Spurious-Free Time-to-Digital Conversion in an ADPLL Using Short Dithering Sequences

We propose an enhancement to the digital phase detection mechanism in an all-digital phase-locked loop (ADPLL) by randomization of the frequency reference using carefully chosen dither sequences. This dithering renders the digital phase detector, realized as a time-to-digital converter (TDC), free from any phase domain spurious tones generated as a consequence of an ill-conditioned sampling of the feedback variable oscillator phase. In modern nanoscale technologies, TDC has a time quantization of 5 to 30 ps. This deadband can potentially result in spurious tones, whenever a near integer-N relationship arises between the oscillator frequency and the TDC sampling process. This work proposes injection of a spectrum-friendly short sequence dither into the reference clock signal to overcome the quantization introduced limit-cycles. This results in robust phase tracking performance and spurious-free operation of the ADPLL, which was verified in a 65-nm CMOS GSM/EDGE transmitter.