JP2011091640A - Low-pass filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain ideal phase flatness by suppressing characteristic variations. <P>SOLUTION: A low-pass filter 1 is obtained by arranging on a rectangular substrate 4, a spiral inductor L which is a series element 2 connected in series between an input terminal 15 and an output terminal 16 and a shunt element 3 which are constituted of a serial circuit of a semiconductor resistor R and a metal capacitor C, whose one ends are grounded. The spiral inductor L, the semiconductor resistor R, and the metal capacitor C are point-symmetrically arranged to the right and left of a diagonal line with the center of the diagonal line of the substrate 4 as an original point on the rectangular substrate 4, and then, each of the elements are connected by lines 5. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a low-pass filter that removes signal components other than a desired band, and in particular, a lumped constant in which a spiral inductor as a series element and a semiconductor resistor and a metal capacitor as shunt elements are arranged on a substrate and the elements are connected by lines. Type low pass filter.

  Conventionally, filters that remove signal components other than the desired band have been used in various fields. Especially in optical receivers used in the field of optical communications, a Bessel-Thomson Low-Pass-Filter having a cutoff frequency of 75% of the signal bit rate is used to remove noise components and prevent ringing. : Hereinafter abbreviated as BT-LPF). For this reason, it is necessary to use BT-LPF in a light receiver also in a measuring device.

  By the way, the BT-LPF is known as a filter having the smallest waveform deterioration among many low-pass filters because the characteristics of the filter itself lead to the accuracy of the measurement result. The reason why the waveform distortion of the BT-LPF is small is that the phase flatness is very high. The ideal BT-LPF has a group delay flatness of 0 ps because the group delay is a constant value up to the cutoff frequency.

  However, it is impossible to actually increase the group delay flatness to 0 ps. For example, in a conventional BT-LPF having a cutoff frequency of 7.5 GHz, an error of about 6 ps (peak-to-peak value) occurs. Therefore, waveform distortion occurs in the signal, and the measurement accuracy deteriorates when used in a measuring instrument. From the above, it has been desired to develop a BT-LPF having a very flat group delay characteristic.

  Further, as a configuration of the low-pass filter, a distributed constant configuration is generally used for a 10 Gbit / s signal. As this type of distributed constant type low-pass filter, for example, a group delay flat type low-pass filter disclosed in Patent Document 1 below is known.

JP 2003-60464 A

  However, when the conventional low-pass filter is configured as a complete distributed constant type, reflection is large and waveform deterioration is likely to occur in the signal. Therefore, it is not suitable for a BT-LPF with improved group delay flatness.

  In addition, in order to solve the above-mentioned reflection problem, it is difficult to analyze the dispersion in the thin film process with a distributed constant type low-pass filter in which a resistor is inserted as an absorber. It was necessary to satisfy the characteristics by adjusting the resistance manually. In other words, in order to suppress variations in the thin film process, the resistance inserted as an absorber is adjusted by manual work such as laser trimming, the characteristics are measured, final adjustment is performed, and the product is shipped after an inspection process. For this reason, there is a problem that not only the cost is increased, but also labor for adjustment is required.

  Furthermore, when the low-pass filter is configured with a lumped constant based on the chip instead of the above-described distributed constant, the desired high-frequency characteristics can be obtained, but the device size tends to be large, and the entire device is affected by the characteristic variation of each chip. There has been a problem in that variations in characteristics occur.

  As described above, in the conventional BT-LPF, it is difficult to suppress the characteristic variation, adjustment is necessary, and the cost is high. The reflection is large and the waveform is easily distorted in the signal. The group delay flatness is about 6 ps ( fc = 7.5 Ghz), and there is a problem that waveform distortion occurs in the signal.

  Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to provide a low-pass filter capable of suppressing characteristic variation and obtaining ideal phase flatness.

In order to achieve the above object, a low-pass filter according to claim 1 of the present invention includes a spiral inductor L as a series element 2 connected in series between an input terminal 15 and an output terminal 16, and one end grounded. In a low-pass filter 1 in which a shunt element 3 consisting of a series circuit of a semiconductor resistor R and a metal capacitor C is arranged on a rectangular substrate 4,
In the rectangular substrate, the spiral inductor, the semiconductor resistor, and the metal capacitor are arranged symmetrically with respect to the left and right of the diagonal line with the center of the diagonal line of the substrate as the origin, and the elements are connected by lines. It is characterized by being.

The low-pass filter according to claim 2 is the low-pass filter according to claim 1,
The spiral inductor L is characterized in that the gaps between all the lines constituting the spiral inductor are the same as the line width.

The low-pass filter according to claim 3 is the low-pass filter according to claim 1 or 2,
All the widths of the lines are the same.

  According to the present invention, a configuration in which spiral inductors, semiconductor resistors, and metal capacitors are arranged symmetrically with respect to the substrate with the center of the diagonal line of the substrate as the origin, and the elements are connected by lines, A flat group delay characteristic can be realized. In addition, the main characteristic variation factor is only the resistance value, and by performing constant design in consideration of variation at the time of constant design, a configuration in which characteristic variation is suppressed can be easily realized.

  This eliminates the need for manual adjustment such as laser trimming as in the prior art, thus reducing costs and reducing the adjustment effort. In addition, there is an advantage that waveform distortion is less likely to occur in the signal with small reflection.

It is a block diagram which shows embodiment of the low-pass filter which concerns on this invention. (A) It is a partial top view of the semiconductor resistance and metal capacitor of the low-pass filter concerning this invention. (B) It is a fragmentary sectional view of (a). (A) It is a partial top view of the spiral inductor of the low-pass filter which concerns on this invention. (B) It is a fragmentary sectional view of (a). It is a figure which shows the equivalent circuit of the low-pass filter which concerns on this invention. It is a figure which shows the group delay characteristic of the low-pass filter which concerns on this invention.

  Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the accompanying drawings.

  In the low-pass filter 1 according to the present invention, a series element 2 composed of a spiral inductor L and a shunt element 3 composed of a series circuit of a semiconductor resistor R and a metal capacitor C are formed in a thin film on a substrate 4, and a line 5 is formed between each element. The lumped constant type BT-LPF connected by the above.

  Here, when the series element 2 (spiral inductor L) and the shunt element 3 (semiconductor resistor R, metal capacitor C) are arranged on the substrate 4, the characteristics greatly change depending on the layout configuration and are not required between the lines. May cause strong electromagnetic coupling.

  Therefore, the low-pass filter 1 according to the present invention has the following features (1) to (6) in order to prevent the above-described unnecessary electromagnetic coupling between the lines, and these features are extremely high groups. Delay flatness is realized.

(1) Using a rectangular (preferably square) substrate 4, a series element 2 (spiral inductor L) and a shunt element 3 (semiconductor resistor R, metal capacitor) C are connected to a diagonal line (dashed line in FIG. 1) of the substrate 4. An BT-LPF having an odd order is configured by symmetrically arranging the center point P on the left and right sides of the diagonal line to prevent uncorrelated electromagnetic field coupling.
(2) A stub is not formed on any line 5 connecting each element (series element 2, shunt element 3), and the line widths W of the lines 5 are all the same, and unnecessary capacitor components are removed.
(3) For the spiral inductor L, the gap between the lines constituting the spiral inductor L is made equal to the line width W.
(4) The distance between the lines 5 connecting the elements (the series element 2 and the shunt element 3) is separated from the line width W by three times or more to prevent unnecessary electromagnetic coupling between the lines 5.
(5) A ground GND is provided around the substrate 4 so as to surround each element (series element 2, shunt element 3), and the ground of the BT-LPF is strengthened.
(6) The distance between the ground GND and each line 5 is about three times the line width W.

  In the layout configuration of (1), the center point P of the diagonal line of the substrate 4 is also applied to the direction of the spiral inductor L, variation in constants, the line 5 connecting each element (series element 2 and shunt element 3), and the ground GND. Symmetric with respect to the center.

  Hereinafter, a specific layout configuration of the low-pass filter 1 according to the present invention having the above-described features (1) to (6) will be described with reference to FIGS.

  As shown in FIG. 1, the low-pass filter 1 according to the present invention uses a square substrate 4 made of a dielectric. The series element 2 comprising the spiral inductor L constituting the low-pass filter 1 and the shunt element 3 comprising the series circuit of the semiconductor resistor R and the metal capacitor C have the diagonal line of the substrate 4 as a boundary line, and the center point of the diagonal line They are arranged point-symmetrically on the left and right sides of the diagonal with P as the origin. In addition, the elements are connected by a line 5 that is point-symmetric to the left and right of the diagonal with the center point P of the diagonal as the origin.

  As shown in FIGS. 2A and 2B, the metal capacitor C includes an upper layer metal 11 formed on the surface of the substrate 4 and a lower layer metal formed on the back surface of the substrate 4 so as to face the upper layer metal 11. 12, and a capacitor is constituted by the facing portions of the upper metal layer 11 and the lower metal layer 12. Note that the upper metal layer 11 formed on the surface of the substrate 4 is connected to a ground GND described later and grounded.

  The semiconductor resistor R is formed as a thin film on the back surface of the substrate 4 as shown in FIGS. 2A and 2B by a thin film process method such as vapor deposition, sputtering, or plating, and both ends thereof are formed on the back surface of the substrate 4. It is connected to the formed lower layer metal 12. The lower layer metal 12 is connected to the upper layer metal 11 on the surface of the substrate 4 through a contact portion 13 in a through hole formed through the substrate 4.

  As shown in FIGS. 3A and 3B, the spiral inductor L1 is formed in a rectangular spiral shape. One end of the spiral inductor L is connected to the upper layer metal 11 on the substrate 4 via an air bridge 14 including a pair of conductive spacers 14a and a line 14b connecting the conductive spacers 14a. The upper layer metal 11 is connected to the semiconductor resistor R. Are connected to the input terminal 15. Similarly, one end of the spiral inductor L <b> 2 is connected to the upper metal layer 11 on the substrate 4 via the air bridge 14, and this upper metal layer 11 is connected to the semiconductor resistor R and the output terminal 16.

  In the spiral inductor L, the gap between the lines constituting the spiral inductor L is set to be the same as the line width W. Further, the line 5 between the input terminal 15 and the spiral inductor L1, the line 5 between the semiconductor resistor R and the spiral inductor L, the line 5 between the spiral inductors L1 and L2, and between the spiral inductor L2 and the output terminal 16. These lines 5 are all set to the same line width W without forming a stub. Thereby, unnecessary capacitor components can be removed.

  A ground GND is formed on the surface of the substrate 4 so as to surround the series element 2 (spiral inductor L) and the shunt element 3 (semiconductor resistor R, metal capacitor C). Thereby, the ground of BT-LPF is strengthened.

  The ground GND intersecting with the input terminal 15 and the ground GND intersecting with the output terminal 16 are conductively connected on the back surface side of the substrate 4 through through holes formed in the substrate 4.

  Further, the ground GND is set to be 3 times (3 W) the line width W from each line 5. In addition, the ground GND has a stepped portion in part facing the elements (L, R, C) in order to escape resonance.

  Here, FIG. 4 shows an equivalent circuit of the low-pass filter 1 having the layout configuration shown in FIG.

  As shown in FIG. 4, the low-pass filter 1 of this example includes a series element 2 composed of two spiral inductors L1 and L2 connected in series between an input terminal 15 and an output terminal 16, an input terminal 15, A series circuit of a semiconductor resistor R1 and a metal capacitor C1 connected between the spiral inductor L1 and a series of a semiconductor resistor R and a metal capacitor C connected in two sets between the two spiral inductors L1 and L2. Circuit (series circuit of semiconductor resistor R2 and metal capacitor C2, series circuit of semiconductor resistor R3 and metal capacitor C3), series of semiconductor resistor R4 and metal capacitor C4 connected between the other spiral inductor L2 and output terminal 16 A fifth-order BT-LPF is constituted by the shunt element 3 formed of a circuit.

  The low-pass filter 1 having the layout configuration of FIG. 1 described above can obtain the group delay flatness as shown in FIG. Specifically, as shown in FIG. 5, a group delay flatness of 1.5 ps (peak-to-peak value) is realized at a cutoff frequency of 7.6 GHz BT-LPF.

  The low-pass filter 1 having the layout configuration shown in FIG. 1 satisfies the group delay flatness of 3.5 ps or less even when the variation in the thin film process is taken into consideration. Can be realized.

  As described above, the low-pass filter 1 according to the present invention includes, on the substrate 4, a spiral inductor L as the series element 2 and a series circuit of the semiconductor resistor R and the metal capacitor C as the shunt element 3. The center is a lumped constant type in which the elements are arranged symmetrically with respect to the left and right of the diagonal line and the elements are connected by a line 5. Thereby, an extremely flat group delay characteristic can be realized. In addition, the size can be reduced and the design can be simplified.

  Further, in the thin film process, the constant error related to the line can be realized extremely small, so that the characteristic variation due to the spiral inductor L and the metal capacitor C whose main characteristics are determined by the line width can be easily suppressed.

  Thereby, in the low-pass filter 1 according to the present invention, the main characteristic variation factor is only the value of the semiconductor resistance R. With regard to the variation in the value of the semiconductor resistance R, it is possible to realize a configuration in which variation in characteristics is suppressed by performing constant design in consideration of variation during constant design.

  Therefore, conventional adjustments such as laser trimming are not required, the adjustment work is saved, and the cost can be reduced. Theoretically, the variation in the group delay characteristics of the designed 7.6 GHz BT-LPF can be suppressed to 2 ps or less at the maximum value, assuming that the resistance causes an error of about ± 10%.

  By the way, in the low-pass filter 1 of the above-described embodiment, the configuration in which the spiral inductor L, the semiconductor resistor R, and the metal capacitor C are arranged on the square substrate 4 has been described as an example, but each element (spiral inductor L, semiconductor resistor R) is described. The metal capacitor C) may be arranged on a rectangular substrate.

  Further, although the BT-LPF having the fifth order has been described as an example of the low-pass filter 1, the order is not limited to the fifth order, and an BT-LPF having an odd order can be realized.

DESCRIPTION OF SYMBOLS 1 Low pass filter 2 Series element 3 Shunt element 4 Board | substrate 5 Line 15 Input terminal 16 Output terminal L (L1, L2) Spiral inductor C (C1, C2, C3, C4) Metal capacitor R (R1, R2, R3, R4) Semiconductor resistance

Claims (3)

A spiral inductor (L) as a series element (2) connected in series between an input terminal (15) and an output terminal (16), a semiconductor resistor (R) having one end grounded, and a metal capacitor (C) In a low-pass filter (1) in which a shunt element (3) composed of a series circuit is arranged on a rectangular substrate (4),
In the rectangular substrate, the spiral inductor, the semiconductor resistor, and the metal capacitor are arranged symmetrically with respect to the left and right of the diagonal line with the center of the diagonal line of the substrate as the origin, and the elements are connected by lines. A low-pass filter characterized by The low-pass filter according to claim 1, wherein the spiral inductor (L) has a gap between all lines constituting the spiral inductor equal to a line width. The low-pass filter according to claim 1 or 2, wherein all the widths of the lines are the same.

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