JP3040947B2 - Low pass filter for high frequency - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a high-frequency low-pass filter using a microstrip line, and more particularly to a high-frequency low-pass filter usable in a quasi-microwave band or a microwave band.

[0002]

2. Description of the Related Art For example, in a radio apparatus used in a car telephone using a frequency in a quasi-microwave band, it is necessary to attenuate a harmonic component by about 70 dB with respect to a transmission signal. For this reason, the wireless device usually includes a low-pass filter for removing harmonic components at the output side of the power amplifier. As such a low-pass filter, a low-pass filter including a hollow shield case and a microstrip line formed on a dielectric substrate provided in the shield case has been known. This conventional low-pass filter (hereinafter, referred to as a "first low-pass filter") is a series connection of four or more three-element π-type low-pass filter circuits each composed of two capacitors and a coil constituted by a microstrip line. It is configured. In this case, since the π-type low-pass filter circuit can obtain about 15 to 20 dB of attenuation per stage, the first low-pass filter can obtain about 70 dB of attenuation as a whole. ing.

On the other hand, at higher frequencies such as quasi-microwaves, as the power of the transmission signal increases, higher harmonic components jump from the input of the low-pass filter to the output through the air, and a desired amount of attenuation is obtained. May not be possible. For this reason, a low-pass filter in which the first low-pass filter is improved so as to obtain a desired amount of attenuation by using a radio wave absorber is also known (hereinafter, referred to as a “second low-pass filter”). In the second low-pass filter, a radio wave absorber is attached to an inner wall of a shield case that is considered to reflect a harmonic component. Is prevented from jumping into the output section side, whereby a desired amount of attenuation is obtained.

[0004]

However, the above-mentioned two conventional low-pass filters have the following problems. That is, in the first low-pass filter, as described above, in addition to the problem that a desired amount of attenuation cannot be obtained due to the intrusion of harmonic components,
Since the π-type low-pass filters are connected in multiple stages, iron loss occurs in the microstrip line, particularly in the inductor portion. For this reason, a pass loss of the in-band signal occurs in this portion, which not only causes a decrease in transmission efficiency, but also causes a problem that the microstrip line may be burned when transmitting large power. There is. In particular, when more π-type low-pass filters are connected in multiple stages to compensate for the reduced amount of attenuation to compensate for the reduced amount of attenuation due to the intrusion of harmonic components as described above, the effect is more remarkable. Problem.

On the other hand, in the second low-pass filter, since the radio wave absorber absorbs harmonic components, heat is generated in that portion, and in some cases, the reliability of other components is impaired. is there.

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and provides a high-frequency low-pass filter capable of reducing the passing loss of the low-pass filter and further attenuating harmonic components. The purpose is to:

[0007]

According to a first aspect of the present invention, there is provided a high-frequency low-pass filter comprising a plurality of π-type low-pass filter circuits each including a capacitor portion and an inductor portion formed by a microstrip line. And a predetermined gap with respect to the dielectric substrate so as to cover the microstrip line constituting the inductor section in one or more (excluding all) of the plurality of low-pass filter circuits. At least one metal plate held on the dielectric substrate by the spacer and electrically connected to the ground pattern formed on the dielectric substrate via the spacer is provided.

In this high-frequency low-pass filter, the inductor portion and the resonance capacitor equivalently constituted by the metal plate cause resonance at an out-of-band frequency such as a harmonic frequency which is an integral multiple of an in-band signal which is an in-band frequency. I do.
Therefore, the high-frequency low-pass filter constitutes a low-pass filter having more resonance points equivalently than the π-type low-pass filter. Therefore, when the attenuation at the out-of-band frequency is configured to be the same, the number of connection stages of the π-type low-pass filter is reduced as compared with the conventional low-pass filter. As a result, the transmission loss of the high-frequency low-pass filter decreases. Further, the harmonic component of the out-of-band frequency is blocked from being absorbed in the high-frequency low-pass filter without being absorbed. For this reason, generation of heat due to absorption of the harmonic component is eliminated, so that there is no influence on other components. Furthermore, since the metal plate covers the microstrip line that forms the inductor portion in the low-pass filter circuit, it prevents the harmonic component signal from jumping from the microstrip line to another microstrip line at the same time. For this reason, it becomes possible to further increase the amount of attenuation at the out-of-band frequency.

In this high-frequency low-pass filter, by adjusting the height of the spacer, the capacitance of the resonance capacitor equivalently constituted by the metal plate changes. For this reason, it becomes possible to remove a harmonic component of an arbitrary frequency.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of a high-frequency low-pass filter according to the present invention will be described below with reference to the accompanying drawings.

A high-frequency low-pass filter 1 (hereinafter, simply referred to as "low-pass filter 1") is attached to an output side of a radio power amplifier (not shown) for a mobile telephone, and is configured to transmit harmonic components of a transmission signal. The cutoff frequency is about 1.8 GHz, and the power that can be input is designed to have an average of 70 W and a peak of 700 W. The low-pass filter 1 has a high-order harmonic rejection ratio of 70 dB or more and a pass loss at a frequency within the band of 0.2 dB or less.

The low-pass filter 1 is shown in FIG.
As shown in (b), the shield case 2 is a hollow rectangular parallelepiped, and the semi-rigid cables 3 and N attached to the input portion and the output portion of the shield case 2, respectively.
Connector 4.

The shield case 2 includes a bottom case 2a, a top cover 2b, and side plates 2c and 2d each having a U-shaped cross section made of aluminum. A printed circuit board (dielectric substrate) 5 made of Teflon material is provided inside the shield case 2, and the printed circuit board 5 is fixed to the bottom case 2a by four screws 6, 6,. I have. A ground pattern is formed on the entire back surface of the printed circuit board 5, and the ground pattern and the bottom case 2a are formed.
Are kept close to each other by the close contact between the two. A microstrip line 7 is formed on the surface of the printed circuit board 5 (on the front side of the drawing in FIG. 2A). As shown in FIG. 2, the microstrip line 7 has an input part pattern 12 and an input part pattern 12 formed at both ends of a power transmission line pattern (power transmission line) 11 (portion indicated by a broken line) formed at the center thereof. Output unit pattern 13 and power transmission line pattern 1
1 and open stub part patterns 14, 15, 16, 17 formed in parallel with each other.

The input portion pattern 12 has a function of connecting the internal conductor (core wire) of the semi-rigid cable 3 to the power transmission line pattern 11 by soldering the output portion pattern 13. By soldering the other end of the conductor whose one end is soldered to the center contact of the connector 4, it has a function of connecting the center contact and the power transmission line pattern 11. FIG. 1 omits illustration of soldered solder.

The open stub part patterns 14 to 17 are
The pattern width (the width in the extending direction of the power transmission line pattern 11) is formed to be approximately λg / 8 (where “λg” indicates a shortened wavelength of the transmission signal on the printed circuit board 5). Therefore, the open stub portion patterns 14, 15, 16, 17 each function as a capacitor. On the other hand, open stub part pattern 1
The power transmission line patterns 11a, 11b, and 11c of the portions sandwiched by 4 to 17 respectively have pattern widths (widths in the length direction of the power transmission line pattern 11) of approximately λg / 8, and each of the coils has a coil width of λg / 8. Function as As a result, the microstrip line 7
A seven-element three-stage π-type low-pass filter circuit 20 (hereinafter simply referred to as “filter circuit 20”) as indicated by a solid line in FIG.
). In this case, the open stub portion patterns 14, 15, 16, 17 correspond to the capacitors 21, 22, 23, 24, respectively, and the power transmission line patterns 11a, 11b, 11c correspond to the coils 25, 26, 27, respectively. In this case, as shown in FIG. 4A, the filter circuit 20 has a maximum attenuation (for example, 75d) at the second harmonic frequency f2 of the in-band frequency.
A low-pass filter having B) is formed. In the filter circuit 20, the amount of attenuation is degraded at, for example, a four-fold higher harmonic frequency f4 due to the intrusion of higher harmonics.

On the other hand, as shown in FIG. 1 (a), a silver plating wire 8 of about 1.2φ is soldered to the central portion of the power transmission line pattern 11.

Further, the printed circuit board 5 is more firmly fixed to the bottom case 2a by spacers 34 made of brass. On the other hand, screws 36, 36,... Are provided on the spacers 34, 34,... So that three metal plates 31, 32, 33 face each other with a gap of about 5 mm from the board surface of the printed board 5. It is screwed by. These metal plates 31 to 33 are provided with spacers 34,
.. Are electrically connected to the ground pattern on the back surface of the printed circuit board 5 through the..., And prevent a harmonic component from entering the output portion pattern 13 from the internal conductor of the semi-rigid cable 3 or the microstrip line 7. In addition to functioning as a shield plate, it also functions equivalently as a capacitor. Specifically, the metal plate 31 prevents the harmonic component from jumping from the inner conductor of the semi-rigid cable 3, which is the portion where the power level of the harmonic component is the highest, to the output portion pattern 13 and the like, and matches the impedance. It also functions as a matching capacitor 40 (shown by a broken line in FIG. 3). In this case, the capacitor 40
Functions as an ideal capacitor having extremely small loss because it is formed by air.

The metal plates 32 and 33 function as a shield plate and a capacitor, respectively. In this case, these capacitors are formed by the surface area of the metal plates 32 and 33 and the spacer 3.
4, 34... Have a capacitance value corresponding to the height. In the present embodiment, these surface areas and heights are set so that these capacitors have capacitance values that resonate with the coils 25 to 27 at out-of-band frequencies such as the third to fifth harmonic frequencies. Designed. Specifically, the metal plate 3
2 functions as a capacitor 41 shown by a broken line in FIG. 3, and the capacitor 41 resonates with the coil 25 at the third harmonic frequency. On the other hand, the metal plate 33 functions as capacitors 42 to 45 indicated by broken lines in FIG. In this case, capacitors 42 and 43 are respectively connected to coil 2
Resonates with 6 and 27 at the 4th and 5th harmonic frequencies. The capacitor 44 resonates with the resonance circuit including the capacitor 42 and the coil 26, the resonance circuit including the capacitor 43 and the coil 27, and the entire circuit including the capacitors 22 to 24 to attenuate the out-of-band frequency. The capacitor 45 functions as an ideal capacitor for impedance matching, similarly to the capacitor 40.

Next, the filter characteristics of the low-pass filter 1 will be described. The low-pass filter 1 is shown in FIG.
As shown in the figure, the metal plates 32 and 33 function as a 7-pole π-type low-pass filter having resonance points at the third harmonic frequency f3, the fourth harmonic frequency f4 and the fifth harmonic frequency f5. I have. For this reason, the attenuation of the out-of-band frequency is extremely high in spite of the fact that the number of stages of the π-type low-pass filter is the same as compared with the filter characteristic in the case where the metal plates 32 and 33 are not provided (see FIG. It turns out that it is big.

When the attenuation at the out-of-band frequency is the same as that of the conventional low-pass filter, π
Since the number of connection stages of the low-pass filter can be reduced, the passage loss of the in-band signal can be reduced. Further, the out-of-band frequency components are not absorbed in the low-pass filter 1 and are blocked from passing therethrough, so that no heat is generated, and as a result, there is no influence on other components.

In the above-described embodiment, the seventh-order π-type low-pass filter has been described as an example. However, the present invention is not limited to this, and the present invention is applied to lower-order and higher-order π-type low-pass filters. be able to.

In this embodiment, the low-pass filter having a cut-off frequency of 1.8 GHz has been described as an example. However, the present invention is not limited to this, and the high-frequency low-pass filter according to the present invention has a cut-off frequency of about 1 GHz. -10 GHz
Can be applied to the low-pass filter.

Further, in this embodiment, three metal plates 3
Although an example using 1-33 has been described, the present invention is not limited to this, and the high-frequency low-pass filter according to the present invention uses one to two or four or more metal plates according to the configuration of the low-pass filter circuit. be able to.

[0024]

As described above, according to the high-frequency low-pass filter of the first aspect, since the inductor portion and the resonance capacitor equivalently constituted by the metal plate resonate at the out-of-band frequency, the out-of-band frequency , When the amount of attenuation is the same, compared with the conventional low-pass filter, π
It is possible to reduce the number of connection stages of the low-pass filter, and thereby reduce the passage loss of the high-frequency low-pass filter. In addition, since the harmonic components of the out-of-band frequencies are not absorbed in the high-frequency low-pass filter, heat generated due to the absorption of the harmonic components is eliminated.
It has no effect on other parts. Furthermore, since the metal plate prevents the harmonic component signal from jumping from the microstrip line to another microstrip line or the like, the attenuation at the out-of-band frequency can be further increased. Further, according to the high-frequency low-pass filter, it is possible to remove a harmonic component of an arbitrary frequency by adjusting the height of the spacer.

[Brief description of the drawings]

FIG. 1A is a plan view of a high-frequency low-pass filter according to an embodiment of the present invention when an upper cover is removed, and FIG. 1B is a cross-sectional view taken along line AA in FIG. .

FIG. 2 is a pattern diagram of a microstrip line in a high-frequency low-pass filter according to an embodiment of the present invention.

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

FIG. 4A is a frequency characteristic diagram showing an attenuation amount with respect to a frequency of a three-stage π-type low-pass filter circuit, and FIG. 4B is a frequency characteristic diagram of a high-frequency low-pass filter according to the embodiment of the present invention; .

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 high-frequency low-pass filter 5 printed board 7 microstrip line 8 silver-plated wire 11 power transmission line pattern 11 a to 11 c power transmission line pattern 14 to 17 open stub section pattern 20 three-stage π-type low-pass filter circuit 21 to 24 capacitor 25 to 27 Coil 31-33 Metal plate 34 Spacer 40-45 Capacitor

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-6-283910 (JP, A) JP-A-62-237801 (JP, A) JP-A-8-335804 (JP, A) JP-A 64-64 54901 (JP, A) Yoshihiro Konishi, "Microwave Integrated Circuits" (Showa 48, Industrial Report), pp. 22-23 (58) Fields investigated (Int. Cl. ⁷ , DB name) H01P 1 / 203 H01P 3/08

Claims (1)

(57) [Claims] 1. A dielectric substrate on which a plurality of π-type low-pass filter circuits including a capacitor section and an inductor section formed by a microstrip line are formed, and one or more (excluding all) of the plurality of The low-pass filter circuit is held on the dielectric substrate by a spacer while having a predetermined gap with respect to the dielectric substrate so as to cover the microstrip line constituting the inductor section in the low-pass filter circuit, and the dielectric substrate A high-frequency low-pass filter comprising: a ground pattern formed thereon; and at least one metal plate electrically connected to the ground pattern via the spacer.