JPH08330805A - Band pass filter - Google Patents

Abstract

PURPOSE: To easily and inexpensively obtain a sharp band pass filter characteristic. CONSTITUTION: Adjacent microstrip lines S1 and S2 are shown by inductors L1 and L2 and a mutual guide coefficient M. A first parallel resonance circuit is constituted by a capacitor C1 and the inductor L1 on an input terminal T1- side, and a second parallel resonance circuit is constituted by the inductor L2 and a capacitor C2 on an output terminal T2-side. The first and second parallel resonance circuits are connected as it were a transformer by the mutual guide coefficience M. Thus, the high frequency signal of an input terminal T is resonated in parallel by the first and second parallel resonance circuits, and is guided to an output terminal T2. Namely, the whole operates as a multiple-tuned circuit having two resonance frequencies. When the necessary number of circuits are cascade-connected by adjusting them to the required frequency characteristics, BPF of the sharp characteristic can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a BPF (bandpass filter: bandpass filter) used in communication equipment and the like.

[0002]

BACKGROUND ART Various types of bandpass filters for communication devices such as mobile phones are already known. VH
In the F band and the UHF band, a filter composed of lumped constant components such as a chip capacitor (multilayer ceramic chip capacitor), a chip inductor, an air core coil, a helical filter, and a SAW filter are used.

FIG. 7 shows a BP in the VHF and UHF bands.
An example of F is shown. A circuit diagram is shown in (A) of the figure. The capacitors CA, CB, CC and the inductor L are shown.
The BPF is composed of A and LB.

FIG. 1B shows a first example of the filter structure.
On both sides (upper and lower sides of the figure) of the substrate 100, GND
Patterns 102 and 104 are formed. The electrode patterns 106 and 108 are formed between the GND patterns 102 and 104. A chip capacitor CA1 and a chip inductor LA1 are provided in parallel between the electrode pattern 106 and the GND pattern 104, and a chip inductor LB1 and a chip capacitor CC1 are provided between the electrode pattern 108 and the GND pattern 104.
Are provided in parallel. In addition, the electrode pattern 106,
A chip capacitor CB1 is provided between 108. Chip capacitors CA1, CB1 and CC1 are shown in Fig. (A).
The capacitors CA, CB and CC of
The chip inductors LA1 and LB1 correspond to the inductors LA and LB, respectively.

FIG. 1C shows chip inductors LA1 and LB.
This is an example in which air core coils LA2 and LB2 are used instead of 1.
The GND pattern, electrode pattern, and chip capacitor are the same as in the example of FIG.

On the other hand, in the microwave band, microstrip lines, those using strip lines,
A filter composed of distributed constant components such as a dielectric resonator is used. FIG. 8 shows an example of the BPF in the microwave band. The BPF 110 whose circuit diagram is shown in FIG. 1A has a microstrip line (or stripline) with a length of λ / 2 (λ is the wavelength of an electromagnetic wave) on a dielectric substrate 122, as shown in FIG. ) 12
4 is formed in multiple stages.

[0007]

However, the above background art has the following disadvantages. (1) VHF and UHF band BPF First, in a BPF using lumped constant components, the circuit is composed of a chip capacitor and a chip inductor.
The Q (quality factor) of parts is low. In particular, the Q of the chip inductor is low and is generally several tens or less. Therefore, it is difficult to obtain steep filter characteristics. On the other hand, for the type using the air-core coil, the air-core coil has a higher Q than that of the chip inductor, so that a filter having a steeper characteristic can be obtained. However, the air-core coil is inferior to the chip component in terms of ease of mounting and mechanical stability. Further, when a plurality of air core coils are used, electromagnetic coupling between the coils becomes a problem.

Next, a helical filter has high performance but is expensive and has a large shape. In particular, it is large in the height direction, and it is difficult to make it thin. Further, the SAW filter has a very high performance and is small in size, but is very expensive and disadvantageous in cost.

(2) Case of Microwave Band BPF First, in a BPF of a type using a microstrip line or a stripline, a multi-stage ½ wavelength coupled line is generally used. Therefore, the occupied area is almost determined by the used frequency, and it is difficult to reduce the size.
Further, in the case of the BPF using the distributed constant circuit, there is a disadvantage that an expensive simulator is used at the time of design or sufficient experience is required, which causes difficulty in design and adjustment.

The present invention focuses on the above points,
The purpose thereof is to provide a stable and cost-effective BPF that can obtain a steep filter characteristic.
Another object is to provide a BPF that is easy to design and can be downsized. Still another object is to use the same circuit configuration for V
B that can be applied to a wide band from the HF band to the microwave band
It is to provide PF.

[0011]

In order to achieve the above object, according to the present invention, a double-tuned circuit is constructed by using a capacitor and a parallel resonant circuit formed by a distributed constant circuit such as a strip line or a micro strip line. The BPF is constructed by using at least one stage of the double tuning circuit.

According to the present invention, the distributed constant circuit connected in parallel with the capacitor functions as an inductor and constitutes a parallel resonance circuit together with the capacitor. On the other hand, the distributed constant circuits are arranged on the substrate so as to face each other, whereby the distributed constant circuits are electromagnetically coupled to each other, so that an M-coupled double tuning circuit is formed as a whole. A BPF having a steep frequency characteristic is formed by matching the required characteristics with the double-tuned circuit and connecting the necessary numbers in cascade.

The above and other objects, features and advantages of the present invention will be apparent from the following detailed description and the accompanying drawings.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS While there may be many embodiments of the present invention, a suitable number of embodiments will now be shown and described in detail.

<First Embodiment> First, a first embodiment will be described with reference to FIG. FIG. 1A shows the first embodiment.
The circuit diagram of is shown. In the figure, the input terminal T1
A parallel circuit of a capacitor C1 and a microstrip line S1 is connected between the and GND. A parallel circuit of a capacitor C2 and a microstrip line S2 is connected between the output terminal T2 and GND.

FIG. 1B shows an equivalent circuit of the first embodiment as described above. As shown in the figure, the microstrip lines S1 and S2 function as inductors L1 and L2 on the one hand, and constitute parallel resonance circuits together with the capacitors C1 and C2. The microstrip lines S1 and S2 are arranged close to each other so as to face each other and be in an electromagnetically coupled state. Therefore, an M (mutual inductive) coupling double-tuned circuit is configured as a whole. In this way, the microstrip lines S1 and S2 are
Is equivalently represented by the inductors L1 and L2 and the mutual induction coefficient M.

FIG. 1C shows an example of an actual circuit configuration. Both sides of the substrate 10 (bottom and top of the figure)
GND patterns 12 and 14 are respectively formed in the. Electrode patterns 16 and 18 are formed between these GND patterns 12 and 14 at appropriate intervals, and microstrip line patterns 20 and 22 are formed at their ends. That is, the electrode pattern 16 and the microstrip line pattern 20 are
The microstrip line pattern 20 is continuously formed of a conductive material so as to have a substantially L shape, and the microstrip line pattern 20 is further continuous with the GND pattern 12. Similarly, the electrode pattern 18 and the microstrip line pattern 22 are continuous so that the whole becomes substantially L-shaped, and the microstrip line pattern 22 is continuous with the GND pattern 12. Further, the microstrip line patterns 20 and 22 are adjacently arranged in parallel so as to be electromagnetically coupled.

Next, chip capacitors C11 and C21 are provided between the GND pattern 12 and the electrode patterns 16 and 18, respectively. The chip capacitors C11 and C21 correspond to the capacitors C1 and C2 shown in FIGS. In the illustrated example, it is provided at a position close to the microstrip line patterns 20 and 22.

The voltage amplitude of the signal becomes maximum at the connection point between the capacitor and the microstrip line. For example, FIG.
If the chip capacitor C11 is placed at the point X3 shown in (A), the voltage amplitude is maximum at this position, so the voltage amplitude from the point X0 to the point X3 is as shown by the graph GA in FIG. . The voltage at the point X1 at this time is Va from the graph GA. Next, if the chip capacitor C11 is placed at the adjacent X2 point shown in FIG. 2 (B), the voltage amplitude is maximum at this position, so the voltage amplitude from the X0 point to the X2 point is shown in FIG. 2 (C). As shown in the graph GB. At this time, the voltage at point X1 changes from graph GB to Vb, which is higher than Va.

As described above, by disposing the capacitor at a position close to the microstrip line, the voltage amplitude at the point X1 which is the end of the microstrip line approaches the maximum amplitude. This makes it possible to reduce the insertion loss of the filter. For this reason, it is preferable to provide the chip capacitors C11 and C21 at the positions shown by the dotted lines in the figure.

Next, the operation of the first embodiment constructed as above will be described. In addition, in order to facilitate understanding, FIG.
The operation will be described with reference to the equivalent circuit of (B). As described above, the first parallel resonant circuit is configured by the capacitor C1 and the inductor L1 on the input terminal T1 side.
On the output terminal T2 side, the inductor L2 and the capacitor C2 form a second parallel resonance circuit. The first and second parallel resonant circuits are connected like a transformer by the mutual induction coefficient M. Therefore, the high frequency signal input to the input terminal T1 is guided to the output terminal T2 and output while being subjected to the parallel resonance action of the first and second parallel resonance circuits.

As described above, according to the present embodiment, the whole is 2
It acts as a double-tuned circuit with one resonance frequency. A BPF is constructed by connecting a required number of these double-tuned circuits in cascade according to the required frequency characteristics. BPF
Specific examples of the above will be described in detail in Examples 2 and 3 described later.

According to this embodiment, there are the following effects. (1) By using the microstrip line as the inductor, the Q of the resonance circuit can be made higher than that when the chip inductor is used. A chip inductor usually has a self-resonant frequency of about several GHz, whereas a microstrip line has several tens of GHz.
Since there is also z, the usable frequency band becomes wider.

(2) Since the inductor is formed as a microstrip line on the substrate, there is no problem in terms of mounting surface and mechanical stability like an air core coil. Moreover, since an inductor as a chip component is not required, a BPF can be obtained at low cost.

(3) Since the resonance circuit is composed of the chip capacitor and the microstrip line, it is not necessary to make the microstrip line a half wavelength as in a general BPF in the microwave band. Therefore, the size can be reduced.

(4) Since the parallel resonant circuit is the basis of the circuit structure, the design is easy.

(5) From the VHF band to the microwave band, the BPF can be constructed with the same circuit configuration.

(6) The usable frequency band (upper limit) is determined by the self-resonant frequency of the chip capacitor, but it can be used up to several GHz by using the high frequency chip capacitor.

<Embodiment 2> This Embodiment 2 is a specific example of a 308 MHz BPF using one double-tuned circuit.
The circuit configuration is shown in FIG. 3. The width W, the length L, and the interval D of the microstrip lines S11 and S21 are W = 1.1.
mm, L = 14.5 mm, D = 1.1 mm. Note that L <λ for the signal wavelength λ of the frequency 308 MHz.
The relationship is / 4. Also, the substrate 10 (see FIG. 1)
As for, a glass epoxy substrate is used, and its thickness t is 1.0 mm.

The microstrip lines S11 and S21 are
It corresponds to the inductors L1 and L2 and the mutual induction coefficient M shown in FIG. The capacitor C12 (36 pF) corresponds to the capacitor C1, and the capacitors C22 (100 pF) and C2
3 (30 pF) corresponds to the capacitor C2. The trimmer capacitor TC (20 to 40 pF) is for fine adjustment of characteristics.

The frequency characteristics of the BPF designed as described above are shown in FIG. In the figure, the vertical axis represents the amount of attenuation,
The horizontal axis is frequency. As shown in this graph, arrow F
A steep and good bandpass characteristic centering around 308 MHz is obtained.

<Third Embodiment> This third embodiment is a specific example of a 308 MHz BPF in which two double-tuned circuits are vertically connected. The circuit configuration is shown in FIG. 5, and the width W, length L, and spacing D of the microstrip lines S12 and S22 are W =
It is 1.5 mm, L = 14.0 mm, and D = 1.0 mm. The same applies to the microstrip lines S13 and S23. In this example as well, the frequency is 308 MHz.
There is a relationship of L <λ / 4 with respect to the signal wavelength λ of z. The substrate 10 is also the same as in the second embodiment.

The microstrip lines S21 and S22 are
It corresponds to the inductors L1 and L2 and the mutual induction coefficient M shown in FIG. The same applies to the microstrip lines S13 and S23. The capacitor C13 (39pF) is the capacitor C1
Corresponding to, the capacitor C24 (39pF) is the capacitor C
Corresponds to 2. Capacitor C14 (100pF), C15
(33pF) corresponds to capacitor C1 and capacitor C2
5 (39 pF) corresponds to the capacitor C2. Similarly, the trimmer capacitor TC (20 to 40 pF) is for fine adjustment of characteristics. CM (2 pF) is a matching capacitor for two double-tuned circuits.

The frequency characteristics of the BPF designed as described above are shown in FIG. The vertical axis and the horizontal axis are the same as in FIG. As shown in this graph, 308 indicated by arrow F
A sharp bandpass characteristic centering around MHz is obtained. It should be noted that, as compared with the second embodiment, the third embodiment has a configuration in which the double-tuned circuit has two stages.
Has a steeper frequency characteristic.

<Other Embodiments> The present invention can be variously modified based on the above disclosure, and includes, for example, the following. (1) In each of the above embodiments, a microstrip line is used, but a strip line, a slot line, a CPW (Coplanar Waveguide) or the like may be used as an inductor to realize M coupling.

(2) In the second and third embodiments, one trimmer capacitor is used for fine adjustment of the characteristics, but a plurality of trimmer capacitors may be used if necessary, and conversely there is a margin for the required performance. If there is, you do not need to use it.

(3) In the third embodiment, the dimensions of the microstrip lines of the vertically connected double tuning circuits are all the same, but it is not always necessary to do so, and different dimension settings may be made between the double tuning circuits. . Further, the length L and the width W of the two strip lines need not be the same even in one double tuning circuit, and may be set appropriately.

(4) In the third embodiment, two double-tuned circuits are vertically connected, but the number of connection stages may be appropriately set as necessary. Also, the numerical values of the circuit elements shown in the embodiments may be set appropriately as necessary.

[0039]

As described above, the present invention has the following effects. (1) Since the distributed constant circuit in the electromagnetically coupled state is used as the inductor, Q can be made higher than that of the chip inductor, and steep filter characteristics can be obtained. Further, it is possible to use at a higher frequency than in the case of the chip inductor, and it is possible to avoid the inconvenience of the air core coil in terms of mounting and mechanical stability.

(2) Since the inductor as a component is not required, the BPF can be constructed at low cost.

(3) Since the resonance circuit is constituted by the chip capacitor and the distributed constant circuit, it is not necessary to make the stripline or the like λ / 2, and therefore the size can be reduced.

(4) Since the design of the parallel resonant circuit is the basis, the design is easy. Further, the BPF can be designed with the same circuit configuration from the VHF band to the microwave band.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of a first embodiment of the present invention.

FIG. 2 is a diagram showing the relationship between the position of a chip capacitor and the voltage amplitude.

FIG. 3 is a circuit diagram showing a configuration of a second embodiment.

FIG. 4 is a graph showing frequency characteristics of the second embodiment.

FIG. 5 is a circuit diagram showing a configuration of a third embodiment.

FIG. 6 is a graph showing frequency characteristics of the third embodiment.

FIG. 7 is a diagram showing a background art of VPF in the VHF and UHF bands.

FIG. 8 is a diagram showing a background art of a microwave band BPF.

[Explanation of reference numerals] 10 ... Substrate 12, 14 ... GND pattern 16, 18 ... Electrode pattern 20, 22 ... Microstrip line pattern C1, C2, C12, C13, C14, C15, C22, C23, C2
4, C25 ... Capacitor CM ... Matching capacitor D ... Microstrip line interval L ... Microstrip line length L1, L2 ... Inductor M ... Mutual induction coefficient S1, S2, S11, S12, S13, S21, S22, S23 … Microstrip line T1… Input terminal T2… Output terminal TC… Trimmer capacitor W… Microstrip line width

Claims (2)

[Claims] 1. A first distributed constant circuit and a second distributed constant circuit arranged so as to be in an electromagnetically coupled state; a first capacitor which forms a parallel resonance circuit with an equivalent inductor of the first distributed constant circuit; and a second distribution. A double tuning circuit including an equivalent inductor of a constant circuit and a second capacitor forming a parallel resonance circuit;
Bandpass filter with at least one stage. 2. The bandpass filter according to claim 1, wherein the double tuning circuit is provided with at least one fine adjustment capacitor.