16.5 A Fractional-N Synthesizer with 110fsrms Jitter and a Reference Quadrupler for Wideband 802.11ax

Abstract

The next-generation 802.11ax WLAN standard improves the throughput by supporting 1024-QAM in a channel bandwidth of 160MHz, demanding extremely low jitter values for the transmitter (Tx) and the receiver (Rx) synthesizers. Recent work has achieved rms jitter below 200fs [1]–[4] with the latest rms jitter reported at 75fs [5]. The work in [5] faces a number of challenges if applied to 802.11ax: (1) with a VCO frequency of 5.7 to 7.3GHz, the circuit is prone to pulling by the PA, especially because it incorporates a single spiral inductor (rather than an 8shaped inductor); (2) it does not provide quadrature outputs, and (3) it relies on a 10b digital-to-time converter (DTC) without calibration of its nonlinearity, which folds high-frequency $\Delta \Sigma-\text { modulator } (\Delta \Sigma M)$ noise unless extremely tight matching is guaranteed. Similarly, the design in [2] is prone to pulling as well.