A Sensitive Method to Measure the Integral Nonlinearity of a Digital-to-Time Converter Based on Phase Modulation

A digital-to-time converter (DTC) produces a time delay based on a digital code. Similar to data converters, linearity is a key metric for a DTC and it can be characterized by its integral nonlinearity (INL). However, measuring the INL of a subpicosecond-resolution DTC is problematic, even when using the best available high-speed oscilloscopes. In this brief we propose a new method to measure the INL of a DTC by applying digital phase modulation and measuring the output spectrum with a spectrum analyzer. The frequency selectivity of this method allows for an improved measurement resolution down to a few femtoseconds and allows measuring an INL below 100 fs. The proposed method is verified by behavioral simulations and is employed to measure the INL of a high-resolution DTC realized in the 65-nm CMOS, with a time resolution of 25 fs and a standard deviation of 27 fs.