RD53 High-Speed Data Transmission Link with Decision Feedback - - PowerPoint PPT Presentation

RD53 High-Speed Data Transmission Link

with Decision Feedback Equalization Veronica Wallangen January 29, 2016

Channel Characterization

◮ RD53 collaboration: produce next generation of pixel readout

chips needed by ATLAS and CMS at the HL-LHC

◮ Need pixel chip readout link: ◮ Currently studying different readout cable options for ITk upgrade ◮ Main requirements for electrical transmission lines:

◮ radiation hardness ◮ low mass ◮ high bandwidth: ≥5 Gbps

◮ Exploring prototype ”hybrid” cable

design consisting of:

◮ 1m custom shielded twisted wire pair

(TWP) for on-stave transmission

◮ 6m twinax for off-stave transmission

◮ Design RD53 receiver circuit for use with ATLAS Phase-II pixel cables

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S-parameter Measurements

◮ Cable characterization using S-parameters: describe response of network

to voltage signals at each port

◮ S21 - transmission coefficient representing voltage detected at port 2 due

to incident signal at port 1

◮ Measure signal attenuation during transmission for wide frequency range

using network analyzer

◮ RD53 chip: Differential signal output through 4-port device ◮ Measured S-parameters of hybrid cable with 50 Ohm SMA connectors

attached directly to cable ends using 4-port VNA

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S-parameter Measurement Results

Need for S-parameters?

◮ Generate SPICE model for hybrid cable to be used in simulations ◮ Achieving high data rates requires channel with low attenuation and

distortion for noise reduction ⇒ Need feedback equalization to compensate for high-frequency signal errors

◮ Industrial equalization technology not currently in use in HEP (use more

primitive driver pre-emphasis) ⇒ more sophisticated solution needed

◮ S-parameters used to determine equalization needed at receiver end to

account for data distortion in order to obtain a proper readout link design

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Signal Degradation

◮ Signal degradation in non-ideal channels

caused by inherent low-pass transfer function characteristics ⇒ affecting amplitude, timing

◮ Intersymbol interference (ISI) at receiver end:

signal smearing leaves trace of previous symbol in next symbol decoding SPICE modeling of hybrid cable

◮ I/O at 1 Gbps: ⇒⇒⇒ ◮ I/O at 5 Gbps:

Equalization essential to obtain higher speeds!

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Concept of Equalization

◮ Compensation by RX equalization: removes ISI, restores signal integrity ◮ Solution to overcome frequency-dependent attenuation: highpass transfer

curve that gives uniform requency response

◮ Simple compensation idea: data processed through transfer function

H−1(s) - ideally inverse of the cable transfer function H(s)

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Equalization Techniques

◮ Many ways to implement equalizer based on communication channel

⇒ need for cable characterization

◮ Main categories:

◮ linear equalization (uses ”pre-decision”) ◮ adaptive equalization: automatically adapts to time-varying

properties of the channel

Adaptive equalization w/ decision feedback

◮ Decision Feedback Equalizer (DFE): use decisions made on basis of

precursors to take care of postcursors

◮ idea: subtract smearing from previous bit

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Motivation: Improved Signal Quality

◮ Eye diagram: overlaying of received bits on time axis ◮ Common indicator of signal quality in high-speed transmissions ◮ Ideal output: ◮ Sample eye diagram without RX equalization (left) and with equalization

implemented (right)

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Design Approach

Test: Linear RX equalization

◮ Simplest equalizer: differential pair with RC HPF ◮ Few parameters ⇒ yet time-consuming tuning!

Berkeley Analog Generator (BAG)

◮ Collaborate with Berkeley Wireless Research Center:

◮ focuses on highly-integrated CMOS implementations ◮ expertise in current data transmission technology

◮ BAG tool: framework for automated analog integrated circuit design

◮ helps avoid repetitive tasks and circuit redesign ◮ integrates design flow steps into single environment ◮ produces input-specific optimized schematics and layouts

◮ Current status: awaiting NDA approval to access BAG

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