KR20100056146A - Low pass filter and method of designing thereof - Google Patents

Abstract

PURPOSE: Low pass filters and a design method thereof are provided to minimize insertion loss in pass band by forming several attenuation multi-poles in pole frequency. CONSTITUTION: A transmission line is arranged between an input terminal on a flat substrate(200) and an output terminal. An open stub is abeam connected to the transmission line. The open stub comprises a first line(L2) and a second line(L4) in which has the width wider than the first line. In order to eliminate the spurious inputted with input terminal, the open stub is selected as the number corresponding to the spurious. An inductance(201) value of the first line and a capacitance value of the second line are calculated.

Description

LOW PASS FILTER AND METHOD OF DESIGNING THEREOF}

The present invention relates to a low pass filter and a design method thereof, and more particularly, to a low pass filter and a design method capable of eliminating spurious that is a frequency component other than the frequency desired in a communication system. .

Filters are typically electronic systems It is one of the most used circuits throughout, and it is a device that can be implemented in various kinds and forms. This filter passes only the desired frequency among the various input frequency components and blocks the rest. Typical types of filters include a low pass filter, a band pass filter, a high pass filter, and a band reject filter.

Among them, the low pass filter is located at the front or the rear of the communication system and is an important component to improve the performance of the communication system by eliminating the unwanted wave generated by the amplifier or mixer. Much research has been done to improve the characteristics of the low pass filter to remove these unwanted waves. As an example of the study on the improvement of the characteristics of the low pass filter, a study is being conducted to make the band (stop band) to be cut off rather than the pass band. Hereinafter, a band to be blocked will be referred to as a stop band.

Meanwhile, the low pass filter is used in the frequency upconverter, and the low pass filter used in the frequency upconverter should be easy to manufacture and convenient to interface with other circuits, so it is mainly implemented in a planar type. do. However, such a low pass filter in the form of a flat plate has a problem of deterioration of the out-of-band rejection characteristic in the second or third harmonics (Harmonic) which is an unnecessary wave. Then, the background art related to the design of the filter having the improved rejection characteristic will be described below.

Korean Patent No. 10-0401124 discloses a high-temperature superconducting lowpass filter for removing broadband harmonics. The low pass filter has a stopband characteristic even at a broadband of 7 to 8 times the cutoff frequency, and can easily remove sub-harmonics inevitably generated in a wireless communication system. However, these low pass filters must use a special substrate called magnesium oxide (MgO) single crystal substrate and undergo a special and complicated process different from the general substrate fabrication method. Therefore, there is a disadvantage that the manufacturing cost of the low pass filter increases, and the harmonic component appears as it is at a frequency of 7 to 8 times or more.

Korean Patent No. 10-0449662 discloses a wideband harmonic rejection bandpass filter. The band pass filter is a resonator type band pass filter, and may improve harmonic rejection characteristics of the band pass filter. In particular, since the small gap between the resonator and the inner groove of the cover protrusion can make a low impedance, the height of the filter can be lowered, making it more compact. This filter is known to be the best filter feature, but the manufacturing cost of the filter increases because the manufacturing is complicated and the tuning process is required because it is manufactured in the form of a mechanism rather than a flat board type, and is tuned with a screw.

In the paper "A Design of the Harmonic Rejection Coupled Line Low-Pass Filter with Attenuation Poles, (Microwave Conference, 1999 Asia Pacific Microwave Conference, 30 Nov. 3 Dec. 1999 Page (s): 682-685 vol. 3)" A small size low pass filter was initiated using a coupled lind structure for removal. The filter shows 20dB of attenuation up to 5GHz. However, it is difficult to realize a sharp skirt characteristic when the pass band and the frequency component to be removed are close.

Accordingly, the present invention provides a low pass filter and a method of designing the same, which can selectively remove unwanted signals, which are unnecessary signal components.

In addition, the present invention provides a low pass filter and its design method which minimizes the insertion loss and maximum reflection loss in the pass band.

The filter according to the present invention is a low pass filter, which has a line width greater than or equal to a predetermined threshold between an input terminal and an output terminal on a flat substrate, and outputs an output signal from which unwanted waves included in the input signal are removed, and is perpendicular to the transmission line. And an open stub for forming an attenuation pole in the band of the unwanted wave to remove the unwanted wave, the open stub including a first line and a second line having a width wider than that of the first line. Is directly connected to the transmission line with a line width above the threshold.

The design method according to the present invention includes a transmission line having a line width greater than or equal to a predetermined threshold value between an input end and an output end on a flat substrate, and an open stub connected to the transmission line at a right angle, wherein the open stub includes a first line and a first line. A method of designing a low pass filter including a second wider line, comprising: a first selecting a number of open stubs by a number corresponding to the unnecessary waves to remove unwanted waves included in an input signal input to an input terminal; And a second process of calculating the inductance value of the first line satisfying the resonance frequency and the capacitance value of the second line by setting the intermediate frequency of the unnecessary wave band to the resonance frequency of the selected open stub for each unwanted wave; Calculates the inductance value and the prototype element value of the capacitance value, and calculates the And a third step of determining the inductance of the transmission line.

Use of the low pass filter of the present invention has the advantage of selectively eliminating unwanted waves that are unnecessary signal components.

In addition, there is an advantage in that insertion loss is minimized in the passband and reflection loss is maximized.

The above objects, features and advantages will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, whereby those skilled in the art may easily implement the technical idea of the present invention. There will be. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

First, in order to explain a method for designing a low pass filter according to an exemplary embodiment of the present invention, a general design method of an elliptic filter and an elliptic filter, which are general low pass filters, will be described first.

1 is an equivalent circuit diagram of an elliptic filter for explaining a method of designing a low pass filter according to a preferred embodiment of the present invention.

Referring to FIG. 1, an elliptic filter is composed of two resistors, a plurality of inductors and a plurality of capacitors. The two resistors represent the input impedance (Z1) 110 and the output impedance (Z0) 150, respectively, and the plurality of inductors represent a plurality of series inductors L ₁ , L ₃ ,..., L _{n + 1} connected in series. ) And a plurality of parallel inductors L ₂ , L ₄ ,..., L _n connected in parallel. Wherein a plurality of series inductor _{(L 1, L 3, ...} , L n + 1) a series inductor (L _{n + 1)} of the final stage of which is determined in accordance with the El-order (Order) of the elliptic filter. The plurality of capacitors C ₂ , C ₄ ,..., C _n are connected in series to the plurality of parallel inductors L ₂ , L ₄ ,..., L _n to form a plurality of resonators. The elliptic filter is suitable for the purpose of improving the skirt characteristics and the out-of-band rejection by forming a multi-pole at the pole frequency.

) 등에 의해 엘립틱 여파기의 차수(order)와 그에 따른 기본요소(Prototype element) 값(

또는

)을 계산한다. 그러면 계산된 차수와 기본요소 값에 의해

와

를 구한다.

와

는 아래의 <수학식 1>과 같은 방법으로 계산할 수 있다.The design method of the elliptic filter includes insertion loss, ripple, skirt and cut off frequencies (

) And the order of the elliptic filter and the corresponding prototype element values (

or

). Then by the calculated order and primitive value

Wow

.

Wow

Can be calculated in the same manner as in Equation 1 below.

와

값을 구한 후, 아래의 <수학식 2>를 이용하여 공진 값인 극(pole) 주파수(

)를 계산한다. 여기서, 극의 형성에 기여하는 차수는 모두 짝수(even)이다. 따라서 도 1에서 극의 형성에 기여하는 소자들은 다수의 병렬 인덕터들(L₂, L₄,…,L_n)과, 다수의 캐패시터들(C₂, C₄,…,C_n)이다.In <Equation 1>

Wow

After calculating the value, use the following Equation 2 to make the pole frequency (the resonance value)

Calculate Here, all orders that contribute to the formation of the pole are even. Accordingly, the elements contributing to the formation of the pole in FIG. 1 are a plurality of parallel inductors L ₂ , L ₄ ,..., L _n , and a plurality of capacitors C ₂ , C ₄ ,..., C _n .

)에서 감쇠 극이 형성되어서 엘립틱 여파기의 대역 외 제거 특성이 향상되는 것이다. 이와 같이, 일반적인 엘립틱 여파기의 설계 방법은 사용자가 원하는 극 주파수(

)의 선정은 불가능하다는 문제가 있다.Pole frequency determined by Equation (2)

), The attenuation pole is formed to improve the out-of-band rejection characteristics of the elliptic filter. As such, the general method of designing an elliptic filter is based on the polar frequency desired by the user.

There is a problem that selection of) is impossible.

)를 선택할 수 있는 본 발명의 바람직한 실시 예에 따른 저역 통과 여파기의 설계 방법을 설명한다.Therefore, in the following, the desired polar frequency (

A method of designing a low pass filter in accordance with a preferred embodiment of the present invention, in which?

)로 설정한다. 본 명세서 상에서, 불요파란 통신 시스템에서 목적으로 하는 주파수 이외의 주파수 성분을 말하며, 불요파에는 고조파(harmonic), 저조파(subharmonic), IMD(Intermodulation Distortion), 기생효과(parastic effect)에 의한 것들을 포함한다. 다음으로, 설정된 극 주파수(

)를 기초로 <수학식 2>를 이용하여

값을 계산한다. 그리고 계산된

값을 만족하는 많은 쌍의

와

를 계산한 후, 많은 쌍의

와

들 각각에 대한

와

를 <수학식 1>을 이용하여 계산한다. 그리고 많은 쌍의

와

값들과 직렬 인덕터들(L₁, L₃,…, L_n+1)의 값들을 최적화 작업을 통해 조절한다. 여기서, 최적화 작업은 일반적인 설계 소프트웨어들을 통해 가능하다. 일반적인 설계 소프트웨어에서의 최적화 작업은 각 변수의 값을 여러 번 바꿔 가면서 그 결과를 앞 단계의 결과 값과 비교하여 더 좋은 결과에 대해서는 메모리에 저장한다. 이러한 과정은 사용자가 원하는 최적의 결과가 나올 때 까지 반복하여 수행함으로써, 최적의 값을 얻을 수 있다.In the method for designing a low pass filter according to an exemplary embodiment of the present invention, first, an intermediate value of a spurious band to be removed by a user is determined by using a pole frequency (

Set to). In the present specification, the unwanted wave refers to a frequency component other than the frequency desired in the communication system, and the unwanted wave includes those due to harmonics, subharmonic, intermodulation distortion, and parasitic effects. do. Next, set pole frequency (

) Using <Equation 2>

Calculate the value. And calculated

Many pairs of values

Wow

After calculating, many pairs

Wow

For each of them

Wow

Is calculated using Equation 1. And many pairs

Wow

The values and the values of the series inductors L ₁ , L ₃ ,..., L _{n + 1} are adjusted through optimization. Here, optimization work is possible through general design software. The optimization work in typical design software changes the value of each variable several times and compares the result with that of the previous step and stores the result in memory for better results. This process may be repeated until the user obtains an optimal result, thereby obtaining an optimal value.

또는

)을 사용하는 일반적인 저역 통과 여파기의 설계 방법과는 다르기 때문에, 완전한 저역 통과 여파기의 특성을 가지지는 못하지만 제 한된 통과 대역에서 삽입 손실을 최소화할 수 있고, 반사 손실(Return Loss)을 최대화할 수 있다.As such, the low pass filter design method according to the preferred embodiment of the present invention is a basic element value (

or

It is different from the general low pass filter design method using), but it does not have the characteristics of a full low pass filter, but it can minimize the insertion loss in the limited pass band and maximize the return loss. .

By using the low pass filter design method according to the preferred embodiment of the present invention, the low pass filter used in the frequency up-converter of the Ka band OBS satellite repeater was designed. Here, the ka band refers to the 26.5 ~ 40GHz band, the ka band satellite repeater is a repeater that receives the 30GHz signal from the uplink, and converts the received signal into a 20GHz signal to transmit the downlink.

Table 1 below shows the input RF frequency, output IF frequency, local oscillation frequency, and unwanted wave frequency of Ka upband OBS satellite repeater. have.

Parameter frequency Input RF Frequency (GHz) 19.8-20.2 Output IF Frequency (GHz) 3.2 to 3.6 LO frequency (GHz) 16.6 Unwanted frequency to remove (GHz) 2IF 6.4 to 7.2 3IF 9.6 to 10.8 1LO 16.6

)는 <표 1>에서 불요파의 중심 주파수로 설정하였다. In order to selectively remove the unwanted components (2IF, 3IF, 1LO) shown in Table 1, a filter design method according to a preferred embodiment of the present invention was used. As a result, as shown in FIG. 2, three open stubs were required to remove three unwanted waves, and 3 to satisfy the characteristics of skirt, flatness, bandwidth, etc. of the low pass filter. More open stubs were needed. And the width and length of all the lines were determined through the optimization work using the design software described above. And pole frequency (

) Is set as the center frequency of the unwanted wave in <Table 1>.

2 is an actual layout diagram of a low pass filter used in a ka band OBS satellite repeater using a filter design method according to an exemplary embodiment of the present invention. The actual size of the designed layout is 7 x 5 mm ² .

Referring to Figure 2, the low pass filter used in the ka band OBS satellite repeater has six open stubs. Thus, the order is sixth order, and n is 12 in the elliptic filter of FIG.

In FIG. 2, the low pass filter used in the ka band OBS satellite repeater includes a transmission line and an open stub having a line width above a predetermined threshold between an input terminal (not shown) and an output terminal (not shown) on the substrate 200. 208, 209, ..., 213).

The substrate 200 has an input terminal and an output terminal. And it is preferable to be in the form of a plate in order to design in a simple etching process.

The transmission line is disposed between the input terminal and the output terminal, and outputs an output signal from which unwanted waves included in the input signal input to the input terminal are removed. This transmission line means a plurality of inductance lines 201, 202, ..., 207 having a predetermined line width and length. These series inductance lines 201, 202, ..., 207 are equivalent to the plurality of series inductors L ₁ , L ₃ , ..., L _{n + 1} shown in FIG. ₁ . The narrower the line width of the plurality of inductance lines 201, 202, ..., 207, the narrower it is, but has a threshold of 0.1 mm. The length of the plurality of inductance lines 201, 202, ..., 207 is determined by the respective inductance value. Determination of the inductance value is determined through the optimization operation described above.

The open stubs 208, 209,..., 213 are respectively connected in parallel with the transmission line and disposed on the substrate 200, and are arranged in the up-down direction with respect to the transmission line. The open stubs arranged in the downward direction of the transmission line are the same as the open stubs 208, 209, ..., 213 arranged in the upper direction of the transmission line, which increase the area of the substrate or the influence of the adjacent pattern or the value of the capacitor. It is for. Accordingly, the open stub disposed in the opposite direction to the first open stub 208 with respect to the first open stub 208 and the transmission line is equivalent to the second inductor 122 and the second capacitor 142 in FIG. 1. to be. In addition, an open stub disposed in a direction opposite to the second open stub 209 based on the second open stub 209 and the transmission line is equivalent to the fourth inductor 124 and the fourth capacitor 144 in FIG. 1. to be. In addition, the open stub disposed in the opposite direction to the third open stub 210 with respect to the third open stub 210 and the transmission line may include a sixth inductor (not shown) and a sixth capacitor (not shown) in FIG. 1. Is equivalent to In addition, the open stub disposed in the opposite direction to the fourth open stub 211 with respect to the fourth open stub 211 and the transmission line may include an eighth inductor (not shown) and an eighth capacitor (not shown) in FIG. 1. Is equivalent to In addition, the open stub disposed in the opposite direction to the fifth open stub 210 with respect to the fifth open stub 212 and the transmission line may include a tenth inductor (not shown) and a tenth capacitor (not shown) in FIG. 1. Is equivalent to In addition, an open stub disposed in a direction opposite to the sixth open stub 213 based on the sixth open stub 213 and the transmission line is equivalent to the twelfth inductor 128 and the twelfth capacitor 146 in FIG. 1. to be.

Each of the open stubs 208, 209,..., 213 is configured by a first line having a line width greater than or equal to a threshold and a second line having a line width thicker than that of the first line. The first lines L2, L4, L6, L8, L10, L12 are equivalent to the parallel inductors 122, 124,... 126 in FIG. 1. The narrower the line width of the first line is, the more preferable it is, but has a threshold of at least 0.1 mm. The length of the first line is determined according to the inductance value according to the above-described optimization work. Each of the open stubs 208, 209, ..., 213 becomes a second line except for the first line, and this second line is connected to the parallel capacitors 142, 144, ..., 146 in FIG. Equivalent The second line also has a predetermined line width and length, but the line width is mainly determined by the value of the capacitance.

The resonant frequencies of some of the open stubs among the open stubs 208, 209,..., 213 are included in the frequency band of the unwanted wave which the user wants to eliminate in advance. Figure 2 is a lowpass filter used in the frequency upconverter for the ka band OBS satellite repeater, so the resonant frequency of three open stubs out of six open stubs (208, 209, ..., 213) is the second order (6.4 to 7.2GHz), 3rd (9.6-10.8GHz) band and 1st local oscillation frequency (16.6GHz) band. The remaining three open stubs are added considering the skirt characteristics, flatness, and bandwidth desired by the user.

Then, reference is made to the graph of FIG. 3 to see if the low pass filter shown in FIG. 2 removes the unwanted wave generated in the frequency upconverter for the ka band OBS satellite repeater.

Figure 3 measures the characteristics of the low pass filter used in the ka-band OBS satellite repeater designed by the filter design method according to an embodiment of the present invention. The horizontal axis represents frequency (Frequency), the vertical axis represents the insertion loss (Insertion Loss) and reflection loss (Return Loss) in dB.

Reference numeral 301 denotes the frequency of the unwanted wave to be removed, and represents harmonics and local oscillation frequency components twice and three times the IF (Intermediate Frequency), which is an unwanted wave generated in the frequency upconverter for the ka band OBS satellite repeater. Reference numeral 302 denotes an insertion loss measurement result, and reference numeral 303 denotes a reflection loss measurement result.

Referring to the graph of FIG. 3, the low pass filter of FIG. 2 exhibits a characteristic of deterioration at a frequency lower than a pass band in the reflection loss 303, and thus does not show a perfect low pass filter. However In the range of 3.2 to 3.6GHz, the passband shows perfect passband characteristics.

Reference numeral 304 is a frequency band including 6.4 to 7.2 GHz and 9.6 to 10.8 GHz, which are 2IF and 3IF, and exhibits high out-of-band rejection characteristics of more than 50 dB. Reference numeral 305 denotes a local oscillation frequency of 16.6 GHz, which shows a high out-of-band rejection characteristic of 40 dB or more. Reference numerals 306, 307, 308, 309 and the like show the positions of the poles optimized by the filter design method according to the preferred embodiment of the present invention, indicating that all the poles are generated near the frequency band to be removed. Can be. In addition, the position of the reference numeral 306 is located very close to the pass band showing a sharp skirt characteristics.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. The present invention is not limited to the drawings.

1 is an equivalent circuit diagram of an elliptic filter for explaining a method of designing a low pass filter according to a preferred embodiment of the present invention.

2 is an actual layout diagram of a low pass filter used in a ka band OBS satellite repeater using a filter design method according to an exemplary embodiment of the present invention.

Figure 3 measures the characteristics of the low pass filter used in the ka-band OBS satellite repeater designed by the filter design method according to an embodiment of the present invention.

Claims (5)

A transmission line having a line width greater than or equal to a predetermined threshold between the input end and the output end of the flat substrate, and an open stub connected to the transmission line at a right angle, wherein the open stub has a width wider than that of the first line and the first line. In the design method of a low pass filter comprising a second line, A first step of selecting the number of the open stubs by the number corresponding to the unnecessary waves to remove the unnecessary waves included in the input signal input to the input terminal; A second process of calculating the inductance value of the first line and the capacitance value of the second line satisfying the resonance frequency by setting the intermediate frequency of the unwanted band for each of the unwanted waves as the resonance frequency of the selected open stub; , And calculating a basic element value of the calculated inductance value and the capacitance value, and determining the calculated basic element value and the inductance value of the transmission line. The method of claim 1, wherein the first process comprises: And selecting the number of the open stubs by the number of the unwanted waves to be removed by the user among the unnecessary waves. In the low pass filter, A transmission line having a line width greater than or equal to a predetermined threshold value between an input terminal and an output terminal on the flat substrate, and outputting an output signal from which unwanted waves included in the input signal are removed; An open stub connected to the transmission line at a right angle and forming an attenuation pole in the band of the unwanted wave to remove the unwanted wave; The open stub comprises a first line and a second line having a width wider than the first line, the first line having a line width above the threshold and directly connected to the transmission line. The method of claim 3, wherein the number of open stubs is Low pass filter, determined by the number of clutter you want to remove. 4. The low pass filter of claim 3, wherein the threshold is 0.1 mm.

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