Automatic frequency control circuit

Abstract

The present invention realizes low power consumption at the time of an automatic frequency control circuit operation. An automatic frequency control circuit includes a mixing unit that generates a modulated signal from a reception signal according to a frequency of a local signal, a demodulation unit that demodulates the modulated signal supplied by the mixing unit, an error evaluation unit that generates a frequency error signal according to a duty of the demodulated signal supplied by the demodulation unit, a holding unit that holds a frequency setting of the local signal and updates the frequency setting according to the frequency error signal supplied by the error evaluation unit, and an oscillation unit that controls a frequency of the local signal according to the frequency setting supplied by the holding unit.

1 . An automatic frequency control circuit comprising:
 a mixing unit that generates a modulated signal from an input signal according to a frequency of a local signal;   an evaluation unit that detects a first ratio of a first period of the modulated signal to a second period of the modulated signal, and generates a value corresponding to a IF frequency of the modulated signal based on a difference between the first ratio and a 50% duty cycle; and   an oscillation unit that controls a frequency of the local signal according to the value,   wherein the first period is a period when the frequency of the modulated signal is higher than a predetermined frequency, and   wherein the second period is a period when the frequency of the modulated signal is lower than the predetermined frequency.   
     
     
         2 . The automatic frequency control circuit according to  claim 1 , wherein
 the predetermined frequency is a target IF frequency, and   when the IF frequency of the modulated signal is equal to the target IF frequency, the first ratio becomes 50%.   
     
     
         3 . The automatic frequency control circuit according to  claim 1 , wherein the evaluation unit comprises:
 a first delay circuit that delays a signal having a duty ratio corresponding to the first ratio based on an input clock;   a second delay circuit that delays an output signal of the first delay circuit based on the clock; and   an adder that adds the output signal of the first delay circuit to an output signal of the second delay circuit, and thereby generates a difference between the first ratio and the 50% duty ratio.   
     
     
         4 . The automatic frequency control circuit according to  claim 3 , wherein
 the evaluation unit comprises a plurality of delay circuits including the first and second delay circuits,   the clock has a cycle obtained by dividing a cycle of the modulated signal, and   a number of the delay circuits corresponds to a number by which the cycle of the modulated signal is divided.   
     
     
         5 . The automatic frequency control circuit according to  claim 4 , wherein when an output of the adder indicates zero, it indicates that the first ratio is equal to 50%. 
     
     
         6 . The automatic frequency control circuit according to  claim 5 , further comprising:
 an adjustment circuit that normalizes the output of the adder based on a scaling coefficient, and thereby generates the IF frequency of the modulated signal;   an subtracter that calculates a difference between the target IF frequency and the IF frequency of the modulated signal output by the adjustment circuit, and thereby generates a corrected value; and   a register that holds the frequency of the local signal and updates the held value based on the corrected value.   
     
     
         7 . An automatic frequency control method comprising:
 generating a modulated signal from an input signal according to a frequency of a local signal;   detecting a first ratio of a first period of the modulated signal to a second period of the modulated signal;   generating a value corresponding to an IF frequency of the modulated signal based on a difference between the first ratio and a 50% duty cycle; and   controlling a frequency of the local signal according to the value,   wherein the first period is a period when the frequency of the modulated signal is higher than a predetermined frequency, and   wherein the second period is a period when the frequency of the modulated signal is lower than the predetermined frequency.