Wave transmission lines and networks – Coupling networks – Wave filters including long line elements
Reexamination Certificate
1999-08-05
2001-10-23
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Wave filters including long line elements
C333S103000, C333S262000
Reexamination Certificate
active
06307448
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a frequency-variable-type filter, an antenna duplexer, and a communication apparatus, for example, which are used in the microwave band.
2. Description of the Related Art
In a conventionally-known frequency-variable-type filter, a resonator is connected to a switching device such as a PIN diode or a variable capacitance diode through a capacitor or the like to perform voltage control thereof so as to vary a resonance frequency (cf. Japanese Unexamined Patent Publication No. 7-321509). When a PIN diode is used, a frequency is switched by switching it on/off, so that two bands including a band in the on-period and a band in the off-period are provided. In the on-period, usually, a positive control voltage is supplied in order to switch the PIN diode on, whereas a negative voltage is supplied in the off-period. The reason why the negative voltage is necessary in the off-period is that when high-frequency signals of large electric power are input, a high-frequency voltage is applied to the PIN diode and it is switched on, which should be avoided. In other words, when a large amount of electric power is input, the PIN diode becomes unstable and thereby the frequency characteristic varies. Consequently, this needs to be avoided.
However, in the conventional frequency-variable-type filter, in order to switch a PIN diode off, a power-supply circuit for generating a negative voltage (approximately −3 through −10 V) is required, so that the circuitry is complicated, leading to an obstacle to miniaturization and cost reduction of a mobile phone or the like.
SUMMARY OF THE INVENTION
To overcome the above describe problems, preferred embodiments of the present invention provide a frequency-variable-type filter, an antenna duplexer, and a communication apparatus, which are small and low-cost.
One preferred embodiment of the present invention provides a frequency-variable-type filter, comprising: a voltage control terminal; at least one resonator; a switching device for being switched on/off by a control voltage supplied to the voltage control terminal; and the switching device being in the off-state, when a control circuit as an external circuit which is electrically connected to the voltage control terminal has high impedance at 0 V.
Another preferred embodiment of the present invention provides a communication apparatus, comprising: a frequency-variable-type filter comprising a voltage control terminal; at least one resonator; and a switching device for being switched on/off by a control voltage supplied to the voltage control terminal; and a control circuit for supplying a control voltage to the voltage control terminal to perform voltage control of the frequency-variable-type filter; and the switching device being in the off-state when the control circuit has high impedance at 0 V.
In the above described frequency-variable-type filter and communication apparatus, a PIN diode may be used as the switching device and a dielectric resonator may be used as the resonator. Additionally, a transistor, a field-effect transistor, or the like, may be used in the control circuit, in which the impedance of the control circuit is 100 K&OHgr; or more when the control circuit is at 0 V.
Furthermore, the anode of the PIN diode may be electrically connected to the resonator through either an inductor or a capacitor, whereas the cathode of the PIN diode may be electrically connected to a ground; and the voltage control terminal may be electrically connected to the anode of the PIN diode through a chalk coil, whereas a noise-cutting bypass capacitor may be electrically connected between the voltage control terminal and a ground. In addition, the impedance of the chalk coil may be set to be 350 &OHgr; or more, and the capacity of the noise-cutting bypass capacitor may be set to be in a range of 10 through 1000 pF.
According to the above described structure and arrangement, even if high-frequency signals of large electric power are input when the switching device is in the off-state, since the control circuit connected to the voltage control terminal has high impedance, a negative voltage is constantly imposed on the switching device. Thus, even if high-frequency signals of large electric power are input to the frequency-variable-type filter, a stable frequency characteristic is obtainable. In addition, since there is no need of a power-supply circuit for generating a negative voltage to form a simple circuit, miniaturization and cost reduction of the communication apparatus can be achieved.
Other features and advantages of the present invention will become apparent from the following description of preferred embodiments of the invention which refers to the accompanying drawings, where like reference numerals indicate like elements to avoid duplicative description.
REFERENCES:
patent: 5227748 (1993-07-01), Sroka
patent: 6085071 (2000-04-01), Yamada et al.
patent: 7321509 (1995-12-01), None
Atokawa Masayuki
Yamada Yasuo
Glenn Kimberly E
Murata Manufacturing Co. Ltd.
Ostrolenk Faber Gerb & Soffen, LLP
Pascal Robert
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