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Re: serdes design vs run length


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.


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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