground 63. It has become. The other configurations and operations are the same as those of the first embodiment, and thus description thereof will be omitted.

 (Third embodiment)

 Next, a third embodiment of the present invention will be described in detail with reference to the drawings. FIG. 8 is a circuit diagram showing a third embodiment of the present invention. In FIG. 8, the components having the same reference numerals as those in FIG. 6 indicate the same components.

 Referring to FIG. 8, in the present embodiment, two parallel resonance circuits 74 and 75 and a multi-path circuit 46 are also configured. Each of the two parallel resonance circuits 74 and 75 is composed of a series circuit of variable capacitance elements 51 and 52 and resonance capacitors 49 and 50, and resonance inductors 71 and 72 connected in parallel to the series circuit. The two resonant inductors 71 and 72 are electromagnetically coupled to form a transformer 73, and the input and output are coupled by their mutual inductance. The multipath circuit 46 also includes a coupling capacitor 56, and variable capacitance elements 54 and 55 inserted between the coupling capacitor 56 and the parallel resonance circuits 74 and 75, and a force.

 [0026] Furthermore, capacitors 61 and 62 are connected in series between the input / output terminals 41 and 42 and the variable capacitance elements 54 and 55. The terminals of the variable capacitance elements 51, 52, 54, 55 which are not directly connected to the ground 63 are connected to the frequency control terminal 43 via the bias resistors 57-60 for inputting the frequency control voltage, respectively! Puru.

[0027] This circuit operates as follows. The two parallel resonant circuits 74 and 75 have variable capacitance elements 51 and 52 (such as varactor diodes) whose capacitance can be changed by the control voltage that is input to the frequency control terminal. It looks like it is open. At this time, the input signal from the input terminal 41 has a small signal path component flowing to the ground 63, and almost 100% of the signal passes through either the mutual inductance path of the transformer 73 or the path of the coupling capacitor 56 of the multipath circuit 46. It passes through and is transmitted to the output terminal 42. Therefore, this resonance frequency determines the pass frequency band. This resonance frequency is changed by changing the capacitance of the variable capacitance elements 51 and 52 in the parallel resonance circuits 74 and 75. Can be changed. On the other hand, as the frequency is further away from the resonance frequency force, the impedance of the parallel resonance circuits 74 and 75 decreases, the ratio of flowing to the ground 63 increases, and the input signal from the input terminal 41 is not transmitted to the output terminal 42.

Further, if the phase difference between the signal passing through the coupling capacitor 56 and the signal passing through the transformer 73 is designed to be 180 degrees at a frequency in the vicinity of the pass frequency band, at this frequency, The signals in the two paths cancel each other out, canceling each other out, and creating a frequency response characteristic. With this dip, the cutoff characteristics can be made steep. Here, the frequency at which this dip occurs can be changed by changing the control voltage from the frequency control terminal 43 and changing the capacitance of the variable capacitance elements 54 and 55 in the multipath circuit.

When these variable capacitance elements 51, 52, 54, 55 are changed, the capacitance is changed by changing the variable capacitance elements 51, 52 of the resonance circuit and the variable capacitance elements 54, 55 of the multipath circuit with the same control voltage. By keeping the ratio constant, the change in the cutoff characteristics is suppressed. When the pass frequency band is changed to the high frequency side, the slope becomes steep in the multi-noss circuit 46 as shown in FIG. 14, and becomes gentle in the parallel resonance circuits 74 and 75 as shown in FIG. By canceling out the changes in the slopes of the two cutoff characteristics, in the filter circuit of the present invention, the cutoff characteristics are maintained in a substantially constant shape even when the pass frequency band changes, as in the frequency characteristics shown in FIG. .

 (Fourth embodiment)

 Next, a fourth embodiment of the present invention will be described. FIG. 9 is a circuit diagram showing a fourth embodiment of the present invention. In FIG. 9, the same components as those in FIG. 8 indicate the same components.

The third embodiment is different from the third embodiment in that, in the parallel resonance circuits 76 and 77, the variable capacitance elements 51 and 52 are connected to the ground 63 and the resonance capacitors 49 and 50 are connected to the signal path. Is different. In order to make the capacitance change due to the bias voltage of the variable capacitance elements 51 and 52 the same as that of the third embodiment, the variable capacitance elements 51 and 52 have a direction in which the anode side of the noctor diode is DC-connected to the ground 63. Have been. The other configurations and operations are the same as those of the third embodiment, and thus description thereof will be omitted.

 (Fifth embodiment)

Next, a fifth embodiment of the present invention will be described in detail with reference to the drawings. Fig 10 FIG. 14 is a circuit diagram showing a fifth embodiment of the present invention. In FIG. 10, the same reference numerals as those in FIG. 6 denote the same components.

 Referring to FIG. 10, in the present embodiment, forces such as two parallel resonance circuits 44 and 45, a multipath circuit 78, and a coupling capacitor 56 are also configured. Each of these two parallel resonance circuits 44, 45 is composed of a series circuit of variable capacitance elements 51, 52 and resonance capacitors 49, 50, and resonance inductors 47, 48 connected in parallel to this series circuit. The multipath circuit 78 includes a coupling inductor 53 and variable capacitance elements 54 and 55 inserted between the coupling inductor 53 and the parallel resonance circuits 44 and 45, respectively.

 Further, capacitors 61 and 62 are connected in series between the input / output terminals 41 and 42 and the variable capacitance elements 54 and 55. The terminals of the variable capacitance elements 51, 52, 54, 55 which are not directly connected to the ground are connected to the frequency control terminal 43 via bias resistors 57-60, respectively.

 [0033] This circuit operates as follows. The two parallel resonance circuits 44 and 45 have variable capacitance elements 51 and 52 (varactor diodes, etc.) whose capacitance can be changed by a control voltage as high as the frequency control terminal 43. I see. Therefore, in this case, the signal from the input terminal 41 has a small signal path force flowing to the ground 63, and almost 100% of the signal passes through the path of the coupling inductor 53 of the multipath circuit 78 or the coupling between the parallel resonance circuits 44 and 45. The signal is transmitted to the output terminal 42 through one of the paths of the capacitor 56. Therefore, this resonance frequency determines the pass frequency band. This resonance frequency can be changed by changing the capacitance of the variable capacitance elements 51, 52 in the parallel resonance circuits 44, 45. On the other hand, as the frequency departs from the resonance frequency, the impedance of the parallel resonance circuits 44 and 45 decreases, the ratio flowing to the ground 63 increases, and the input signal is not transmitted to the output terminal 42.

Here, at a frequency near the pass frequency band, if the phase difference between the signal passing through the coupling capacitor 56 and the signal passing through the coupling inductor 53 is designed to be 180 degrees, At that frequency, the signals on the two paths cancel each other out and are offset, creating a dip in the frequency response. With this dip, the cutoff characteristics can be made steep. Here, the frequency at which this dip occurs varies the control voltage from the frequency control The capacitance can be changed by changing the capacitance of the variable capacitance elements 54 and 55 in the circuit 78.

 When these variable capacitance elements 51, 52, 54, 55 are changed, the capacitance is changed by changing the variable capacitance elements 51, 52 of the resonance circuit and the variable capacitance elements 54, 55 of the multipath circuit with the same control voltage. By keeping the ratio constant, the change in the cutoff characteristics is suppressed. When the pass frequency band is changed to the high frequency side, the slope becomes steep in the multi-noss circuit 78 as shown in FIG. 14 and conversely becomes gentle in the parallel resonance circuits 44 and 45 as shown in FIG. By canceling out the changes in the slopes of the two cutoff characteristics, in the filter circuit of the present invention, the cutoff characteristics are maintained in a substantially constant shape even when the pass frequency band changes as in the frequency characteristics shown in FIG.

 (Sixth embodiment)

 Next, a sixth embodiment of the present invention will be described. FIG. 11 is a circuit diagram showing a sixth embodiment of the present invention. In FIG. 11, the same components as those in FIG. 10 indicate the same components.

In this embodiment, in the parallel resonance circuits 44 and 45, the variable capacitance elements 51 and 52 are connected to the ground and the resonance capacitors 49 and 50 are connected to the signal path. Is different from In order to make the capacitance change due to the noisy voltage the same as in the fifth embodiment, the variable capacitance elements 51, 52, 54, and 55 are oriented so that the anode side of the nodal diode is DC-connected to the ground 63. I have. The other configurations and operations are the same as those of the fifth embodiment, and thus the description is omitted.

 Although the first to sixth embodiments have been described above, in implementing the present invention, varactor diodes can be used for the variable capacitance elements 51, 52, 54, and 55. A nonode diode is an element that applies the fact that the reverse capacitance of the PN junction or Schottky junction of the diode changes with the applied voltage. When a varactor diode is used in the embodiment shown in FIG. 6 to FIG. 11, the operation which is the object of the present invention can be realized by applying a positive voltage to the frequency control terminal and changing the voltage value.

 [0038] Instead of a varactor diode, as shown in FIG.

A variable capacitance element using Electro Mechanical Systems) technology and a variable capacitance element utilizing the nonlinearity of the dielectric constant of a ferroelectric can also be used. FIG. 12 shows a seventh embodiment of the present invention. It is a circuit diagram showing a state. In FIG. 12, components having the same reference numerals as those in FIG. 6 indicate the same components. This embodiment is different from the first embodiment in that variable capacitance elements 151, 152 and 154, 155 formed by the MEMS technology are used in the parallel resonance circuits 84, 85 and the multipath circuit 86. The other configurations and operations are the same as those of the first embodiment, and thus description thereof is omitted. Needless to say, such a variable capacitance element formed by the MEMS technology can be applied to the second to sixth embodiments.

 Further, in order to obtain a desired frequency characteristic, a parallel capacitance can be further added to the parallel resonance circuits 44, 45, 64, 65, 74—77, 84, 85, or a multipath variable capacitance element It is also possible to connect capacitors in series with 54,55.

 In the embodiment of the present invention, a chip capacitor may be used as a capacitor, and a chip inductor may be used as an inductor. It is also possible to use a printed circuit board or a ceramic substrate or a capacitor formed on the substrate, a spiral inductor, or a transmission line having an electrical length of 1Z4 wavelength or less.

 Further, the number of parallel resonance circuits is not limited to two, but may be further increased, and these parallel resonance circuits may be connected by reactance.

 Further, according to the present invention, a first signal transmission path is formed by coupling a plurality of parallel resonance circuits with reactances (capacitors, inductors, transformers, etc.), and the first signal transmission path is formed in parallel with the first signal transmission path. In this configuration, a second signal transmission path (multipath circuit) is formed by connecting reactances (capacitors, inductors, transformers, etc.). When changing the resonance frequency, the variable capacitance element is changed to create a dip in the frequency characteristics so that the phase difference between the signals passing through these two paths becomes 180 degrees at a frequency near the pass frequency band. . The force of the first signal transmission path and the multi-noss circuit of the second signal transmission path are also cut off even if the pass frequency band is changed by changing the resonance frequency by canceling the change in the slope of the change in the cutoff characteristic. The characteristics are kept almost constant. Therefore, it is apparent that the present invention is not limited to the above embodiment, and can be appropriately modified within the scope of the technical idea of the present invention.

 Industrial applicability

As an application example of the present invention, a multi-band mobile phone that covers the 800 MHz band to the 2 GHz band is used. It can be expected to be applied to multi-band portable data communication terminals that cover the frequency range of mobile telephones in the 800 MHz band and wireless LAN frequencies in the 5 GHz band.

Claims

The scope of the claims  [1] two parallel resonance circuits that are arranged between the signal path between the input and output and the ground and that can change the resonance frequency with the variable capacitance element for the parallel resonance circuit;  A first reactance coupling between the two parallel resonant circuits,  A second reactance connected between the input and output,  A band control circuit that connects each of the connection points of the first reactance and the two parallel resonance circuits to the input and the output, respectively, and that can be varied to generate a dip near a pass frequency band of the resonance frequency. Filter circuit having a variable capacitance element for use in the filter.  2. The filter circuit according to claim 1, further comprising: means for controlling the capacitance of the variable capacitance element for the parallel resonance circuit and the variable capacitance element for the band control circuit so that the capacitance changes while maintaining a constant ratio.  3. The filter circuit according to claim 1, wherein the capacitance of the variable capacitance element for the parallel resonance circuit and the variable capacitance element for the band control circuit are changed by the same control voltage. 4. The filter circuit according to claim 1, wherein the first reactance is an inductor, and the second reactance is a capacitor. 5. The filter circuit according to claim 1, wherein the first reactance is a capacitor, and the second reactance is an inductor. 6. The filter circuit according to claim 1, wherein the first reactance coupling is a mutual inductance of a transformer.  7. The filter circuit according to claim 1, wherein the parallel resonance circuit is configured by connecting a series circuit of a capacitor and the variable capacitance element for the parallel resonance circuit in parallel with an inductor.  8. The filter circuit according to claim 1, wherein the parallel resonance circuit is configured by connecting a series circuit of a capacitor and the variable capacitance element for the parallel resonance circuit in parallel with an inductor and a capacitor.  9. The filter circuit according to claim 1, wherein the variable capacitance element for the parallel resonance circuit and the variable capacitance element for the band control circuit are a non-acting diode. [10] The variable capacitance element for the parallel resonance circuit and the variable capacitance element for the band control circuit, 2. The filter circuit according to claim 1, wherein the filter circuit is a MEMS variable capacitance element. 11. The filter circuit according to claim 1, wherein the variable capacitance element for the parallel resonance circuit and the variable capacitance element for the band control circuit are elements utilizing nonlinearity of voltage dependence of a dielectric constant of a ferroelectric.  [12] A filter circuit for transmitting a signal from an input to an output,  Two parallel resonance circuits each including a variable capacitance element for a parallel resonance circuit are connected in series with a first reactance, and the connection point is connected to the input and the output via a variable capacitance element for a band control circuit. The parallel resonance circuit may be configured to change a resonance frequency with the variable capacitance element for the parallel resonance circuit, and the variable capacitance element for the band control circuit may generate a dip in a pass frequency band near the resonance frequency. A first signaling path, which can change to  A filter circuit having a second signal transmission path configured by connecting a second reactance between the input and the output in parallel with the first signal transmission path.  13. The filter circuit according to claim 12, further comprising: means for controlling the capacitance to change while maintaining a constant capacitance ratio between the variable capacitance element for the parallel resonance circuit and the variable capacitance element for the band control circuit.  14. The filter circuit according to claim 12, wherein the capacitance of the variable capacitance element for the parallel resonance circuit and the variable capacitance element for the band control circuit are changed by the same control voltage. 15. The filter circuit according to claim 12, wherein the first reactance is an inductor, and the second reactance is a capacitor. 16. The filter circuit according to claim 12, wherein the first reactance is a capacitor, and the second reactance is an inductor. 17. The filter circuit according to claim 12, wherein the first reactance coupling is a mutual inductance of a transformer.  18. The filter circuit according to claim 12, wherein the parallel resonance circuit is configured by connecting a series circuit of a capacitor and a variable capacitance element for the parallel resonance circuit in parallel with an inductor. 19. The filter according to claim 12, wherein the parallel resonance circuit is configured by connecting a series circuit of a capacitor and the variable capacitance element for the parallel resonance circuit in parallel with an inductor and a capacitor. Lutha circuit.  20. The filter circuit according to claim 12, wherein the variable capacitance element for a parallel resonance circuit and the variable capacitance element for a band control circuit are a non-acting diode. [21] The variable capacitance element for the parallel resonance circuit and the variable capacitance element for the band control circuit, 13. The filter circuit according to claim 12, which is a MEMS variable capacitance element. 22. The filter circuit according to claim 12, wherein the variable capacitance element for the parallel resonance circuit and the variable capacitance element for the band control circuit are elements utilizing the non-linearity of the voltage dependence of the dielectric constant of the ferroelectric.