JP2006254281A - Filter circuit and wireless apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To secure a bandwidth in a passing band to reduce insertion loss of a band-pass filter and increase the amounts of attenuation in the outside band in a prevented area other than the passing band. <P>SOLUTION: A filter circuit has an input port IN which can input signals in a plurality of frequency bands, a plurality of output ports OUTa, OUTb and OUTc which can output signals in different frequency bands, respectively. In addition, the filter circuit also has a plurality of band-pass filters 1a, 1b and 1c which are connected to the output ports, a plurality of band prevention filters 2a, 2b and 2c which are connected between the band-pass filters and the input port IN, and a controller 3 which variably controls the input impedances of the band prevention filters. In the previous step of the plurality of band-pass filters, the passing bands of which are mutually different, a plurality of band-preventing filters 4a, 4b and 4c are connected, and impedances of the band-preventing filters are variably controlled, depending on passing bands of the band-pass filters that makes a signal pass through. Consequently, only the signals in a desired frequency band can be fetched from the output ports, and the amounts of attenuation in the outside band can be increased so that the selection of signals is improved. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a filter circuit including a plurality of band pass filters that allow signals of different frequency bands to pass therethrough and a radio apparatus.

  A wireless device such as a mobile phone or a cordless phone includes a filter that allows a signal in a specific communication band to pass in order to obtain a signal in a specific communication band from a plurality of frequency components.

  A wireless device that handles a plurality of communication bands connects a semiconductor switch or a micro-electro-mechanical system (MEMS) switch before or after the filter, or both, and a signal is transmitted to a filter having a pass frequency in a desired communication band. A filter circuit is used that switches this switch so that it is transmitted and obtains a signal in a specific communication band from a plurality of frequency components.

  For example, the conventional filter circuit disclosed in Non-Patent Document 1 selects any one of a plurality of bandpass filters having different pass frequencies with a switch, and passes a signal in a desired communication band with low loss. It is said.

The conventional band pass filter has a large out-of-band attenuation in a frequency band slightly separated from the pass band, but tends to decrease the out-of-band attenuation as the distance from the pass band increases. Although there is a method of increasing the out-of-band attenuation outside the passband, there is a problem that the passband width becomes narrow, which is not preferable.
2003 IEEE ULTRASONICS SYMPOSIUM pp.1726-1729

  An object of the present invention is to provide a filter circuit and a radio that can secure a bandwidth in a pass band, reduce loss loss of a band pass filter, and increase an out-of-band attenuation in a stop band other than the pass band. A device is provided.

  According to one aspect of the present invention, an input port to which signals in a plurality of frequency bands are input, a plurality of output ports that output signals in different frequency bands, and the plurality of output ports are provided. A plurality of band-pass filters that pass signals of different frequency bands and supply them to corresponding output ports, and are provided corresponding to the plurality of band-pass filters, each of which can individually vary the input impedance, Provided is a filter circuit including a plurality of band rejection filters that switch whether or not a signal input to an input port is supplied to a corresponding bandpass filter.

  According to the present invention, it is possible to secure the bandwidth in the pass band, reduce the loss loss of the band pass filter, and increase the out-of-band attenuation in the stop band other than the pass band.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

(First embodiment)
FIG. 1 is a block diagram showing a schematic configuration of a filter circuit according to the first embodiment of the present invention.

  The filter circuit of FIG. 1 includes an input port IN that can receive signals in a plurality of frequency bands, a plurality of output ports OUTa, OUTb,..., OUTn that can output signals in different frequency bands, and these output ports OUTa. .., 1n connected to .about.OUTn, and a plurality of band rejection filters 2a, 2b,..., 2n connected between these bandpass filters 1a-1c and the input port IN. And a control unit 3 that variably controls the input impedance of these band rejection filters 2a to 2n. FIG. 1 shows an example in which n band-stop filters, band-pass filters, and output ports are provided. However, the number of n is not particularly limited as long as n is 2 or more.

  The band pass filters 1a to 1n pass signals of different frequency bands and supply them to the corresponding output ports OUTa to OUTn. The band rejection filters 2a to 2n change the input impedance according to the signal from the control unit 3 and switch whether to supply the signal input to the input port IN to the corresponding band pass filters 1a to 1n.

  The bandpass filters 1a to 1n preferably have a circuit configuration in which a plurality of resonators are connected in a ladder shape. As an example of the resonator, a bulk acoustic wave (SAW) resonator or a bulk acoustic wave (BAW) resonator can be considered. Or you may use the resonator which is comprised with an inductor and a capacitor and has a function similar to a bulk acoustic wave.

  The band rejection filters 2a to 2n are desirably filters that can be regarded as having an open input impedance at a certain frequency (stopping frequency), such as an LC parallel resonance circuit in which an inductor element and a capacitor element are connected in parallel or a Chebychev filter. In order to make the input impedance of the band rejection filters 2a to 2n variable, it is desirable to use a variable reactance element such as a variable capacitor element or a variable inductor element.

  FIG. 2 is a block diagram showing a specific example of a filter circuit using an LC parallel resonant circuit as the band rejection filters 2a to 2n. The configuration and operation of this embodiment will be described below using the filter circuit of FIG. 2 as an example.

  The filter circuit of FIG. 2 includes three LC parallel resonance circuits 4a, 4b, and 4c that function as band-stop filters connected to the input port IN, and three bands that are connected to these LC parallel resonance circuits 4a, 4b, and 4c. The filter includes pass filters 1a, 1b, and 1c and three output ports OUTa, OUTb, and OUTc connected to the band pass filters 1a, 1b, and 1c. The number of the LC parallel resonance circuits 4a, 4b, 4c, the band pass filters 1a, 1b, 1c and the output ports OUTa, OUTb, OUTc is not limited to three, and may be two or four or more.

  The three bandpass filters 1a, 1b, and 1c have different passbands. For this reason, signals of specific passbands that have passed through the corresponding bandpass filters 1a, 1b, and 1c are output from the output ports OUTa, OUTb, and OUTc, respectively.

The LC parallel resonant circuits 4a, 4b, 4c have a variable capacitor element 5 and an inductor element 6 connected in parallel. The parallel resonance frequency fa of the LC parallel resonance circuits 4a, 4b, and 4c, that is, the blocking frequency fa is expressed by equation (1), where C is the capacitance of the variable capacitor element 5 and L is the inductor of the inductor element 6.
fa = 1 / (2π / √ (LC)) (1)

  In the present embodiment, the control unit 3 controls the capacitance of the variable capacitor element 5. By varying the capacitance of the variable capacitor element 5, the blocking frequency fa of the LC parallel resonance circuits 4a, 4b, 4c can be set to an arbitrary frequency. Further, at the stop frequency fa, the LC parallel resonance circuits 4a, 4b, and 4c perform parallel resonance, and the input impedance near the stop frequency fa is almost open.

  For example, consider a case where a signal is passed only through the band pass filter 1c. In this case, the band rejection filters 4a and 4b before the band pass filters 1a and 1b adjust the capacitance of the variable capacitor element 5 therein so that the rejection frequency matches the pass band of the band pass filter 1c. Further, the band rejection filter 4c in the previous stage of the bandpass filter 1c sets the rejection frequency to a frequency completely different from the passband by increasing the capacitance of the variable capacitor element 5 therein.

  In this case, since the stop frequencies of the band stop filters 4a and 4b coincide with the pass band of the band pass filter 1c, the input impedance is opened for signals in this frequency band, and the band stop filters 4a and 4b are connected to the input port. Block the signal input to IN. On the other hand, the stop frequency of the band stop filter 4c is a value that hardly affects the pass band of the band pass filter 1c, and the input signal from the input port IN is output via the band stop filter 4c and the band pass filter 1c. Is transmitted to the port OUTc.

  As described above, the filter circuit of FIG. 2 is equivalent to a circuit in which only the LC parallel resonance circuit 4c and the band pass filter 1c are connected between the input port IN and the output port OUTc. That is, for the signal in the pass band of the band pass filter 1c, the influence of the band pass filters 1a and 1b can be almost ignored, and only the signal that has passed through the band pass filter 1c is supplied to the output port OUTc.

  Similarly, when a signal is passed through the band pass filter 1b, the stop frequencies of the band stop filters 4a and 4c are made to coincide with the pass band of the band pass filter 1b, and the stop frequency of the band stop filter 4b is set to that of the band pass filter 1b. Set to a value that has little effect on the passband. When passing the signal through the band pass filter 1a, the stop frequencies of the band stop filters 4b and 4c are matched with the pass band of the band pass filter 1a, and the stop frequency of the band stop filter 4a is passed through the band pass filter 1a. Set to a value that has little effect on bandwidth.

  FIG. 3 is a circuit diagram showing an example of a specific configuration of the bandpass filters 1a, 1b, and 1c. The band pass filters 1a, 1b, and 1c in FIG. 3 have a plurality of resonators connected in a ladder shape. More specifically, the band-pass filters 1a, 1b, and 1c in FIG. 3 include a plurality of series arm resonators 7 connected in series between the input and output terminals of the filters, and the input and output nodes of these resonators and the ground. And a plurality of parallel arm resonators 8 respectively connected between the terminals.

  Both the antiresonance frequency of the series arm resonator 7 and the resonance frequency of the parallel arm resonator 8 are set to the center frequency of the passband. Thereby, the band pass filters 1a, 1b, and 1c having the characteristics shown in FIG. 4 are obtained. In FIG. 4, the horizontal axis represents frequency, and the vertical axis represents signal attenuation. A dotted line 41 in FIG. 4 indicates a pass band, and a stop band having a very large signal attenuation exists in the vicinity thereof (dotted line 42). It shows that the characteristics of the band pass filters 1a, 1b, and 1c are better as the signal attenuation of the cut-off band is larger.

  In the case of the ladder-shaped bandpass filters 1a, 1b, and 1c as shown in FIG. 3, the characteristics of the bandpass filters 1a, 1b, and 1c are improved by changing the number of series arm resonators 7 and parallel arm resonators 8. In this case, an additional inductor or an additional capacitor may be inserted.

  The circuit configurations of the bandpass filters 1a, 1b, and 1c are not limited to those illustrated in FIG. FIG. 5 is a circuit diagram showing a modification of the bandpass filters 1a, 1b, and 1c. The band-pass filters 1a, 1b, and 1c in FIG. 5 have a plurality of resonators connected in a ladder shape, but have a connection form different from that in FIG. The band pass filters 1a, 1b, and 1c of FIG. 5 also have a series arm resonator 7 and a parallel arm resonator 8, and take a balanced input / output form.

  FIG. 6 is a diagram showing the propagation characteristics of the bandpass filters 1a, 1b, and 1c. FIG. 6A shows the S11 characteristic of the Smith chart showing the input impedance of the filter, and FIG. 6B shows the S21 characteristic showing the signal passing amount.

  As shown in FIG. 6B, the pass band is about 1.95 GHz, and as shown in FIG. 6A, the input impedances of the band pass filters 1a, 1b, 1c are short-circuited around 1.9 GHz. A short circuit appears as the frequency becomes higher than the passband. When the center frequency changes, the frequency at which the input impedance appears to be a short circuit also changes. Therefore, in the filter circuit of FIG. 2, the band rejection filters 4a, 4b, and 4c can be regarded as being simply short-circuited at a frequency at which the input impedances of the band-pass filters 1a, 1b, and 1c are almost short-circuited.

  FIG. 7 is an equivalent circuit diagram when the stop frequencies of the band-stop filters 4a and 4b are made to coincide with the pass band of the band-pass filter 1c for the purpose of passing the signal through the band-pass filter 1c. FIG. 7 shows an example in which the input impedance of the bandpass filter 1a is a value that can be regarded as a short circuit. In this case, the output of the band rejection filter 4a can be regarded as being directly grounded.

  In FIG. 7, the band rejection filter 4a in the previous stage of the band pass filter 1a functions as a band pass filter 1c having the same pass frequency as the band pass filter 1c. For this reason, the signal input to the input port IN is filtered by the one-dot chain line portion 9 in FIG. 7, and then input to the bandpass filter 1c via the band rejection filter 4c and further filtered. Thus, equivalently, the filtering is performed twice, and the out-of-band attenuation is increased as compared with the band-pass filter 1c alone. Similarly, the same applies to the passband of the other bandpass filter 1a or 1b. By adopting the circuit configuration of FIG. 2, the out-of-band attenuation can be increased.

  FIG. 8 shows that the pass frequency of the band pass filter 1a shown in FIG. 2 is 0.8 GHz, the pass frequency of the band pass filter 1b is 1.7 GHz, the pass frequency of the band pass filter 1c is 2.0 GHz, and the band pass filters 1a and 1b. In the case where the stop frequency is 2.0 GHz, the frequency dependence of the signal output from the output port OUTc connected to the band pass filter 1c and the pass characteristics of the band pass filter 1c alone, that is, the conventional band pass filter 1c. It is the figure which compared the frequency dependence of the passage characteristic of.

  As can be seen from FIG. 8, according to the filter circuit of the present embodiment, the out-of-band attenuation is larger than that of the conventional filter circuit except for some frequency regions.

  FIG. 9 is a block diagram showing a schematic configuration of a wireless device (for example, a mobile phone) incorporating the filter circuit of FIG. The wireless device in FIG. 9 includes an antenna 11, a directional coupler 12, a transmission device 13, and a reception device 14.

  The transmission device 13 includes a baseband processing unit 21 (BB processing unit) that performs baseband processing on an audio signal captured from the microphone 20, a modulator 22 that modulates the baseband processed signal, and a modulator 22. A band-pass filter 23 that extracts only a desired frequency component included in the output modulation signal and a high-frequency power amplifier 24 that amplifies the signal that has passed through the band-pass filter 23 are included.

  The receiver 14 amplifies the received signal that has passed through the directional coupler 12, and a filter having the same configuration as in FIG. 2 that extracts only a desired frequency component contained in the signal amplified by the low noise amplifier 31. A circuit 32, an amplifier 33 that amplifies the signal that has passed through the filter circuit 32, a demodulator 34 that demodulates the signal amplified by the amplifier 33, and a baseband processing unit 35 that performs baseband processing on the demodulated signal (BB) And a speaker 36 that outputs the signal after the baseband processing as sound.

  As described above, the filter circuit of the present embodiment can switch the pass band of the signal under the control of the control unit 3, and thus is suitable for a plurality of wireless systems using a plurality of different frequency bands. Therefore, the present embodiment can be applied to a multi-system mobile phone.

  As described above, in the first embodiment, a plurality of band rejection filters 4a, 4b, and 4c are connected in front of a plurality of bandpass filters 1a, 1b, and 1c having different passbands to allow signals to pass therethrough. Since the impedance of the band rejection filter is variably controlled in accordance with the pass band of the signal, only signals in a desired frequency band can be extracted from the output ports OUTa, OUTb, and OUTc, and the out-of-band attenuation can be increased. The selectivity is very good.

(Second Embodiment)
In the second embodiment, a switch is provided between the bandpass filters 1a, 1b, and 1c and the output ports OUTa, OUTb, and OUTc.

  In the first embodiment described above, a signal is output from an output port to which a bandpass filter having a desired passband is connected, but a corresponding bandpass filter is also output from an output port to which other bandpass filters are connected. The signal that passed through is output. For this reason, there is a possibility of adversely affecting the circuit on the subsequent stage of the filter circuit.

  Therefore, in this embodiment, a signal is output only from an output port connected to a band pass filter having a desired pass band.

  FIG. 10 is a block diagram showing a schematic configuration of a filter circuit according to the second embodiment of the present invention. In FIG. 10, the same reference numerals are given to components common to FIG. 2, and differences will be described below.

  The filter circuit of FIG. 10 includes a plurality of switches 30a, 30b, and 30c connected between the plurality of band-pass filters 1a, 1b, and 1c and the corresponding output ports OUTa, OUTb, and OUTc. These switches 30a, 30b, and 30c are turned on / off by a signal from the control unit 3.

  These switches 30a, 30b, and 30c are each a single-pole single-throw switch that inputs and outputs, and are preferably realized by switches that can be formed in a chip, such as semiconductor switches and MEMS switches.

  Of the plurality of switches 30a, 30b, and 30c, only the switch connected to the band pass filter that passes the signal is turned on, and the other switches are turned off. As a result, even if a signal leaks from a bandpass filter other than the bandpass filter that allows the signal to pass therethrough, the corresponding switch is off, so there is no possibility that the signal is transmitted to the circuit on the subsequent stage side.

  Insertion of the switches 30a, 30b, and 30c causes an insertion loss. However, leakage signals from the other bandpass filters 1a, 1b, and 1c can be reliably cut off, so that frequency selectivity is improved.

(Third embodiment)
The third embodiment is a modification of the second embodiment, in which one ends of a plurality of switches 30a, 30b, and 30c are connected in common and connected to the amplifier 33 at the subsequent stage.

  FIG. 11 is a block diagram showing a schematic configuration of a filter circuit according to the third embodiment of the present invention. In FIG. 11, the same reference numerals are given to components common to FIG. 10, and the differences will be mainly described below.

  The filter circuit of FIG. 11 includes an output terminal 31 in which one ends of a plurality of single-pole single-throw switches 30a, 30b, and 30c are connected in common. The output terminal 31 is input to the amplifier 33 in FIG. Of the plurality of band-pass filters 1a, 1b, and 1c, only one passes a signal. Of the plurality of switches 30a, 30b, and 30c, only one switch is turned on. For this reason, as shown in FIG. 11, even if one ends of the plurality of switches 30a, 30b, and 30c are connected in common, there is no possibility that the signals collide with each other.

  In the configuration of FIG. 10 described above, it is necessary to connect the amplifier 33 to each of the plurality of output ports OUTa, OUTb, and OUTc. However, in the configuration of FIG. 11, only one amplifier 33 is required. However, as the amplifier 33, it is necessary to use a broadband amplifier that covers all the passbands of the plurality of bandpass filters 1a, 1b, and 1c.

  As described above, in the third embodiment, by connecting one end of the plurality of switches 30a, 30b, and 30c to the output terminal 31, the number of amplifiers 33 can be reduced, and the circuit scale, power consumption, and Parts cost can be reduced.

(Fourth embodiment)
The fourth embodiment uses different types of switches 30a, 30b, and 30c from the second and third embodiments.

  FIG. 12 is a block diagram showing a schematic configuration of a filter circuit according to the fourth embodiment of the present invention. In FIG. 12, the same reference numerals are given to the components common to FIG. 11, and the differences will be mainly described below.

  The filter circuit of FIG. 12 includes only one three-pole single-throw switch 32 instead of a single-pole single-throw switch. One end of the switch 32 is connected to the three band pass filters 1 a, 1 b, 1 c, and the other end is connected to the output terminal 31.

  The switch 32 in FIG. 12 is switched to the output side of the bandpass filters 1 a, 1 b, 1 c through which the signal passes under the control of the control unit 3, and transmits the output of the filter to the output terminal 31. Since this switch 32 turns on only one of the three connection paths, the three systems of signals can be switched by only one switch 32.

  Thus, in the fourth embodiment, by providing the three-pole single-throw switch 32, the number of switches can be reduced, and the circuit scale can be further reduced as compared with the third embodiment.

  In the fourth embodiment, when four or more band pass filters 1a, 1b, and 1c are provided, a multipole single throw switch 32 corresponding to the number of band pass filters may be provided. Alternatively, a plurality of multipole single throw switches 32 may be provided, or a combination of multipole single throw switches 32 and single pole single throw switches may be used.

1 is a block diagram showing a schematic configuration of a filter circuit according to a first embodiment of the present invention. The block diagram which shows the specific example of the filter circuit which used LC parallel resonance circuit as band stop filter 2a, 2b, 2c. The circuit diagram which shows an example of the specific structure of band pass filter 1a, 1b, 1c. The figure which shows the characteristic of a band pass filter. The circuit diagram which shows the modification of bandpass filter 1a, 1b, 1c. The figure which shows the propagation characteristic of bandpass filter 1a, 1b, 1c. The equivalent circuit diagram at the time of making the stop frequency of band stop filter 4a, 4b correspond with the pass band of the band pass filter 1c. The figure which compared the passage characteristic with 1st Embodiment and a prior art example. The block diagram which shows schematic structure of the radio | wireless apparatus incorporating the filter circuit of FIG. The block diagram which shows schematic structure of the filter circuit by the 2nd Embodiment of this invention. The block diagram which shows schematic structure of the filter circuit by the 3rd Embodiment of this invention. The block diagram which shows schematic structure of the filter circuit by the 4th Embodiment of this invention.

Explanation of symbols

1a, 1b, 1c Band pass filter 2a, 2b, 2c Band stop filter 3 Control unit 4a, 4b, 4c LC parallel resonant circuit 5 Variable capacitor element 6 Inductor element

Claims (10)

An input port for receiving signals of multiple frequency bands;
A plurality of output ports each outputting a signal in a different frequency band;
A plurality of band pass filters that are provided corresponding to the plurality of output ports and pass signals of different frequency bands to supply the corresponding output ports;
Provided in correspondence with the plurality of band-pass filters, each of which can individually change the input impedance, and a plurality of band blocking for switching whether or not the signal input to the input port is supplied to the corresponding band-pass filter And a filter circuit. A signal of a specific frequency band included in the signal input to the input port is supplied to the output port through one of the band pass filters,
The band rejection filter corresponding to the band pass filter through which the signal of the specific frequency passes is set to have an input impedance smaller than other band rejection filters,
The filter circuit according to claim 1, wherein the other band rejection filter performs a resonance operation at the specific frequency.   The filter circuit according to claim 1, wherein the band rejection filter has a variable reactance element capable of varying an input impedance. The bandpass filter is
A plurality of first resonators connected in series between the input and output terminals;
The filter circuit according to claim 1, further comprising a second resonator connected between an input / output node of the plurality of first resonators and a reference voltage terminal.   The at least one of the first resonator and the second resonator is a surface acoustic wave (SAW) resonator or a bulk acoustic wave (BAW) resonator. 5. The filter circuit according to 4.   2. A plurality of connection switchers provided corresponding to the plurality of output ports and capable of individually switching between passing or blocking a signal output from the corresponding output port. 6. The filter circuit according to any one of 5.   The filter circuit according to claim 6, wherein the plurality of connection switching units allow signals in the frequency band of the corresponding band-pass filter to pass and block signals in other frequency bands. An output terminal connected in common to one end side of the plurality of connection switchers;
8. The filter circuit according to claim 6, wherein the other end side of the plurality of connection switching units is connected to a corresponding output port. A multipole connection selector for selecting any one of the plurality of output ports;
The filter circuit according to claim 1, further comprising: an output terminal that outputs a signal from an output port selected by the multipole connection switching device. An antenna,
An amplifier for amplifying a high-frequency signal received by the antenna;
A filter circuit that passes a signal of a predetermined frequency band included in the high-frequency signal amplified by the amplifier;
A demodulator that performs demodulation processing on the signal that has passed through the filter circuit;
A baseband processing unit that performs baseband processing on the signal demodulated by the demodulator,
The filter circuit is
An input port for receiving signals of multiple frequency bands;
A plurality of output ports each outputting a signal in a different frequency band;
A plurality of band pass filters that are provided corresponding to the plurality of output ports and pass signals of different frequency bands to supply the corresponding output ports;
Provided in correspondence with the plurality of band-pass filters, each of which can individually change the input impedance, and a plurality of band blocking for switching whether or not the signal input to the input port is supplied to the corresponding band-pass filter And a filter.

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