JP2008172456A - High-frequency band-pass filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-frequency band-pass filter which is compact and has low loss and a steep skirt characteristic. <P>SOLUTION: The high-frequency band-pass filter includes: a high-frequency circuit board 3 having a ground layer formed on the reverse surface of a dielectric; a resonator 5 having a microstrip coupling line 7 having two microstrip lines 6 and 6 having a wavelength 1/2 time as long as a wavelength at a passing center frequency, disposed in parallel on the top surface of the dielectric of the high-frequency circuit board 3; and an input/output line 11 connected to side portions of the two microstrip lines 6 and 6 and inputting and outputting high frequencies, the two microstrip lines 6 and 6 being characterized in that the lengths from connection positions to both ends are in predetermined proportion based upon a wavelength at an attenuation frequency provided nearby the center frequency. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a band-pass filter that passes a high-frequency signal in a specific frequency band among frequency components of a high-frequency signal and attenuates other high-frequency signals, and more particularly, a high-frequency band-pass filter using a microstrip coupling line. About.

Conventionally, as the high-frequency bandpass filter, for example, as shown in FIG. 5, a plurality of λ / 2 microstrip line resonators 21 ₁ , 21 ₂ , 21 ₃ ,..., 21 _n−1 , 21 _n and input / output micros A multi-stage microstripline resonator filter in which striplines 22 and 23 are coupled with a length of λ / 4 is known. Here, λ is a wavelength, and input / output ports are connected to the input / output microstrip lines 22 and 23, respectively.

The multi-stage microstrip line resonator filter has microstrip line resonators 21 ₁ , 21 ₂ , 21 ₃ ,..., 21 _n−1 , 21 _n each having a low loss in the pass band, but a skirt in the stop band. The characteristics are gentle. For this reason, if the skirt characteristic is steep, the number of microstrip line resonators 21 ₁ , 21 ₂ , 21 ₃ ,..., 21 _n−1 , 21 _n further increases, and there is a problem that it is difficult to reduce the size.

As a high-frequency bandpass filter using a microstrip line, both ends of a λ / 2 microstrip line resonator 25 are electrically connected to the ground as shown in FIG. There has been proposed one in which an input / output line 26 is connected to an intermediate position by tap coupling (for example, see Non-Patent Document 1). Such a tap-coupled high-frequency bandpass filter can function as a stub from the connection position of the microstrip line 25 to both ends. FIG 6 (b) and (c), the frequency on the horizontal axis (unit GHz), takes a frequency characteristic (in dB) on the vertical axis, shows the pass characteristic _{S 21} and the reflection characteristic _{S 11} of the high frequency band pass filter.
IEICE Transactions, Vol. 89, No. 6, (2006), p. 372

However, the tap-coupled high-frequency bandpass filter has the electrical length l ₁ and the electromagnetic length l from the connection position of the microstrip line 25 to both ends, assuming that the electrical length of the microstrip line resonator 25 is l = l ₁ + l ₂ = π. If ₂ is a [pi / 2, passing characteristic S _21, as shown in FIG. 6 (b) it can not be a steep skirt characteristic. Further, when the electrical length l ₁ and the electrical length l ₂ are other than π / 2, as shown in FIG. 6C, the pass characteristic S ₂₁ generates an attenuation frequency only at a frequency higher than the pass center frequency. There is a problem that a steep skirt characteristic cannot be realized even with a small size and low loss.

  The present invention was created in view of the above-described problems of the prior art, and an object of the present invention is to provide a high-frequency bandpass filter having a small size, low loss, and steep skirt characteristics.

In order to solve the above problems, the present invention provides the following high-frequency bandpass filter.
That is, the invention described in claim 1 is a high-frequency circuit board in which a ground layer is formed on the lower surface of a dielectric, and two microstrip lines having a wavelength approximately half the pass center frequency on the upper surface of the dielectric of the high-frequency circuit board. Are provided with a resonator having a microstrip coupling line arranged in parallel, and an input / output line for inputting and outputting a high-frequency signal electrically connected to the sides of the two microstrip lines. The length from the connection position of the input output line to both ends is set to a predetermined ratio based on the wavelength of the attenuation frequency provided in the vicinity of the center frequency.

  With the above configuration, when a high frequency signal is input from the connection position of the input side microstrip line of the microstrip coupling line via the input line, the high frequency bandpass filter has a high frequency of the pass center frequency among the frequency components of the high frequency signal. The input-side microstrip line resonates with the signal, the output-side microstrip line is excited, and a high frequency with a passing center frequency is output to the output line. At this time, attenuation frequencies can be generated on the high frequency side and the low frequency side near the center frequency.

  According to a second aspect of the present invention, in the above, the microstrip coupling line provides a high-frequency bandpass filter in which both longitudinal ends of two microstrip lines are electrically connected to the ground layer. It is.

  According to the invention described in claim 3, in the above, the microstrip coupling line provides a high-frequency bandpass filter in which the interval between two microstrip lines is arbitrarily set. Thereby, the space | interval of the two microstrip lines which comprise a microstrip coupling line can be narrowed, the coupling degree of these microstrip lines can be raised, and it can be made low loss.

  According to a fourth aspect of the present invention, in the above, the microstrip coupling line provides a high-frequency bandpass filter in which an input / output line is tapped at an arbitrary position of two microstrip lines. Thereby, the attenuation frequency can be generated on the low frequency side and the high frequency side near the center frequency depending on the connection position of the input / output line to the two microstrip lines constituting the microstrip coupling line.

  According to a fifth aspect of the present invention, there is provided a high-frequency bandpass filter in which the high-frequency circuit board is a PTFE (polytetrafluoroethylene) substrate, a glass epoxy substrate, or an alumina substrate.

ただし、前記マイクロストリップ線路の入出力線路の接続位置から両端側への電気長は、偶モードでθ_ｅ１、θ_ｅ２、奇モードでθ_ｏ１、θ_ｏ２、マイクロストリップ線路の偶モードインピーダンスはＺ_ｅ、奇モードインピーダンスはＺ_ｏである。 According to a sixth aspect of the present invention, there is provided a high-frequency bandpass filter in which the pass center frequency is set by the following equation.

However, the electrical length from the input / output line connection position to both ends of the microstrip line is θ _e1 and θ _{e2 in} the even mode, θ _o1 and θ _o2 in the odd mode, and the even mode impedance of the microstrip line is Z _e. The odd mode impedance is Z _o .

  According to the seventh aspect of the present invention, there is provided a high-frequency bandpass filter in which the center frequency of the resonator is 5 GHz.

  According to the high-frequency bandpass filter of the present invention, a small-sized, low-loss, high-frequency bandpass filter having a steep skirt characteristic can be realized by the configuration and operation as described above.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The embodiment is not limited to the following.

  As shown in FIG. 1, the high-frequency bandpass filter of this embodiment has a box-shaped metal case 1, and a rectangular high-frequency circuit board 3 is fixed to the metal case 1 with a plurality of screws 4. Input / output terminals 2 and 2 are attached to the input / output terminal attachment holes on the side surface of the case 1.

As shown in FIG. 3, the high-frequency circuit board 3 has a ground layer 3b formed on the lower surface of the dielectric 3a. A plurality of through holes (not shown) are formed in the high-frequency circuit board 3.
In FIGS. 1 and 2, microstrip lines 6 and 6 on the input side and the output side having a half wavelength (electrical length of 180 ° at the pass center frequency) having a pass center frequency of 5 GHz are provided at substantially the center of the upper surface of the dielectric 3a. The microstrip lines 9 and 9 are electrically connected to both ends of the microstrip lines 6 and 6. A microstrip coupling line 7 is composed of the microstrip lines 6 and 6 on the input side and the output side.

  In the microstrip lines 9 and 9, a plurality of connection holes 8 are formed in the vicinity of the ends of the microstrip lines 6 and 6 on the input side and the output side, and these connection holes 8. And is electrically connected to the ground layer 3b.

  Input / output lines 11 and 11 for inputting and outputting a high frequency signal are electrically connected to both sides of the input side and output side microstrip lines 6 and 6. The input / output lines 11 and 11 formed of microstrip lines have one end formed in a step shape connected to the connection position of the input and output microstrip lines 6 and 6 by tap coupling, and the other end connected to the input / output terminals 2 and 2. 2 is electrically connected. The input / output lines 11 and 11 and the central conductors of the input / output terminals 2 and 2 are arranged in a straight line and are substantially orthogonal to the two microstrip lines 6 and 6. The lengths of the two microstrip lines 6 and 6 from the connection position of the input / output lines 11 and 11 to both side ends have a predetermined ratio based on the wavelength of the attenuation frequency provided near the center frequency.

  The high-frequency bandpass filter according to the present embodiment, when a high-frequency signal is input from the connection position of the input-side microstrip line 6 of the microstrip coupling line 7 via the input line 11, has a center frequency among the frequency components of the high-frequency signal. The input-side microstrip line 6 resonates with a high-frequency signal of 5 GHz, and the output-side microstrip line 6 is excited to output a high-frequency signal with a center frequency of 5 GHz to the output line 11. At this time, attenuation frequencies are respectively generated on the high frequency side and the low frequency side near the center frequency of 5 GHz.

  Therefore, the high frequency bandpass filter according to the present embodiment can make the microstrip lines 6 and 6 on the input side and the output side of the microstrip coupling line 7 at most approximately 1/2 wavelength of the pass center frequency. . Thereby, it can be made smaller than the conventional high-frequency bandpass filter.

  In the high-frequency bandpass filter according to the present embodiment, the distance between the microstrip lines 6 and 6 on the input side and the output side of the microstrip coupling line 7 is narrowed to increase the degree of coupling between the microstrip lines 6 and 6. And low loss. Therefore, the loss can be reduced as compared with the conventional high-frequency bandpass filter.

  Furthermore, the high-frequency bandpass filter according to the present embodiment has a low frequency band near the center frequency depending on the connection positions of the input / output lines 11 and 11 to the input and output microstrip lines 6 and 6 of the microstrip coupling line 7. Attenuation frequencies can be generated on the high side and the high side. Therefore, the high-frequency bandpass filter can obtain a steep skirt characteristic in the stop band.

  Next, the high-frequency bandpass filter of this embodiment will be specifically described. The electric field distribution at the pass frequency and the electric field distribution at the stop frequency of the high-frequency bandpass filter of the present embodiment were analyzed by electromagnetic field analysis. At the pass frequency, the input power appears at the output terminal with little loss. On the other hand, the input power hardly appears at the output terminal at the stop frequency.

なお、マイクロストリップ線路６，６の入出力線路１１，１１の接続位置から両端側への電気長は、偶モードでθ_ｅ１、θ_ｅ２、奇モードでθ_ｏ１、θ_ｏ２、偶モードインピーダンスはＺ_ｅ１、Ｚ_ｅ２、奇モードインピーダンスはＺ_ｏ１、Ｚ_ｏ２として、２つの結合線路が等しい場合、Ｚ_ｅ１＝Ｚ_ｅ２＝Ｚ_ｅ、Ｚ_ｏ１＝Ｚ_ｏ２＝Ｚ_ｏである。
高周波帯域通過フィルタの通過周波数では、Ｓ_１１＝０、Ｓ_２１＝１が成りたつので、下記の設計式が求まる。
Ｓ_１１＝０より

ここで、特性インピーダンスＺ_０＝５０Ωとする。 Next, a description will be given of circuit analysis of the high-frequency bandpass filter until a design formula is derived.
As shown in FIG. 1, the high-frequency bandpass filter of the present embodiment has the input / output line 11 attached from the middle of the coupled line. When S ₁₁ and S ₂₁ of the high-frequency bandpass filter of the present invention are obtained, the following equation is obtained.

The electrical length from the connection position of the input / output lines 11 and 11 of the microstrip lines 6 and 6 to both ends is θ _e1 and θ _{e2 in} the even mode, θ _o1 and θ _o2 in the odd mode, and the even mode impedance is Z _{e1, Z e2,} odd mode impedance as _{Z o1, Z o2,} if the two coupled lines are _equal, a _{Z e1 = Z e2 = Z e} , Z o1 = Z o2 = Z o.
Since S ₁₁ = 0 and S ₂₁ = 1 are satisfied at the pass frequency of the high-frequency bandpass filter, the following design equation is obtained.
From S ₁₁ = 0

Here, it is assumed that the characteristic impedance Z ₀ = 50Ω.

Next, a high-frequency bandpass filter is designed using a design formula, compared with actual measurement values, and the evaluation results are described.
The high-frequency circuit board 3 is a 47 × 80 mm rectangular PTFE substrate, which is subjected to electroless gold plating, has a gold plating thickness of 3 μm or more, a relative dielectric constant ε _r = 2.2, a dielectric loss tangent tan δ = 0.000, a dielectric The thickness b is 0.381 mm, the center frequency f ₀ is 5 GHz, and one wavelength λg is 54.2 mm.
The microstrip lines 6 and 6 are in a band shape having a width of 2 mm, the length from the connection position of the input / output line 11 to the side end is 19.2 mm and the length of 2.6 mm, and the gap g = 0 between the microstrip lines 6 and 6 .2 mm.
The microstrip lines 9, 9 are in the form of a strip having a width of 1.9 mm and a length of 12.8 mm, and 12 square holes 8... Having a side of 0.6 mm are formed.
The input / output lines 11 and 11 have a line width of 1.1 mm, one end is step-shaped and has a width of 0.2 mm, and the tip base is formed to have a width of 0.6 mm and a length of 0.5 mm.
From the above values, the odd-mode impedance Z _o = 28.2835Ω, even-mode impedance Z _e = 36.2298Ω, θ _e1 = 162.9 °, θ _o1 = 154.8 °, θ _e2 = Calculate 21.8 ° and θ _o2 = 20.64 °.

上記計算の結果、設計値が−１．０３３であったのに対して、理論値が−１で、相対誤差が３．３％であった。 Next, the validity of the design equation is verified by substituting the numerical value of the high-frequency bandpass filter produced in the design equation.

As a result of the calculation, the design value was −1.033, whereas the theoretical value was −1 and the relative error was 3.3%.

Next, an actual measurement result of the frequency characteristics of the high-frequency bandpass filter according to the present invention will be described with reference to FIG.
The horizontal axis represents frequency (unit: GHz), and the vertical axis represents frequency characteristics (unit: dB).
4, the reflection characteristic S ₁₁ is sharp attenuation characteristic is obtained in the low frequency side and high frequency side in the vicinity of the frequency 5 GHz, passing characteristic S ₂₁ is a low loss passband frequency 5 GHz, stopband And steep skirt characteristics are obtained.

  The preferred embodiments of the present invention have been described above, but the above-described embodiments do not limit the present invention. Those skilled in the art can make various modifications and changes in specific embodiments without departing from the technical idea and technical scope of the present invention. For example, in the above-described embodiment, both ends of the two microstrip lines 6 and 6 of the microstrip coupling line 7 are short-circuited. Instead, the two microstrip lines 6 and 6 of the microstrip coupling line 7 are short-circuited. Both ends of 6 may be opened. When both ends of the two microstrip lines 6 and 6 are grounded and open, only the line length is increased, and the frequency characteristics are not changed.

  The high-frequency bandpass filter of the present invention is small, has low loss, and has a steep skirt characteristic. Yes, it has high industrial utility value.

It is a figure which shows the structure of the high frequency band pass filter in one Embodiment of this invention. It is explanatory drawing to which the principal part of the high frequency band pass filter shown in FIG. 1 was expanded. It is sectional drawing of a high frequency circuit board and a microstrip line. It is a frequency characteristic figure by the high frequency band pass filter by the present invention. It is a figure explaining the conventional high frequency band pass filter. It is a figure explaining the conventional high frequency band pass filter.

Explanation of symbols

3 High Frequency Circuit Board 3a Dielectric 3b Ground Layer 5 Resonator 6 Microstrip Line 7 Microstrip Coupling Line 11 Input / Output Line

Claims (7)

A high-frequency circuit board having a ground layer formed on the lower surface of the dielectric;
A resonator having a microstrip coupling line in which two microstrip lines having approximately a half wavelength of a passing center frequency are arranged in parallel on the dielectric upper surface of the high-frequency circuit board;
An input / output line for inputting and outputting a high frequency, electrically connected to a side portion of the two microstrip lines,
The two microstrip lines have a predetermined ratio based on a wavelength of an attenuation frequency provided in the vicinity of the center frequency from the connection position of the input / output line to both side ends. 2. The high-frequency bandpass filter according to claim 1, wherein both ends of the microstrip coupling line in the longitudinal direction of two microstrip lines are electrically connected to the ground layer. The high frequency bandpass filter according to claim 1 or 2, wherein the microstrip coupling line has an arbitrary interval between two microstrip lines. 4. The high-frequency bandpass filter according to claim 1, wherein the microstrip coupling line has an input / output line tapped at an arbitrary position of two microstrip lines. 5.   5. The high-frequency bandpass filter according to claim 1, wherein the high-frequency circuit board is a PTFE substrate, a glass epoxy substrate, or an alumina substrate. The high-frequency bandpass filter according to any one of claims 1 to 5, wherein the pass center frequency is set by the following equation.

However, the electrical length from the input / output line connection position to both ends of the microstrip line is θ _e1 and θ _{e2 in} the even mode, θ _o1 and θ _o2 in the odd mode, and the even mode impedance of the microstrip line is Z _e. The odd mode impedance is Z _o .   The high-frequency bandpass filter according to claim 6, wherein a center frequency of the resonator is 5 GHz.

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