JPS62101A - Branching filter circuit - Google Patents

Abstract

PURPOSE:To decide properly the length of a distributed constant line not affecting the mutual frequency pass band in the design stage in advance by selecting the length of the 1st and 2nd distributed constant lines to a value different from lambda/4 and the input impedance viewed from the input terminal as infinite at a prescribed frequency. CONSTITUTION:The input impedance Z10 of one transmission line 31 and a band-pass filter 12 should be sufficiently high in the frequency pass band of the other line 41 and a band-pass filter 22. Thus, the lengths L10, L20 of transmission lines 31, 41 are decided in the stage of design in a way that they differ from lambda/4 and the impedance of the transmission lines 31, 41 viewed from the input terminals 1-1, 1-2 is infinite taking the characteristic impedance of the band-pass filters 12, 22 into account. Thus, the effect of the band-pass filters 22, 12 in each frequency pass band is prevented simply and surely.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a demultiplexing circuit for separating or combining signals of different frequencies according to frequency in wireless communication or the like.

(Prior Art) Conventionally, as a technology in this field, there has been one as shown in FIG. 1, for example. The configuration will be explained below.

FIG. 2 is a block diagram showing a configuration example of a conventional branching circuit. In FIG. 0, 1-1.1-2 is a signal input terminal, and 2-1.2-2.3-1.3-2 is the signal output terminal.

A transmission line 11 having a length Ll and a bandpass filter I2 are connected between one signal input/output terminal 1-1.1-2.2-1.2-2, and a signal output terminal 2-1 A load resistor 13 is connected between .2 and 2. A transmission line 2 of length L2 is connected between the other signal input/output terminal 1-1゜1-2.3-1.3-2.
1 and the bandpass filter 22 are connected, and furthermore, the signal output terminal 3
A load resistor 23 is connected between -1° and 3-2. The bandpass filter 12.22 is composed of, for example, inductance and capacitance. Also, load resistance 13.23
is set, for example, to lΩ.

The transmission lines 11 and 21 are usually configured with distributed constant circuits. For example, the F matrix Fl (matrix) of one transmission line 11 is as shown in the following equation.

...(1) Here, the phase θ! , line length L1. The relationship among the signal wavelength λ, the signal frequency f, and the signal speed V is as follows: 01=β1·Ll β1=2π/input=v/f v=3×1010cm#ec.

Next, the F matrix F! of the bandpass filter 12 including the load resistor 13 will be described. 11 (matrix) as shown in the following equation.

Then, the transmission line! ! And the F matrix Fl (matrix) of the bandpass filter 12 is expressed as a linear expression.

F1= Therefore, the input impedance Zl of the transmission line 11 and the bandpass filter 12 is as shown in the following equation.

Here, bandpass filter 1 whose frequency passbands do not overlap
In the parallel connection configuration of 2.22, it is necessary to configure the configuration so that there is no influence of the other bandpass filter 12.22 in each frequency pass band.

Therefore, in the conventional branching circuit, one transmission line 11 and band filter 12 are connected to the other transmission line 21 and band filter 2.
2, the other transmission line 21
Since it is necessary that the input impedance of one transmission line 11 and the bandpass filter 12 be sufficiently high at the center frequency of the passband of the bandpass filter 22, the above formula (4
), θl=π/2 (i.e. L1=λ/4)
The phase θ1 was determined so that In other words, the input impedance of the bandpass filter 12 was assumed to be zero.

In the above formula (4), if θ1=π/2, then the formula (4
) is as follows.

As is clear from this equation (5), @ input terminal 1-1.
Input impedance Z on the transmission line 11 side as seen from 1-2
1 is the reciprocal of the impedance of the bandpass filter 12. As a result, the input impedance of the bandpass filter 12 becomes zero and the input impedance Z1 becomes infinite, and one transmission line 11 and bandpass filter 12 influence the other transmission line 21 and bandpass filter 22. will not be given.

(Problems to be Solved by the Invention) However, in the branching circuit with the above configuration, the transmission line 11
．． It is assumed that the length of 21 is set to L1=L2=π/2 and that the input impedance of bandpass filter 12.22 is zero, but in reality, the input impedance of bandpass filter 12 is zero. Therefore, since the input impedance Zl does not become infinite, the transmission line 11 and band filter 12 on one side and the transmission line 21 and band filter 22 on the other side influence each other, resulting in large characteristic deterioration. It was a factor. In order to prevent this, conventionally there was a problem in that the distance between the line lengths Ll and L2 had to be determined mainly through experiments.

The present invention provides a branching circuit that solves the problem of the prior art in that the lengths of the two transmission lines must be determined through experiments.

(Means and effects for solving the problem) In order to solve the problem, the present invention provides a first distributed constant line having different frequency passbands, a first bandpass filter, and a second distributed constant line. In a branching circuit in which a line and a second bandpass filter are connected in parallel, the lengths of the first and second distributed constant lines are
A value different from λ/4 (where input is the wavelength), and the first
and a second distributed constant line connected to the first and second distributed constant lines, respectively.
The input impedance including the bandpass filter is set to a value that becomes infinite at a predetermined frequency. As a result, the frequency pass bands of the first distributed constant line and first bandpass filter and the second distributed constant line and second bandpass filter are
They no longer influence each other, and the first and second
The length of the distributed constant line can be determined. Therefore, the above problem can be eliminated.

(Embodiment) FIG. 1 is a block diagram of a configuration of a branching circuit showing an embodiment of the present invention. Note that the same elements as those in FIG. 2 are given the same reference numerals.

The difference between this embodiment and the conventional one is that the lengths LIO and L20 of the transmission line 31.41 provided between the signal input terminal 1-1.1-2 and the bandpass filter 12.22 are The difference is that the means of deciding are different.

That is, the input impedance 210 of one transmission line 31 and bandpass filter 12 must have a sufficiently high impedance value in the frequency pass band of the other transmission line 41 and bandpass filter 22.

Now, since the bandpass filter 12 is in the attenuation range, the input impedance 210 is a pure imaginary number as shown in the following equation.

210 = ±ja ... (6) However, a
;210 imaginary part.

Here, Zlo = +ja, ZIG = -ja, 2
Consider two cases.

(1) In the case of 210 = +ja From the above equation (4), the input impedance 210 becomes as shown in the following equation.

In this equation (7), in contrast to the conventional 01=π/2,
If the length LIO of the transmission line 31 is appropriately calculated in consideration of the characteristic impedance of the bandpass filter 12 so that CQSθ1>O, the input impedance 210 can be made infinite, and thereby one of the transmission lines 31 and bandpass filter 1
2 to the other transmission line 41 and bandpass filter 22 can be prevented.

(2) When 210 = -ja From the above formula (0), the input impedance 210 becomes as shown in the following formula.

In this equation (8), C
If the length LIO of the transmission line 31 is appropriately determined in consideration of the characteristic impedance of the bandpass filter 12 so that aSO3<O, one transmission line 31 and The influence from the bandpass filter 12 on the other transmission line 41 and the bandpass filter 22 can be prevented.

According to this embodiment, each transmission line viewed from the input end 1-1. 31.
The lengths LIO and L20 of the transmission lines 31 and 41 can be determined in advance at the design stage so that the input impedance on the 41 side becomes infinite. This makes it possible to easily and accurately prevent the influence of other bandpass filters 22.12 in each frequency passband.

(Effects of the Invention) As described above in detail, according to the present invention, the lengths of the first and second distributed constant lines are different from λ/4, and the lengths of the first and second distributed constant lines are different from λ/4. Since the input impedance seen from the input end of the constant line is set to a value that becomes infinite at a given frequency, the length of the distributed constant line can be accurately determined in advance at the design stage so as not to affect the mutual frequency passband. .

[Brief explanation of drawings]

FIG. 1 is a block diagram of the structure of a branching circuit showing an embodiment of the present invention, and FIG. 2 is a block diagram of the structure of a conventional branching circuit. 12.22...Band filter, 31.41...
...Transmission line. Applicant's agent Yasushi Kakimoto/2,22:
Figure 1 Figure 2

Claims (1)

[Claims] A first distributed constant line with different frequency passbands and a first
In a branching circuit in which a bandpass filter, a second distributed constant line, and a second bandpass filter are connected in parallel, the lengths of the first and second distributed constant lines are expressed as λ/ 4 (where λ is the wavelength), and the first
and a second distributed constant line connected to the first and second distributed constant lines, respectively.
A branching circuit characterized in that each input impedance including a bandpass filter is set to a value that becomes infinite at a predetermined frequency.