AFC circuit, carrier recovery circuit and receiver device...

Television – Receiver circuitry – Automatic frequency control

Reexamination Certificate

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Details

C375S326000

Reexamination Certificate

active

06490010

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an AFC circuit, a carrier recovery circuit and a receiver for use in satellite digital television broadcasting and more particularly to an AFC circuit, a carrier recovery circuit and a receiver for regenerating a carrier even at the time of low C/N ratio.
SUMMARY OF THE INVENTION
For digital transmission through a satellite, hierarchical transmission method capable of data transmission to some extent even at the time of a low C/N ratio by adaptive transmission by time division based on modulation method having different numbers of constellation points by considering a deterioration of the C/N ratio due to rain attenuation or the like has been invented. In such a transmission method, it is very difficult to obtain a reference signal necessary for carrier recovery from a period of modulation wave having many constellation points at the time of low C/N ratio, and therefore, a carrier recovery method which is an ordinary carrier recovery method for continuously regenerating the carrier, cannot be used.
Accordingly, an object of the present invention is to achieve carrier recovery by disposing periodically a modulated signal period with few constellation points which is modulated by, for example, BPSK modulation method or QPSK modulation method and from which a reference carrier signal having a considerable CN rate can be obtained, and fetching out phase/frequency error information periodically. Further, because, according to a method for observing a phase error signal periodically, with a frequency of a predetermined period, the same phase error signal is obtained, so-called false lock phenomenon in which the frequency is apparently synchronized with a different frequency from its proper carrier frequency occurs. To avoid this phenomenon, a modulated signal with few constellation points which is modulated by, for example, BPSK modulation method or QPSK modulation method is set at a predetermined interval and in the pseudo synchronizing state, a frequency of a difference from the proper carrier frequency is observed by using such a fact that the reception signal phase is rotated in a predetermined direction so as to control the VCO (Voltage Controlled Oscillator) thereby making it possible to synchronize with the proper frequency. In the modulation period having few constellation points, detection of the pseudo synchronizing state and synchronization with a desired frequency are enabled using a statistical characteristic of an observed signal.
BACKGROUND ART
In a conventional transmission method of transmitting a modulated signal having many constellation points continuously or a method in which the number of constellation points is changed by time division system, if carrier recovery is carried out continuously, when the C/N ratio drops, a stabilized carrier recovery signal cannot be obtained in a modulation period having many constellation points. For this reason, even if a modulated signal having few constellation points exists, stable demodulation is disabled.
Further, in a method for carrying out carrier recovery periodically using only a period having a few constellation points for such a modulated signal, there is a problem that false lock occurs by observing the phase periodically. Therefore, a wide capture range cannot be achieved. Thus, because a very high frequency stabilization accuracy is demanded in the transmission system containing the frequency conversion portion, the receiver device becomes very expensive.
For the reason, according to the method for transmitting modulated signals having different numbers of constellation points in time division system, if the conventional carrier recovery system is utilized, the carrier recovery is disabled when the C/N ratio is low.
Then, although a method for controlling the VCO or NCO (Numerical Controlled Oscillator) can be considered by measuring the phase with only a period having a few constellation points, there is a problem that a wide capture range cannot be achieved because of false lock phenomenon caused by observing the phase periodically.
The present invention has been achieved in viewpoints of the above problems and therefore it is an object of the invention to provide an AFC circuit capable of regenerating a carrier signal synchronous with the inputted signal in terms of the frequency while preventing an occurrence of false lock even when a reference signal period or a modulated signal period having a few constellation points which can be used for carrier recovery, contained in an input signal, is short or even when noise is mixed in the inputted signal.
Another object of the invention is to provide a carrier recovery circuit capable of regenerating the carrier signal stably in a wide capture range by transmitting modulations signal having different numbers of constellation points by time-division system and carrying out carrier synchronization using phase and frequency error information obtained periodically even if the C/N ratio is low when a modulated signal is received and regenerated.
Still another object of the invention is to provide a receiver device capable of regenerating information contained in digitally modulated signal by carrying out carrier synchronization using phase and frequency error information obtained periodically and regenerating the carrier signal stably in a wide capture range even if the C/N ratio is low when digitally modulated signal provided with a reference signal period or digitally modulated signal period having a few constellation points to be useful for carrier recovery at a predetermined time interval is received and regenerated.
DISCLOSURE OF INVENTION
To achieve the above object, the AFC circuit for detecting a frequency difference between two inputted signals and zeroing said frequency difference between the inputted signals according to the detection result, comprises: a frequency difference detecting portion for detecting a phase difference between the inputted signals and generating a frequency correction signal according to the phase difference or a time differential value of the phase difference; and a frequency difference correcting portion for rotating the phase of the inputted signal according to the frequency correction signal outputted from the frequency difference detecting portion so as to zero the frequency difference between the inputted signals.
In the AFC circuit for detecting a frequency difference between two inputted signals and zeroing the frequency difference between the inputted signals according to the detection result, a frequency difference detecting portion detects a phase difference between the inputted signals and generates a frequency correction signal according to the phase difference or a time differential value of the phase difference and a frequency difference correcting portion rotates phase of the inputted signals according to the frequency correction signal outputted from the frequency difference detecting portion so as to zero the frequency difference between the inputted signals. As a result, even when the reference signal period modulated signal period having few constellation points which is usable for regenerating a carrier contained in the inputted signals is short or even when noise is mixed in the inputted signals, a carrier signal synchronous with the inputted signals is regenerated while preventing an occurrence of false lock.
Another feature of the invention is an AFC circuit for detecting a frequency difference between two inputted signals and zeroing said frequency difference between the inputted signals according to the detection result, the AFC circuit comprising: a correlation computing portion for detecting a phase difference between the inputted signals and computing an autocorrelation coefficient of the phase difference; and a frequency difference correcting portion for counting a number of peaks in a waveform of the autocorrelation coefficient obtained by the correlation computing portion and rotating phase of the inputted signals according to the count result so as to zer

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