Telecommunications – Receiver or analog modulated signal frequency converter – Local control of receiver operation
Reexamination Certificate
1997-11-14
2001-05-01
Eisenzopf, Reinhard J. (Department: 2745)
Telecommunications
Receiver or analog modulated signal frequency converter
Local control of receiver operation
C455S132000, C455S258000, C375S344000
Reexamination Certificate
active
06226505
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an automatic frequency correction apparatus and method for a radio calling system and, more particularly, to an automatic frequency correction apparatus and method which can be suitably used for a mobile station as a constituent of a radio calling system.
Some conventional radio calling systems of this type have automatic frequency correction apparatuses installed in mobile stations. For example, Japanese Patent Laid-Open No. 3-70335 discloses an automatic frequency correction apparatus installed in a mobile station. The automatic frequency correction apparatus described in this reference detects the error between a reception frequency and a local oscillation frequency in a specific interval of reception data from a transmitting base station, and selects correction data corresponding to the detected error, thereby controlling a reference oscillator. With this operation, the reference oscillator is matched with the reception frequency from the transmitting base station to allow the mobile station to perform accurate, stable reception and transmission control.
In a radio calling system of this type, however, to make the local oscillation frequency accurate and stable, each mobile station must use a high-precision oscillator such as a TCXO (Temperature Compensated X'tal Oscillator) and an automatic frequency correction circuit including special components such as a frequency meter and an A/D converter. For this reason, to improve the precision of the local oscillation frequency in each mobile station, the cost and the size of a control circuit inevitably increase, resulting in difficulty in satisfying demands for economical, small-size apparatuses.
In addition, since automatic frequency correction is executed on the basis of the reception frequency from a transmitting base station, a stable reception frequency cannot be obtained in a situation in which the reception ratio of reception signals is low. If a high-precision oscillator is used for this situation, a problem is posed in terms of cost performance.
Furthermore, when the reception ratio of reception signals is low, an automatic frequency correction section cannot execute a stable operation.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an automatic frequency correction apparatus and method which can obtain a stable reception ratio in a wide area.
It is another object of the present invention to provide an automatic frequency correction apparatus and method which allows a reduction in circuit scale.
In order to achieve the above objects, according to the present invention, there is provided a frequency correction apparatus for a radio calling system, comprising reception means for receiving call signals comprising a frame synchronization portion and a data portion, local oscillation means for generating local oscillation frequencies, demodulation means for demodulating the received call signals after the call signals are converted into reception intermediate frequencies by using the local oscillation frequencies from the local oscillation means, and frequency correction means for correcting the local oscillation frequency of the local oscillation means on the basis of frequencies of the frame synchronization portions of demodulated signals output from the demodulation means.
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Japanese Office Action, dated Jan. 12, 1999, with English language translation of Japanese Examiner' comments.
Japanese Office Action, dated Jun. 30, 2000, with English language translation of Japanese Examiner's comments.
Appiah Charles N.
Eisenzopf Reinhard J.
McGuireWoods LLP
NEC Corporation
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