Design of clock and data recovery circuits for energy-efficient short-reach optical transceivers
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Date
2022-12-19
Authors
Khanghah, Meysam M.
Journal Title
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Publisher
University College Cork
Published Version
Abstract
Nowadays, the increasing demand for cloud based computing and social media
services mandates higher throughput (at least 56 Gb/s per data lane with 400
Gb/s total capacity 1) for short reach optical links (with the reach typically less
than 2 km) inside data centres. The immediate consequences are the huge
and power hungry data centers. To address these issues the intra-data-center
connectivity by means of optical links needs continuous upgrading.
In recent years, the trend in the industry has shifted toward the use of more
complex modulation formats like PAM4 due to its spectral efficiency over the
traditional NRZ. Another advantage is the reduced number of channels count
which is more cost-effective considering the required area and the I/O density.
However employing PAM4 results in more complex transceivers circuitry due
to the presence of multilevel transitions and reduced noise budget. In addition,
providing higher speed while accommodating the stringent requirements
of higher density and energy efficiency (< 5 pJ/bit), makes the design of the
optical links more challenging and requires innovative design techniques both
at the system and circuit level.
This work presents the design of a Clock and Data Recovery Circuit (CDR) as
one of the key building blocks for the transceiver modules used in such fibreoptic
links. Capable of working with PAM4 signalling format, the new proposed
CDR architecture targets data rates of 50−56 Gb/s while achieving the required
energy efficiency (< 5 pJ/bit).
At the system level, the design proposes a new PAM4 PD which provides a better
trade-off in terms of bandwidth and systematic jitter generation in the CDR. By
using a digital loop controller (DLC), the CDR gains considerable area reduction
with flexibility to adjust the loop dynamics.
At the circuit level it focuses on applying different circuit techniques to mitigate
the circuit imperfections. It presents a wideband analog front end (AFE),
suitable for a 56 Gb/s, 28-Gbaud PAM-4 signal, by using an 8x interleaved, master/
slave based sample and hold circuit. In addition, the AFE is equipped with
a calibration scheme which corrects the errors associated with the sampling
channels’ offset voltage and gain mismatches. The presented digital to phase
converter (DPC) features a modified phase interpolator (PI), a new quadrature
phase corrector (QPC) and multi-phase output with de-skewing capabilities.The DPC (as a standalone block) and the CDR (as the main focus of this work)
were fabricated in 65-nm CMOS technology. Based on the measurements, the
DPC achieves DNL/INL of 0.7/6 LSB respectively while consuming 40.5 mW
power from 1.05 V supply. Although the CDR was not fully operational with
the PAM4 input, the results from 25-Gbaud PAM2 (NRZ) test setup were used
to estimate the performance. Under this scenario, the 1-UI JTOL bandwidth
was measured to be 2 MHz with BER threshold of 10−4. The chip consumes 236
mW of power while operating on 1 − 1.2 V supply range achieving an energyefficiency
of 4.27 pJ/bit.
Description
Keywords
Wireline and optical communication , Clock and data recovery , PAM4 clock and data recovery , Optical transceivers , Digital to phase converters , Data centers , Short-reach optical communication
Citation
Khanghah, M. M. 2022. Design of clock and data recovery circuits for energy-efficient short-reach optical transceivers. PhD Thesis, University College Cork.