adaptive equalizers for LVDS data over CAT5

So what's an LVDS equalizer? The equivalent of what is called "AGC" in audio - some kind of normalizer that brings the signal back to some useable amplitude at the receiving end?

robert

it also increases the HF gain see fig 4

Martin

LVDS sends digital data as a differential current, usually between chips on boards, using differential pc traces, or between boxes, on twisted pairs. The signals lose high-frequency components due to transmission line lossses (skin and dielectric losses) and also suffer dispersion, where different frequencies travel at different speeds. Both soften up edges and smear bits into one another, with the smearing being called ISI, intersymbol interferance. Since the process is linear, in theory an equalizing network can undo the distortions. Amplitude isn't the real problem, since the receiver is essentially a comparator, and gain is cheap.

Some LVDS receiver chips include fixed equalization, like with four pin-selectable steps, that is essentially high-frequency boost. Some have adaptive equalizers whose algorithms are not well explained. I doubt that the IC equalizers do much more than HF boost, so probably don't fix dispersion very well.

Modems and such can have very sophisticated equalization, based on FIR filtering, that fix all sorts of stuff, echoes even.

John

John, in order to have distributed strictly synchroneous operation, I guess you'd need a master controller and equal cable lengths to all units. Once you have the cables equally long, you can take a 100EP196 to delay one signal against the other. Then feed two channels, one arbitrarily delayed onto a mixer and adjust until the integral is zero. This way you can adjust the timeing to subnanosecond precision.

Rene

Thanks, John, for the detailed answer. I know little more about LVDS than what can be inferred from expanding the acronym, but I'm following this thread because I'm thinking about a serializer/deserializer application in a "dirty" environment.

Right now it's a DOS PC with some old software and three full-size IDE driver boards that output step/direction signals for eighteen stepper and servo motors. The signals go into the power drive boxes via some ribbon cables, from which lead 12 meters worth of power cables to the motors in a camera robot.

I'd like to replace the ribbons by a SerDes combo via CAT5 and move the power drives into the robot. Then it would only have to drag an AC power cord and some cheap CAT5 cable across the studio rather than an arm-thick bundle of heavy, unwieldy, expensive (and in many other ways bad) cables.

Max step frequency is only 250kHz, but I don't want to tap into some clock on the boards, so I'm considering free-running the 36-bit SerDes at some higher frequency like 2MHz. That's 72Mbit/s. Nothing compared to what these things can do, but it's still some 20 meters in an electrically dirty environment.

Except that I have no time at all to implement this shit, that's why I haven't done any serious research about it. National's "LVDS owner's manual" seems to be a pretty thorough source.

What's a recommended SerDes part combo? Slow, rugged, cheap, big (no BGA). This would be pretty much an one-off, so I could design it around samples from Maxim even.

That sounds like a good use for a serdes, but I haven't used any so I can't suggest a part. We just work inside FPGAs, since we have them anyhow. If you're driving motor coils, the occasional data error won't matter, at the speeds these things run!

John