JPH0738306A - High frequency filter - Google Patents

Abstract

PURPOSE:To improve the frequency selection characteristic, and also, to miniaturize this high frequency filter. CONSTITUTION:This high frequency filter is provided with a first multi-terminal pair circuit 1 (called a minute transmission line periodic structure) constituted by connecting plural pieces of minute electric length transmission lines having about 1/10 wavelength or below in a mesh-like or a linear (cascade-like) or shape of combination of both shapes, and a second multi-terminal pair circuit 2 having the same constitution as that of the circuit 1, and between both terminal pairs and between plural sets of terminals, a circuit consisting of an inductance or a capacitor is connected, and each one terminal of the circuits 1 and 2 is constituted as an input terminal or an output terminal. This filter becomes a characteristic being near an LC parallel resonance circuit in a low frequency band, and loss in a passing area being comparatively lower than its antiresonance frequency becomes small, and can be made roughly flat. On the other hand, in a high frequency band, a capacity value can be increased together with a capacity value by frequency dependency of an equivalent capacity of a filter input/output terminal, therefore, the attenuation quantity of an obstructing area can be enlarged.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high frequency filter suitable for being incorporated in an integrated circuit or a multilayer substrate.

[0002]

2. Description of the Related Art Since a filter is generally based on a low pass filter, a low pass filter will be described below as an example. The low-pass filter is one of the important circuits for removing unnecessary waves in the transmitter / receiver. Figure 7
(A) And (b) is a structural example of the low pass filter in the conventional microwave integrated circuit. FIG. 7A shows a configuration using distributed constant lines, 31 and 32 are input / output terminals, 33, 35 and 37 are low impedance transmission lines, and 34 and 36 are high impedance transmission lines. It is configured by connecting ¼ wavelength transmission lines in cascade, and realizes low-pass filter characteristics by sequentially performing impedance conversion with high and low impedances. This equivalent circuit is shown in FIG. FIG. 7B is a general expression of a low pass filter, which is realized as a lumped constant low pass filter using a lumped constant inductor (L) and a capacitor (C). Depending on the case, a capacitor may be used as an open transmission line (open stub).
Or by using a high impedance transmission line as the inductor, the performance is finely adjusted. Here, FIG.
Elements that roughly correspond to each other in (a) and (b) have the same numbers.

FIG. 8 shows an example of a low-pass filter using a parallel resonant circuit (generally, a band elimination filter), which is simple and small in size, and can realize greater attenuation than a filter of FIG. 7 at a specific frequency. . 41 and 42 are input / output terminals. Reference numeral 43 is an inductor (L), and 44 is a capacitor (C), which exhibits a very large attenuation amount in the vicinity of these anti-resonance frequencies f ₀ . 45 and 46 are parallel capacitors (Cs)
Therefore, the impedance becomes smaller as the frequency becomes higher. As a result, as shown in FIG. 9, the anti-resonance frequency f ₀ tends to pass signals in the following, will be the signal throughput is suppressed at f ₀ above, a low-pass filter.

[0004]

However, when the low pass filter shown in FIG. 7 is applied to a microwave integrated circuit, particularly a monolithic microwave integrated circuit, there are some problems. FIG. 7A has a drawback that the total length is several times as long as a quarter wavelength, a wide low impedance line is required, and the occupied area becomes large in inverse proportion to the frequency. Therefore, it was practically difficult to apply in a frequency band lower than the millimeter wave.

In FIG. 7 (b), the width of the metal conductor forming the inductor is generally as narrow as 10 to 20 microns, so that the influence of the loss of the inductor is large, and the loss increases with the increase of frequency in the pass band. This tendency becomes remarkable when the number of stages is increased for the purpose of improving the stopband attenuation performance, and there is a drawback that unacceptable passage loss is exhibited as the cutoff frequency design value (1 / π√LC) is approached.

In the low pass filter of FIG. 8, the larger the parallel capacitance (Cs) is, the larger the stop band attenuation amount can be.
Due to the influence of the parallel capacitance, a pass frequency band much lower than the notch frequency (1 / 2π√LC) determined by the inductor (L) and the capacitor (C) was obtained. This is due to the use of fixed parallel capacitance. That is,
The low-pass filters of FIGS. 7 and 8 have a drawback that they are large in shape or have low frequency selectivity in a monolithic integrated circuit. The present invention solves these drawbacks and provides a small high-frequency filter having excellent frequency selectivity.

[0007]

The high frequency filter of the present invention comprises a plurality of minute electrical length transmission lines each having a length of about one-tenth wavelength or less, in a mesh shape, a linear shape or both shapes. First connected to a combination of shapes
A multi-terminal pair circuit and a second multi-terminal pair circuit having the same configuration as the first multi-terminal pair circuit, and an inductor or a capacitor is provided between a plurality of pairs of terminals between the pair of terminals. A circuit is connected, and one terminal of the first multi-terminal pair circuit is used as an input terminal (or an output terminal), and one terminal of the second multi-terminal pair circuit is used as an output terminal (or an input terminal). .

According to the present invention, the first and second multi-terminal pair circuits have a structure in which a plurality of minute transmission lines that are sufficiently short with respect to the wavelength are connected in a mesh shape, a linear shape (cascade shape), or a combination of both shapes. (Hereinafter referred to as a periodic structure), and its characteristics shift from a point (lumped constant) to a distributed constant as the frequency increases. Further, a low pass filter, a band pass filter, etc. can be realized by combining inductors and capacitors connected between a plurality of sets of terminals in a multi-terminal pair circuit. Connect inductor L, between some terminals,
A configuration in which the capacitor C is connected between the other terminals serves as a low pass filter.

The operation will be described by taking this low-pass filter as an example. Since the transmission line periodic structure can be regarded as a point at a frequency lower than the LC anti-resonance frequency, a simple parallel-connected LC resonance circuit is obtained, and a loss in the pass band is small and a relatively flat characteristic can be obtained. it can. At the LC anti-resonance frequency, the impedance between the input and output terminals becomes very large (infinity), so the signal is greatly attenuated in this vicinity, and L
In the frequency region higher than the C anti-resonance frequency, a capacitance having a frequency characteristic is effectively generated between the ground due to the effect of the periodic structure, and the capacitance value (= a / [1-bω ² ]; a, b
Is a constant, and 1> bω ² ) rapidly increases as the frequency ω increases. Therefore, in the stop band, the signal pass is suppressed more than in the conventional low pass filter of FIG. 7 or 8. can do.

In the present invention, since the signal suppression effect in the attenuation band increases as the frequency increases, the stop band attenuation amount can be increased while keeping the pass band wide, resulting in a small filter with good frequency selectivity. The drawbacks can be overcome. In the case of a bandpass filter in which terminals are connected by an inductor-capacitor series circuit, the frequency dependence of the capacitance is maintained.

[0011]

1 is a diagram showing the configuration of a first embodiment of a low-pass filter according to the present invention. In the figure, 10 and 1
Reference numeral 1 is an input / output terminal, and 1 and 2 are 4-terminal pair circuits (terminal names: A; B; C; D; and a; b; c; d) having a periodic structure in which minute transmission lines are combined in a square shape. is there.
Here, the characteristic impedance of the minute transmission line is Z, the physical length is I, and the electrical length 2π × I / λg (λg is the wavelength) is θ. The minute transmission line means a physical length I << λg (wavelength) or an electrical length θ << π, and here, I≈λg / 10 or I <λg / 10. 3 and 4 are terminals A, respectively
-A and a capacitor (Cp connected between terminals C and c)
), 5 and 6 are between terminals B-b and terminals D-d, respectively.
It is an inductor (Lp) connected in between.

Since the circuit of the present invention is symmetric with respect to the broken line, it can be represented by the equivalent lattice circuit shown in FIG. Here, the short impedance Zs and the open impedance Zf are respectively defined by the following (1)
Expression (2) is expressed.

[0013]

[Equation 1] Where ω is the angular frequency, k = 1 + 3θ / ZCp ω. The first term in the equation (1) is the inductance (3/4) Zθ /
The parallel resonance circuit N ₁ having ω and the capacitance 2Cp, and the second term is the parallel resonance circuit N ₂ having the inductance Lp / 2 and the capacitance 2kCp. Equation (2) is a parallel resonance circuit of the inductance (3/4) Zθ / ω and the capacitance 6θ / Zω, and 2> 9θ ^{2 in} the used frequency band. In the low frequency region, the capacitance can be approximated by 6θ / Zω. As will be described later in a specific example, the resonance frequency of the parallel resonance circuit N ₂ is a fraction of the resonance frequency of the parallel resonance circuit N ₁ . Where θ / ω is a constant value,
If the velocity of the electromagnetic wave propagating through the minute transmission line 20 is vg, then θ / ω = I / vg.

Further, when this is transformed into a π type equivalent circuit, it becomes as shown in FIG. Reference numeral 21 is a circuit corresponding to the parallel resonant circuit N ₂ of FIG. 2, 22 is a circuit corresponding to the parallel resonant circuit N ₁ of FIG.
Reference numeral 23 is an inductor resulting from the change from the lattice type to the π type equivalent circuit, and the series resonant circuits of 24 and 25 operate as a parallel capacitance in the operating frequency band. In a frequency region much lower than the resonance frequency of the parallel resonance circuit 22 (having the same resonance frequency as the parallel resonance circuit N ₁ ), the notch frequency ω n of the circuit of FIG. 3 is the parallel resonance circuit formed by the parallel resonance circuit 21 and the inductor 23. Depends on. The admittance Y of the parallel circuit of the inductor 23 and the capacitor 2 kCp is

[0015]

[Equation 2] Therefore, the angular frequency ωn at which this and the inductor Lp / 2 resonate is 1 / √LpCp. That is, the notch frequency ωn of the embodiment of the present invention is calculated as shown in FIG.
Are almost equal to the resonance frequencies of Lp and Cp of 3 to 6.
The passband is on the low frequency side of this ωn.

Series resonant circuits 24 and 25 at both ends of FIG.
The impedance Zf of the

[0017]

[Equation 3]Since the electrical length θ is proportional to the frequency, the capacitance Cf is
Frequency increases, 9θ ²→ Rapid increase while approaching 2
To do. Since θ / ω is a constant value, Cf is a quadratic function of θ
It is represented by the reciprocal of. Therefore, the low-pass filter passes
Almost parallel capacitance up to near over frequency and notch frequency
It becomes constant, and the parallel is used in the frequency range higher than the notch frequency.
2 terminal pair circuit (small transmission line period
If you design (Structure) 1 and 2, pass near the notch frequency
You can widen the range and increase the attenuation in the stop range.
It That is, frequency selectivity can be improved. The capacity
Cf has nothing to do with Lp and Cp.

The operation will be described below with reference to a specific example. Figure 1
Set the parameters of each element that makes up as follows. I = 0.45 mm, Z = 30Ω, Lp = 3 nH, Cp =
0.3 pF, εr = 3.3 where εr is the relative permittivity of the substrate forming the minute transmission line. By using these constants, FIG. 3 to FIG. 4 are obtained. From the parameter values of FIG. 4, the above description of the present embodiment can be understood. In FIG. 5, the solid line indicates the frequency characteristic of the low pass filter of FIG. 4, and the broken line indicates the parallel capacitance value Cf of 2p.
It is the frequency characteristic of the conventional example fixed to F. It can be seen that the present invention significantly improves frequency selectivity.

The capacitance Cf in the equation (4) is the inductor L
Since it is determined independently of p and the capacitor Cp, it can be generated and used when other filters are realized. Other Embodiments FIG. 6 is a diagram showing the configuration of a second embodiment of the low-pass filter of the present invention. In the figure, 1 and 2 are 2-terminal pair circuits having a field-shaped minute transmission line periodic structure, 3 and 4 are capacitors, and 5 and 6 are inductors. Since these are the same as those described in FIG. 1, the same reference numerals are given. 7 is a dielectric film, 8 is a ground conductor, and 9a to 9
Reference numeral d is a portion where the ground conductor is partially removed, and the terminals of the two-terminal pair circuits 1 and 2 are connected through this portion. 12
Is a dielectric substrate or a semiconductor substrate. The dielectric film 7 is formed or stuck on the dielectric substrate or the semiconductor substrate 12. The inductors 5 and 6 and the capacitors 3 and 4 may be formed on the dielectric film 7 or may be formed on the substrate 12. (A) According to the configuration of this embodiment, since the two-terminal pair circuit having the minute transmission line periodic structure is formed on the upper and lower sides, the inductor L and the capacitor C can be easily connected. That is, the distance between the terminals is extremely small, and it is not necessary to use an unnecessary wiring line when connecting with L or C. Therefore, it is not necessary to consider the transmission line characteristics, the stray C, the stray inductance, the parasitic resistance, etc. in the terminal connection portion.
This is suitable for realizing the performance as analyzed in the embodiment. (B) Further, if the dielectric film 7 is realized by a thin film of about 10 μm, for example, when the characteristic impedance of the minute transmission line is 30Ω, the line width is 20 μm to 40 μm.
Since it can be realized as thin as a micron, the dimension of the line intersection can be made sufficiently small with respect to the wavelength. Therefore, the design error can be eliminated. (C) Further, in the periodic structure, a plurality of signal flow paths are arranged in parallel, so that the insertion loss in the low pass filter pass band due to the loss of the transmission line conductor can be significantly suppressed.

The characteristic of the embodiment of FIG. 1 shown in FIG. 5 is calculated by including the transmission line loss, but the measured values are almost the same. It should be noted that the loss is small in the pass band and has a flat characteristic, and the loss in the high frequency attenuation region is larger than that of the conventional characteristic due to the frequency characteristic of the equivalent capacitance Cf. Even in the configuration in which the dielectric film 7 is replaced with a dielectric substrate, the advantages (a) to (c) can be maintained. In this case and when the above-mentioned dielectric film 7 is used, since the configuration itself of the low-pass filter is such that the input / output terminals are arranged above and below the dielectric substrate, different circuits are used for the two-layer dielectric substrate. It can be formed on the upper and lower surfaces. For example, if a high frequency circuit is formed on the upper surface and a low frequency circuit is formed on the lower surface, it is possible to make the receiver and the transmitter compact.

In the embodiment described above, the first and second multi-terminal pair circuits (minute transmission line periodic structure) are formed into a square-shaped two-terminal pair circuit. However, the present invention is not limited to this. Not limited to this, a plurality of minute transmission lines may be connected in an arbitrary mesh form, may be formed in a cascade form, or a multi-terminal circuit having a combination of both forms may be used to similarly configure a filter.

[0022]

As described above, according to the present invention, by connecting a circuit including an inductor or a capacitor between the terminals of the first and second multi-terminal pair circuits having the minute transmission line periodic structure, the low frequency region is connected. Has a characteristic close to that of the LC parallel resonant circuit, can reduce the loss in the pass band relatively lower than the anti-resonant frequency and can be made substantially flat, and in the high frequency region, due to the frequency dependence of the equivalent capacitance Cf,
Since the capacitance value can be increased with the frequency, the attenuation amount in the stop band can be increased. Therefore, it is possible to provide a compact high frequency filter having excellent frequency selectivity.

Further, by forming a multilayer structure in which the circuits having the periodic structure are vertically stacked, the circuit can be made more compact and the circuit composed of L or C between the upper and lower terminals can be wired in a short distance, thus improving the designability. The effect is obtained. Furthermore, since the size of the surface occupied by the plurality of minute transmission lines is still sufficiently small with respect to the wavelength, it can be realized in a smaller size than the conventional high frequency filter.

[Brief description of drawings]

FIG. 1 is a principle configuration diagram showing an embodiment of the present invention.

2 is a lattice type equivalent circuit diagram of the embodiment of FIG. 1. FIG.

FIG. 3 is a π-type equivalent circuit diagram of the embodiment of FIG.

FIG. 4 is a view showing a π-type equivalent circuit in a specific design example of the embodiment of FIG.

5 is a diagram showing the frequency characteristic of FIG. 4 in comparison with the characteristic of the conventional example of FIG.

FIG. 6 is a diagram showing the configuration of another embodiment of the present invention.

FIG. 7 is a diagram showing a configuration of a conventional high-frequency band low-pass filter.

FIG. 8 is a diagram showing the configuration of another conventional low-pass filter in the high frequency band.

9 is a diagram showing a characteristic example of the conventional high-pass low-pass filter shown in FIG. 8;

Claims (1)

[Claims] 1. A plurality of minute electrical length transmission lines having a length of about one-tenth wavelength or less are connected in a mesh shape, a linear shape (cascade shape), or a combination of these shapes. A first multi-terminal pair circuit, and a second multi-terminal pair circuit having the same configuration as the first multi-terminal pair circuit, and an inductor or a capacitor between a plurality of sets of terminals between the both terminal pairs. Characterized in that one circuit of the first multi-terminal pair circuit is used as an input terminal (or output terminal) and one circuit of the second multi-terminal pair circuit is used as an output terminal (or input terminal). And a high frequency filter.