A method, apparatus, and energy metering system obtains mains samples of a mains power line signal, performs non-white noise (NWN) filtering of the mains power line signal, obtains adjustable clock source samples of an adjustable clock signal of an adjustable clock oscillator, determines a difference based on the mains samples and the adjustable clock source samples, adjusts an adjustable clock source frequency of the adjustable clock oscillator based on the difference, and applies the adjustable clock source frequency to an analog to digital converter (ADC) to determine a conversion rate of the ADC.