Wave transmission lines and networks – Plural channel systems – Having branched circuits
Patent
1998-01-08
2000-11-21
Lee, Benny
Wave transmission lines and networks
Plural channel systems
Having branched circuits
333115, 333116, 333204, 455114, H01P 518, H01P 1203, H04B 104
Patent
active
061508985
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to low-pass filters with directional couplers suitable for use in the transmission circuits of cellular phones used for mobile communications, and cellular phones employing such low-pass filters with directional couplers.
BACKGROUND OF THE INVENTION
FIG. 10 is a block diagram of the transmission system of an ordinary cellular phone. The monitor signal is coupled out from the power amplified by a power amplifier 1 through a capacity-coupling capacitor 2. An isolator 3 and then a low-pass filter 4 are connected in the system, and the signal is transmitted from the antenna 6 after removing second harmonic spurious and third harmonic spurious energy in the transmission system when a mode switch 5 is turned to the transmission side.
In the above configuration, however, the number of poles in the low-pass filter 4 may have to be increased to fully attenuate amplified second harmonic spurious energy and third harmonic spurious energy in the system. In addition, the isolator 3 connected for preventing reflection signals, regardless of the input position of the mode switch 5, results in a higher price.
DISCLOSURE OF THE INVENTION
The present invention offers a small and inexpensive low-pass filter with directional coupler and a cellular phone employing such a low-pass filter for attenuating high-frequency signals, in particular, second harmonic spurious and third harmonic spurious energy in the system.
A low-pass filter of the present invention eliminates the use of an isolator and connects a stub line to the main transmission line of the directional coupler for coupling out the monitor signal. With this configuration, a specified frequency band can be attenuated with the same line length as directional couplers of the prior art, thereby reducing the number of components in the transmission system of cellular phones.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective of a low-pass filter with directional coupler in accordance with a first exemplary embodiment of the present invention.
FIG. 2 is a perspective of a low-pass filter with directional coupler in accordance with a second exemplary embodiment of the present invention.
FIGS. 3A and 3B are explanatory views of the relation between a main transmission line and a stub line in accordance with the second exemplary embodiment of the present invention.
FIG. 4 is a perspective of a low-pass filter with directional coupler in accordance with a third exemplary embodiment of the present invention.
FIG. 5 is a perspective of a low-pass filter with directional coupler in accordance with a fourth exemplary embodiment of the present invention.
FIG. 6 is a perspective of a low-pass filter with directional coupler in accordance with a fifth exemplary embodiment of the present invention.
FIG. 7 is a perspective of a low-pass filter with directional coupler in accordance with a sixth exemplary embodiment of the present invention.
FIG. 8 is a perspective of a low-pass filter with directional coupler in accordance with a seventh exemplary embodiment of the present invention.
FIG. 9 is a block diagram of a transmission system in a cellular phone employing a low-pass filter with directional coupler of the present invention.
FIG. 10 is a block diagram of a transmission system in a cellular phone of the prior art.
FIGS. 11, 12, and 13 are perspective views of a low-pass filter with differently configured stub lines, respectively.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A first exemplary embodiment of the present invention is explained with reference to drawings.
FIG. 1 shows a low-pass filter with directional coupler in the first exemplary embodiment of the present invention which is used in the 900 MHz frequency band. In FIG. 1, a main transmission line 105 which has terminals 101 and 102 at its ends and a sub transmission line 106 which has terminals 103 and 104 at its ends are disposed in parallel on a dielectric board 107 whose bottom face is a shield electrode. The main transmission line 105 and the sub
REFERENCES:
patent: 4555681 (1985-11-01), Dominick et al.
patent: 4706049 (1987-11-01), Dydyk
patent: 4999593 (1991-03-01), Anderson
patent: 5075646 (1991-12-01), Morse
patent: 5212815 (1993-05-01), Schumacher
Fujihashi Yoshikuni
Hashimoto Koji
Kushitani Hiroshi
Yuda Naoki
Lee Benny
Matsushita Electric - Industrial Co., Ltd.
Summons Barbara
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