Hey Doug (and All!), Your point is well made. It has been my experience that most "costly" margin work is done on the tail end...not on the front end. If boards are designed right (even the dreaded two layer flavor)you will not have a margin issue. Bandaides cost big bucks, and when you are leaking from every pulsating capillary...what 'cha gonna do? Do the design right in the first place...take control over where traces are laid...add knock down resistors...manage the power grid...it can be done fairly inexpensively.
My $.02 Dave Spencer Doug McKean wrote: > > At the risk of being solidly flamed, I'd ask to raise the > dB Margin discussion one more time and if need be off-line. > See my email address below. If you're going to flame me, do it quick > to the bone and I'll be done with it. > > My discussion is going to: > > 1. Assume an awful lot and may be unacceptable to some, > 2. Stay somewhat nonrigorous statistically. > > I'll readily admit to these flaws up front for the sake of discussion > and not so much for actual fact. The facts are dependent upon your > own experience, the designers you work with, and your equipment, etc... > > It has been my own experience that the difference between a -6dB > margin and a -10dB margin for a product 'could be' the difference between > 100's of dollars of 'fixes' and 1000's of dollars of 'redesign' respectively > in manufacturing quantities. (Remember I say 'could be'.) > > Somewhere in the back of everyone's mind with whatever margin > you should pick should be a thing called statistical 'variance' of data. > And dependent upon variance is another thing called 'confidence' margin. > > Let's say also that the 80% of the products made works out to be 10 samples > to be > tested (just for ease of discussion). 80% of the samples passing means 8 > pass/2 fail. > > Let me assume that someone picks -6dB as the goal below the standard. > Let's say the limit of the standard is 50dB. > > For demonstration purposes, 8 samples test 3dB below the limit (now 44dB) > and 2 test 3dB above the limit. > > Samples 1 - 8 = 41dB, Samples 9 and 10 = 47dB. > Average = 42.2 > Variance = 6.4 > > The following confidence levels are: > > % Confidence Variance Lower limit Upper limit > 80% Confidence +/- 2.59 39.61 44.79 > 95% Confidence +/- 3.97 38.23 46.17 > 99% Confidence +/- 5.21 36.99 47.41 > > In other words, even at 99% confidence (1% unaccounted for) > we have less than 3dB 'headroom'. > > Now, let's pick the -10dB below standard and assume 50dB is the standard. > Our limit is now 40dB. Assuming 8 samples 3dB below and 2 samples 3db again > we have the following: > > Samples 1 - 8 = 37dB, Samples 9 and 10 = 43dB. > Average = 38.2 > Variance = 6.4 > > % Confidence Variance Lower limit Upper limit > 80% Confidence +/- 2.59 35.61 40.79 > 95% Confidence +/- 3.97 34.23 42.17 > 99% Confidence +/- 5.21 32.99 43.41 > > Here, even at 99% confidence (1% unaccounted for) > we have less than 6dB 'headroom' where before we had less than 3dB. > > For those of us not familiar with all this, first notice how the > variances didn't change (as long as I was assuming the same type of data > spread around the limit) and how the upper/lower limits fit > around the 'average' with respect to the variances. > > Now, I won't conclude which is the better. It could be argued that > -6dB is neccessary and -10dB is sufficient. As I said before, > this could result in a small amount of money or alot of money > being used to comply. That's for you to decide. > > This example has been crudely done I admit, but I thought to > start somewhere. There's a high possibility someone out there > is much smarter than me to shoot holes in this entire discourse. > I'm open for any suggestions. > > ******************************************************* > Doug McKean > [email protected] > ------------------------------------------------------- > The comments and opinions stated herein are mine alone, > and do not reflect those of my employer. > ------------------------------------------------------- > *******************************************************
