JP2008219748A - Attenuation characteristic variable filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an attenuation characteristic variable filter which is in size and low in cost having a simple constitution, and has attenuation characteristics which are switchable, while maintaining satisfactory filter characteristics. <P>SOLUTION: A line is formed between two input/output terminals IN and OUT; a resonance circuit SR for a trap is shunt-connected to the line; one-end sides of resonance circuits RS1 and RS2 for attenuation characteristic adjustment are connected in common; and diodes D1 and D2 are switching elements interposed between a common connection point and the ground; and other end sides of the resonance circuits RS1 and RS2 for attenuation characteristic adjustment are connected to the line. Furthermore, capacitors C12 and C21 are connected to the line in series, and an inductor L12 is shunt-connected to the line to constitute a high-pass filter HPF. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an attenuation characteristic variable filter in which an attenuation characteristic can be switched using a switch element.

  In a communication system using different frequency bands, Patent Document 1 discloses a communication system in which a plurality of filters having passbands corresponding to each frequency band are prepared and switched by a switch. In this communication system, a switch is disposed at each of the input and output ends of two signal lines that pass through filters having different characteristics, the pass band is changed by switching the switch, and a predetermined frequency is set in the pass band frequency of the first filter. The attenuation characteristic is obtained.

  In addition, a bandpass filter and a band which connect a plurality of resonators with transmission lines, inductors and capacitors and adjust their element values so that attenuation characteristics and pass characteristics can be set without being restricted by the characteristics of the resonator. A blocking filter is shown in US Pat.

  Further, Patent Document 3 discloses a filter corresponding to two frequency bands by combining a voltage controllable switching element such as a diode or FET with a resonator and switching them.

FIG. 1 is a configuration example of a filter disclosed in Patent Document 3. As shown in FIG. 1, a diode D and a capacitor C are connected between one end of each resonance element Re and the ground, and the other ends of adjacent resonance elements Re are connected by a capacitor Cab. In addition, an input terminal IN and an output terminal OUT are provided via a capacitor Ce. The capacitor Cab acts to couple adjacent resonant elements, and this filter acts as a band pass filter as a whole.
Japanese Patent Laid-Open No. 10-289342 Japanese Patent No. 3482957 JP 2000-101380 A

  However, in the system shown in Patent Document 1, it is necessary to prepare a plurality of filters in order to obtain a predetermined attenuation characteristic, and the transmission / reception module becomes large or the system configuration becomes complicated. In addition, when using a plurality of communication systems in one device, it is important to secure attenuation near the passband when the frequencies used by them are close to each other. To obtain such characteristics, In addition to a filter that passes a desired frequency, another circuit such as a trap for obtaining an attenuation characteristic is required, which increases the module size.

  In the filter shown in Patent Document 2, the filter characteristics can be adjusted by changing the element to be used, but instantaneous characteristic switching by voltage control is impossible.

  Furthermore, in the filter of Patent Document 3, since the resonance frequency of the resonator is changed by voltage control, a switching element is required for each resonator when a plurality of resonators are used, and the attenuation amount in a specific frequency band It is not suitable for the purpose of adjusting.

  Further, both Patent Documents 2 and 3 cannot obtain a stable high-pass characteristic that allows a higher frequency band to pass. Therefore, there is a problem that it is difficult to adapt to a system that needs to pass a high frequency band of the attenuation band with a low loss and to ensure a large amount of attenuation in the attenuation band.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an attenuation characteristic variable filter that solves the above-described problems, reduces the size and costs with a simple configuration, and allows the attenuation characteristic to be switched while maintaining good filter characteristics. It is to provide.

The variable attenuation characteristic filter of the present invention is configured as follows.
(1) A line (signal propagation path) is provided between two input / output terminals, a trap resonance circuit is connected to the line with a shunt, and a plurality of attenuation characteristic adjusting resonance circuits are shunted to the line. And connecting (arranging) a switch element between the common connection point and the ground, or connecting the ground end sides of the plurality of attenuation characteristic adjusting resonance circuits in common. The opposite ends are connected in common, and a switch element is connected (arranged) between the common connection point and the line.

  (2) A capacitor is inserted into the series with respect to the line, and an inductor is connected to the shunt with respect to the line, and a high-pass filter unit is configured by the capacitor and the inductor.

(3) The switch element is an FET.
(4) The switch element is mounted on the surface of the multilayer substrate, and the trap resonance circuit and the plurality of attenuation characteristic adjustment resonance circuits are mounted on or formed in the surface of the multilayer substrate.

According to the present invention, the following effects can be obtained.
(1) By switching on / off the switching element, a state where a plurality of resonance circuits are equivalently connected to the shunt and a state where they are disconnected from the shunt are switched, so that the trap connected to the shunt with respect to the line It is possible to selectively obtain the characteristics of the resonance circuit for use and the combined characteristics of the plurality of resonance characteristic adjustment resonance circuits and the trap resonance circuit.

  In addition, since the attenuation characteristic variable filter can be configured with a small number of parts as a whole, the size and cost can be reduced.

  Further, since the characteristics of the filter are changed using the switch element, it is not necessary to configure a plurality of paths and filters as in Patent Document 1, and the total number of elements can be reduced.

  In addition, since the attenuation amount in the specific frequency band can be adjusted as necessary, insertion loss in the pass band can be reduced when a large attenuation amount is unnecessary.

  In addition, since the characteristics can be switched using a single switch element, a bias circuit for applying a single control voltage to the switch element may be provided, the control circuit is simplified, and the entire circuit is Miniaturization can be achieved.

  (2) By configuring the high-pass filter portion with a capacitor inserted in series with the line and an inductor connected to the shunt, characteristics capable of passing up to a high frequency band on the high frequency side of the attenuation band can be obtained.

  (3) By using an FET (Field Effect Transistor) as the switch element, the current flowing when turned on can be reduced, and the power consumption can be reduced and the size can be reduced accordingly.

  (4) By configuring the switch element, the trap resonance circuit, and the plurality of attenuation characteristic adjustment resonance circuits using a multilayer substrate, a single small component that can be surface-mounted on a circuit board of an electronic device. Easy to configure.

<< First Embodiment >>
The attenuation characteristic variable filter according to the first embodiment will be described with reference to FIGS.
FIG. 2 is an overall circuit diagram of the attenuation characteristic variable filter according to the first embodiment. In FIG. 2, a line (signal propagation path) is provided between two input / output terminals IN and OUT, and a trap resonance circuit SR composed of a series circuit of an inductor Lrs and a capacitor Crs is connected to the shunt. ing.

  In addition, two attenuation characteristic adjusting resonance circuits RS1 and RS2 are connected to the shunt with respect to the line. These attenuation characteristic adjusting resonance circuits RS1 and RS2 are constituted by series circuits of SAW resonators Re1 and Re2 and inductors L11 and L21.

  In addition, the ground end sides of the two attenuation characteristic adjusting resonance circuits RS1 and RS2 are connected in common, and a series circuit of diodes D1 and D2 as switch elements is connected between the common connection point and the ground. Yes. A bias circuit including a resistor R and a capacitor Cc is formed between the diodes D1 and D2 and a control terminal Vc that applies a bias voltage (control voltage).

  In addition, a capacitor C21 is inserted between the line resonance resonance circuit SR and the attenuation characteristic adjustment resonance circuit RS2, and a capacitor C12 is inserted between the two attenuation characteristic adjustment resonance circuits RS1 and RS2. An inductor L12 is connected to the shunt between the output terminal OUT and the ground. This shunt-connected inductor L12 and series-connected capacitors C12 and C21 constitute a high-pass filter section HPF having an LC configuration.

  3A is an equivalent circuit when the diodes D1 and D2 are on in the circuit shown in FIG. 2, and FIG. 3B is a circuit when the diodes D1 and D2 are off in the circuit shown in FIG. Is an equivalent circuit.

  By applying a predetermined positive voltage to the control terminal Vc shown in FIG. 2, both the diodes D1 and D2 are turned on. In this state, since the inductance components of the diodes D1 and D2 are sufficiently smaller than the inductors L11 and L21 of the attenuation characteristic adjusting resonance circuits RS1 and RS2, the attenuation characteristic adjusting resonance circuit RS1, as shown in FIG. The ground end side of RS2 is equivalently grounded.

  In this way, by connecting the inductors L11 and L21 in series with the SAW resonators Re1 and Re2 and increasing the inductance components of the attenuation characteristic adjusting resonance circuits RS1 and RS2, the resonance caused by 1.3 GHz is caused. Improves attenuation of pass characteristics.

  If the voltage applied to the control terminal Vc shown in FIG. 2 is 0 V or a negative voltage, the diodes D1 and D2 are turned off, and the diodes D1 and D2 are equivalent to extremely small capacitance components, as shown in FIG. In addition, the attenuation characteristic adjusting resonance circuits RS1 and RS2 are not connected to the line.

  4A shows the pass characteristics in the state shown in FIG. 3A, and FIG. 4B shows the pass characteristics in the state shown in FIG. 3B.

  When the diodes D1 and D2 shown in FIG. 2 are in an OFF state, the trap resonance circuit SR connected to the line is shunted, and as shown in FIG. An attenuation pole is generated, showing a steep attenuation characteristic from 0.8 GHz to this attenuation pole, and a characteristic with a very small insertion loss at a high frequency of 0.8 GHz or higher.

  When the diodes D1 and D2 shown in FIG. 2 are in the ON state, the action of the three resonance circuits of the trap resonance circuit SR connected to the line in a shunt and the attenuation characteristic adjustment resonance circuits RS1 and RS2 are superimposed. Characteristics. As shown in FIG. 4A, the attenuation poles of the trap resonance circuit SR and the attenuation characteristic adjustment resonance circuits RS1 and RS2 are generated with a predetermined bandwidth in the vicinity of 0.7 GHz. Therefore, a steeper attenuation characteristic than that shown in FIG. 4B is obtained from 0.8 GHz to this attenuation band. In the band of 0.8 GHz or more, the characteristic that the insertion loss increases from 1.0 to 1.7 GHz is shown. This is a result of connecting a plurality of resonators to the shunt with respect to the line.

  Therefore, as shown in FIG. 4B, when obtaining a low insertion loss characteristic over a wide band from the attenuation band to the high band side, the diodes D1 and D2 are turned off to secure the attenuation band width and from the pass band. In order to obtain a steep characteristic toward the attenuation region, the diodes D1 and D2 may be turned on.

  In addition, by connecting a plurality of diodes in series in this way, the equivalent capacitance component at the time of OFF can be reduced, and the frequency shift amount due to ON / OFF of the diodes D1 and D2 can be further increased. Further, since the fluctuation of the voltage applied per diode is reduced, the harmonic characteristics of the high-pass filter part HPF are improved.

  FIG. 5 is an enlarged external perspective view of the attenuation characteristic variable filter according to the first embodiment. In FIG. 5, diodes D1 and D2, SAW resonators Re1 and Re2, inductors L11 and L21, and a resistor R are mounted on the upper surface of the multilayer substrate 10, respectively.

  In the multilayer substrate 10, the remaining inductors and capacitors shown in FIG. 2 are formed. The terminals of the input / output terminals IN and OUT, the ground terminal, and the control terminal Vc are formed on the back surface of the multilayer substrate 10, respectively. If the desired characteristics can be obtained, the remaining inductors and capacitors shown in FIG. 2 may be surface-mounted, or the inductors L11 and L21 may be formed inside the multilayer substrate 10.

<< Second Embodiment >>
FIG. 6 is a circuit diagram of an attenuation characteristic variable filter according to the second embodiment. In this example, an FET is used as the switch element. Thereafter, an FET Q is inserted between the common connection point of the attenuation characteristic adjusting resonance circuits RS1 and RS2 and the ground, and a bias circuit including a resistor R and a capacitor Cc is connected to the gate thereof.

  In FIG. 6, when the FET Q is an N-channel FET, the FET Q1 is turned on by applying a predetermined positive voltage to the control terminal Vc. Further, by setting the applied voltage of the control terminal Vc to 0 V or a predetermined negative voltage, the FET Q1 is turned off. When Q1 is in the ON state, the ground end side of the resonance circuit for attenuation characteristic adjustment RS1 and RS2 is grounded with a very small inductance component, which is equivalent to the state shown in FIG. When Q1 is in the off state, the state is equivalent to that shown in FIG. Therefore, as in the case of the first embodiment, the center frequency of the attenuation band can be switched by the control voltage.

  As described above, by using the FET as the switch element, the current value for the switch element on / off control can be made extremely small, and the power consumption can be reduced and the size can be reduced accordingly.

<< Third Embodiment >>
FIG. 7 is a circuit diagram of an attenuation characteristic variable filter according to the third embodiment. Unlike the circuit shown in FIG. 2 in the first embodiment, the inductors L11 and L21 are connected to the ground end side of the resonance circuit for attenuation characteristic adjustment RS1 and RS2, respectively, and the end opposite to the ground end side is connected. A common connection is made, and a diode D as a switching element is connected between the common connection point and the line. A capacitor C21 is inserted in series with the line between the diode D and the trap resonance circuit SR. Further, a capacitor C12 is inserted in series between the inductor L12 connected to the line in a shunt and the output terminal OUT.

Thus, the switch element (diode D) may be provided on the line side instead of the ground side.
A bias circuit comprising a resistor R and a capacitor Cc is provided between the diode D and the ground. When a predetermined positive voltage is applied to the control terminal Vc, a bias current flows through a path of resistance R → diode D → inductor L12, and the diode D is turned on.

  In this example, the high-pass filter section HPF is configured by the capacitors C21 and C12 connected in series and the inductor L12 connected to the shunt. As described above, the insertion position and the number of capacitors in the high-pass filter section are not limited to those in the first to third embodiments, and various modifications can be considered. The connection position of the trap resonance circuit SR may be not only the input end IN side but also the output end OUT side, and may be connected to a shunt at a predetermined location on the line.

<< Fourth Embodiment >>
FIG. 8 is a circuit diagram of an attenuation characteristic variable filter according to the fourth embodiment. In the first to third embodiments, the resonance circuit for adjusting the attenuation characteristic connected to the shunt with respect to the line has two stages, but in the fourth embodiment, it has three stages.

  In FIG. 8, a line (signal propagation path) is provided between two input / output terminals IN and OUT, and a trap resonance circuit SR composed of a series circuit of an inductor Lrs and a capacitor Crs is connected to the shunt. is doing.

  In addition, three attenuation characteristic adjusting resonance circuits RS1, RS2, and RS3 are connected to the shunt with respect to the line. These resonance circuit RS1, RS2 and RS3 for adjusting the attenuation characteristics are constituted by a series circuit of SAW resonators Re1, Re2 and Re3 and inductors L11, L21 and L31.

  In addition, the ground end sides of the three attenuation characteristic adjusting resonance circuits RS1, RS2, and RS3 are connected in common, and a series circuit of diodes D1 and D2, which are switching elements, is connected between the common connection point and the ground. is doing. A bias circuit including a resistor R and a capacitor Cc is formed between the diodes D1 and D2 and a control terminal Vc that applies a bias voltage (control voltage).

  Further, a capacitor C21 is inserted between the resonance circuit SR for trapping the line and the resonance circuit RS2 for adjusting the attenuation characteristic, and a capacitor C12 is inserted between the resonance circuits RS1 and RS2 for adjusting the attenuation characteristic, thereby adjusting the attenuation characteristic. A capacitor C11 is inserted between the resonance circuits RS2 and RS3. An inductor L12 is connected to the shunt between the output terminal OUT and the ground. The shunt-connected inductor L12 and the series-connected capacitors C21, C12, and C11 constitute a high-pass filter portion HPF having an LC configuration.

  In this way, the characteristics of the attenuation band (the attenuation curve and the attenuation bandwidth from the pass band to the attenuation band) can be switched even when three or more resonance circuits for adjusting the attenuation characteristics are provided.

  In addition to the SAW resonator, a BAW resonator or a dielectric resonator may be used as the resonator provided in the attenuation characteristic adjusting resonance circuit.

  Further, an LC resonance circuit similar to the trap resonance circuit may be used without using the resonator. Furthermore, the connection position of the resonance circuit for trapping, the resonance circuit for adjusting attenuation characteristics, and the line may be appropriately changed in order to obtain desired characteristics. At this time, predetermined filter characteristics can be obtained by changing the values of series-connected capacitors and shunt-connected inductors.

FIG. 6 is a circuit diagram of a filter disclosed in Patent Document 3. It is a circuit diagram of an attenuation characteristic variable filter according to the first embodiment. It is an equivalent circuit of the attenuation characteristic variable filter according to the on / off state of the diode. It is a figure which shows the passage characteristic of the circuit shown in FIG. It is an expansion appearance perspective view of an attenuation characteristic variable filter. It is a circuit diagram of an attenuation characteristic variable filter according to a second embodiment. It is a circuit diagram of an attenuation characteristic variable filter according to a third embodiment. It is a circuit diagram of an attenuation characteristic variable filter according to a fourth embodiment.

Explanation of symbols

IN, OUT- I / O terminal SR-trap resonance circuit HPF-high-pass filter section RS1, RS2-resonance adjustment resonance circuit Re1, Re2-SAW resonator D1, D2-diode (switch element)

Claims (4)

Provide a line between the two input and output ends,
A trap resonant circuit is connected to the shunt with respect to the line,
A plurality of attenuation characteristic adjusting resonance circuits for the line are connected to the shunt,
The ground end sides of the plurality of attenuation characteristic adjusting resonance circuits are connected in common, and a switch element is connected between the common connection point and the ground, or the end opposite to the ground end side is shared. And a switching element connected between the common connection point and the line.   The attenuation characteristic variable filter according to claim 1, wherein a capacitor is inserted in series with respect to the line, and an inductor is connected to the shunt with respect to the line, and the capacitor and the inductor constitute a high-pass filter unit.   The variable attenuation characteristic filter according to claim 1, wherein the switch element is an FET.   5. The switch element is mounted on a surface of a multilayer substrate, and the trap resonance circuit and the plurality of attenuation characteristic adjustment resonance circuits are mounted on or formed in the surface of the multilayer substrate. The attenuation characteristic variable filter described.

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