Model-free adaptive frequency calibration for voltage-controlled oscillators

The operating frequency accuracy of the local oscillators is critical for the overall system performance in the communication systems. However, the high-precision oscillators could be too expensive for civil applications. In this paper, we propose a model-free adaptive frequency calibration framework for a voltage-controlled crystal oscillator (VCO) equipped with a time to digital converter (TDC), which can significantly improve the frequency accuracy of the VCO thus calibrated. The idea is to utilize a high-precision TDC to directly measure the VCO period which is then passed to a model-free method for working frequency calibration. One advantage of this method is that the working frequency calibration employs the system history of input/output (I/O) data, instead of establishing an accurate VCO voltage-controlled oscillator model. Another advantage is the lightweight calibration method with low complexity such that it can be implemented on an MCU with limited computation capabilities. Experimental results show that the proposed calibration method can improve the frequency accuracy of a VCO from ±20 ppm to ±10 ppb, which indicates the promise of the modelfree adaptive frequency calibrator for VCOs.