JPS633210Y2 - - Google Patents

Description

[Detailed Description of the Invention] This invention relates to a low-pass filter, and particularly to a low-pass filter using a distributed constant line.

An LPF (low pass filter) as shown in FIG. 1 is used in the transmitting stage of a communication device for ultra-high frequency bands above the UHF band. In the figure, distributed constant lines 1 to 1 using microstrip lines etc.
3 and capacitive elements 4 to 7 provided between each terminal of these distributed constant lines 1 to 3 and a reference potential point (ground point), respectively. this
The LPF can be viewed as a configuration in which three third-order π-type filters each consisting of a distributed constant line and a pair of capacitive elements provided between both ends of the line and ground are connected in cascade. In this case, capacitive elements 5 and 6 will be shown as parallel equivalent capacitances of the capacitive elements of each π-type filter.

Distributed constant lines 1 to 3 in this circuit are generally used microstrip lines, whose characteristic impedance is Zo, propagation constant is β, transmission line wavelength is λg, and line length is l.
These lines 1 to 3 operate as an inductance L around the cutoff frequency c, and the following relational expression holds true.

2πcL=Zotanβl …(1) Now, when designing the seventh-order Butterworth characteristic with input and output impedance of Ro (Ω), the impedance ratio of each element 4, 1, 5, 2, 6, 3, 7 Since is 1:2:2:2:2:2:1, each constant is as follows.

C ₄ = C ₇ = 1/ωcR ₀ …(2) C ₅ = C ₆ = 2/ωcR ₀ …(3) l=λg/2πtan ^-1 (2R ₀ /Zo) …(4) Here, C ₄ , C ₅ , C ₆ and C ₇ are each capacitive element 4,
The capacitance values of 5, 6 and 7 are shown, and ωc=2πc.

The frequency vs. attenuation characteristics of such an LPF are shown in Figure 2, which shows that it operates as an ideal LPF around the cutoff frequency c, but at higher frequencies ₁ , ₂ ,...
It is known that at frequencies 1, 2, . . . , it operates as a capacitor, and zero points occur in the attenuation characteristics as a function of frequency, resulting in a pass band at frequencies ₁ , ₂ , and so on.

Since the conventional LPF has the above-mentioned characteristics,
If the harmonics of the transmitter match frequencies such as ₁ , ₂ , etc., it will no longer function as an LPF. Therefore, in view of the fact that the frequency characteristics of a single transmitter decrease as the frequency increases, a method may be considered in which frequency ₁ is shifted to a higher region. Generally, this frequency ₁ can be increased by increasing the ratio Zo/R ₀ between the characteristic impedance Zo and the input/output impedance R ₀ or by increasing the order of the filter.

However, as communication equipment has become smaller and more modular in recent years, the size of the circuit board has become limited, and the order and
It is becoming difficult to increase Zo/R ₀ . Characteristic impedance of microstrip line Z ₀
is determined by the dielectric constant εr of the dielectric, the thickness h of the dielectric substrate, the conductor width W, etc., but when miniaturizing and modularizing, a substrate with εr = around 10 is generally selected, so for example Zo = The substrate width W, which is 50Ω, has a value approximately equal to the substrate thickness h. Since h is generally around 1 mm and the minimum value of W is 2 to 3 mm,
When the maximum value of Zo is limited and R ₀ = 50Ω, Zo/
R ₀ will be about 1.5 to 2, and in the worst case it will be around 1. As described above, when modularization and miniaturization are adopted, there is a drawback that it is difficult to increase ₁ using conventional methods.

The present invention was developed to eliminate the above-mentioned drawbacks of the conventional technology, and its purpose is to compensate for the pass band that appears in the pass blocking area of the filter by eliminating it as much as possible using an extremely simple configuration. An object of the present invention is to provide an LPF whose characteristics are improved by using the same method.

The LPF according to the present invention is an LPF in which a plurality of filters having a π-type configuration each consisting of a distributed constant line and a capacitive element provided between both ends of the distributed constant line and a reference potential point are connected in series. The present invention is characterized in that at least a portion of the distributed constant line is a lumped constant inductor to improve the characteristics of the pass blocking region of the filter.

The present invention will be explained below using the drawings.

FIG. 3 is a circuit diagram of an embodiment of the present invention.
Parts equivalent to those in the figures are designated by the same reference numerals. 7
A part of the microstrip line 3, which is a distributed constant line in the next LPF, is connected to a lumped constant inductor 8.
The other configuration is the same as that of FIG. 1. In this configuration, if an LPF with seventh-order Butterworth characteristics is designed, each constant will be the same as equations (2) to (4), but the inductance L ₈ of the inductor 8 will be L ₈ = 2R ₀ /Wo- Zotan (2πl ₃ /λg) …(5) is given as: Here, l ₃ indicates the length of the microstrip line 3 portion.

The frequency vs. attenuation characteristics of this circuit are shown in Figure 6, and due to the presence of the lumped constant 8, the frequency ₁ ,
₂ , . . . etc. are increased compared to the conventional example shown in FIG. 1, and the characteristics can be improved.

Microstrip line has each frequency ₁ , ₂ ,
In order to show capacitance in ..., etc., the circuit of Fig. 1 has a zero point at these frequencies as shown in Fig. 2, but by making at least a part of the microstrip line a lumped constant inductor, this can be achieved. It is possible to compensate for the zero point.
The smaller l ₃ in equation ( ₅ ), the greater the attenuation at ₁ , ₂ , .

5 and 6 are diagrams showing specific configurations of the microstrip line 3 and the lumped constant inductor 8. In FIG. 5, 20 is a dielectric, 21
is the lower conductor, 22 is the upper conductor, and the dielectric 20
Cut off a part of it and connect it with a conductor 8,
This is made to operate as a lumped constant inductor. In FIG. 6, dielectric 2
A portion of the conductor above 0 is formed into a loop, and this loop portion is operated as a lumped constant inductor 8, and 23 is a bypass capacitor.

In the above embodiment, the lumped constant inductor 8 is inserted into a part of the third stage microstripline 3, but other microstriplines 1 and 2 are inserted.
It may be inserted into a plurality of microstrip lines, or the same effect will be obtained even if it is inserted into a plurality of microstrip lines. Moreover, it is not limited to the 7th order LPF.

As mentioned above, according to the present invention, a lumped constant inductor is inserted in series in a part of the distributed constant line, so it can have an extremely simple configuration.
This makes it possible to easily improve the characteristics of the LPF's pass blocking region. Characteristic impedance
Even if the ratio between Zo and the input/output impedance R ₀ is small, the amount of attenuation in the passband outside the band can be increased, allowing for miniaturization and modularization.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of a conventional LPF, and Figure 2 is a circuit diagram of a conventional LPF.
3 is a circuit diagram of an embodiment of the present invention, FIG. 4 is a characteristic diagram of the circuit in FIG. 3, and FIGS. 5 and 6 are examples of the configuration of a lumped constant inductor. It is a diagram. Explanation of the symbols of the main parts: 1-3...microstrip line, 4-7...capacitive element, 8... lumped constant inductor.

Claims (1)

[Claims for Utility Model Registration] (1) Multiple π-type filters connected in cascade, each consisting of a distributed constant line and a capacitive element provided between both ends of the distributed constant line and a reference potential point. 1. A low-pass filter comprising: a low-pass filter, characterized in that at least a portion of the distributed constant line is an inductor to improve characteristics of a pass-blocking region of the filter. (2) The low-pass filter according to claim 1, wherein the distributed constant line is a microstrip line.