Re: serdes design vs run length
Joel,
This is in response to your run length question a couple days ago.
Bill Woodruff did a good job describing the PLL design tradeoffs.
I could add a few technical comments (e.g., the need for larger
valued DC blocking caps with good high frequency response, and
the implicit minimum BW requirement to track low-frequency wander
below several hundred kHz), but I don't think that's the main issue.
My connection here is integrated CMOS serdes, so that's where this
response is coming from. If "cost" means unit cost of a product
still to be developed, then I don't have much to say. The cost
I would worry about is development-related -- costs of bringing
such a product to market -- technical risks which translate into
schedule risks and, therefore, into missed market opportunities.
To my (admittedly limited) knowledge, most existing 1-3G serdes
assume 8b10b code. And they owe their existence to a long process
of solving non-trivial design issues. To my (admittedly biased)
mind, an existing, proven solution counts for more than a few
comparison points against a concept that may or may not have some
small, theoretical technical advantage.
Regards,
Mike
Joel Goergen wrote:
>
> Howdy all,
>
> I was looking through some notes from Albuquerque and came across some
> remarks I have heard and wanted to bring it up on the reflector.
>
> I had heard some folk pointing out that there are some of us worried
> about the relative board costs, relative emi costs, relative systems
> cost, but ignoring relative costs directed at the serdes. So, to remedy
> this, let me ask the following to those of you whom design serdes
> devices:
>
> What impact does the encoding or run length have on the relative cost of
> a serdes device at 2.5gigabit and 3.125gigabit for 8b10b, Scrambled, and
> 64b66b? Does the extended run length really drive the cost, or have we
> just not employed new methods into phase lock control that might reduce
> the run length impact?
>
> Or are there other serdes issues that one coding scheme has over the
> other that impact relative cost more?
>
> Thanks and take care
> Joel Goergen
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