Variable filter and communication apparatus

A variable filter includes, on a dielectric substrate including ground conductor, first resonator including a transmission line connected to input terminal, second resonator including a transmission line connected to output terminal, and coupling portion including a transmission line having one end connected to the first and second resonators and another end being an open end, or structure having one end connected to the first and second resonators, including a serial connection of a transmission line and a variable capacitor, another end of the variable capacitor connected to the ground conductor, and adjusting means capable of changing electric length, in the first and second resonators and the coupling portion, wherein pass band width can be changed by changing ratio of electric transmission length of the coupling portion to electric transmission lengths of transmission line including the coupling portion, and the first and second resonators.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2011-054681, filed on Mar. 11, 2011, the entire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a variable filter to be used for band pass of a high frequency signal, and a communication apparatus using this filter.

BACKGROUND OF THE INVENTION

A market of mobile communication including portable phones is expanding, and high performance of its service is under progress. As frequency band used by mobile communication gradually shifts to a frequency band higher than giga hertz (GHz), there is a tendency for mobile communication to become multi-channel. A possibility of future introduction of software radio (SDR:software-defined-radio) is being studied vigorously. In order to realize software radio, a wider adjustment range of circuit characteristics is desired.

FIG. 4is a circuit diagram of a conventional frequency variable filter100j. The variable frequency filter100jhas a plurality of channel filters101a,101b,101c. . . , and switches102aand102b. By switching the switches102aand102b, any one of the channel filters101a,101b, and101c. . . is selected to change the frequency band. A high frequency signal input from an input terminal103is filtered by the selected channel filter101and is output from an output terminal104.

The frequency variable filter100jhas channel filters corresponding in number to the number of channels. A multi-channel increases the number of channel filters, complicates the structure, and increases the size and cost. A possibility of realizing SDR is small.

Attention has been paid recent years to a compact micro machine device using MEMS (microelectromechanicalsystems). An MEMS device (micro machine device) using MEMS is able to have a high Q (quality factor) and be applied to a high frequency band variable filter (Patent Documents 1 and 2, Non-Patent Documents 1, 2, and 3). Since an MEMS device is compact and has a low loss, it is often used for a CPW (coplanarwaveguide) distributed constant resonator.

Non-Patent Document 3 discloses a filter having the structure that a plurality of variable capacitors of MEMS devices ride over a three-stage distributed constant line. In this filter, a control voltage Vb is applied to a drive electrode of a MEMS device to displace a variable capacitor, change a gap to a distributed constant line, and change an electrostatic capacitance. Change in the electrostatic capacitance changes the pass band of the filter.

SUMMARY OF THE INVENTION

Although a conventional filter is able to make variable the center frequency of a pass band, it is not able to change largely a pass band width.

According to one aspect, a variable filter includes:a dielectric substrate having a ground conductor therein;an input terminal formed on the dielectric substrate;an output terminal formed on the dielectric substrate;

a first resonator including a transmission line whose one end is connected to the input terminal;

a second resonator including a transmission line whose one end is connected to the output terminal;

a coupling portion including a transmission line whose one end is connected to other ends of the first and second resonators and whose another end is an open end, or a structure whose one end is connected to other ends of the first and second resonators, including a serial connection of a transmission line and a variable capacitor, another end of the variable capacitor being connected to the ground conductor; and

adjusting means capable of changing an electric length, in the first and second resonators and the coupling portion;

wherein a pass band width is able to be changed by changing a ratio of electric transmission length of the coupling portion to electric transmission lengths of transmission line including the coupling portion, and the first and second resonators.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and are not restrictive of the invention, as claimed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1Ais an equivalent circuit diagram of a variable filter of the first embodiment. Serial connection of a first variable capacitor C1and a distributed constant type first variable transmission line L1is connected to an input terminal IN, serial connection of a second variable capacitor C2and a distributed constant type second variable transmission line L2is connected to an output terminal OUT, and a distributed constant type third variable transmission line LC1is connected as a coupling portion to the other ends of the transmission lines L1and L2. It can also be said that as viewed from the coupling portion of the transmission line LC1, a first branch portion of the transmission line L1and a second branch portion of the transmission line L2are connected by using one end of the transmission line LC1as a coupling portion, and the other end of the transmission line LC1is an open end. An inter-stage variable capacitor Cm is connected between the input terminal IN and output terminal OUT, although this capacitor is not an indispensable component. The transmission lines L1, L2, and LC1constitute resonators having variable electric lengths. The variable filter is formed on a dielectric substrate such as an LTCC (low temperature co-fired ceramics).

The variable capacitors C1and C2are able to provide impedance matching with external. The inter-stage variable capacitor Cm forms attenuation poles on both sides of the pass band to make steep the shape of the pass band. The electric lengths of the first variable transmission line L1, second variable transmission line L2, and coupling portion variable transmission line LC1are (λ/4)−x, (λ/4)−x, and (λ/4)+x, respectively. The variable filter passes a high frequency signal having a wave length of λ from the input terminal IN to output terminal OUT.

A high frequency signal input from the input terminal IN passes through an impedance adjusting capacitor C1, thereafter propagates to the transmission line L1of the first branch portion, and the transmission line LC1of the coupling portion, and is reflected at the open end of the transmission line LC1. The reflected high frequency signal propagates the transmission line LC1reversely, and reenters the transmission line L1from the coupling portion. The reentered high frequency signal is reflected at the C1side end of the first transmission line L1to propagate the transmission line L1reversely. Namely, the state similar to the initial state resumes. Similar operations are repeated thereafter. At least a portion of the high frequency signal propagates the transmission line LC1reversely enters the second transmission line L2of the second branch portion. If the transmission lines have the above-described electric lengths, almost all the high frequency signal having a wave length of λ is supplied to the second transmission line.

FIGS. 1B and 1Care a top view and a cross sectional view illustrating the structure of making variable an electric length of a variable transmission line.

As illustrated inFIG. 1B, movable electrodes ME are disposed above a line L. The number of movable electrodes ME may be increased or decreased when necessary. One movable electrode may be used.FIG. 1Cis a cross sectional view taken along line IC-IC ofFIG. 1Btraversing a pair of movable electrodes ME. As illustrated inFIG. 1C, A transmission line L made of, e.g., copper is formed on a dielectric substrate20. The transmission line L has a bottom portion wider than a top portion extending on both sides, and spaces for accommodating the movable electrodes ME of variable capacitors VC are secured above the extending portions. This structure may be formed by two plating steps using resist patterns with an opening for defining the external shape. The extending portions of the transmission line constitute the fixed electrode FE of the variable capacitor VC. An insulating layer27is formed on the upper surface of the extending portion to prevent short circuit and improve an effective dielectric constant. The insulating layer may be made of inorganic material or organic material. The insulating layer may be omitted in some cases.

The movable electrode ME is formed on a dielectric substrate20, and is supported by a cantilever structure CL made of, e.g., copper. It may be considered that the top end portion of the cantilever CL constitutes the movable electrode ME. This structure may be formed by a plating process using a resist pattern with three dimensional structure, or by two plating processes using an opening for defining an external shape. A driving electrode DE is formed on the dielectric substrate20under the movable portion of the cantilever CL. The driving electrode may be formed at the same time when the extending portion of the transmission line is formed. The driving electrode may be formed of different metal material from the material of the transmission line in a different process. In this case, another process such as sputtering may be used.

The dielectric substrate20has such structure that a conductive metal layer22made of Ag or the like is formed on a ceramics layer21and another ceramics layer23is formed on the conductive metal layer22. This structure may be formed by laminating a ceramics green sheet layer, a conductive layer (wiring layer), and a ceramics green layer in position alignment and sintering the lamination. The ceramics layer is further formed with metal vias for interlayer connection, and a high impedance resistor via for preventing leakage of a high frequency signal to a DC bias path. The dielectric constant of ceramics material may be selected in a range from about 3 to about 100. Via conductors are buried under the support portion of the cantilever CL, and under the drive electrodes DE. The cantilever CL is connected to the ground layer22, and the drive electrode DE is connected to a terminal26formed on the bottom surface of the dielectric substrate20via a through via conductor25. Pads for inputting and outputting an RF signal and a DC drive signal may be formed on the bottom surface of the dielectric substrate. These pads are connected to the structures on the substrate surface or wirings in the substrate via metal vias and high impedance resistor vias in the substrate.

In the structure illustrated inFIG. 1C, the movable electrode ME is connected to the ground layer. A DC voltage of about 10 V to 100 V is applied to the drive electrode DE. An electrostatic force attracts the movable electrode ME to the fixed electrode FE. An electric length of the transmission line L is determined by a variable capacitance of the variable capacitor VC and a circuit constant of the transmission line L. The electric length is able to be elongated by making the variable capacitance large.

FIG. 1Dis a cross sectional view illustrating an example of the structure of variable capacitors C1, C2and Cm connected to a communication line. A lower electrode line L01having a projecting electrode on a bottom and an upper electrode line L02having a projecting electrode on a top constitute a variable capacitor with the projecting electrodes being overlapped. A drive electrode DE is formed under the projecting electrode of the upper electrode line L02. An insulating film28is formed on the upper surface of the projecting electrode of the lower electrode line L01. The drive electrode DE is connected via the through via conductor25to a terminal26on the bottom surface of the dielectric substrate20. The projecting electrode of the upper electrode line L02has a cantilever structure, and is displaced downward by generating an electrostatic attraction force upon application of a DC voltage to the drive electrode. As an example of the variable capacitor, although an MEMS capacitor is illustrated inFIGS. 1B,1C, and1D, the variable capacitor is not limited to an MEMS capacitor.

FIG. 1Eillustrates a variable capacitor using a varactor. A capacitance of a varactor diode BD is changed with a reverse bias. Inductors L11and L12for applying a reverse bias and blocking a high frequency signal are connected to the anode and cathode of the varactor diode. Capacitors C11and C12are connected to the anode and cathode of the varactor diode BD to flow a high frequency signal through the varactor and cut a DC bias.

The MEMS variable capacitor is not limited to a cantilever structure. A variety of structures are possible.

FIG. 1Fillustrates an example of the structure of a variable filter of a both-side supported lever type. A pair of conductive support pillars PL is formed on a dielectric substrate20, and a lever structure movable electrode ME is formed between the support pillars. A transmission line L is disposed on the dielectric substrate20under the movable electrode ME. Drive electrodes DE are formed on the dielectric substrate20on opposite sides of the transmission line L. Dielectric layers27and29are formed on the transmission line L and drive electrodes DE. The dielectric layers27and29on the drive electrode DE may be omitted. The structure inside the dielectric substrate20is similar to that of the structure illustrated inFIG. 1C.

FIG. 2Ais a graph illustrating a change in the pass characteristics of a variable filter when an electric length of the transmission line is elongated by applying a DC voltage to the variable capacitors of the transmission lines L1, L2, and LC1in the structure ofFIG. 1A. The abscissa represents a frequency in the unit of GHz, and the ordinate represent a transmission factor in the unit of dB. One example illustrates the filter pass characteristics when an applied voltage is increased from 0 V to 80 V at a step of 20 V. The center frequency of the pass band changes from about 4.4 GHz to about 2.06 GHz.

FIG. 2Bis a graph illustrating a change in the pass band of a variable filter of the structure illustrated inFIG. 1Awhen a coupling coefficient k is changed. The coupling coefficient k is a ratio of x to a quarter wave length (λ/4), k=x/(λ/4), when an electric length of the coupling line is (λ/4)+x, and the electric lengths of the lines L1and L2are (λ/4)−x. As the coupling coefficient k becomes small from 0.1 to 0.02, the pass band width becomes narrow.

FIG. 2Cis a graph illustrating a change in a −3 dB band width relative to a change in the coupling coefficient k. The −3 dB band width is a width of a band indicating a −3 dB change from the peak. It indicates that as the coupling coefficient k increases, the band width increases linearly.

It is seen from these graphs that the center frequency and band width of the pass band are able to be controlled by changing the coupling capacitances of the transmission lines L1, L2, and LC1of the circuit ofFIG. 1A. For example, it is easy to know a drive voltage to be applied to a drive electrode to obtain a desired center frequency and band width by using a lookup table indicating the center frequency and band width of a pass band as a function of an application voltage to obtain each coupling capacitance or capacitance value of the transmission lines L1, L2, and LC1.

It is possible to adjust both the center frequency and pass band width of a pass band.

In the first embodiment, the electric length of the coupling portion transmission line LC1is (λ/4)+x having a long physical length of the transmission line. It is preferable if a more compact structure is possible.

FIG. 3Ais an equivalent circuit of a variable filter of the second embodiment. Description will be made mainly on different points from the first embodiment. The transmission line LC1with the open end of the first embodiment is replaced with a serial connection of a coupling portion third transmission line LC2, a variable capacitor Cc and a line VIA constituted of a via conductor. The other end of the line VIA is grounded. A total electric length of the coupling portion is (λ/4)+x. The branch portion is similar to the first embodiment, and an electric length of each resonator is (λ/4)−x. By introducing the variable capacitor Cc, the electric length of the transmission line LC2is able to be shortened.

FIG. 3Bis a perspective top view of an example of the structure realizing the circuit ofFIG. 3A. A serial connection of a variable capacitor C1and a transmission line L1is connected to an input terminal IN. A serial connection of a variable capacitor C2and a transmission line L2is connected to an output terminal OUT. Electrodes of a variable capacitor Cm connect the variable capacitors C1and C2. The transmission lines L1and L2are connected to a transmission line LC2of a coupling portion. The other end of the coupling portion transmission line is grounded via a variable capacitor Cc and a via conductor. A variable capacitor is formed at upper five positions of each of the transmission lines L1, L2, and LC2. A cross section along line A-A has, e.g., the structure ofFIG. 1D. A cross section along line B-B has, e.g., the structure ofFIG. 1C. The structure of the variable capacitor C1, C2has, e.g., the structure ofFIG. 1D.

A glass epoxy substrate may be used in place of a ceramics substrate. All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts constituted by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification related to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.