JP2005203980A - Filter circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem wherein it is difficult to realize a small-sized, high-performance balun-incorporated type filter circuit, since the occupancy area of filter components and the number of elements are large. <P>SOLUTION: The filter circuit is provided with two functions of a band-pass filter which passes a signal in a specified band (2.4 GHz band) and attenuates a signal in an adjacent band (lower than or equal to 1.9 GHz) and a balanced-to-unbalanced transforming circuit. The resonance circuit, consisting of L1 and C1 of an attenuation circuit part 2 forms an attenuation electrode of 1.9 GHz; but since L1 and C2 resonate in a signal pass band 2.4 GHz and capacitance can be replaced nearly with the C1, to realize its band-pass filter. Consequently, no capacitor element is no longer necessary to be installed at a node ND, and the value of the L1 can be made small. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention includes a filter circuit (hereinafter referred to as a balance filter) having first and second coupling lines that are electromagnetically coupled to each other and having both a so-called balun (Bulun: balanced and unbalanced conversion circuit) function and a band-pass filter function. ).

Conventionally, there are various types of balance filters using a coupled line, and all of them include an attenuation circuit that attenuates the signal level in the adjacent frequency band of the pass band.
For example, a dielectric substrate is laminated as a filter having a characteristic of passing a 2.4 GHz band signal and attenuating the frequency band of a radio wave of another communication system adjacent to the 1.9 GHz band and having a balun function. A filter having a structure is known (for example, see Patent Document 1).

In the technique described in Patent Document 1, a resonator is formed by a ¼ wavelength coupled line as a band-pass filter unit, and three vertical coupled lines (strip lines) are also used in the balun unit. Therefore, the necessary occupation area is wide. Further, in order to reduce the area, it has a structure in which a total of about 10 dielectric substrates are laminated.
Therefore, the number of high-frequency circuit elements is large, and the size in the height direction is large.
JP 2003-087008 A

  The problem to be solved is that the area occupied by the filter part formed by the resonator of the 1/4 wavelength coupled line is large, and the number of high frequency circuit elements is large. It is difficult to realize a balun built-in bandpass filter circuit with a small area and a small area.

  The filter circuit according to the present invention includes first and second coupling lines that are electromagnetically coupled to each other, and includes an unbalanced signal input / output from the first coupling line side and an input / output from the second coupling line side. A filter circuit having both a function of balanced / unbalanced conversion for mutually converting a balanced signal to be converted and a function of a band-pass filter having a characteristic of attenuating an adjacent frequency band of a pass band. A capacitance circuit that is connected and has an attenuation circuit unit that attenuates a signal component in an adjacent frequency band of the pass band, constitutes a parallel resonance circuit with the inductance of the first coupled line, and has a capacitance for realizing the function of the band pass filter; This is realized by the impedance of the attenuation circuit section at the frequency of the pass band.

In the present invention, it is preferable that the attenuation circuit unit includes an inductor and a capacitor connected in series between the input / output terminal of the unbalanced signal and a common potential line, and an attenuation pole is provided in an adjacent frequency region of the passband. A series resonance circuit having a non-equilibrium signal, and an input terminal for the unbalanced signal, and one end of the first coupling line, which are connected in series to attenuate a signal component on the opposite side of the passband of the attenuation pole. A series resonance circuit in which the blocking capacitor and the inductor of the series resonance circuit resonate in the passband.
In the present invention, preferably, the length of the coupling portion of the two coupled lines is set to be shorter than ¼ of the center frequency wavelength of the pass band, and the band pass filter is set according to the difference between the lengths. The size of the capacitance for realizing the function is set.

In the present invention, preferably, at least an inductor of the attenuation circuit section is formed from two conductive layers, each of which has at least one main surface formed on the main surface of the substrate made of an insulator, and is laminated via an interlayer dielectric layer. Is configured.
In the present invention, preferably, the first and second coupling lines are formed of the same conductive layer (edge coupling or lateral coupling). Alternatively, the first and second coupling lines are formed from different conductive layers (broadside coupling or vertical coupling).

  In the filter circuit according to the present invention, the input may be an unbalanced signal and the output may be a balanced signal, or conversely, the input may be a balanced signal and the output may be an unbalanced signal. Mutually balanced / unbalanced conversion is possible.

  The operation of the present invention will be described below when the input is unbalanced and the output is balanced. An attenuation circuit unit, a band pass filter, and a balanced unbalanced conversion unit (first and second coupled lines) are connected to a signal transmission path from the input / output terminal of the unbalanced signal to the input terminal of the balanced signal. These are formed from two conductive layers formed on an insulator on the surface of a substrate via an interlayer dielectric layer. An LC series resonance circuit having an attenuation pole in the adjacent frequency band of the pass band is connected between the signal transmission path and the reference potential line, and a blocking capacitor is connected in series to the transmission path. An LC parallel resonance circuit constituting a band pass filter is connected to the blocking capacitor. The inductance of the LC parallel resonance circuit corresponds to the inductance component of the first coupled line. The first coupled line is coupled to the second coupled line by vertical or lateral coupling, and the second coupled line is connected to an input / output terminal for an unbalanced signal.

The signal input from the unbalanced terminal is converted into a balanced signal through the attenuation circuit unit, the band pass filter, and the balanced unbalanced conversion unit, and is output from the input / output terminal of the balanced signal. At this time, the constant is set so that the impedance of the LC parallel resonant circuit is low at the frequency of the pass band of the LC parallel resonant circuit constituting the band pass filter. In addition, at this passband frequency, another LC series resonant circuit composed of an inductance and a blocking capacitance of an LC series resonant circuit having an attenuation pole in the adjacent frequency band of the passband satisfies the resonance condition, and its impedance is low. These constants are set so that Therefore, the capacitance of the LC series resonance circuit having the attenuation pole in the adjacent frequency band of the pass band and the first coupled line are equivalently high-frequency shorted. Therefore, the impedance of the attenuation circuit unit at this time becomes capacitive, and functions as a capacitance component of the band pass filter (LC parallel resonant circuit). That is, in the present invention, the attenuating circuit unit also has the function of a capacitor of the band pass filter, and a dedicated capacitor is omitted from the band pass filter.
On the other hand, in the adjacent frequency band of the pass band, the signal attenuation by the band pass filter is large, and in order to perform sufficient attenuation, the LC series resonance circuit having the attenuation pole oscillates and its impedance is lowered, so that the signal component Most of this is dropped to ground potential. That is, the LC series resonance circuit having the attenuation pole functions as a trap circuit having a specific frequency. Further, a cutoff capacitor functions in a frequency region further away from the pass band than the attenuation pole, thereby blocking the signal in this frequency region and preventing input to the coupled line.

Since the filter circuit of the present invention has both the band-pass filter and the balanced / unbalanced conversion function in the first and second coupled lines, the occupied area can be reduced as compared with the case where the coupled lines are independently provided. In addition, the capacitance constituting the bandpass filter can be realized by an attenuation circuit unit that is capacitive in the passband, and it is not necessary to provide an element that realizes the capacitance independently, and the occupied area is further reduced accordingly. Yes. As a result, the inductance of the LC series resonance circuit having the attenuation pole in the adjacent frequency region of the pass band is also reduced, and the occupied area can be further reduced accordingly.
Since it has such a small area configuration, for example, a filter circuit can be realized from two conductive layers whose layers are insulated on a substrate such as a semiconductor substrate.
As described above, the present invention can provide a balun built-in bandpass filter that is small in area and size in the height direction and has practically sufficient characteristics.

1A and 2A are circuit diagrams of a filter circuit according to an embodiment of the present invention, and FIGS. 1B and 2B show cross-sectional configurations.
The filter circuit (balance filter) 1 includes an attenuation circuit unit 2, a coupled line 3, and an impedance matching unit 4. Of these, the coupling line 3 is formed of the same conductive layer, so-called edge coupling or lateral coupling, and the type is called so-called broad-side coupling or longitudinal coupling, in which a dielectric layer is sandwiched between different conductive layers. There are two embodiments with the type to be. 1A and 1B show the lateral coupling type, and FIGS. 2A and 2B show the vertical coupling type. Both types have the same structure except for the structure of the coupled line 3.

The attenuation circuit section 2 is called a so-called trap circuit, and includes an LC series resonance circuit 21 having an attenuation pole near a specific frequency, for example, 1.9 GHz, and a low-frequency cutoff capacitor 22 (capacitance: C2). The trap circuit 21 includes an inductor (inductance: L1) 23 and a capacitor (capacitance: C1) 24 connected between the input / output terminal Tu for the unbalanced signal and a reference potential (ground potential). The low-frequency cutoff capacitor 22 is connected in series to the signal line, and suppresses or cuts off a low-frequency signal level of, for example, 1.5 GHz or less.
The coupled line 3 includes a first coupled line CL1 and a second coupled line CL2. The first coupling line CL1 is connected between the low-frequency cutoff capacitor 22 and the ground potential, and the second coupling line CL2 is connected between the two balanced signal input / output terminals Tb1 and Tb2.
Although the impedance matching unit 4 includes a resistor and the like, FIG. 1 shows only a capacitor (capacitance: C3) 41 connected between the input / output terminals Tb1 and Tb2 for two balanced signals.

  As shown in FIGS. 1B and 2B, the balance filter 1 having such a structure includes an insulating layer 11 formed on, for example, a semi-insulating semiconductor substrate or a substrate 10 having a low conductivity. The conductive layer is formed of two conductive layers formed on an insulating layer 11 with an interlayer dielectric layer 12 interposed therebetween. FIGS. 1B and 2B show only a portion where the inductor 23 and the coupled line 3 are formed, but all other structures are formed from one conductive layer, for example, the upper conductive layer. The inductor 23 is a spiral type and cannot be formed as a single layer because the area occupied is as small as possible. It is necessary to connect the capacitor 24 (C1) or 22 (C2) formed of the layers. At this time, a lower connection layer can be formed from a lower conductive layer to reduce the parasitic capacitance at the intersection.

  In the laterally coupled type shown in FIG. 1B, for example, the first coupled line CL1 and the second coupled line CL2 are formed from an upper conductive layer, and are formed side by side at a predetermined interval. On the other hand, in the vertical coupling type shown in FIG. 2B, the first coupling line CL1 is formed from an upper conductive layer, and the second coupling line CL2 is formed from a lower conductive layer. In any type, the coupled line width is arbitrary depending on the design, and both may be the same or different. If the coupling is too strong, the coupling lines may be shifted from each other in the width direction.

  In addition to the cross-sectional structure, a planar pattern can be changed. For example, the second coupled line CL2 can be separated at the center of its length and both ends thereof can be free open ends.

Next, the coupled line length will be described. FIG. 3 is an explanatory diagram of the coupled line length.
A coupled line for balanced / unbalanced conversion is usually designed to have a line length of ¼ wavelength (λ / 4) of the center frequency of a high-frequency signal to be handled. However, as shown in the figure, the size of the first coupling line CL1 to which the unbalanced signal is input is reduced to the ground potential by the wavelength shortening capacitor 100 (capacitance: C4). Depending on C4, the length of each of the coupled lines CL1 and CL2 can be shortened.
More specifically, the shorter the coupled line, the greater the signal loss (decrease in conversion efficiency) from the frequency band shifted from the center frequency. However, since the band-pass filter characteristic can be provided by the parallel resonance circuit of the capacitance C4 and the inductance component of the first coupling line CL1, if there is little loss in the passband near the center frequency, a large loss is caused in other frequency regions. There is no practical problem even if this occurs. Thus, by adding the capacitor 100 (C4), the characteristics can be maintained at a practical level in a necessary frequency region even if the wavelength is apparently shortened, so that a coupled line with a small occupation area can be realized.

  The present embodiment has a significant feature in that the wavelength shortening capacitor 100 is omitted as a circuit configuration. The constant C4 of the capacitor 100 for shortening the wavelength is optimized by comprehensively considering the band of the filter, the conversion efficiency of the coupled line, the occupied area of the coupled line portion, and the like. On the other hand, it is necessary to design the constants of the trap circuit 21 and the low-frequency cutoff capacitor 22 according to their required characteristics. In a normal design method, after investigating the characteristics for each of these circuit units, the characteristics when the circuit units are integrated are estimated, and then fine adjustment is performed as appropriate so that the final target characteristics can be obtained.

  For example, in the trap circuit 21, the frequency f for trapping the signal component at the attenuation pole is given by the following equation (1), and the constants L1 and C1 of the inductor 23 and the capacitor 24 for the trap circuit correspond to the trapping frequency f. Various combinations are possible.

[Formula 1]
f = 1 / 2π√LC (1)

  However, when each part is designed individually as described above, the inductance L1 of the trap circuit 21 is set so as to have a function (blocking function) for reducing the influence on other parts in a band other than the trapping frequency. Make it as large as possible. However, in general, an inductor having a high Q value for a filter occupies a large area, and increasing the inductance L1 is an obstacle to circuit miniaturization. Further, if the above-described capacitance C4 for shortening the wavelength of the coupled line portion is too large, the effect of shortening the coupled line length is significantly hindered, which becomes an obstacle to circuit miniaturization.

  In the present embodiment, each part of the balance filter with a built-in balun is not designed individually, but the circuit constants of the trap circuit 21, the low-frequency cutoff capacitor 22 (C2), and the coupling lines CL1 and CL2 are comprehensively determined. Optimize to. As a result, the inductor 23 (L1) and the coupling lines CL1 and CL2 are downsized without adding the wavelength shortening capacitor 100 (C4), and the occupied area of the entire balance filter is reduced.

  In order to realize this, first, the function of the wavelength shortening capacitor 100 (C4) is replaced by the attenuation circuit unit 2 shown in FIGS. 1 (A) and 2 (A). That is, when viewed from the node ND where the unbalanced signal is input to the first coupling line CL1 to which the capacitor 100 is connected in FIG. 3, the two capacitors 22 (C2) and 24 (C1) are connected to the ground line. Are connected, and an inductor 23 (L1) is connected between them. Of these, the inductor 23 (L1) and the capacitor 22 (C2) constitute an LC series resonance circuit of this path, and therefore the resonance frequency of the band-pass filter provided in the first coupling line CL1 (for example, The constants L1 and C2 of the inductor 23 and the capacitor 22 are set so as to be adapted to the 2.4 GHz band. As a result, in the band of the band pass filter, the impedance of the LC series resonance circuit composed of the inductor 23 and the capacitor 22 becomes low, so that the node ND and the capacitor 24 of the trap circuit seem to be close to a high frequency short circuit. Therefore, if the constant C1 of the capacitor 24 is optimized in consideration of the loss of the LC series resonance circuit for high frequency short circuit, this functions in place of the capacitor 100 (C4) for wavelength short circuit (or for the passband filter). .

At this time, the capacitor 22 needs to play a role of low-frequency cutoff.
In general, a low-frequency cutoff capacitor does not act on a specific frequency unlike a trap circuit, and therefore has a high degree of freedom in setting a constant. Further, the inductor 23 (L1) of the trap circuit is a series resonance circuit that brings the node ND and the capacitor 24 close to a high frequency short circuit, and its constant is determined, and its inductance L1 changes in a small direction. At this time, in order to make the Q values of the inductances L1 equal, by adjusting the low-frequency cutoff capacitor 22 having a high degree of freedom in setting constants, it is possible to optimize so that attenuation characteristics in the adjacent frequency range can be obtained comprehensively. . As a result, the inductor 23 (L1) can be reduced in size, and the capacitor 100 (C4) can be omitted as a circuit component.

FIG. 4 shows a circuit design example with the capacitor 100 (C4) added and a circuit design example omitted. FIG. 4B shows a specific numerical example of the constant of the circuit in which the capacitor 100 (C4) is omitted by the adjustment method described above based on the constant circuit shown in FIG.
Capacitance C1, which is an alternative to capacitance C4 = 2pF in the signal passband, has a value of approximately the same level as 2.5pF. At this time, the values of the inductance L1 and the capacitance C1 for realizing the high-frequency short circuit are changed from 9 nH to 2.5 nH and from 1 pF to 2.5 pF, respectively. As a result, the capacitor 100 (C4) is not required, and the area of the inductor 23 (L1) having a large occupied area can be greatly reduced. Although the value of other capacitors is increased from 2.5 times to 3.5 times, the area of the capacitor is smaller than that of the inductor, so that the total area increase is slight.

FIG. 5A shows the frequency characteristics of the circuit shown in FIG. 4A, and FIG. 5B shows the frequency characteristics (simulation results) of the circuit shown in FIG. 4B.
These graphs show that the characteristics of the two are almost the same, such as the loss of the signal passband in the 2.4 GHz band, the polar attenuation width of the adjacent frequency band near 1.9 GHz, and the low frequency attenuation width of 1.5 GHz or less. It has been.

According to the embodiment of the present invention, the following effects can be obtained.
First, since the inductor 23 (L1) for the trap circuit can be reduced in size, the area of the balance filter can be greatly reduced. For example, in the above example, the inductance L1 of the trap circuit is 1/3 or less of the conventional method. In the inductance, when the Q values are the same, the inductance value is substantially proportional to the area. In this example, the area of the inductor 23 (L1) of the trap circuit can be reduced to 1/3 or less.

  Second, the wavelength shortening capacitor 100 (C4) becomes unnecessary, and the area occupied by the balance filter can be reduced accordingly. In addition, the value of another capacitor may increase like the said example. However, since the capacitors 22 (C2) and 24 (C1) need to separate the inductor 23 from the coupling lines CL1 and CL2, the space for their arrangement is originally sufficient. Even if the capacitance values C1 and C2 increase slightly, the entire capacity is increased. It does not become an increase factor of On the other hand, elements are densely packed around the wavelength shortening capacitor 100 (C4) arranged in the vicinity of the coupling line CL1, and if the capacitor 100 (C4) can be omitted, the entire area can be reduced accordingly. .

Third, since the inductor 23 (L1) for the trap circuit is reduced in size and the wavelength shortening capacitor 100 (C4) is not required, the inductor 23 (L1) having a higher Q factor can be used. As a result, the filter performance can be improved.
Fourth, since the inductor 23 (L1) for the trap circuit is reduced in size and the wavelength shortening capacitor 100 (C4) is not required, the coupling lines CL1 and CL2 can be made larger by that much. As a result, the filter performance can be improved.
Fifth, since the number of circuit elements is reduced, the degree of freedom in element arrangement is improved.

1A is a circuit diagram of a balance filter according to an embodiment of the present invention, and FIG. 1B is a cross-sectional view of a laterally coupled type coupled line and an inductor. 1A is a circuit diagram of a balance filter according to an embodiment of the present invention, and FIG. 1B is a cross-sectional view of a longitudinally coupled type coupling line and an inductor. It is explanatory drawing of coupled line length. (A) is a figure which shows the circuit design example which added the capacitor for wavelength shortening, (B) is a figure which shows the circuit design example which abbreviate | omitted the said capacitor. FIG. 5A is a graph showing the frequency characteristic (simulation result) of the circuit shown in FIG. 4A, and FIG. 5B is a graph showing the frequency characteristic (simulation result) of the circuit shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Balance filter, 2 ... Attenuation circuit part, 3 ... Coupling line part, 4 ... Impedance adjustment part, 10 ... Board | substrate, 11 ... Insulating layer, 12 ... Interlayer insulation layer, 21 ... Trap circuit, 22 ... Low-pass cutoff Capacitor, 23... Inductor, 24, 41... Capacitor, 100.

Claims (6)

First and second coupling lines electromagnetically coupled to each other, and an unbalanced signal input / output from / to the first coupling line side and a balanced signal input / output from the second coupling line side to each other A filter circuit having both a function of balanced / unbalanced conversion for conversion and a function of a band pass filter having a characteristic of attenuating an adjacent frequency band of the pass band,
An attenuation circuit unit connected to the first coupling line, for attenuating a signal component in an adjacent frequency band of the pass band;
A filter circuit that forms a parallel resonance circuit with the inductance of the first coupled line, and that realizes the function of the band-pass filter by the impedance of the attenuation circuit unit at the frequency of the pass band. The attenuation circuit section is
A series resonant circuit including an inductor and a capacitor connected in series between the input / output terminal of the unbalanced signal and a common potential line, and having an attenuation pole in an adjacent frequency region of the passband;
A blocking capacitor that is connected in series between the input / output terminal of the unbalanced signal and one end of the first coupled line, and that attenuates the signal component on the side opposite to the passband of the attenuation pole;
The filter circuit according to claim 1, wherein the blocking capacitor and the inductor of the series resonance circuit constitute a series resonance circuit that resonates in a pass band. The length of the coupling part of the two coupled lines is set to be shorter than ¼ of the center frequency wavelength of the passband, and the function of the bandpass filter is realized according to the difference between the lengths. The filter circuit according to claim 1, wherein the magnitude of the capacitance is set. At least one main surface is formed on the main surface of a substrate made of an insulator, and at least an inductor of the attenuation circuit unit is constituted by two conductive layers stacked via an interlayer dielectric layer. 2. The filter circuit according to 1. The filter circuit according to claim 4, wherein the first and second coupled lines are formed of the same conductive layer. The filter circuit according to claim 4, wherein the first and second coupling lines are formed of different conductive layers.

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