01. February 2016

A 40 Gbit/s backplane for cable TV infrastructure

Modern cable television networks rely on the video, audio and internet telephony services being bundled and transmitted via a cable. In the latest infrastructure developments for cable television, Internet protocol-based services are converged on a single platform known as CCAP (Converged Cable Access Platform). This article describes the development of a 40 Gbit/s backplane that provides the connectivity for this platform and permits extremely fast data transfer rates.

Introduction

The demands on the infrastructure of a cable television network are steadily growing. In particular, extremely high data rates are required since the infrastructure must support multiple applications. Looking around at other industries with the same bandwidth challenges, the telecom industry uses base station platform standards which offer these high transmission bandwidths: in particular, AdvancedTCA® (Advanced Telecom Computing Architecture) has generated high acceptance.

The development of a 40 Gbit/s backplane requires the selection of appropriate connectors and a suitable base material for printed circuit boards. Design rules first must be defined on the basis of computer simulation with these materials to achieve a balance between cost and performance.  In order to ensure the backplane meets the relevant platform standard, the connector selection for the backplane and daughter card is determined by the requirements in the base specification and the application.

In this instance, HARTING worked closely with the PICMG (PCI Industrial Computer Manufacturers Group) to establish the interconnect limit values for the signal transmission parameters in the specification via a series of signal-integrity simulations. The 40 Gbit/s backplane was then laid out and assembled by HARTING Integrated Solutions in readiness for its performance verification via extensive testing.

The backplane characterisation was performed on the basis of the PICMG’s 3.1 R2.0 ‘Ethernet/Fiber Channel for AdvancedTCA® Systems’ specification. One of the main tasks was to define the backplane physical layer interface, which needs to support 10GBASE-KR and 40GBASE-KR4 Ethernet characteristics. During the associated testing, HARTING performed the bulk of these signal-integrity simulations within the PICMG.

PICMG 3.0 is the AdvancedTCA® base specification, which includes board design guidelines targeting 3.125 Gbit/s serial data rates. Signal-integrity simulations performed by HARTING and the PICMG sub-committee were part of the process, described below, to define the requirements and limits for the test fixtures and backplane assembly utilised by HARTING for high-speed backplanes.

Interconnect characteristics

The interconnect parameters for the backplane Ethernet implementation were obtained from the IEEE802.3-2010 standard. The interconnect characteristics in Annex 69B of this standard are informative limits and the testing is based on S-parameters. The analysis can be done based on measured or simulated S-parameters or a mix of both.

Key interconnect parameters are:

  • Fitted attenuation (FA)
  • Insertion loss (IL)
  • Insertion loss deviation (ILD)
  • Insertion loss to crosstalk ratio (ICR)
  • Return loss (RL)
  • Crosstalk (XT)
  • Power sum differential near-end crosstalk (PSNEXT)
  • Power sum differential far-end crosstalk (PSFEXT)
  • Power sum differential crosstalk (PEXT)

Backplane design rules

Design rules cover aspects such as backplane material, backdrill maximum stub length and routing guidelines. The stub length is the height of the plated through-holes connecting signal traces on different layers. The connector footprint also has a strong impact on the results, and it is important to assign the layers (signal and ground power) properly and to define the backdrill levels to minimise the stub length of the plated through-hole.

Channel simulation

Channel simulation analyses the entire transmission path between defined test points and calculates interconnect parameters. The design rules are defined by simulation. The setup is complex and typically consists of many subcircuits with S-parameter files or other models. Each file has to be consistent and causal, and it was decided to program templates for the processing of the parameters to avoid time-consuming manual handling of the raw data and errors arising from this.

Backplane samples were manufactured so that verifications of the simulations and design parameters could be performed. These were both required before releasing the final backplane design and manufacturing data. The interconnect parameters of the backplanes were measured using a 4-port network analyser.

Test fixtures were required for the backplane measurements, developed using the ADFplus connector. The test cards have a strong impact on the results and the connector launches are optimised with 3D field solvers.

Two test fixtures have been developed for ATCA Backplane testing. The 10-layer fixtures have 16 coaxial SMA connector launches, allowing each signal contact to be tested or terminated with precision resistors. The difference between the card types lies in the mounting layer of the SMA-connectors (one with SMAs on the top side and one with SMAs on the bottom side). This allows two adjacent or nearby configurations to be tested (the distance between the slots does not allow the use of right angled adapters).

About HARTING Integrated Solutions

HARTING Integrated Solutions (HIS) designs and manufactures backplanes and backplane systems for customer-specific applications. The group's focus is on backplane PCB design simulation, design validation and signal integrity testing, as well as comprehensive system testing. HIS is active as a fully integrated system developer and manufacturer, including series production. HIS operates a global manufacturing footprint, with factories on three continents: Europe, Asia and North America. Each site has common equipment, tooling and procedures to provide seamless service to all global customers.


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