Patent Application
20160064791
This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2014-178328, filed Sep. 2, 2014, the entire contents of which are incorporated herein by reference.
Embodiments described herein relate to an impedance converter in which a plurality of characteristic impedance elements, each of which has an electric length corresponding to a particular frequency, are disposed.
For high-frequency circuits, an impedance converter is used to perform impedance matching and reduce attenuation of a high-frequency signal. This impedance converter needs to maintain frequency characteristics of the fundamental frequency f0 of the high-frequency signal.
When a high-frequency signal of the fundamental frequency f0 is subjected to impedance conversion at the impedance converter, harmonic signals of odd frequency multiples of the fundamental frequency f0, such as threefold, fivefold, and sevenfold frequencies 3·f0, 5·f0, and 7·f0, are also subjected to impedance conversion, and are allowed to pass through the impedance converter. If the harmonic signals of odd frequency multiples pass through the impedance converter, the harmonic signals influence the high-frequency signal of the fundamental frequency f0 and, for example, distort the high-frequency signal of the fundamental frequency f0.
Hereinafter, an impedance converter according to the present embodiment will be described in detail with reference to the drawings. In the following embodiment, the elements which perform the same operations will be assigned the same reference numerals, and redundant explanations will be omitted.
An impedance converter is required to maintain frequency characteristics of a high-frequency signal of the fundamental frequency f0, and to reflect and attenuate harmonic signals of odd frequency multiples of the fundamental frequency f0.
According to one embodiment, an impedance converter includes a plurality of disposed characteristic impedance elements and at least one stub. The disposed characteristic impedance elements each has an electric length corresponding to a particular frequency. The at least one stub is formed on a characteristic impedance element formed on a signal input side among the plurality of characteristic impedance elements, and has an impedance value which suppresses passage of a signal having a predetermined multiple of a fundamental frequency.
Hereinafter, an embodiment will be described with reference to the drawings.
The impedance converter 1 has an electric length corresponding to a particular frequency as shown in
Through the impedance converter 1, for example, a high-frequency signal in an ultra high frequency (UHF) band passes as a high-frequency signal. The high-frequency signal is input to characteristic impedance element 101, passes through characteristic impedance elements 102 and 103, and is output from characteristic impedance element 104. Accordingly, characteristic impedance element 101 is the signal input side, and characteristic impedance element 104 is the signal output side.
Impedance values Z1, Z2, Z3, and Z4 of the characteristic impedance elements of the transmission line have the following magnitude relationship:
Z
1
≦Z
2
≦Z
3
≦Z
4 (1)
ZA, ZB, ZC, ZD, and ZE represent impedance values at certain points in the impedance converter 1. Impedance value ZA represents an impedance value on a low impedance side of characteristic impedance element 101. Impedance value ZB represents an impedance value on a high impedance side of characteristic impedance element 101. Similarly, impedance value ZC represents an impedance value on a low impedance side of characteristic impedance element 102, and impedance value ZD represents an impedance value on a high impedance side of characteristic impedance element 103. Impedance value ZE represents an impedance value on a high impedance side of characteristic impedance element 104.
L1, L2, L3, and L4 represent line lengths of the characteristic impedance elements 101, 102, 103, and 104, respectively. L1 is a line length of characteristic impedance element 101, L2 is a line length of characteristic impedance element 102, L3 is a line length of characteristic impedance element 103, and L4 is a line length of characteristic impedance element 104. The line lengths L1, L2, L3, and L4 of the characteristic impedance elements 101, 102, 103, and 104 are nearly equal to a quarter wavelength (A/4) of fundamental frequency f0.
L
1
,L
2
,L
3
,L
4
L≈λ/4 at f0 (2)
A specific example of the impedance conversion of the impedance converter 1 will be described. When the impedance converter 1 is ideal, impedance conversion from, for example, an input impedance value to an output impedance value (50Ω) is performed. Actually, the impedance converter 1 converts an impedance value 2.08Ω into 48.5Ω.
Specifically, at the first-stage characteristic impedance element 101, impedance value ZA(=approximately 2.08Ω) on the input side is converted into impedance value ZB(4.22Ω) on the output side.
At the second-stage characteristic impedance element 102, impedance value ZB(4.22Ω) on the input side is converted into impedance value ZC(12.9Ω) on the output side.
At the third-stage characteristic impedance element 103, impedance value ZC(12.9Ω) on the input side is converted into impedance value ZD(34.1Ω) on the output side.
At the fourth-stage characteristic impedance element 104, impedance value ZD(34.1Ω) on the input side is converted into impedance value ZE(48.5Ω) on the output side.
The impedance converted values of the characteristic impedance elements 101, 102, 103, and 104 of respective stages are mere examples, and may be other impedance values.
In the impedance converter 1, a plurality of stubs, for example, two stubs including a first stub S1 and a second stub S2, are formed on characteristic impedance element 101 disposed on the signal input side among the plurality of characteristic impedance elements 101, 102, 103, and 104.
The first and second stubs S1 and S2 have impedance values Z5 and Z6 which suppress passage of a high-frequency signal having a predetermined frequency multiple of the fundamental frequency f0, such as a threefold frequency 3·f0, which is an odd frequency multiple.
The points where the stubs S1 and S2 are formed, i.e., points on the transmission line in which the characteristic impedance elements 101-104 are connected in series, are points where the impedance value is 4Ω or smaller on the transmission line. This impedance value (4Ω or smaller) is an impedance value for maintaining the characteristics of the high-frequency signal of the fundamental frequency f0.
Specifically, the first stub S1 is formed at an end portion Z1a on the signal input side (low impedance Za side) of the first-stage characteristic impedance element 101, for example, at a point where the impedance value is 2.08Ω. The stub S1 is formed to partly overlap characteristic impedance element 101 at the end portion Z1a on the signal input side, as shown in
The second stub S2 is formed at a point where the impedance value is 4Ω or smaller between characteristic impedance elements 101 and 102 including an end portion Z1b on the signal output side (high impedance ZB side) of the first-stage characteristic impedance element 101. Since the impedance value ZB on the output side of the first-stage characteristic impedance element 101 is 4.22Ω as described above, the second stub S2 is formed at, for example, an end portion Z1b of characteristic impedance element 101 as a point where the impedance value is 4Ω or smaller. Like the first stub S1, the second stub S2 is formed to partly overlap characteristic impedance element 101 at the end portion Z1b on the signal input side, as shown in
The second stub S2 is not necessarily on the high impedance side (ZB side) of characteristic impedance element 101, and may be at any point where the impedance value is 4Ω or less, for example, on a transmission line between characteristic impedance elements 101 and 102 or on characteristic impedance element 101 or 102 where the impedance value is 4Ω or less.
The characteristic impedances Z5 and Z6 of the stubs S1 and S2 are five or more times larger than the impedance value of characteristic impedance element 101 on which the stubs S1 and S2 are formed. Based on expression (1), the impedance values of the stubs S1 and S2 and those of the characteristic impedance elements 101, 102, 103, and 104 have the following magnitude relationship:
Z
1
≦Z
2
≦Z
3
≦Z
4
≦Z
5
≦Z
6 (3)
The line lengths L5 and L6 of the stubs S1 and S2 are each set based on a high-frequency signal having a threefold frequency 3·f0 of the fundamental frequency f0 and a particular frequency, such as frequency 3·f0. The line lengths L5 and L6 of the stubs S1 and S2 can be obtained based on the following expression:
L
5
,L
6≈λ/4 at 3·f0 (4)
Line length L6 of the stub S2 is longer than line length L5 of the stub S1. Namely, the line lengths L5 and L6 of the stubs S1 and S2 have the following relationship:
L
5
≦L
6
The characteristic impedances Z5 and Z6 of the lines of the stubs S1 and S2 that suppress passage of a high-frequency signal at a threefold frequency 3·f0 of the fundamental frequency f0 are determined by a reflection coefficient Γ that satisfies the following expressions (5) and (6) at the fundamental frequency:
Γ=(Zc-3rd−Zf0)/Zc-3rd+Zf0) (5)
Γ≧0.67 (6)
, where Zc-3rd is an impedance value of the stubs S1 and S2 at Z1a and Z1b, and Zf0 is an impedance value ZA and ZB of the fundamental wave.
The stubs S1 and S2 may have lengths L5 and L6 corresponding to different predetermined frequency multiples, such as threefold and fivefold frequencies.
The first stub S1 is formed on the low impedance side (ZA side) of characteristic impedance element 101, and has line length L5 which suppresses passage of a high-frequency signal having a threefold frequency 3·f0 of the fundamental frequency.
The second stub S2 is formed on the high impedance side (ZB side) of characteristic impedance element 101, and has line length L6 which suppresses passage of a low-frequency signal included in the high-frequency signal having a threefold frequency 3·f0 of the fundamental frequency.
When the impedance converter 1 is used for impedance conversion at, for example, a high-frequency amplifier circuit using a wide band Doherty amplifier, the impedance converter 1 of the wide band Doherty amplifier is configured on the assumption that two substrates 10 and 11 are used as shown in, for example,
Of the substrates 10 and 11, one substrate 10 constitutes characteristic impedance elements 101 and 102 on the low impedance side. On the substrate 10, stripline 12 of characteristic impedance elements 101 and 102 is formed. Stripline 12 is made of, for example, copper.
The other substrate 11 constitutes characteristic impedance elements 103 and 104 on the high impedance side. On substrate 11, stripline 13 of characteristic impedance elements 103 and 104 is formed. Stripline 13 is made of, for example, copper foil.
The dielectric constant of substrate 10 is higher than that of substrate 11. Substrate 10 is thicker than substrate 11.
Since the impedance converter 1 is provided with the first stub S1 formed on the lower impedance side (ZA side) of characteristic impedance element 101, and having line length L5 which suppresses passage of a high-frequency signal having a threefold frequency 3·f0 of the fundamental frequency and the second stub S2 formed on the high impedance side (ZB side) of characteristic impedance element 101, and having line length L5 which suppresses passage of a low-frequency signal included in the high-frequency signal having a threefold frequency 3·f0 of the fundamental frequency, the impedance converter 1 has gain-frequency characteristics shown in
In contrast, in the frequency band K3 that includes a threefold frequency 3·f0 of the fundamental frequency f0, the reflection characteristics Q are higher and the transmission characteristics R are lower than in the frequency band K0 that includes the fundamental frequency f0. The frequency band K3 includes the threefold frequency 3·f0, for example, 1700 MHz, 1905 MHz, and 2055 MHz. This shows that, in the frequency band K3 that includes the threefold frequency 3·f0, a high-frequency signal in the frequency band K3 that includes the threefold frequency 3·f0 is reflected and attenuated.
Accordingly, the impedance converter 1 of the present embodiment shown in
In addition, the characteristic impedances Z5 and Z6 of the stubs S1 and S2 are five or more times larger than the impedance value of characteristic impedance element 101 on which the stubs S1 and S2 are formed, and Since the reflection coefficient Γ satisfies Γ≧0.67, as shown in expressions (5) and (6), degradation in the reflection characteristics Q in the frequency band K0 that includes the fundamental frequency f0 can be suppressed.
Since the impedance converter 1 is provided with the first stub S1 formed on the lower impedance side (ZA side) of characteristic impedance element 101, and having line length L5 which suppresses passage of a high-frequency signal having a threefold frequency 3·f0 of the fundamental frequency and the second stub S2 formed on the high impedance side (ZB side) of characteristic impedance element 101, and having line length L6 (≧L5) which suppresses passage of a low-frequency signal included in the high-frequency signal having a threefold frequency of the fundamental frequency, the setting of the line lengths L5 and L6 can also improve frequency characteristics in the frequency band K0 including the fundamental frequency f0. For example, as shown in
In contrast, if the line length of the second stub S2 is not L6 (≧L5), the gain at the frequency of 800 MHz is −25.89 dB as shown in
In the above embodiment, the case where two stubs S1 and S2 are provided is described. However, the number of the stubs is not limited to two, and for example, only one stub S1 may be formed.
Accordingly, if one stub S1 is provided, the reflection characteristics Q are low and the transmission characteristics R are high at a frequency corresponding to the line length L5 of the stub S1, for example, 1905 MHz in the frequency band K3 that includes the threefold frequency 3·f0.
L
10
,L
11
,L
12, and L13≈λ/4 at 3·f0 (7)
The line lengths L10, L11, L12, and L13 correspond to frequencies in the frequency band K3 including the threefold frequency 3·f0 in which a high-frequency signal is attenuated.
As shown in
While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-178328 | Sep 2014 | JP | national |
