High Speed Reconfigurable NRZ/PAM4 Transceiver Design Techniques

Abstract

While the majority of wireline standards use simple binary non-return-to-zero (NRZ) signaling, four-level pulse-amplitude modulation (PAM4) standards are emerging to increase bandwidth density. This dissertation proposes efficient implementations for high speed NRZ/PAM4 transceivers. The first prototype includes a dual-mode NRZ/PAM4 serial I/O transmitter which can support both modulations with minimum power and hardware overhead. A source-series-terminated (SST) transmitter achieves 1.2Vpp output swing and employs lookup table (LUT) control of a 31-segment output digital-to-analog converter (DAC) to implement 4/2-tap feed-forward equalization (FFE) in NRZ/PAM4 modes, respectively. Transmitter power is improved with low-overhead analog impedance control in the DAC cells and a quarter-rate serializer based on a tri-state inverter-based mux with dynamic pre-driver gates. The transmitter is designed to work with a receiver that implements an NRZ/PAM4 decision feedback equalizer (DFE) that employs 1 finite impulse response (FIR) and 2 infinite impulse response (IIR) taps for first post-cursor and long-tail ISI cancellation, respectively. Fabricated in GP 65-nm CMOS, the transmitter occupies 0.060mm² area and achieves 16Gb/s NRZ and 32Gb/s PAM4 operation at 10.4 and 4.9 mW/Gb/s while operating over channels with 27.6 and 13.5dB loss at Nyquist, respectively. The second prototype presents a 56Gb/s four-level pulse amplitude modulation (PAM4) quarter-rate wireline receiver which is implemented in a 65nm CMOS process. The frontend utilize a single stage continuous time linear equalizer (CTLE) to boost the main cursor and relax the pre-cursor cancelation requirement, requiring only a 2-tap pre-cursor feed-forward equalization (FFE) on the transmitter side. A 2-tap decision feedback equalizer (DFE) with one finite impulse response (FIR) tap and one infinite impulse response (IIR) tap is employed to cancel first post-cursor and longtail inter-symbol interference (ISI). The FIR tap direct feedback is implemented inside the CML slicers to relax the critical timing of DFE and maximize the achievable data-rate. In addition to the per-slice main 3 data samplers, an error sampler is utilized for background threshold control and an edge-based sampler performs both PLL-based CDR phase detection and generates information for background DFE tap adaptation. The receiver consumes 4.63mW/Gb/s and compensates for up to 20.8dB loss when operated with a 2- tap FFE transmitter. The experimental results and comparison with state-of-the-art shows superior power efficiency of the presented prototypes for similar data-rate and channel loss. The usage of proposed design techniques are not limited to these specific prototypes and can be applied for any wireline transceiver with different modulation, data-rate and CMOS technology.