JP2003163503A - Lowpass filter, and multistage lowpass filter, multilayer rf package and multilayer rf module using lowpass filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a low-loss and lowpass filter having steep attenuating characteristics. <P>SOLUTION: The lowpass filter has a series capacitor circuit 6 with two terminals, an input line 4a connected to one of the two terminals, an output line 4b connected to the other one of the two terminals, and a transmission line 5 with one end connected to the input line 4a and the other to the output line 4b. Characteristic impedance of the transmission line 5 is set to be lower than the characteristic impedance of the input line 4a and the output line 4b. Combination of the characteristic impedance and electrical length of the transmission line 5 is selected so that the lowest passing zero point and the second lowest passing zero point substantially or nearly correspond. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low pass filter, and more particularly to a low pass filter used in the VHF band, the UHF band, the microwave band and the millimeter wave band. Further, the present invention relates to an RF package and an RF module incorporating the low pass filter.

[0002]

11 and 12 show a conventional strip line type low pass filter, for example. A low-pass filter of this type is disclosed in, for example, Japanese Patent Laid-Open No. 2000-18.
It is described in Japanese Patent No. 3603. In these figures, 101a and 101b are ground conductors and 102a and 102b.
Is a dielectric substrate, 103 is a strip conductor, 104a is an input line, 104b is an output line, 105 is a transmission line in a filter circuit, and 106 is a series capacitance circuit.
Reference numeral 06b is a strip conductor forming the series capacitance circuit 106.

Next, the operation of this conventional low pass filter will be described. The transmission line 105 functions as a series inductance, and is usually composed of a line having a characteristic impedance sufficiently higher than that of the input line 104. Therefore, the width of the strip conductor of the transmission line 105 is narrower than the width of the strip conductor of the input line 104a. Since the conductor width is narrow, the current flowing on the strip conductor of the transmission line 105 is concentrated and it is easy to obtain a large inductive property. As a result, the electrical length of the transmission line 105 can be shortened, and the filter can be made compact. On the other hand, the strip conductor 10
6a and 106b are arranged substantially parallel to each other on the dielectric substrate 102b, and capacitively couple between them to function as a series capacitance circuit. Thus, the equivalent circuit of the conventional low pass filter is as shown in FIG. The parallel capacitance Cp in FIG. 13 is mainly the capacitance formed between the strip conductors 106 a and 106 b and the ground conductor 101. The transmission line 105 and the strip conductor 106 are designed such that each element in the equivalent circuit is equivalent to an element of an original low-pass filter having a desired pass band characteristic (for example, Chebyshev response) at the filter cutoff frequency. To be done. FIG. 14 shows an equivalent circuit of a third-order original low-pass filter.
The transmission line 105 and the two strip conductors 106
In the parallel resonant circuit configured by the combination of the series capacitance circuits configured by a and 106b, the parallel resonant circuit is equivalent to the series inductance element L of the original filter at the cutoff frequency, and at the stopband or near the stopband. Designed to resonate. At the resonating frequency, the parallel resonant circuit is open, so that a passing zero point is formed. As described above, the conventional low-pass filter has a set of parallel resonance circuits and is a third-order polar low-pass filter having one pass zero point at which the pass loss becomes infinite in or near the stop band. Operate. By forming the pass zero point using the parallel resonance circuit, the conventional low-pass filter has a characteristic that a steep attenuation characteristic can be obtained as compared with a filter of the same order without the parallel resonance circuit.

[0004]

By the way, in order to obtain a steeper attenuation characteristic in the conventional low pass filters of FIGS. 11 and 12, the most effective method is to add one parallel resonance circuit. That is. This allows
Two passing zeros are obtained, the attenuation characteristic becomes steeper, and a large amount of attenuation can be secured over a certain fixed bandwidth depending on the arrangement of the two passing zeros. However, according to this method, if the Q value of the resonance circuit cannot be made large, it is unavoidable that the insertion loss in the pass band greatly increases. In a filter that pursues miniaturization, the Q value of the resonant circuit is often small in principle. Also, when the loss of the dielectric material forming the filter is not small, the Q value of the resonance circuit also becomes small. In particular, LTCC (Low Temp) which has been rapidly used in high frequency circuits in recent years.
erature Co-fired Ceramics, low-temperature firing ceramics) has the characteristic that a high-frequency circuit having a stripline structure can be easily configured in a small size because a multilayer structure can be easily configured, but it does not contain a glass component to lower the firing temperature. Partly because of the influence of this, the dielectric loss is by no means small. That is, when a plurality of resonance circuits are formed in the filter to obtain steep attenuation characteristics, there is a problem that the insertion loss increases significantly.

In addition, in the LTCC, since the strip conductor formed between the dielectric layers is formed of a thick film, the minimum strip conductor width that can be formed is relatively large. Therefore, having a high-impedance line in the filter causes an increase in ground conductor spacing, which causes a problem of increasing the thickness of the filter. LTCC
When a filter is built in the multilayer structure of a multi-layer package composed of etc. and an active device such as MMIC is mounted on the upper surface of the package, the thickness of the filter circuit causes an increase in the thermal resistance of the package and the heat dissipation of the active device. It will interfere.

The present invention has been made to solve the above problems, and has a low-pass filter having steep attenuation characteristics and low loss, and a multistage low-pass filter using the low-pass filter. The purpose is to obtain a pass filter, RF package, RF module.

[0007]

According to the present invention, there is provided a series capacitance circuit having two terminals, an input line connected to one of the terminals, and an output line connected to the other of the terminals.
A low frequency band having a transmission line having one end connected to the input line and the other end connected to the output line, wherein the characteristic impedance of the transmission line is lower than the characteristic impedances of the input line and the output line. It is a pass filter.

Further, the series capacitance circuit is composed of first and second strip conductors, and the first and second strip conductors are provided.
Strip conductors are provided so as to face each other with the dielectric substrate interposed therebetween.

The input line and the output line are arranged on the main surface of the dielectric substrate on the side where the first strip conductor is provided, and the second strip conductor and the input line are connected to each other. The spaces are connected to each other through a connecting means having conductivity which is provided so as to penetrate the dielectric substrate.

Further, the combination of the characteristic impedance and the electrical length of the transmission line is selected so that the lowest passing zero point and the next lowest passing zero point substantially match or approach each other.

The angle formed by the direction from the tip of the first strip conductor to the input line and the direction from the tip of the second strip conductor to the output line is approximately 90 degrees. In this way, the tip end portion of the first strip conductor is projected from the edge of the second strip conductor facing through the dielectric layer, and the tip end portion of the second strip conductor is covered with the dielectric layer. The first transmission conductor and the first transmission conductor are arranged so as to project from the edges of the first strip conductors that are opposed to each other, and to surround the series capacitance circuit.

A multi-stage low-pass filter having two low-pass filters according to the present invention, in which the output line side of one low-pass filter and the input line side of the other low-pass filter are cascaded. While connecting, the conductor patterns of the two low pass filters are arranged so as to be substantially point-symmetrical about the connection point of the two low pass filters, and the second low pass filter of the one low pass filter is arranged. The strip conductor and the second strip conductor of the other low-pass filter are arranged in the same plane.

Further, the present invention is a multi-layer RF package having a multi-layer structure incorporating the low-pass filter of the present invention, wherein an active device serving as a heat source is mounted on the multi-layer structure.

Further, the present invention is a multi-layer RF module having a multi-layer structure incorporating the low-pass filter of the present invention, wherein an active device serving as a heat source is mounted on the multi-layer structure.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1. The first embodiment according to the present invention is shown in FIGS. FIG. 1 is an exploded perspective view showing a configuration of a strip line type low pass filter according to the present embodiment. FIG. 2 is a top view of the dielectric substrate in the low pass filter of FIG. 1 and is a diagram for explaining the pattern shape of the strip conductor. In these figures, 1
a, 1b are ground conductors, 2a, 2b, 2c are dielectric substrates, 3
a and 3b are strip conductors, 4a is an input line, and 4b is an output line. Reference numeral 5 denotes a transmission line in the filter circuit, one end of which is connected to the input line 4a and the other end of which is connected to the output line 4b. Here, the in-filter transmission line 5 is composed of a line having a characteristic impedance lower than those of the input line 4a and the output line 4b. Therefore, as is clear from FIG. 2, the width W2 of the strip conductor forming the in-filter transmission line 5 is larger than the width W1 of the strip conductor forming the input line 4a and the output line 4b.
Further, the electrical length of the in-filter transmission line 5 is relatively large, which is about a fraction of the wavelength of the cutoff frequency of the low pass filter. Reference numeral 6 denotes a series capacitance circuit having two terminals, and 6a and 6b are series capacitance circuits 6
Is a strip conductor. As shown in FIG. 1, one terminal of the series capacitance circuit 6 is connected to the input line 4a, and the other terminal of the series capacitance circuit 6 is connected to the output line 4b. The strip conductor 6a is provided on the dielectric substrate 2b, and the strip conductor 6b is provided on the dielectric substrate 2b.
c, ie strip conductors 6a and 6b
Are provided so as to be parallel to the dielectric substrate 2b so as to face each other with the dielectric substrate 2b interposed therebetween, and an electrostatic capacitance is formed between the two. Reference numeral 7 is a connecting means for connecting the strip conductors 3a and 3b in the vicinity of the output line 4b. The connecting means 7 is composed of a via (through hole) provided so as to penetrate the dielectric substrate 2b as shown in the figure. Further, the present invention is not limited to this, and any one may be used as long as it is provided to penetrate the dielectric substrate 2b and electrically connects the strip conductors 3a and 3b. For example, the dielectric substrate 2b. You may make it comprised from the columnar type or the prismatic type electroconductive member provided so that it might penetrate. In the case of using a conductive member, for example, a mask is formed on the dielectric substrate 2b, a through hole is formed by exposing and developing, and a conductive paste is filled in the through hole. In the following description, for the sake of clarity, the via 7 is used as an example instead of the connecting means 7.

Next, the electrical operation of the above low pass filter of the present invention will be described. First, FIG. 3 shows an equivalent circuit of the low-pass filter according to the present embodiment. In FIG. 3, since the transmission line 5 in the filter circuit has a large electric length, it is described as a transmission line having a characteristic impedance Zt and an electric length θt. In addition, the capacitance value of the series capacitance circuit 6 is C
s, and the parasitic capacitance between the series capacitance circuit 6 and the ground conductors 1a and 1b is represented by Cp. Next, FIG.
An equivalent circuit of the transmission line 5 in the filter circuit is shown in (a). The equivalent circuit of the transmission line can be represented by a π-type circuit including a circuit having a series reactance and a circuit having a parallel susceptance. The reactance value Xt and the susceptance value Bt of each circuit in FIG. , (2)
It is known to be represented as.

[0017]

[Equation 1]

Here, ω is the frequency, ωc is the cutoff frequency of the filter, and θc is the electrical length of the transmission line 5 at the cutoff frequency. That is, the electrical length θt of the transmission line 5 is 180
If the frequency is smaller than the frequency, both Xt and Bt in the circuit of FIG. 4A have positive values, the series reactance circuit is equivalent to the series inductance element, and the parallel susceptance circuit is equivalent to the parallel capacitance element. Become. That is, if the electrical length θt of the transmission line 5 is a frequency smaller than 180 degrees, the transmission line portion of the equivalent circuit of FIG. 3 is represented by a series inductance element and a parallel capacitance element. The low-pass filter is similar to the conventional low-pass filter described above, as shown in FIG.
It can be seen that it is possible to operate as a polarized low-pass filter (the filter order is the third order) having one set of parallel resonant circuits having the equivalent circuit shown in (b).

As can be seen from the above equation (1), Xt is a sine function with the frequency ω as an argument. This means that the electrical length θ of the line at the cutoff frequency
It is shown that the period of the sine function becomes large when c is small, and the sine function is ω near the cutoff frequency.
Asymptotically to a linear function of, and Xt increases almost monotonically with respect to ω. This corresponds to the fact that Xt exhibits a lumped constant (inductance) property. On the other hand, when θc is large, the cycle of the sine function becomes small, the maximum value of Xt approaches the cutoff frequency, and Xt is no longer a monotonically increasing function with respect to the frequency ω. In a low pass filter,
The magnitude of θc depends on the fact that Xt has a predetermined value (reactance value of the corresponding inductance element in the original filter) at the cutoff frequency.
c is small and θc is large when Zt is small.

Focusing on this point, let us consider a parallel resonant circuit constituted by a combination of the transmission line 5 and the series capacitance circuit 6. FIG. 5 shows a transmission line 5 and a series capacitance circuit 6.
The susceptance value of each is overwritten. The susceptance value of the parallel resonance circuit is zero, that is, the susceptance value of the transmission line 5 is also shown as a positive value in order to make the position of the resonance frequency easy to understand.
In FIG. 5, what is indicated by the alternate long and short dash line 51 is the susceptance of the series capacitance circuit, and the frequency ω increases monotonically. On the other hand, solid lines 52 and 53 show the susceptance value (reciprocal of Xt) of the transmission line 5 in two cases where Zt is different (when θc is small and when θc is large). Thus, when Zt is large (solid line 52 (Zt = 90 oh
m)), the susceptance value monotonically decreases near the cutoff frequency, and the susceptance of the series capacitance circuit is 1
It intersects at a point, that is, it resonates only at one frequency ωp1.
On the other hand, when Zt is small (solid line 53 (Zt = 3
In the case of (0 ohm)), 1 / Xt has a minimum value in the vicinity of the cutoff frequency. Therefore, depending on the method of selecting the values of Zt and θc, the chain line 51 has two intersections, that is,
It is clear that they can resonate at two frequencies ωp1 and ωp2.

The low-pass filter according to the present embodiment utilizes the above-mentioned property, and obtains two passing zero points in a filter having one set of parallel resonant circuits.
These two passing zero points are almost the same or close to each other,
Good damping characteristics are obtained. Here, the design of the filter according to the present embodiment will be briefly described. The cutoff frequency ωc, the first zero-point frequency ωp1, and the characteristic impedance Zt of the transmission line 5 are prepared in advance and the following simultaneous equations are formed.

[0022]

[Equation 2]

Here, L in the equation (3) is the value of the series inductance of the original low-pass filter that gives a desired characteristic. Equations (3) and (4) are solved using the electrical length θc of the transmission line 5 and the capacitance value Cs of the series capacitance circuit 6 at the cutoff frequency as unknowns. Then, by applying the obtained θc and Cs to the following equation, the second zero-point frequency is immediately obtained.

[0024]

[Equation 3]

The value of the obtained second zero-point frequency ωp2 is ωp1
The above calculation may be repeated using the characteristic impedance Zt of the transmission line 5 as a parameter so that they substantially match or are sufficiently close to each other. In this way, the electrical length θc of the transmission line 5, the characteristic impedance Zt, and the capacitance value Cs of the series capacitance circuit are determined as the circuit components of the third-order low-pass filter having two desired zero-point frequencies.

In FIG. 6, the characteristic of the present embodiment is shown by a thick line 61. The characteristic of the conventional low-pass filter is also shown by a thin line 62. These two filters are designed so that the first zero-point frequencies ωp1 of the two filters coincide with each other in order to facilitate comparison of characteristics. It can be seen that the low-pass filter of the present embodiment has a steeper attenuation characteristic than the conventional filter. At the same time, it is found that not only the frequency bandwidth in which a certain amount of attenuation can be obtained is widened, but also extremely large amount of attenuation is obtained. Further, since the two resonance circuits are not used as means for obtaining the two zero points, the loss of the pass band is equivalent to that of the conventional low pass filter using only one resonance circuit. The increase in loss does not occur as a negative effect of improving the damping characteristics.

By the way, based on the above design method, the characteristic impedance Zt of the transmission line 5 and the second zero point frequency ωp2
When the relationship of is obtained, it becomes as shown in FIG. Note that FIG. 7 shows the result of calculation with a ripple value of 0.05 dB. Figure 7
The horizontal axis represents the Zt standardized by the characteristic impedance Z0 of the input line 4, and the vertical axis represents the ratio of ωp2 and ωp1. The parameter is the position of ωp1 with reference to the cutoff frequency. Looking at this relationship, the characteristic impedance Zt of the transmission line 5 must be set in order to bring the two zero points close to each other.
It can be seen that is a required condition that is smaller than Z0. If the first zero ωp1 is set to 2.5 times the cutoff frequency or more, it is possible to bring the two zeros close to each other even if the characteristic impedance Zt of the transmission line is equal to Z0. When the cutoff frequency is separated by 2.5 times or 3 times, the amount of attenuation increases even without depending on the passing zero point. Therefore, a circuit configuration with only one passing zero point like a conventional low pass filter is used. It becomes possible to obtain necessary and sufficient damping characteristics. That is, in the low-pass filter of the present embodiment, the value of the characteristic impedance Zt of the transmission line 5 may be considered to be equal to or less than the characteristic impedance Z0 of the input line 4a or the output line 4b. In this respect, since it is common sense in the conventional low-pass filter that the characteristic impedance of the transmission line 5 is set to be high to reduce the electrical length, the appearance of the low-pass filter of the present embodiment is clear. It is one of the features.

Further, the fact that a line having a characteristic impedance higher than the characteristic impedance of the input line or the output line is not required in the filter circuit means that when the circuit is constituted by strip lines as in this embodiment, the ground conductor is used. 1a
This leads to the possibility of reducing the distance between 1 and 1b, that is, reducing the thickness of the filter circuit. This is effective when a multilayer circuit structure is made of a material such as LTCC. As described above, in LTCC, since the strip conductor formed between the dielectric layers is formed of a thick film, the minimum strip conductor width that can be formed is relatively large. Therefore, the minimum strip width is often assigned to the line having the highest characteristic impedance in the filter circuit. That is, the smaller the value of the highest characteristic impedance in the filter circuit, the smaller the ground conductor spacing,
The filter circuit becomes thinner. As described above, the present embodiment has the effect of reducing the thickness of the filter circuit. This means that when the filter of the present embodiment is built in the multilayer structure of a multilayer package composed of LTCC and the like, and an active device such as MMIC is mounted on the upper surface of the package, the package thickness under the MMIC Therefore, there is an effect that the thermal resistance of the package can be reduced and a package having excellent heat dissipation performance can be provided.

In the description of the operation of this embodiment, the frequency of the second zero point is explained as being coincident with or sufficiently close to the first zero point. However, for example, ωp1 is approximately twice the cutoff frequency. Of course, the frequency may be set and ωp2 may be arranged at approximately 1.5 times ωp1 (approximately 3 times the cutoff frequency). An application of the filter having such a zero point is a spurious suppression filter of an amplifier mounted on the output side of the HPA. That is, the present embodiment provides excellent low loss characteristics and excellent attenuation characteristics even as a spurious suppression filter for an amplifier.

As described above, in the present embodiment,
A series capacitance circuit 6 having two terminals, an input line 4a connected to one of the terminals, an output line 4b connected to the other of the terminals, one end connected to the input line 4a and the other end It is a low-pass filter including a transmission line 5 connected to the output line 4b, and the characteristic impedance of the transmission line 5 is set lower than the characteristic impedances of the input line 4a and the output line 4b. The lowest resonance frequency of the parallel resonance circuit formed of the combination of the transmission lines 5 and the next lowest resonance frequency can be brought close to each other, that is, the two passing zero points can be placed close to each other. As a result, it becomes possible to obtain the characteristics of a conventional low pass filter using two parallel resonant circuits with one set of parallel resonant circuits. In other words, a steep attenuation characteristic can be obtained as compared with the conventional low-pass filter in which the transmission line 5 has a high characteristic impedance and the transmission line 5 has a short length. Moreover, since it is not necessary to use a plurality of parallel resonant circuits in the filter in order to obtain steep attenuation characteristics, there is an effect that a low-pass low-pass filter can be obtained.
This point is extremely effective when the loss of the resonance circuit is not small, for example, when a low loss material cannot be used as the dielectric material or conductor forming the filter circuit.

In addition, the present embodiment is a low-pass filter having a strip line structure, which includes a series capacitance circuit 6
As the so-called parallel plate type capacitance circuit, two strip conductors 6a and 6b are provided in parallel with the dielectric substrate 2b and face each other with the dielectric substrate 2b interposed (interposed) therebetween. Therefore, there is an effect that a filter that can be easily manufactured with a dielectric material such as LTCC that easily forms a multilayer structure can be obtained. In addition, since the line having high characteristic impedance is not necessary, it is possible to reduce the distance between the ground conductors 1a and 1b, and this point is particularly limited to the minimum value of the conductor pattern width that can be manufactured like LTCC. In some cases, it is effective for thinning the filter circuit.

Further, in the present embodiment, as the low pass filter having the strip line structure, as the series capacitance circuit 6, the first and second strip conductors 6a,
A so-called parallel plate type capacitance circuit is used in which 6b are opposed to each other with the dielectric substrate 2b interposed therebetween, and the input line 4 is formed on the main surface of the dielectric substrate 2b on which the first strip conductor 6a is formed.
a and the output line 4b are arranged, and the input line 4a and the second strip conductor 6b are connected via a via 7 penetrating the dielectric substrate 2b. Therefore, the input line 4a and the output line 4b are in the same plane (in the same layer). As a result, there is an effect that the connection of the external circuit becomes easy.

Further, since the characteristic impedance and the electrical length of the transmission line 5 are selected so that the two passing zero points substantially match or are sufficiently close to each other, a large amount of attenuation can be obtained in the local frequency band. In addition to being possible, there is an effect that a low-pass filter having an extremely steep attenuation characteristic can be obtained.

Embodiment 2. Embodiment 2 according to the present invention is shown in FIGS. FIG. 8 is an exploded perspective view showing the configuration of the strip line type low pass filter representing the present embodiment. FIG. 9 is a top view of the dielectric substrate provided in the low pass filter of FIG. 8 and is a diagram for explaining the pattern shape of the strip conductor. In these figures,
4 and 7 are the same as those in the first embodiment. 15
Is a transmission line in the filter circuit, and 16 is a series capacitance circuit. The transmission line 15 is arranged so as to circulate around the series capacitance circuit 16, and strip conductors 16a and 16b forming the series capacitance circuit 16 are orthogonal to each other. In addition to the above arrangement, as shown in FIG. 9, the tip of one strip conductor is formed to be larger than the edge of the other strip conductor by a dimension Δ. The in-filter transmission line 15 is similar to that of the first embodiment in that the transmission line 15 has a characteristic impedance lower than that of the input line 4a and the output line 4b. Also, the electrical length of the transmission line 15 is relatively large, which is about a fraction of the wavelength of the cutoff frequency of the filter, which is similar to the first embodiment.

The filter of this embodiment is the same as that of the first embodiment.
Same operation as. Therefore, the same effect as the low pass filter of the first embodiment is obtained. In the low pass filter of the present invention, the electrical length of the transmission line 15 is relatively large, but the transmission line 15 is laid out so as to circulate around the series capacitance circuit 16. Therefore, it is possible to efficiently lay out the transmission line 15 having a large electric length in area, and it is possible to compactly obtain a polar type low loss low pass filter having two pass zeros.

Further, the strip conductors 16a and 16b forming the series capacitance circuit 16 are orthogonal to each other, and the shape of the strip conductor is determined so as to protrude from the other edge by a dimension Δ as shown in FIG. The dimension Δ is a dimension corresponding to the crossing of misalignment between the conductor patterns provided in different layers in the multilayer structure. For this reason,
Even if the conductor patterns on the different layers are displaced by Δ due to a manufacturing error, the opposing areas of the two strip conductors do not change, and the error in the capacitance in the series capacitance circuit is small. That is, there is an effect that there is little variation in the filter characteristics due to the positional deviation between the conductor patterns. The LTCC or the like has a problem that the positional deviation between the conductor patterns is not small, and the low-pass filter according to the present embodiment is effective when a multilayer circuit is made of such a material.

As described above, in the present embodiment,
In the series capacitance circuit 16 of the low-pass filter having the strip line structure, the direction from the tip of the first strip conductor 16a toward the input line 4a and the second strip conductor 1
The angle between the tip of 6b and the direction toward the output line 4b is set to about 90 degrees, and the tip of the first strip conductor 16a is connected to the dielectric substrate (dielectric layer) 2b.
Of the first strip conductor 16a that protrudes from the edge of the second strip conductor 16b that faces the first strip conductor 16b via the dielectric substrate (dielectric layer) 2b. Since the transmission line 5 is arranged so as to surround the series capacitance circuit 6 while protruding from the edge, the transmission line 5 is arranged between the first strip conductor 16a and the second strip conductor 16b provided in different layers. However, even if some displacement occurs, the area of the portion where they face each other does not change. Therefore, there is an effect that a low-pass filter in which the characteristic deterioration due to the positional deviation is small can be obtained. This point is effective when the filter is made of a material, such as LTCC, which is not a little misaligned between layers. Further, since the transmission line 15 is arranged so as to circulate around the series capacitance circuit 16, there is an effect that the filter circuit in which the electric length of the transmission line 15 is large can be downsized in area.

Embodiment 3. Embodiment 3 according to the present invention is shown in FIG. FIG. 10 is a diagram illustrating a pattern shape of a strip conductor of a low pass filter having a multilayer structure similar to that of the second embodiment. In FIG.
7 and 15 and 16 are the same as those in the second embodiment. The low pass filter of this embodiment is the same as that of the second embodiment.
Two low-pass filters shown in are connected in cascade to form a multistage. That is, two low-pass filters shown in the second embodiment are prepared, and as shown in FIG. 10, the output line side of one filter and the input line side of the other filter are connected in cascade and two low-pass filters are connected. The conductor patterns of the two filters are arranged so as to be substantially point-symmetric with respect to the connection point (via 7) of the filters. Further, the second strip conductor 16b of one filter and the second strip conductor 16d of the other filter are arranged in the same plane as shown by the broken line in FIG. As described above, in the present embodiment, the strip conductor 16b (16d) and the transmission line 15 which form the series capacitance circuit.
It is characterized in that it has a substantially point-symmetrical conductor pattern with the via 7 connecting a and 15b as the center point. Further, 16b and 16d are provided in a common layer and are formed in a strip conductor pattern integrated with the via 7 interposed (16b = 16d).

The filter of the present embodiment is the same as that of the second embodiment.
The same operation is performed and the same effect is obtained. The two low-pass filters of the second embodiment are cascade-connected, and since a total of four pass zeros are provided in the close frequency bands, the low-pass filter has an extremely steep attenuation characteristic.

In the present embodiment, the filter shown in the second embodiment has a point-symmetrical shape in which the vias 7 are connected in cascade with the via 7 as the center, and 16b and 16d are provided in a common layer and the via 7 is formed. Since the strip conductor patterns are formed so as to be sandwiched and integrated with each other, when a positional deviation occurs between the conductor patterns provided on different layers, the directions of changes in the capacitance values of the two series capacitance circuits are reversed. Direction.
Therefore, the frequency changes at the passing zero points are in opposite directions. Therefore, to the frequency side where the stop band is unilaterally low,
Alternatively, it does not shift to a higher frequency band. That is, in the low-pass filter according to the present embodiment, like the low-pass filter according to the second embodiment, the characteristic variation due to the positional displacement between the conductor patterns is originally small, and the positional displacement between the conductor patterns is caused. The shift of the stop band is small, and the stop attenuation amount can be stably obtained.

As described above, in the present embodiment,
The output line side of the first low-pass filter described in the second embodiment and the input line side of the second low-pass filter are connected in cascade, and the connection point of the two filters is substantially centered. The conductor patterns of the two filters are arranged in point symmetry, and the second strip conductor 16b of the first filter and the second strip conductor of the second filter are arranged.
Since the strip conductors 16d are arranged in the same plane, the area of the part where the strip conductors 16d face each other does not change even if a displacement occurs between the conductor patterns provided in different layers. The direction of change of the capacitance value in the two series capacitance circuits is opposite, and the frequency change of the passing zero point becomes
Since the directions are opposite to each other, there is an effect that the blocking attenuation amount can be stably obtained without the characteristic deterioration due to the position shift.

It should be noted that any of the low pass filters described in the first to third embodiments is used to form a multi-layer structure in which the low pass filter is built-in, and an active material serving as a heat source on the multi-layer structure. A multi-layer RF package or a multi-layer RF module in which the device is mounted can be produced. As described above, the built-in low-pass filter does not require a line having a high characteristic impedance, so that it is possible to reduce the distance between the ground conductors 1a and 1b, and a conductor pattern that can be manufactured like LTCC. When a package or module is made of a material with a limited minimum width, the package can be made thinner, which lowers the thermal resistance of the package and reduces the active device such as MMIC mounted on the surface of the package. There is an effect that a package or module advantageous in terms of heat dissipation can be obtained.

[0043]

According to the present invention, a series capacitance circuit having two terminals, an input line connected to one of the terminals, an output line connected to the other of the terminals, and one end connected to the input line. And a transmission line having the other end connected to the output line, and the characteristic impedance of the transmission line is a low-pass filter in which the characteristic impedance is lower than the characteristic impedance of the input line and the output line. The lowest resonance frequency of the parallel resonance circuit composed of the combination of the capacitance circuit and the transmission line and the next lowest resonance frequency can be brought close to each other, that is, the two passing zero points can be placed close to each other. As a result, it is possible to obtain steep attenuation characteristics, and it is not necessary to use multiple parallel resonant circuits in the filter to obtain steep attenuation characteristics. Since an effect of a low-loss low-pass filter is obtained.

Further, the series capacitance circuit is composed of first and second strip conductors, and the first and second strip conductors are provided.
Since the strip conductors of 1 are provided so as to face each other with the dielectric substrate interposed therebetween, there is an effect that a filter that is easy to manufacture with a dielectric material such as LTCC that is easy to form a multilayer structure can be obtained. In addition, it is possible to reduce the distance between the ground conductors because a line with high characteristic impedance is unnecessary,
This point is particularly effective for reducing the thickness of the filter circuit when the minimum value of the conductor pattern width that can be manufactured is limited, as in LTCC.

Further, the input line and the output line are arranged on the main surface of the dielectric substrate on the side where the first strip conductor is provided, and the second strip conductor and the input line are connected to each other. The input lines and the output lines are in the same plane (in the same layer) because the spaces are connected to each other through the conductive connecting means provided through the dielectric substrate. . As a result, there is an effect that the connection of the external circuit becomes easy.

Further, since the combination of the characteristic impedance and the electrical length of the transmission line is selected so that the lowest passing zero point and the next lowest passing zero point substantially match or approach each other, the local passing In addition to being able to obtain a large amount of attenuation in the frequency band, there is an effect that a low-pass filter having an extremely steep attenuation characteristic can be obtained.

The angle formed by the direction from the tip of the first strip conductor to the input line and the direction from the tip of the second strip conductor to the output line is approximately 90 degrees. In this way, the tip end portion of the first strip conductor is projected from the edge of the second strip conductor facing through the dielectric layer, and the tip end portion of the second strip conductor is covered with the dielectric layer. The first strip conductor and the second strip conductor provided on different layers are arranged so as to project from the edges of the first strip conductor facing each other through the transmission line and to circulate the series capacitance circuit. Even if a positional deviation occurs between the strip conductor and the strip conductor, the area where the two parts face each other does not change, so a low-pass filter with less characteristic deterioration due to the positional deviation can be obtained. A. Further, since the transmission line is arranged so as to circulate around the series capacitance circuit, there is an effect that the filter circuit having a large electric length of the transmission line can be downsized in area.

A multi-stage low-pass filter having two low-pass filters according to the present invention, wherein the output line side of one low-pass filter and the input line side of the other low-pass filter are cascaded. While connecting, the conductor patterns of the two low pass filters are arranged so as to be substantially point-symmetrical about the connection point of the two low pass filters, and the second low pass filter of the one low pass filter is arranged. Since the strip conductor and the second strip conductor of the other low-pass filter are arranged in the same plane, if a displacement occurs between the conductor patterns provided in different layers, two The capacitance values of the series capacitance circuit change in opposite directions, and the passing zero points change in frequency in opposite directions. Therefore, to the frequency side where the stop band is unilaterally low,
Alternatively, it does not shift to a higher frequency band. That is, the characteristic variation due to the positional deviation between the conductor patterns is originally small, and the stop band shift due to the positional deviation between the conductor patterns is small, so that the blocking attenuation amount can be stably obtained.

Further, if a multilayer RF package having a multi-layer structure in which the low-pass filter of the present invention is built in and an active device serving as a heat source is mounted on the multi-layer structure, the built-in filter has a characteristic impedance of Since high lines are not required, it is possible to reduce the distance between the ground conductors, and if the package is made of a material that has a limit to the minimum value of the conductor pattern width that can be formed, the package can be made thinner. The thermal resistance value of the package can be reduced, and an advantageous package can be obtained in terms of heat dissipation of the active device mounted on the upper surface of the package.

Further, if a multilayer RF module having a multi-layer structure incorporating the low-pass filter of the present invention, and an active device serving as a heat source is mounted on the multi-layer structure, the built-in filter has a characteristic impedance of Since high lines are not required, it is possible to reduce the distance between ground conductors, and if the module is made of a material that has a limit to the minimum conductor pattern width that can be formed, the package can be made thinner, The thermal resistance value of the package can be reduced, and an advantageous module can be obtained in terms of heat dissipation of the active device mounted on the upper surface of the package.

[Brief description of drawings]

FIG. 1 is an exploded perspective view showing a configuration of a low pass filter according to a first embodiment of the present invention.

FIG. 2 is an explanatory view showing a strip conductor pattern shape of the low pass filter of FIG.

FIG. 3 is a circuit configuration diagram of a low-pass filter according to the first embodiment of the present invention.

FIG. 4 is a circuit configuration diagram showing (a) an equivalent circuit of a transmission line 5 and (b) an equivalent circuit of a low-pass filter according to the first embodiment of the present invention.

FIG. 5 is an explanatory diagram showing frequency characteristics of the susceptances of the transmission line and the series capacitance circuit in the low pass filter according to the first embodiment of the present invention.

FIG. 6 is an explanatory diagram showing reflection and pass characteristics of the low pass filter according to the first embodiment of the present invention.

FIG. 7 is an explanatory diagram showing a relationship between a transmission line and a second zero point frequency according to the first embodiment of the present invention.

FIG. 8 is an exploded perspective view of a low pass filter according to a second embodiment of the present invention.

9 is an explanatory diagram showing a strip conductor pattern shape of the low pass filter of FIG. 8. FIG.

FIG. 10 is an explanatory diagram showing a strip conductor pattern shape of a low pass filter according to a third embodiment of the present invention.

FIG. 11 is an exploded perspective view showing a conventional low pass filter.

12 is an explanatory diagram showing a strip conductor pattern shape of the low-pass filter of FIG. 11. FIG.

FIG. 13 is a circuit configuration diagram showing an equivalent circuit of a conventional low pass filter.

FIG. 14 is a circuit configuration diagram showing a third-order original low-pass filter corresponding to a conventional low-pass filter.

[Explanation of symbols]

1a, 1b, 101a, 101b Ground conductor, 2a, 2
b, 2c, 102a, 102b Dielectric substrate, 3a, 3
b, 103 strip conductor, 4a, 104a input line, 4b, 104b output line, 5, 15, 15a, 1
5b, 105 transmission lines, 6, 16, 16A, 16B,
106 series capacitance circuit, 6a, 6b, 16a, 16b,
16c, 16d, 106a, 106b Strip conductor, 7 Connection means.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Hiroshi Ikematsu             2-3 2-3 Marunouchi, Chiyoda-ku, Tokyo             Inside Ryo Electric Co., Ltd. F term (reference) 5J006 HB05 HB13 HB22 JA03 JA31                       LA03 LA07 LA23 NA08 NB08                       NB10 NC03

Claims (8)

[Claims] 1. A series capacitance circuit having two terminals, an input line connected to one of the terminals, an output line connected to the other of the terminals, and one end connected to the input line and the other. A low-pass filter comprising: a transmission line having an end connected to the output line, wherein a characteristic impedance of the transmission line is lower than characteristic impedances of the input line and the output line. 2. The series capacitance circuit is composed of first and second strip conductors, and the first and second strip conductors are provided so as to face each other with a dielectric substrate interposed therebetween. The low pass filter according to claim 1. 3. The input line and the output line are arranged on the main surface of the dielectric substrate on the side where the first strip conductor is provided, and the second strip conductor and the input line are connected to each other. The low-pass filter according to claim 2, wherein the spaces are connected to each other through a connecting means having conductivity which is provided so as to penetrate the dielectric substrate. 4. The combination of the characteristic impedance and the electrical length of the transmission line is selected such that the lowest passing zero and the next lowest passing zero substantially match or approach each other. 4. The low-pass filter according to any one of 1 to 3. 5. The angle formed by the direction from the tip of the first strip conductor toward the input line and the direction from the tip of the second strip conductor toward the output line is approximately 90 degrees. In this way, the tip portion of the first strip conductor is projected from the edge of the second strip conductor which faces the second strip conductor via the dielectric layer,
In addition, the tip end portion of the second strip conductor is projected from the edge of the first strip conductor facing through the dielectric layer, and the transmission line is arranged so as to surround the series capacitance circuit. The low pass filter according to any one of claims 2 to 4, which is characterized in that. 6. The low-pass filter according to claim 5,
A multistage low-pass filter provided with one, wherein the output line side of one low-pass filter and the input line side of the other low-pass filter are connected in cascade, and the connection point of the two low-pass filters. The conductor patterns of the two low-pass filters are arranged so as to be substantially point-symmetrical with respect to the center, and the second strip conductor of the one low-pass filter and the second strip conductor of the other low-pass filter are arranged. A multi-stage low-pass filter, wherein the strip conductor and the strip conductor are arranged in the same plane. 7. A multi-layer RF package having a multi-layer structure incorporating the low-pass filter according to claim 1, wherein an active device serving as a heat source is mounted on the multi-layer structure. 8. A multi-layer RF module comprising a multi-layer structure incorporating the low-pass filter according to claim 1, wherein an active device serving as a heat source is mounted on the multi-layer structure.