Copper Cable Assemblies Challenge Optical Performance

To tackle data transfer rates of 10 Gb/s or higher at longer distances, cable-assembly manufacturers are combining advances in copper conductor technology with improved dielectric materials and high-performance connectors. As a result, they can offer flexible, reliable, efficient, and economical alternatives to fiber-optic modules and cables for applications like high-speed computing, networking, and network storage. To further extend the reach of these copper cable assemblies without compromising performance, manufacturers also are leveraging the benefits of silicon. Specifically, suppliers like W.L. Gore & Associates (), Molex, Inc., Amphenol Interconnect Products Corp., and Tyco Electronics are targeting 10 Gigabit Ethernet (10GbE) and 8G and 10G fibre channel systems with copper cables. By doing so, they are enhancing small-form-factor pluggable (SFP+) copper cable assemblies to satisfy the size, flexibility, and performance needs of these systems. These enhancements also help meet the electrical, mechanical, and physical specifications established by standards like SFF-8431 as well as the emerging SFF-8461.

For example, Gore has developed an enhanced SFP+ copper cable assembly that complies with the SFF-8431 standard while offering reduced jitter, low latency, and a maximum signal eye opening. Despite their good performance, both the size and flexibility of copper cables have been a concern in dense-networking and high-speed-computing environments. To meet the needs of compact, higher-port-count switch installations, Gore has unveiled a very-low-profile SFP+ direct-attach copper cable assembly. Although fiber-like in size, it boasts a higher degree of flexibility (Fig. 1).

The new cable is designed for 0.5-to-3-m assemblies. The 2.10-mm round profile cable is rated for lengths to 2 m and the 2.80-mm round profile cable is tailored for 3 m. According to Gore, the smaller, low-profile cable is one-half the cross-sectional area of OM3 fiber-optic cable and one-third the size of the 62.5/125 optical cable with a significantly reduced bend radius. By comparison, the standard SFP+ copper cable assembly in these lengths utilizes cables that are roughly 4.4 mm in diameter with a typical bend radius of 30 mm. The reduced bend radius and smaller diameter of the low-profile cable permits tighter routing as well as cleaner dressing of cables in networking and computing applications, according to the manufacturer.

Russ Hornung, Gore's Product Manager, states, "With fiber-like size and flexibility, the new low-profile cable technology expands the market for SFP+ copper interconnects." Now, the supplier is working toward the emerging SFF-8461 standard, which addresses longer lengths with higher reliability and integrity. The SFF-8431 was created as a very conservative specification for SFP+ copper cables, notes Hornung. In fact, it started as an optical specification. Copper was added as an addendum to this specification (addendum E) under a multi-source agreement (MSA) by a group of diverse companies to ensure compatibility amongst various suppliers. The SFF-8461 offers passive copper enhancements, adds Hornung.

According to Amphenol's Business Development Manager, Greg McSorley, "SFF-8431 is an optic-centric standard. We are working toward making SFF-8461 copper-centric. SFF-8461 is looking to improve the specs on direct-attach SFP+ copper cable assemblies that will take advantage of the silicon available today." Some 15 companies comprising host, chip, and cable suppliers are involved in generating the SFF-8461 specifications. The first draft is expected to be generated by year end.

Consequently, Hornung notes that active components are being combined with traditional assemblies to double the useful length of the copper cable assembly. For instance, analog signal processing and RF noise-cancellation developer Quellan, Inc. has readied CMOS-based analog-signal-processing chips for active copper SFP+ assemblies that provide longer lengths at one-fifth the power. By embedding active devices inside the connector, the firm minimizes performance impairments like jitter, attenuation, group delay, and crosstalk, thereby enabling copper cables to propagate signals at higher speeds over longer distances with integrity. To compensate for all of the interconnect impairments, Quellan's patented analog-signal-processing technology, which is called Q:Active, provides amplitude equalization, group delay equalization, skew correction, and crosstalk reduction.

For high-density data-center switching, Gore and Quellan have jointly demonstrated active QSFP cable that can handle 35 Gb/s of data over 15 m of cable. Late last year, Gore added a QSFP copper cable assembly to its line of products aimed at Infiniband and 40-GbE applications. The partners also have demonstrated an active SFP+ cable assembly reaching 10 m. Data-center cable management and power consumption are becoming increasingly critical as interconnect densities rise, states Tony Stelliga, Quellan's Chairman and CEO. Yet passive cable assemblies are inherently unable to resolve these issues. "By removing cable impairments and attenuation with active, low-power analog signal processing, thinner, lighter, and longer cabling can be used. This reduces weight, increases room for airflow, and saves power by avoiding the need for optics," states Stelliga.

Implementing its Q:Active technology in silicon using a CMOS process, Quellan has developed chips for both single- and quad-channel cable assemblies. The QLx111GRx is designed for single-channel SFP+ copper cables with the ability to support data rates to 11.3 Gb/s. In contrast, the QLx411GRx quad-channel version targets QSFP copper interconnects. Functionally, the QLx111GRx is a settable, single-channel, receive-side equalizer followed by a limiting amplifier. It compensates for channel loss and restores signal fidelity as demonstrated in the open eye diagram (Fig. 2). In addition, the built-in signal-detection function indicates the absence of a received signal. Operating on a single 1.2-V supply, the QLx111GRx consumes less than 135 mW with latency below 500 ps. On 28 AWG cable, it extends that reach to 10 m.

Similarly, the QLx411GRx is a settable quad-receive-side equalizer with four equalizers and limiting amplifiers in a single 4-x-7-mm QFN package. Consuming less than 135 mW per channel, it operates on a single 1.2-V supply. Like the QLx111GRx, the QLx411GRx extends its reach to 10 m on 28 AWG cable.

To simplify cable manufacturers' jobs, Quellan also has readied active cable-assembly modules as turnkey solutions for both SFP+ and QSFP cables. The SFP+ module QLx1000M comprises the lane extender QLx111GRx and the driver QLx111GTx on a single board. In contrast, the QSFP module QLx4000M uses the quad chip along with the drivers.

To handle higher data rates and longer reach, Gore's SFP+ and QSFP copper cable assemblies employ patented conductor technology called EYE-OPENER with low dielectric material. When combined with silicon, Gore asserts that this technology will extend three times further or use a smaller diameter cable than passive interconnects. To increase the range from 8.5 to 15 m or to decrease the cable size by 60 percent for SFP+ copper cable assemblies, active equalization is implemented into the connector ends.

More SFP+ Sources
Molex also is promoting copper cable interconnects as an alternative to fiber optics for 10G and beyond data rates. Like Gore, this company has readied both passive and active SFP+ pluggable interconnect solutions. Although passive SFP+ copper cable assemblies are inexpensive, the host has to do the work of driving it properly. In contrast, active versions cost more but are comparable to optical modules.

With a reach to 3 m and less than 1 mW power consumption, Molex's 10G SFP+ direct-attached passive copper cable assemblies promise to provide superior electromagnetic-interference (EMI) performance with fiber-optic flexibility. As a result, the same cable can be used in 10G Ethernet, 8G FC, 4G, 2G, and 1G applications, says Greg Walz, Molex's Product Manager for Pluggable I/Os. Additionally, these cables employ a new pull-to-release latch design (Fig. 3). Aside from making it easy to access the latch in belly-to-belly and stacked-port configurations, this design eliminates the use of cable extractors. It also prevents the accidental removal of adjacent channels.

With transmit pre-emphasis and adaptive receive equalization incorporated, the active version of this cable assembly offers enhanced signal integrity and longer cable length. Other benefits include transmit disable, loss-of-signal (LOS) detection, interrupts, and a management interface. The active SFP+ copper cable assemblies come in medium- and long-reach versions. The medium-reach cable offers length to 7 m with the ability to handle an 11.3-Gb/s data rate. To achieve that performance, the 74753 series medium-range cables implement limiting post amplifiers at transmit and receive in both directions. In contrast, the 111038 series long-reach cables, which range to 20 m in length, use post amplifiers with pre-emphasis at transmit and active equalization at receive in both directions. The two hot-pluggable varieties contain 0.039-in.-thick printed-circuit board (PCB) that mates with a 20-position SMT host connector.

For applications that require aggregate data of 40 Gb/s, Molex has developed transition cable assemblies that convert QSFP cables into four lanes of SFP+ at 10 Gb/s data speed. Designated the 74764 series, these assemblies come in various lengths with a variety of copper gauges and shielding options. Likewise, Amphenol Interconnect Products offers both passive and active SFP+ copper cable assemblies with both push-to-release and pull-to-release latch designs. Mimicking optical transceivers, the active SFP+ cable assemblies feature transmit disable and LOS detection capabilities. By exceeding Gigabit Ethernet and Fibre Channel requirements for performance and reliability, the manufacturer asserts that the SFP+ copper interconnects offer a cost-effective alternative to optical solutions for 15-m applications.

Also in this race is Tyco Electronics, which offers both passive and active SFP+ copper-cable-assembly offerings as low-cost alternatives for short-reach applications. These assemblies can handle serial data transmission to 12 Gb/s in each direction. To ensure that EMI radiation is sufficiently suppressed, the cables incorporate a 360-deg. cable braid crimp with an enhanced EMI skirt design. They also utilize a new pull-to-release latch. In addition, the copper interconnects implement an EEPROM signature that permits the host to differentiate between a passive/active copper cable assembly and a fiber-optic module. According to Dave Stonfer, Tyco's Product Manager for High-Speed Cable Assemblies, the active design includes signal amplification and equalization in the assembly. It also features LOS detection and transmit disable. Stonfer points out that the active cable's low power consumption makes it economical for rack or rack-to-rack applications.

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