Wave transmission lines and networks – Coupling networks – Balanced to unbalanced circuits
Reexamination Certificate
2001-10-16
2003-11-11
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Balanced to unbalanced circuits
C333S034000
Reexamination Certificate
active
06646518
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a balun used to connect a balanced transmission line to an unbalanced transmission line and a semiconductor device provided with the balun.
2. Description of the Prior Art
A balun that operates in a low frequency band and is used to connect a balanced transmission line to an unbalanced line consists of a concentrated constant component such as a transformer, whereas a balun that operates in a high-frequency microwave band consists of a distributed constant component. Since most of baluns each of which consists of a distributed constant component include a quarter-wavelength matching element or are transformers whose size is determined according to usable wavelengths, a disadvantage to them is that their frequency bands are fundamentally narrow.
FIG. 23
is a perspective view showing the structure of a prior art balun
100
which is in practical use and operates in a microwave band, the balun having small wavelength dependence and a large frequency band. In the figure, reference numeral
101
denotes a first conductive layer that is tapered, and reference numeral
102
denotes a second conductive layer that is tapered.
As shown in
FIG. 23
, the first conductive layer
101
is tapered from a maximum width at an end portion
101
a
thereof to a minimum width at another end portion
101
b
thereof, and the second conductive layer
102
is tapered from a maximum width at an end portion
102
a
thereof to a minimum width at another end portion
102
b
thereof. The taper of each of the first and second conductive layers
101
and
102
can be a linear taper. As an alternative, the taper of each of the first and second conductive layers
101
and
102
can be, as to shifting characteristic impedance, an exponential taper, a triangular taper, a Klopfenstein taper, or any other taper which can reduce the amount of reflection while transforming the characteristic impedance of a balanced transmission line into the characteristic impedance of an unbalanced transmission line over a large frequency band. Furthermore, in order to hold the spacing between the first and second conductive layers
101
and
102
, they are usually formed on both sides of a dielectric substrate such as a printed board (not shown in the figure), respectively.
In operation, the balun
100
connects an unbalanced line coupled to the end portions
101
a
and
102
a
of the first and second conductive layers
101
and
102
to a balanced line coupled to the other end portions
101
b
and
102
b
, and also transforms the characteristic impedance of the unbalanced line to the characteristic impedance of the balanced line. In order to minimize the amount of reflection due to changes in the characteristic impedance of the balun, the taper of each of the first and second conductive layers
101
and
102
can be optimized.
By the way, it is possible to easily connect a coaxial connector to the end portions
101
a
and
102
a
on the unbalanced-line side of the balun
100
shown in
FIG. 23
because the pair of the end portions
101
a
and
102
a
is a normal microstrip line, but since the end portions
101
b
and
102
b
on the balanced-line side of the balun are formed on the both sides of a substrate not shown in the figure, respectively, it is difficult to physically connect them to a pair of balanced terminals, which is formed on an electronic component, such as an IC, and which is arranged in the same plane of the substrate.
Therefore, in order to connect an electronic component, such as an IC, which has a pair of balanced output terminals in the same plane, to another electronic component having a pair of unbalanced input terminals, one of the pair of balanced output terminals is connected to a grounded surface of the substrate by way of a termination. On the other hand, the other one of the balanced output terminal pair is connected to one unbalanced input terminal of the other electronic component by way of a microstrip line.
FIG. 24
is a plan view showing the structure of a prior art balun
200
which can be incorporated into a power amplifier for use with television broadcasting transmitters as disclosed in Japanese patent application publication (TOKKAIHEI) No. 9-46106.
FIG. 25
is a bottom view of the balun
200
. In the figure, reference numeral
201
denotes a first conductive layer formed on a top surface of a printed board
220
, and reference numeral
202
denotes a second conductive layer formed on a bottom surface of the printed board
220
. These conductive layers
201
and
202
form a broadside-coupling-type line and constitute an isolation transformer
203
. The first and second conductive layers
201
and
202
, which constitute the isolation transformer
203
, both have a predetermined width. Reference numeral
204
denotes a high-frequency signal input terminal to which an end of the first conductive layer
201
is connected, reference numeral
205
denotes an output terminal to which another end of the first conductive layer
201
is connected, reference numeral
206
denotes a output terminal to which the end of the second conductive layer
202
is electrically connected via a through hole
210
, reference numerals
207
a
and
207
b
denote third and fourth conductive layers with a ground potential, reference numeral
208
denotes a fifth conductive strip layer that connects the first conductive layer
201
to the third conductive layer
207
a
and that functions as an inductance, and reference numeral
209
denotes a sixth conductive layer that is formed in the form of a strip and that connects the second conductive layer
202
to the fourth conductive layer
207
b
and functions as an inductance. A push-pull circuit transistor (not shown in the figure) for use in power amplifiers is connected to the pair of output terminals
205
and
206
.
In the prior art balun constructed as shown in
FIGS. 24 and 25
, a high-frequency signal, which is applied to the high-frequency signal input terminal
204
by way of a microstrip line which is an unbalanced line, flows through the first and second conductive layers
201
and
202
which constitute the isolation transformer
203
, as a pair of two equal-amplitude currents 180 degrees out of phase with each other. One of the electric current pair is supplied from the first conductive layer
201
, by way of the output terminal
205
, to one terminal of a push-pull circuit transistor (not shown in the figure) for use in power amplifiers, and the other one of the electric current pair is supplied from the second conductive layer
202
, by way of the through hole
210
and the output terminal
206
, to another terminal of the push-pull circuit transistor.
A problem with prior art baluns that operate in a microwave band constructed as above is that although conductive layers have to be tapered in order to provide small wavelength dependence and a large frequency band, it is difficult to make a physical connection between a balun including such conductive layers and an electronic circuit, such as an IC, having a pair of balanced terminals, as previously mentioned. Another problem is that when one of the pair of balanced terminals is connected to a ground by way of a termination, the efficiency is reduced because the other one of the pair of balanced outputs is not used, and the load on a differential circuit that generates a pair of balanced outputs becomes unbalanced because of an inductance included in the termination which increases with increasing frequency, which results in a malfunction of the differential circuit.
Furthermore, a problem with the prior art balun as disclosed in Japanese patent application publication No. 9-46106 shown in
FIGS. 24 and 25
is that the pattern of the conductive layers is complex and the balun is not suitable for use with a connection of a balanced line with an unbalanced line over a wide frequency band requested by 40 Gbps optical communication.
SUMMARY OF THE INVENTION
The present invention is proposed to solve the above-mentioned proble
Burns Doane , Swecker, Mathis LLP
Mitsubishi Denki & Kabushiki Kaisha
Pascal Robert
Takaoka Dean
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