A fast AFC technique with self-calibration for fast-locking PLLs

Abstract

A fast adaptive frequency calibration (AFC) technique with self-calibration for fast-locking phase-locked loops is presented with frequency-selecting switches. The proposed AFC directly calculates the proper switch states of the voltage-controlled oscillator (VCO). It requires only six clock cycles of the reference oscillator regardless of the number of VCO switches to reach the final switch state in the ideal case. The proposed method counts the number of VCO cycles per reference clock period for the minimum VCO frequency (MIN) and the maximum VCO frequency (MAX) during the first four-clock periods. For the following two-clock periods, the proper states of the VCO switches are set to the calculated value from MIN, MAX and the desired division ratio for a target frequency (EST). A frequency synthesiser with the proposed AFC was implemented on a 0.18 µm CMOS process. The AFC time decreased from 40 to 0.4 µs employing the proposed scheme such that the total lock time is 40 µs with the loop bandwidth of 40 kHz.