[BP] Follow-up to Friday's ad-hoc call - SDD21 curve
Joel,
As a follow-up to Friday's channel ad-hoc, I'd like to reiterate my concern with lowering the SDD21 curve beyond what's needed to hit the task force objectives. Those objectives are of course to go across 40" of "improved FR4." It was set primarily for cost and system configuration reasons and I absolutely agree with the objective. From the July meeting it's been my understanding that the main area of concern with the existing curve is at frequencies below 3GHz. There were 3 proposals from yourself, John D., and Steve A. to address this. There were 2 straw polls taken, the first favoring Steve's proposal and the 2'nd favoring yours.
The primary advantage of your proposal is that it provides the most relief at the lower frequencies, but the primary disadvantage is that it pulls the curve down more than necessary at higher frequencies. This is done to maintain the definition of the curve with a single equation, which I consider an aesthetic objective. The main disadvantage however is that lowering the curve will increase transceiver power and potentially area (although power is clearly the main concern). The attenuation is increased and the SNR relative to xtalk is decreased. The changes won't break the bank, but they curves define a minimum criteria a transceiver must design to and it will increase power regardless of the signaling method chosen. Power is just as significant a concern for system vendors as cost, but I'm concerned it's taken too much of a back seat in the channel analysis. What's needed in a channel model is the capabi!
lity to manufacture a cost effective system (with improved FR4 materials) under the assumption that well skilled engineering resources are applied to the design. It would be desirable to pickup a reasonable percentage of legacy backplanes, but that's not a driving objective. A system vendor has many levers to pull to stay above the SDD21 curve - trace length, trace width, via design, etc. Many vendors, particularly early adopters, will go to even better materials to get additional margin above the line or to go additional distance. However all channels above the line pay the power that is set by transceivers designed to the defined standards curve. The curve should therefore be set to the minimum needed to meet the objective, without addition for margin, to capture legacy channels, or for aesthetics. Transceiver designers can target higher limits, and offerings will certainly be available that do, but the standard should not go beyond the!
minimum needed or it forces a cost/power tradeoff beyond what!
's neces
sary.
I would propose the following options to obtain the additional margin at the lower frequencies with minimal impact to higher frequencies:
- Use John's curve. He withdrew his proposal, I'll reinstate it here. His curve provides additional margin at the lower frequencies while maintaining a single equation to define the curve.
- Use Steve's curve at the lower frequencies until it intersects the existing curve (somewhat above 3GHz), and use the existing curve above that point. That provides slightly more relief at the lower frequencies than John's curve and also has a relatively smooth transition point between curves.
- Take your new curve below 3GHz and the existing curve above that. This results in the most relief at the lower frequencies but does have a step at the 3GHz point. One could argue that the step isn't a serious problem because the curve is defined to include all discontinuities so real channels will not follow a smooth line along it anyway. However, I consider that much of a step to be highly undesirable and fully expect most others will as well.
Thanks, Joe
Joe Abler abler@us.ibm.com
IBM Microelectronics Division 919-254-0573
Technical Marketing & HSS Applications 919-254-9616 (fax)
3039 Cornwallis Road
Research Triangle Park, NC 27709