Wave transmission lines and networks – Coupling networks – Delay lines including long line elements
Reexamination Certificate
2000-07-21
2003-03-04
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Delay lines including long line elements
C333S026000
Reexamination Certificate
active
06529099
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a 180° phase shift circuit having an improved isolation characteristic and, more particularly, to a 180° phase shift circuit having an improved isolation characteristic as well as phase shift characteristics.
(b) Description of the Related Art
A 180° phase shift circuit is a balance-unbalance converter which converts an unbalanced signal, input between an unbalance terminal and the ground, into a pair of balanced signals having an equal amplitude and a phase difference of 180° therebetween by using a power distribution, and thus delivers the pair of balanced signals through a pair of balanced signal output terminals. The principal part of the 180° phase shift circuit is called “balun” in this technical field In the field of microwave circuits, 180° phase shift circuits are widely used as a power distribution/synthesis circuit for a power amplifier, a balanced modem circuit, a mixer, and a phase shift device. A variety of proposals have been made for improving the characteristics of the 180° phase shift circuit.
Patent Publication JP-A-7-131277 describes a 180° phase shift circuit, such as shown in
FIG. 1
, which compensates degradations in an amplitude-difference characteristic and a phase-difference characteristic between the pair of balanced signals
FIG. 1
shows an example of the 180° phase shift circuit used for an amplifier block, wherein the 180° phase shift circuits
51
A and
51
B are used as an input circuit and an output circuit, respectively, for the amplifier block
43
having a pair of balanced signal input terminals T
21
and T
31
and a pair of balanced signal output terminals T
22
and T
32
.
Each of the 180° phase shift circuits
51
A and
51
B is connected at the pair of balanced signal terminals T
21
and T
31
or T
22
and T
32
to the terminals of the amplifier block
43
after an impedance matching of the 180° phase shift circuits
51
with the amplifier block
43
. Thus, the amplifier block
43
receives input balanced signals from the 180° phase shift circuit
51
A to deliver output balanced signals to the 180° phase shift circuit
51
B. In this configuration, the 180° phase shift circuit
51
B delivers an output unbalanced signal having a reduced distortion within a wide band, which improves the characteristics of the amplifying system as a whole including the amplifier block
43
and the pair of 180° phase shift circuits
51
A and
51
B.
Each of the 180° phase shift circuits
51
A and
51
B includes a balun
41
and an amplitude/phase correction circuit
42
. The balun
41
in the 180° phase shift circuit
51
A converts the unbalanced signal supplied through an input port P
1
to deliver a pair of balanced signals through the output ports P
2
and P
3
. The balun
41
in the 180° phase shift circuit
51
B converts balanced signals supplied through the input ports P
2
and P
3
to deliver an unbalanced signal through an output port P
1
thereof. The amplitude/phase correction circuit
42
, connected between the output port P
2
and balanced signal output terminal T
21
, is implemented by a distributed parameter line having a specific characteristic impedance and a specific length for compensating or correcting the characteristics of the 180° phase shift circuit for the amplitude difference and the phase difference between both the balanced signal.
In the 180° phase shift circuit, if a reflected wave is generated due to an impedance mismatching on one of the pair of balanced signal terminals, the reflected wave is transferred though the 180° phase shift circuit to the other of the pair of balanced signal terminals as a leakage signal. The leakage signal generates an adverse effect on the function of the circuitry unless the 180° phase shift circuit has an excellent isolation characteristic. In the described circuitry, however, the isolation characteristic is not considered on the premise that a sufficient impedance matching is attained in the circuitry.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a 180° phase shift circuit having improved phase shift characteristics and an improved isolation characteristic.
The present invention provides a 180° phase shift circuit including a first through third signal terminals, a balun including first and second ports for receiving/delivering a pair of balanced signals and a third port for delivering/receiving an unbalanced signal, a first impedance matching line connected between the first port and the first signal terminal, a second impedance matching line connected between the second port and the second signal terminal, and a serial branch including first and second resistors and a &lgr;g/2 distribution parameter line connected between the fist resistor and a second resistor, the serial branch being connected between a first node connecting together the first impedance matching line and the first signal terminal and a second node connecting together the second impedance matching line and the second signal terminal.
In accordance with the 180° phase shift circuit of the present invention, reflected signal generated outside the 180° phase shift circuit due to impedance-mismatching and entering the 180° phase shift circuit through one of the balanced signal terminals cannot pass through the other of the balanced signal terminals. Thus, the isolation characteristic of the 180° phase shift circuit can be improved without degrading the phase shift characteristics of the 180° phase shift circuit.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.
REFERENCES:
patent: 5416451 (1995-05-01), Kaltenecker et al.
patent: 5467063 (1995-11-01), Burns et al.
patent: 5684430 (1997-11-01), Yamamoto
patent: 5809409 (1998-09-01), Itoh et al.
patent: 6128479 (2000-10-01), Fitspatrick et al.
patent: 7-131277 (1995-05-01), None
patent: 2000-4109 (2000-01-01), None
Gordon J. Laughlin, “A New Impedance-Matched Wide-Band Balun and Magic Tee.”, IEEE Transactions on Microwave Theory and Techniques, vol. MTT-24, No. 3, pp. 135141, Mar. 1976.
Chang Joseph
Foley & Lardner
NEC Compound Semiconductor Devices Ltd.
Pascal Robert
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