Impact of PI Nonlinearity on High-Resolution Frequency-to-Digital Converter

Abstract

The performance of Delta Sigma frequency-to-digital converters (FDCs) play crucial role in implementing low-noise fractional-N phase locked loops (PLLs), so various structures of FDCs have been proposed. In this paper, an FDC with a linear phase interpolator (PI), one with a nonlinear PI, and one without PI are implemented, and their performance in terms of the in-band signal-to-noise ratio (SNR) is compared. In this experiment, the focus is on whether the PI-based FDC still shows superior performance compared to the conventional FDC even in non-ideal case, especially when the PI includes nonlinearity. We show that the PI-based FDC performance improves compared to the conventional FDC, but even small nonlinearity (0.5LSB INL) in the PI increases the in-band noise power by around 10dB.