Patent ID: 6571088
Filing Date: 2003-05-27
Classification: H03J,H04L

Abstract:
An AFC circuit for use in a receiver in matching the frequency of a reference clock signal in said receiver with the frequency of a reference clock signal in a transmitter, said circuit comprising:an orthogonal demodulator supplied with a reception signal received from said transmitter for carrying out orthogonal demodulation by the use of a local signal to convert said reception signal into an analog baseband signal; an A/D converter supplied with said analog baseband signal produced by said orthogonal demodulator for carrying out A/D conversion to produce a digital baseband signal; a delay profile/search circuit responsive to said digital baseband signal produced by said A/D converter for producing profile data represents profile of data of the digital baseband signal and a frame timing correction amount, said profile data representing a profile of data including in the digital baseband signal; a timing generating circuit for generating an ideal frame timing on the basis of the reference clock signal in said receiver and for adding said frame timing correction amount to said ideal frame timing to produce a frame timing signal, said timing generating circuit being furthermore for calculating, with reference to said frame timing correction amount, a first frequency error representative of a difference in frequency between the reference clock signal in said receiver and the reference clock signal in said transmitter and for judging that an AFC operation is put into an unlocked state to produce an unlock signal if an absolute value of said first frequency error becomes equal to or greater than a first predetermined value; a frequency offset estimating circuit for calculating, with reference to said frame timing signal, a second frequency error representative of a difference in frequency between the reference clock signal in said receiver and that reference clock signal in said transmitter which is contained in said digital baseband signal from said A/D converter, said frequency offset estimating circuit being furthermore for judging that the AFC operation is put into a locked state to produce a lock signal if an absolute value of said second frequency error becomes equal to or smaller than a second predetermined value not greater than said first predetermined value, said frequency offset estimating circuit being turned off and on under control of a control signal; a CPU for controlling said frequency offset estimating circuit by the use of said control signal so that said frequency offset estimating circuit is turned off when said CPU is supplied with said lock signal and that said frequency offset estimating circuit is turned on when said CPU is supplied with said unlock signal; an accumulator which produces a control voltage and which is for adding to a current value of the control voltage the frequency error calculated by said frequency offset estimating circuit to produce a new value of the control voltage; a reference clock generating circuit for generating, as the reference clock signal in said receiver, a controlled reference clock signal having an oscillation frequency controlled by said new value of the control voltage; and a PLL circuit for producing said local signal on the basis of said controlled reference clock signal.