Electronic Design

More On Driving Twisted Pairs

Although the product described here is an ASSP rather than an enhanced amplifier, I must note that an obvious potential new application area for driving Ethernet cable is parallel 10-Gigabit Ethernet.

Last summer, a startup called SolarFlare Communications demonstrated a high-performance integrated analog front end for handling 10-Gbase-T transmission over copper. The company's AFE integrated four 10-bit, 1-Gsample/s ADCs, each with a dedicated programmable gain amp and PLL. A separate PLL synchronized traffic.

The second part of the demo used SolarFlare's digital chip, which it had previously shown with a discrete analog front end. The demo drove 70 m of CAT6 cable with an impressively low bit error rate. Company VP Ron Cates noted that the ideal cable was not CAT5, but CAT6 or CAT7, which provide more tightly controlled twisted pairs. For at least the next couple of years, 10G Ethernet will follow the parallel model.

Meanwhile, I can't leave the subject of twisted pair without thinking of digital subscriber loop (DSL) and its challenges, which brings the topic back to amplifiers. Traditionally, DSL system designers have used amps that run off ±12-V supplies. Legerity, however, has a line of amplifiers that that run from the central office's -48-V "battery" voltage. That concept can be a little tricky in some installations, because legacy specs for certain telephone networks envisioned scenarios in which battery voltages would sag to as low as -36 V.

Modern installations have made that more of a regulatory issue than a practical consideration, however. Telephone system engineers are less concerned about deep battery-discharge events, and in China and India, the adoption rate for the 48-V parts has been high.

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