Frequency locking method and circuit for phase-locked loop

A frequency locking method for a phase-locked loop, comprising:

• S1:
• outputting, by a frequency control module (C), a minimum output frequency control word min_code to a numerically controlled oscillator (A) to obtain a maximum output frequency, and outputting, by the frequency control module (C), a maximum output frequency control word max_code to the numerically controlled oscillator (A) to obtain a minimum output frequency;
• S2:
• outputting, by a time-to-digital converter (B), count information to the frequency control module (C) according to the minimum output frequency to obtain a minimum frequency ratio min_ratio, and outputting, by the time-to-digital converter (B), count information to the frequency control module (C) according to the maximum output frequency to obtain a maximum frequency ratio max_ratio;
• S3:
• obtaining an ideal frequency control curve according to the minimum output frequency control word min_code, the maximum output frequency control word max_code, the minimum frequency ratio min_ratio, and the maximum frequency ratio max_ratio, and calculating both a first frequency control word target_code and a first frequency ratio target_ratio based on the ideal frequency control curve;
• S4:
• applying a Newton's iterative method according to the first frequency control word target_code and the first frequency ratio target_ratio to calculate and obtain a new frequency control word ctrl_code;
• S5:
• sending the new frequency control word ctrl_code obtained in step S4 to the numerically controlled oscillator (A) and, obtaining a new frequency ratio target_ratio 1 by using the time-to-digital converter (B) and the frequency control module (C);
• and
• S6:
• ending, by the frequency control module (C), a iteration and stably outputting the new frequency control word ctrl_code when an error between the new frequency ratio target_ratio 1 obtained in step S5 and a target frequency ratio div_ratio is within a set range;
• otherwise, updating a value of the first frequency control word target_code to make it equal to the new frequency control word ctrl_code obtained in step S4, and repeating step S4 to recalculate the new frequency control word ctrl_code.