Low-Profile QSFP Cables Handle Faster Data Rates Over Longer Distances

Sept. 22, 2009
To fulfill the interconnect needs of high-speed applications like Infiniband quad-data-rate (QDR) and 40-Gb/s Ethernet (GbE), many cable manufacturers have been pushing quad-small-form-factor-pluggable (QSFP) copper cable assemblies to new ...

To fulfill the interconnect needs of high-speed applications like Infiniband quad-data-rate (QDR) and 40-Gb/s Ethernet (GbE), many cable manufacturers have been pushing quad-small-form-factor-pluggable (QSFP) copper cable assemblies to new levels of performance. At the same time, they have reduced the cable diameter to minimize its profile. These cable makers, which include W.L. Gore & Associates, Tyco Electronics, Amphenol Interconnect Products Corp., and Molex, Inc., also have been improving the flexibility of these QSFP cables for greater bend radius. To increase cable length with fiber-like size and data rates, they have been exploiting the benefits of active analog-signal-processing chips.

W.L. Gore, for instance, has released a very-low-profile QSFP direct-attach copper cable assembly with a diameter of 0.170 in. To realize smaller-diameter QSFP cables with the same 10-Gb/s or higher electrical performance of larger-diameter cables, Gore designers leveraged the proprietary expanded-PTFE (ePTFE) dielectric material. According to the company, its low dielectric constant of 1.3 enables smaller-diameter cables with fiber-like size. Compared to typical industry offerings, Gore asserts that the cross-section savings for 4X channel, 8-pair cable employing this low-profile QSFP copper cable is about 58 percent. Application Engineer Chris Ericksen states, "We see the consistency and extremely low-dielectric loss tangent (0.0004) of ePTFE having a more pronounced impact when compared to conductor loss, as data rates increase and support frequencies move further into the microwave range."

Shielding plays an important role in signal integrity as data rates climb. As a result, Gore has developed novel ways to helically wrap an aluminum-polyester combination around each twinax cable of the QSFP assembly. The result is reduced loss and extended bandwidth using cheaper material, asserts Russ Hornung, Product Manager at Gore. The firm has compared the performance of the QSFP using the latest shielding versus both an improved cable and a legacy cable (Fig. 1). It found that a legacy cable with a 5-to-8-Gb/s data rate (or 2.5-to-4.0-GHz fundamental frequency) is not functional at 10 GHz. Although the improved cable has similar loss at 5 GHz, it now has the bandwidth to handle a 10- to 12-Gb/s data rate or support the fundamental frequency of 5 to 6 GHz.

By comparison, each twinax of the QSFP cable has better bandwidth with the ability to support at least a 32-Gb/s data rate on a single pair or above 20 GHz fundamental frequency. In addition, there is a significant improvement in differential loss even at the lower frequencies or bit rates. At 5 GHz or a 10-Gb/s bit rate, the QSFP cable boasts a 3-dB improvement over the improved cable. "This results in a gauge-size improvement. In other words, the state-of-the-art 26AWG looks like an improved 24AWG," states Hornung. With improved shielding materials and techniques, one can achieve a 3-dB improvement in loss or shrink an entire gauge size, Hornung asserts.

Another proponent of QSFP cable assemblies is Tyco Electronics. With a programmed EEPROM signature, Tyco's QSFP family enables the host to differentiate between a copper cable assembly and fiber-optic module. It also incorporates equalization to meet the signal-integrity requirements of the QSFP multi-source agreement (MSA). By improving the isolation between conductors via mechanical dimensioning of materials in the backshell, Development Engineer Jason Ellison states that the company has been able to achieve an order-of-magnitude improvement in crosstalk for its assemblies. Simultaneously, return loss has been improved by 5 dB.

The supplier also has implemented many protective features in the mechanical assembly, such as a rib to prevent damage to the board latch. When the plug is installed upside down, a stop will prohibit any destruction of the PT connector. In addition, a stainless-steel tongue protects the printed-circuit board (PCB) and resists bending or breaking while a 360-deg. crimp ferrule suppresses electromagnetic interference (EMI).

While Tyco's QSFP cable assemblies come in standard 26AWG and 28AWG gauges, the company is working closely with Madison Cable to go to smaller gauges for those who need longer lengths, says Product Manager Dave Stonfer. Using passive techniques, he notes that Tyco offers fully equalized, partially equalized, and unequalized QSFP cable assemblies. Fully equalized cables do not rely on the host system to assist with minimizing the intersymbol interference (ISI). In contrast, partially equalized cable assemblies work in conjunction with the host system to tackle ISI.

Recently, Amphenol Interconnect Products Corp. added a rail bus to its QSFP cable assemblies that prevents the corner of the assembly from protruding outside of the cageespecially outside the area of retention latch, says Mike Wingard, AIPC's Director of Engineering. Wingard adds that it prevents the latch from bending when one comes at it at an angle. Using passive equalization techniques, the supplier is working on improving the length of its cables without increasing the cost. It also is employing better impedance matching to obtain less than 3-percent crosstalk with 100- differential impedance.

On the connector side, Molex's 38-circuit iPass connector and cable assembly has been adopted by Infiniband as the QSFP 4xQDR standard. The iPass connector features a surfacemount design that enables placement on both sides of the PCB. With a pitch of 0.8 mm, the design permits easy routing of traces on both sides of the PCB while standoffs allow easy cleaning after soldering. In essence, the QSFP interface also replaces a copper-only standard with a pluggable copper and optical solution. Aside from copper or fiber assembly options, Molex's QSFP iPass pluggable solution includes loopbacks.

The manufacturer also has readied cable assemblies for Infiniband DDR applications. Molex's LaneLink DDR Infiniband assemblies are capable of supporting up to 40 Gb/s over eight differential cable pairs or four channels. For applications that require transition from QSFP cables into four lanes of SFP+ at a 10-Gb/s data rate per channel, Molex has developed transition cables as well. Similarly, a variety of such transition cables called hybrids have been developed by W.L. Gore and Tyco Electronics.

QSFP cable-assembly manufacturers also are tapping the advantages of active devices to propagate data at higher speeds and over longer lengths with high signal integrity. Embedded in the connector side of the assembly, the active signalprocessing chips minimize interconnect impairments like jitter, attenuation, group delay, and crosstalk. By overcoming the limitations of passive cable assemblies, they deliver thinner, lighter, and longer cabling for high-speed applications. To provide amplitude and group-delay equalization in addition to skew correction and crosstalk reduction, Quellan, Inc. (www. quellan.com)an Intersil company (www. intersil.com)has developed 130-nm, CMOS-based analog-signal-processing technology called Q:Active. Utilizing this patented technology, Quellan is offering single- and quad-channel solutions for copper cable assemblies. For QSFP, the developer has readied QLx411GRx, which is a settable quad-receive-side equalizer with four equalizers and limiting amplifiers in a single QSN package. For the transmit side, the supplier offers a quadchannel driver dubbed the QLx411GTx. By combining the quad chip with drivers, Quellan also has prepared active cableassembly modules as turnkey solutions.

Last year, Quellan used its quad signal- processing chip with Gore's cable assembly to demonstrate the industry's first active QSFP copper cable assembly capable of handling 40 Gb/s data over 15 m. The active cable is compliant with SFF- 8436 and Infiniband QDR and DDR specifications. "We have passed all the compliance and interoperability tests on the receive side," affirms Quellan's Marketing Engineer, Gourgen Oganessyan. Products that successfully pass rigorous testing at the Infiniband Technology Association's (IBTA) Plugfest event are included in the IBTA integrator's list (see Table). Speaking of thinner copper cables, Quellan has shown that its active cables can support a QDR rate with 32AWG-gauge wire to 5 m. Efforts are underway to further boost the speed capability of active 32AWG cable without sacrificing power, notes Oganessyan.

Like Gore, AIPC, Molex, and Tyco Electronics are reaping the benefits of active solutions. Texas Instruments and Maxim Integrated Products are AIPC's primary chip suppliers. Yet the cable manufacturer also is working with others to cost effectively address the equalization needs of thin QSFP copper cable assemblies. Although optical QSFP transceivers are expensive, silicon devices add to the cost of copper cables, explains Wingard. Hence, there is a crossover point. For less than 20 m, Wingard points out, analysis shows that active copper design looks viable. The developer is working on improvements to push the length between 15 and 20 m without the use of expensive low-dielectric- constant material like PTFE.

Active QSFP cable assemblies also are on Tyco's drawing board. Using standard limiting equalizers on the assembly's receive side, the maker is developing a half-active cable assembly with plans to go into production in the fourth quarter. According to Ellison, a full-active assembly solution also is in the works. While the half-active version is expected to extend the reach of QSFP copper cable assembly to 12 m, the full-active one is projected to handle Infiniband bit rates to 15 m.

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