Fast frequency calibration of VCO´s in phase-locked loops

This paper presents a novel technique to achieve fast calibration of the voltage-controlled oscillator (VCO) into the optimum subband of operation. The proposed technique is pre-dominantly digital thus exhibiting low power/area requirements, in addition to greatly reduced calibration times, suitable for application in lower technology nodes. To verify the proposed technique the VCO and corresponding calibration circuitry is modelled as part of a charge-pump phase locked loop (CP-PLL) in UMC´s 90nm process. VCO operation is split into 16 different subbands (i.e. 4-bit calibration) with each subband exhibiting a gain (KVCO) of ≈ 50MHz/V to achieve an overall tuning range of ≈ 10%. Through application of the proposed technique, calibration into the optimum subband of VCO operation is shown to occur in minimal time.