Chemistry-Inspired Charge-Pump Frequency Synthesiser

Phase-Locked Loops (PLLs) are frequently used for frequency synthesis. The working principle consists in minimising the phase difference between a reference frequency and the synthesised tone. A periodic pulse, with a relatively high constant frequency and a modulated width, pulls up or down a Charge Pump that adjusts the voltage at the input of a Voltage Control Oscillator and thus compensates the phase difference. In this paper we propose a novel, Chemistry-inspired approach that reviews the working principle of traditional PLLs: the charge pump is corrected with constant-width pulses that are scheduled with a rate that is a function of the frequency error. Such an approach brings two main benefits. First, comparison spurs do not appear as the correction happens with long time intervals and thus spurs are buried in the output tone. Second, there is no theoretical limitation in the resolution of the output frequency, and de facto, Chemistry-inspired frequency synthesisers have the same advantage <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{of}$</tex> fractional PLLs in terms of frequency resolution but have no drawbacks in terms of RF spectral purity.