JPH04306004A - Band pass filter - Google Patents

Abstract

PURPOSE:To attain a high Q, to reduce an insertion loss, to make the size small and to adjust its input and output impedance optionally. CONSTITUTION:Three coil electrode patterns 41, 42, 43 formed to one major side of a board 1 made of a dielectric material both form a loop. Then the electrodes 41, 42, 43 are arranged in a magnetically coupled state and electrode extract patterns 7a, 7b and ground terminal patterns 6a, 6b are led out with an interval offering a prescribed impedance from the coil electrode patterns 41, 42 toward the end of the board 1. On the other hand, a ground terminal pattern 6c is led out from the coil electrode pattern 43 located in the middle toward the end of the said board 1.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is applicable to
The present invention relates to a dielectric laminated bandpass filter used in portable radio equipment and the like with a frequency of .

0002

2. Description of the Related Art Conventional resonators can be roughly divided into those using strip lines and those using coil patterns. When a bandpass filter is manufactured using such resonators, the structure is such that a plurality of resonators are magnetically coupled.

Here, as shown in FIG. 19, there is a 1/2 wavelength resonator in which both ends of the line are open, and as shown in FIG. 20, one end of the line is open. There is a 1/4 wavelength resonator which is open and the other end is short-circuited. On the other hand, as a method using a coil pattern, as previously proposed by the applicant (see FIG. 21), a spiral coil pattern 201 and a ground pattern 203 are formed on both sides with a dielectric layer 202 in between. There are things like that.

0004

[Problems to be Solved by the Invention] However, the above-mentioned conventional bandpass filters using resonators each have the following problems. ■Using a strip line (a) A resonator with a resonant frequency of 2 to 3 GHz is quite large. Particularly, a bandpass filter having a structure in which a plurality of resonators are coupled causes a significant increase in size. This is due to the reasons shown below.

That is, the length of the strip line L10, L
11 is determined as shown in Equation 1 (1/2 wavelength resonator) and Equation 2 (1/4 wavelength resonator).

[0006]

[Math 1]

[0007]

[Math 2]

[0008] In the above equations 1 and 2, λ is the wavelength, and ε
is the dielectric constant of the dielectric laminate sheet. At present, the dielectric constant of a dielectric laminate sheet that can be co-fired with silver or copper and has excellent temperature characteristics cannot be made very large.
ε≒about 10. Therefore, in Equation 1 and Equation 2 above, if ε=10, L10=15.8mm, L1
1=7.9 mm, which is extremely long, leading to an increase in the size of the resonator (bandpass filter) as described above.

(b) In a bandpass filter,
It is desirable to adjust the input/output impedance depending on the device to be incorporated (matching the impedance between the bandpass filter and the device). However, in the case of a stripline type, since the input/output impedance has a unique value for each stripline, adjustment cannot be performed even if the extraction position is changed, and matching is not possible. ■Those using a coil pattern Since the coil pattern is spiral, magnetic flux influences each other between adjacent patterns, making it difficult for current to flow. Therefore, the effective resistance increases and the Q becomes low.

For example, in FIG. 21, the pattern piece 20
1a and pattern piece 201b, the current flows in the same direction (they both flow in the D direction), so the magnetic fields cancel each other out and the magnetic flux becomes coarse. As a result, it becomes difficult for the current to flow, and there is no substantial Resistance increases. When the Q becomes low in this way, there is a problem in that the insertion loss of the bandpass filter becomes large.

The present invention has been made in view of the current situation, and provides a bandpass filter that has a high Q and low insertion loss, can be miniaturized, and can arbitrarily adjust input and output impedance. The purpose is to

0012

[Means for Solving the Problems] In order to achieve the above object, the present invention has three or more first electrodes arranged on one main surface of a plate made of a dielectric material, while the other This is a bandpass filter in which a second electrode is arranged on the main surface, and the first electrode has a loop shape, and adjacent first electrodes are arranged in a magnetically coupled state, and at both ends. A take-out terminal and a ground terminal are respectively drawn out from the first electrode located toward the end of the plate at intervals having a predetermined impedance, while the other first electrode is pulled out toward the end of the plate. The second electrode has a planar shape, and the ground terminal is drawn out toward the end of the plate.

Three or more first electrodes are arranged between one main surface of two plates made of dielectric material, while at least one of the other main surfaces of the two plates is arranged between two main surfaces of the two plates. This is a bandpass filter in which a second electrode is arranged on the main surface, and the first electrode has a loop shape, and adjacent first electrodes are arranged in a magnetically coupled state, and at both ends. A take-out terminal and a ground terminal are respectively drawn out from the first electrode located toward the end of the plate at intervals having a predetermined impedance, while the other first electrode is pulled out toward the end of the plate. The second electrode has a planar shape, and the ground terminal is drawn out toward the end of the plate.

Three or more first electrodes are arranged between one main surface of two plates made of dielectric material, while a second electrode is arranged on the other main surface of the two plates. A bandpass filter in which electrodes are arranged respectively, the first electrodes forming a loop shape, adjacent first electrodes being arranged in a magnetically coupled state, and first electrodes located at both ends. A take-out terminal and a ground terminal are respectively drawn out from the first electrode toward the end of the plate at intervals having a predetermined impedance, while a ground terminal is drawn out from the other first electrode toward the end of the plate. Furthermore, at least one of the second electrodes is divided into two parts so as to have a slightly larger shape than the first electrode, and a ground terminal is connected from each of the two parts to the end of the plate. It is characterized by being drawn out.

[0015] A third electrode having the same shape as the first electrode is formed between the first electrode and at least one of the second electrodes.

[0016]

[Operation] With the above configuration, the reason is that the first electrode and the second electrode are in a so-called strip line structure facing each other, and the pattern pieces of the first electrode are adjacent to each other like a spiral coil pattern. Because of this, Q is dramatically improved, insertion loss is reduced, and skirt characteristics become sharper.

Furthermore, since the first electrode has a loop shape, the dimensions of the element are reduced. In addition, the impedance can be adjusted by simply changing the distance between the lead terminal of the first electrode and the ground terminal, making it extremely easy to adjust the impedance.

[0018]

[Example] (First Example) The first example of the present invention is shown in FIG.
This will be explained below based on ~FIG. 11. 1 and 2 are diagrams showing the structure of a bandpass filter according to a first embodiment of the present invention, in which FIG. 1 is a plan view, FIG. 2 is an exploded perspective view, and FIG.
is a plan view of a dielectric sheet used in the present invention, FIG. 4 is a plan view showing a state in which a coil electrode pattern is formed on the dielectric sheet of FIG. 3, and FIG. 5 is a plan view showing a state in which a ground electrode pattern is formed on the dielectric sheet of FIG. A plan view showing the state, FIGS. 6 and 7 are views when dielectric sheets are laminated, and FIG. 6 is a side view.
FIG. 7 is a front view, FIG. 8 is a front view when the laminate is crimped,
FIG. 9 is a front view when external electrodes are formed, FIG. 10 is an equivalent circuit diagram of the band-pass filter, and FIG. 11 is a graph showing the frequency characteristics of the band-pass filter.

As shown in FIG. 2, the bandpass filter of the present invention includes a dielectric layer 1 made of a plurality of dielectric sheets 101, and protective layers 2 provided above and below this dielectric layer 1.
・It has 3. One main surface 1 of the dielectric sheet 101 located at the top among the dielectric sheets 101...
01a has three coil electrode patterns 41, 42, 4.
3 is formed. Then, these are fired and integrated with each other.

The specific structure of the coil electrode pattern 41 is that pattern pieces 41 are arranged in a straight line and facing each other.
a and 41b are connected to one end of these pattern pieces 41a and 41b via a linear pattern piece 41c, and the other end of pattern piece 41b is parallel to pattern piece 41c. The structure is such that a pattern piece 41d extending in the direction of the pattern piece 41a is formed.

[0021] Here, the total length L3 of the coil electrode pattern 41 is configured to be as shown in Equation 3 below.

[0022]

[Math 3]

[0023] λ is the wavelength, and ε is the dielectric constant. Note that the distance L1 (see FIG. 1) between the pattern piece 41d and the pattern piece 41a is preferably equal to or less than the width L2 of the pattern piece 41a (the same applies to the pattern pieces 41b to 41d). Furthermore, in the following embodiments, the gap between the pattern pieces 41a and 41d will be referred to as a gap 30.

On the other hand, the coil electrode patterns 42 and 43
has substantially the same shape as the coil electrode pattern 41 having the above structure. However, the gap parts 31 and 32 (configured to have the same length as the gap part 30) are 42a and 42, respectively.
They are different in that they are formed between 43a and 43c and between 43a and 43c. Then, the coil electrode pattern 41
- The structure is such that the pattern pieces 41d and 43c of the 43 patterns and the pattern pieces 42c and 43d of the coil electrode patterns 42 and 43 are provided adjacent to each other and are magnetically coupled to each other.

The coil electrode patterns 41 and 42 have the following:
The earth terminal pattern 6a or 6b and the extraction electrode pattern 7a or 7b are connected, respectively, and these earth terminal patterns 6a and 6b and the extraction electrode pattern 7a
- The end of 7b extends to the side surface A of the bandpass filter. In addition, the above-mentioned both coil electrode patterns 43 include:
A ground terminal pattern 6c is connected, and the end of this ground terminal pattern 6c is connected to the side surface A of the bandpass filter.
It has been extended to.

Next, a ground electrode pattern 5 is formed on the surface 3a of the protective layer 3 on the dielectric layer 1 side.
The size of this ground electrode pattern 5 is larger than the outer periphery of the coil electrode patterns 41, 42, and 43, and is formed on substantially the entire surface of the surface 3a. Furthermore, at positions corresponding to the ground terminal patterns 6a, 6b, and 6c on the surface 3a, ground terminal patterns 8a and 8b are provided, one end of which is connected to the ground electrode pattern 5 and the other end of which is extended to the side surface A of the bandpass filter.・8c is formed. Then, the ground terminal pattern 6a and the ground terminal pattern 8a
is connected to the external ground electrode 9a, the ground terminal pattern 6b and the ground terminal pattern 8b are connected to the external ground electrode 9b, and the ground terminal pattern 6c and the ground terminal pattern 8c are connected to the external ground electrode 9c, respectively. The electrode pattern 7a is connected to the external extraction electrode 10a, and the extraction electrode pattern 7b is connected to the external extraction electrode 10b. Both external earth electrodes 9a, 9b, 9c mentioned above
Both the external electrodes 10a and 10b are formed on the side surface of the bandpass filter so as to have a U-shaped cross section.

[0027] Here, a bandpass filter having the above structure was manufactured according to the following procedure. First, a pattern as shown in FIG. 4 (the coil electrode patterns 41, 4
2, 43 and both terminal patterns 6a, 6b, 6c, 7a,
Apply copper paste or the like so that the pattern 12 is the same as 7b. Meanwhile, in parallel with this, a pattern as shown in FIG. 5 (the earth electrode pattern 5 and the same pattern as the ground terminal patterns 8a, 8b, and 8c) Copper paste or the like is applied so as to form a pattern 13.

Next, as shown in FIGS. 6 and 7, the protective sheet 2 is placed so that the pattern 12 and the pattern 13 are placed opposite each other with the sheet layer 16 made of the dielectric sheet 101 interposed therebetween. Sheet layer 16 and protective sheet 11 (
A layered product 15 is produced by laminating the protective sheet 2 (with a similar structure to that of the protective sheet 2) and further press-bonding the protective sheet 2. After this, exposed parts 17a, 17b, 17c to 19a, 19b, 1 of the paste layer shown in FIG.
As shown in FIG. 9, paste layers 20a, 20b, 20c, and 21a are formed by printing or applying copper paste or the like on the parts corresponding to 9c (the parts that will become the external ground electrodes 9a and 9b and the external lead-out electrodes 10a and 10b), as shown in FIG.・Produce 21b. Thereafter, the laminate is fired to integrate the dielectric sheets, thereby producing a bandpass filter. Incidentally, the firing of the laminate and the paste layers 20a and 2
The baking of 0b, 21a, and 21b may be performed in a separate process.

Incidentally, although the bandpass filter manufactured as described above does not have a capacitor pattern formed therein, it has an equivalent circuit as shown in FIG.
(M in the figure indicates magnetic coupling). This is due to the following two reasons. ■Coil electrode pattern 41・42・
43 is at the same potential as the ground electrode pattern 5 (that is, in a grounded state). ■Coil electrode pattern 41・
Since the dielectric layer 1 is interposed between 42 and 43 and the ground electrode pattern 5, stray capacitance occurs.

In this manner, a capacitor is formed, and the pattern pieces 41 of the coil electrode patterns 41 and 43 are
d and 43c and between the pattern pieces 42c and 43d of the coil electrode patterns 42 and 43, an equivalent circuit as shown in FIG. 10 is obtained. Note that the above stray capacitance is mainly caused by the coil electrode pattern 41.
・Since it occurs between the coil electrode patterns 41, 42, 43 and the ground electrode pattern 5, the capacitor capacity changes by moving the coil electrode patterns 41, 42, 43 and the ground electrode pattern 5 closer to or away from each other, and the frequency of the pass band can be changed. becomes possible. Specifically, the coil electrode patterns 41, 42, 4
3 and the ground electrode pattern 5 (dielectric sheet 10
If the thickness of the dielectric sheet 101 is decreased, the capacitor capacitance becomes larger and the frequency of the pass band becomes lower. If it is increased (increased), the capacitance of the capacitor becomes smaller, and the frequency of the passband becomes higher. Further, the stray capacitance can also be changed by changing the dielectric constant of the dielectric layer 1 and the thickness of the coil electrode patterns 41, 42, and 43. For example, coil electrode pattern 4
If the width L2 of 1, 42, and 43 is increased, the stray capacitance increases and the frequency of the passband can be lowered, so that further miniaturization can be achieved. However, if the spacing between each pattern piece becomes too narrow, the waveform will deteriorate, so the width L2 of the coil electrode patterns 41, 42, 43 should be made larger than necessary.
It is not desirable to make it thick.

The bandwidth of the bandpass filter is changed by changing the distance L6 between the pattern pieces 41d and 43c or the distance L7 between the pattern pieces 42c and 43d. Specifically, when the distance L6 or L7 is narrowed, the bandwidth becomes wider, but when the distance L6 or L7 is narrowed, the bandwidth becomes wider.
When 7 is increased, the bandwidth becomes narrower. However, distance L6
Alternatively, if L7 is made narrower than necessary, bimodal characteristics will occur, which is not preferable.

The input/output impedance can be changed by changing the distance L8 between the ground terminal pattern 6a and the extraction electrode pattern 7a.
Or earth terminal pattern 6b and extraction electrode pattern 7b
This is done by changing the distance L9. According to experiments, by adjusting the dielectric constant and thickness of the dielectric layer 1, the area of the coil electrode pattern 4, etc., it was possible to increase the applicable frequency of the bandpass filter of the present invention from several 100 MHz to several GHz. An example of this is shown in the experiment below. [Experiment] The frequency characteristics of the bandpass filter having the above structure were investigated, and the results are shown in FIG. [Other matters] ■ When mounting the bandpass filter of the present invention, the electrodes on the printed circuit board are aligned with the external ground electrodes 9a, 9b, 9c and the external lead-out electrodes 10a, 10b. Just place the bandpass filter and solder it. At this time, since the outside is covered with the protective layers 2 and 3, the coil electrode pattern 41
- It is possible to prevent damage to 42, 43 and the ground electrode pattern 5. (2) The dielectric layer 1 is not limited to a structure in which a plurality of thin dielectric sheets 101 are stacked, but a dielectric sheet formed in advance to a predetermined thickness may be used. ■It is not necessary to manufacture the products of the present invention one by one.
Multiple coil electrode patterns 4 on one wide dielectric sheet
1, 42, and 43, and the same number of ground electrode patterns 5 are formed on a similar dielectric sheet, and after laminating the pieces in this state, cutting them into pieces one by one and firing them. It may be.

(Second Embodiment) A second embodiment of the present invention will be described below with reference to FIGS. 12 and 13. FIG. 12 is an exploded perspective view of a bandpass filter according to a second embodiment of the present invention, and FIG. 13 is a graph showing frequency characteristics of the bandpass filter shown in FIG. 12. As shown in FIG. 12, the structure is the same as that of the first embodiment, except that the dielectric layer 1 and the ground electrode pattern 5 are sequentially provided not only on one side but on both sides of the coil electrode patterns 41, 42, and 43. be. [Experiment] The frequency characteristics of the bandpass filter having the above structure were investigated, and the results are shown in FIG.

As is clear from FIG. 13, it is recognized that the frequency of the passband is slightly higher. this is,
This is thought to be due to the fact that stray capacitance is formed not only on one side but on both sides of the coil electrode patterns 41, 42, and 43, so that the capacitance of the bandpass filter increases.

(Third Embodiment) A third embodiment of the present invention will be described below with reference to FIGS. 14 and 15. 14 and 15 are diagrams showing a bandpass filter according to a third embodiment of the present invention, in which FIG. 14 is an exploded perspective view and FIG. 15 is a plan view. As shown in FIGS. 14 and 15, one ground electrode pattern (upper pattern in FIG. 14)
The configuration is the same as that of the bandpass filter shown in the second embodiment, except that the shape of the bandpass filter is different. Specifically, the ground electrode pattern is divided into three, and each pattern 51.
2 and 53 are configured to be one size larger than the coil electrode patterns 41, 42, and 43, and the ground terminal patterns 8a, 8b, and 8c are connected to each of the ground electrode patterns 51, 52, and 53, respectively. It has a unique structure.

With such a configuration, the pattern piece 41
A part of the ground electrode patterns 51 and 53 corresponding to d and 43c (for example, the part indicated by the two-dot chain line B in FIG. 15)
Since the stray capacitance can be adjusted by simply cutting the wires, the frequency can be easily adjusted. Although the above adjustment is possible with the ground electrode patterns of the first and second embodiments (formed almost on the entire surface), the cutting length becomes longer, so if you want to adjust the frequency, , it is desirable to adopt the configuration of this embodiment.

(Fourth Embodiment) A fourth embodiment of the present invention will be described below based on FIG. 16. FIG. 16 is an exploded perspective view of a bandpass filter according to a fourth embodiment of the present invention. As shown in FIG. 16, coil electrode patterns 41 and 42
・Coil electrode patterns 41, 42, 4 are placed on the dielectric sheet 101 adjacent to the dielectric sheet 101 on which 43 is formed.
The configuration is the same as that of the bandpass filter shown in the second embodiment, except that floating electrode patterns 35, 36, and 37 having the same shape as those of Embodiment 3 are formed.

Although not shown, it has been experimentally confirmed that by adopting such a structure, the center frequency of the passband becomes lower than that of the bandpass filter shown in the second embodiment. This is the coil electrode pattern 41
- This is thought to be due to the fact that stray capacitance is formed between 42 and 43 and the floating electrode patterns 35, 36, and 37, increasing the capacitance of the bandpass filter. [Other matters] ■Coil electrode pattern 41, 42, 4
The structure of No. 3 is not limited to those shown in the various embodiments described above, and may be, for example, a loop-shaped structure as shown in FIGS. 17 and 18. ■The formation positions of the extraction electrode pattern and the ground terminal pattern are not limited to the positions shown in the above embodiment. For example, both patterns 6b and 7b of the coil electrode pattern 42 are placed on the C end surface side (see FIG. 2).
You can also pull it out. (2) The number of coil electrode patterns is not limited to three, and may be four or more. In this case, an extraction electrode pattern and a ground terminal pattern are drawn out from the coil electrode patterns located at both ends, and ground terminal patterns are drawn out from the other coil electrode patterns.

[0039]

[Effects of the Invention] As explained above, according to the present invention, the Q can be dramatically improved because of the so-called strip line structure and because the pattern pieces of the first electrode are not adjacent to each other. becomes possible. As a result, the insertion loss of the bandpass filter is reduced, and the skirt characteristics become sharper.

Furthermore, since the first electrode has a loop shape, the dimensions of the element are reduced. In addition, the impedance can be adjusted by simply changing the distance between the lead terminal of the first electrode and the ground terminal, making it extremely easy to adjust the impedance. Because of these factors, insertion loss is small,
Moreover, it is possible to provide an extremely excellent band-pass filter that is small in size and whose input/output impedance can be arbitrarily adjusted.

[Brief explanation of drawings]

FIG. 1 is a plan view of a bandpass filter according to a first embodiment of the present invention.

FIG. 2 is an exploded perspective view of a bandpass filter according to a first embodiment of the present invention.

FIG. 3 is a plan view of a dielectric sheet used in the present invention.

4 is a plan view showing a state in which a coil electrode pattern is formed on the dielectric sheet of FIG. 3; FIG.

5 is a plan view showing a state in which a ground electrode pattern is formed on the dielectric sheet of FIG. 3; FIG.

FIG. 6 is a side view when dielectric sheets are laminated.

FIG. 7 is a front view when dielectric sheets are laminated.

FIG. 8 is a front view when the laminate is crimped.

FIG. 9 is a front view when external electrodes are formed.

10 is an equivalent circuit diagram of the bandpass filter shown in FIG. 1. FIG.

11 is a graph showing frequency characteristics of the bandpass filter shown in FIG. 1. FIG.

FIG. 12 is an exploded perspective view of a bandpass filter according to a second embodiment of the present invention.

13 is a graph showing frequency characteristics of the bandpass filter shown in FIG. 12. FIG.

FIG. 14 is an exploded perspective view of a bandpass filter according to a third embodiment of the present invention.

FIG. 15 is a plan view of essential parts of a bandpass filter according to a third embodiment of the present invention.

FIG. 16 is an exploded perspective view of a bandpass filter according to a fourth embodiment of the present invention.

FIG. 17 is a plan view of essential parts showing a modification of the bandpass filter of the present invention.

FIG. 18 is a plan view of main parts showing another modification of the bandpass filter of the present invention.

FIG. 19 is an explanatory diagram showing a conventional stripline dielectric resonator.

FIG. 20 is an explanatory diagram showing a conventional stripline dielectric resonator.

FIG. 21 is an explanatory diagram showing a conventional coil pattern dielectric resonator.

[Explanation of symbols]

1 Dielectric layer 5 Earth electrode pattern 6a　Earth terminal pattern 6b Ground terminal pattern 6c　Earth terminal pattern 7a Exit electrode pattern 7b Extraction electrode pattern 8a　Earth terminal pattern 8b Ground terminal pattern 8c Ground terminal pattern 35 Floating electrode pattern 36 Floating electrode pattern 37 Floating electrode pattern 41 Coil electrode pattern 42 Coil electrode pattern 43 Coil electrode pattern 51 Ground electrode pattern 52 Ground electrode pattern 53 Earth electrode pattern 101 Dielectric sheet

Claims (4)

[Claims] 1. A bandpass filter in which three or more first electrodes are arranged on one main surface of a plate made of a dielectric material, and a second electrode is arranged on the other main surface. The first electrode has a loop shape, and adjacent first electrodes are arranged in a magnetically coupled state, and
A take-out terminal and a ground terminal are drawn out from the first electrodes located at both ends toward the end of the plate at intervals having a predetermined impedance, while the other first electrodes are drawn out from the end of the plate. The ground terminal is pulled out towards
Furthermore, the band pass filter is characterized in that the second electrode has a planar shape, and a ground terminal is drawn out toward an end of the plate. 2. Three or more first electrodes are arranged between one main surface of two plates made of a dielectric material, while at least one of the other main surfaces of the two plates is arranged between two main surfaces of the two plates. A bandpass filter in which a second electrode is arranged on one main surface, the first electrode forming a loop shape, and adjacent first electrodes being arranged in a magnetically coupled state, A take-out terminal and a ground terminal are drawn out from the first electrodes located at both ends toward the end of the plate at intervals having a predetermined impedance, while the other first electrodes are drawn out from the end of the plate. A band pass filter characterized in that a ground terminal is drawn out toward an end of the plate, the second electrode has a planar shape, and a ground terminal is drawn out toward an end of the plate. 3. Three or more first electrodes are arranged between one main surface of two plates made of a dielectric material, while a first electrode is arranged on the other main surface of the two plates. a bandpass filter in which two electrodes are respectively arranged;
The electrodes have a loop shape, and the adjacent first electrodes are arranged in a magnetically coupled state, and the first electrodes located at both ends are arranged in a state of magnetic coupling.
An extraction terminal and a ground terminal are respectively drawn out from the first electrode toward the end of the plate at intervals having a predetermined impedance, while a ground terminal is drawn from the other first electrode toward the end of the plate. Furthermore, at least one of the second electrodes is divided into two parts so as to have a slightly larger shape than the first electrode, and each of the second electrodes is divided into two parts from the divided electrodes toward the end of the plate body. A bandpass filter characterized by having a ground terminal drawn out. 4. A third electrode having the same shape as the first electrode is formed between the first electrode and at least one of the second electrodes. Section 3
Bandpass filter as described.