Automatic frequency control device for satellite communications ground system

An automatic frequency control device of a satellite communications system having a frequency downconverter, a demodulator and a first local oscillator, located at a receiving set of an earth station system, for compensating for frequency error by supplying a second local oscillator of the frequency downconverter with a difference frequency between a carrier frequency received from the demodulator and a first frequency generated by the first local oscillator, comprising;

• a phase detecting means for detecting a first phase difference between a central frequency of a signal applied to the demodulator and the first frequency generated by the first local oscillator by using both I(Inphase) data and Q(Quadrature) data applied from the demodulator;
• an integrating means connected to said phase detecting means, for generating control voltage by integrating second and third phase differences applied from said phase detecting means and sweep voltage applied from a sweep voltage generation and control means, respectively;
• a control voltage range conversion means for adjusting a central voltage of the control voltage as applied to a voltage controlled oscillator by changing the control voltage from said integrating means to be made within a control voltage range of the voltage controlled oscillator;
• said voltage controlled oscillator for generating an appropriate second frequency according to the control voltage applied from said control voltage range conversion means;
• a carrier lock alarm means for detecting a state of carrier locking from the I and Q data and for generating a first alarm;
• a control voltage limitation alarm means for generating a second limitation alarm in order to repeatedly make the sweep voltage in case the control voltage applied from said integrating means is out of a predetermined voltage range;
• and
• a sweep voltage generation and control means for generating the sweep voltage upon receiving the first alarm from said carrier lock alarm means and for controlling the sweep voltage to be repeated within a prescribed automatic frequency control range.