Automatic frequency control system with improved argument approximation
AssigneeTexas Instruments Incorporated
InventorShigenori Kinjo
Life of this patent
Estimating the term…
Abstract
An argument approximation method is used to estimate the frequency offset for an automatic frequency control (AFC) circuit. An angle of rotation can be calculated as arctan(Q/I), where Q is the imaginary component of the rotation and I is the real component of the angle. Rather than use the arctangent calculation, which is resource intensive, to calculate the angle of rotation, the angle of rotation is calculated as the sine of the angle of rotation, which is easily calculated from Q and I, and the sine approximation is offset by a correction factor of 1−cos (θ) in order to reduce approximation errors.
Claims9
An automatic frequency control circuit comprising:
- a controllable oscillator for generating a signal having a frequency responsive to an input;
- circuitry for comparing the output of the controllable oscillator to a carrier frequency;
- circuitry for adjusting said input to said controllable oscillator, said adjusting circuitry comprising:
- circuitry for determining real and imaginary components (I and Q) of an angle of rotation (θ) between said carrier frequency and the frequency output by the controllable oscillator, where θ=arctan(Q/I);
- circuitry for generating an approximation of the angle of rotation (θ′), wherein θ′=sin (θ)+(1−cos (θ)) for sin (θ)>0 and θ′=sin (θ)−(1−cos (θ)) for sin (θ)<0.
A method of controlling the frequency of a controllable oscillator, comprising the steps of:
- determining real and imaginary components (I and Q) of an angle of rotation (θ) between a carrier frequency and output of the controllable oscillator, where θ=arctan(Q/I);
- generating an approximation of the angle of rotation (θ′), wherein θ′=sin (θ)+(1−cos (θ)) for sin (θ)>0 and θ′=sin (θ)−(1−cos (θ)) for sin (θ)<0;
- and
- adjusting the controllable oscillator responsive to said enhanced approximation.
Dependencies are inferred from the claim text.