"Steve Underwood" <steveu@coppice.org> wrote in message news:80bddbd7.0403072150.563e829d@posting.google.com...> Richard Lamb <n6228l@earthlink.net> wrote in messagenews:<404B70BE.86DD8383@earthlink.net>...> > I believe the quote is "Better is the mortal enemy of good".... > > I believe the reality is trite slogans are the mortal enemy of > rational analysis. At least, that seems to be their main application,Steve, Well, I surely agree with the reality you suggest particularly as it applies to current political speeches. However, I'd also defend *this* slogan as one very useful - because it requires one to stop and think about what they're doing. In my experience, that's how it's used. In case the nuance was lost, I made a little comment about "worst case analysis" to suggest it's best to know what you're doing (rational analysis). The emphasis is on "good enough" which obviously leaves plenty of room for interpretation. In many circumstances I'd say a design isn't good enough unless the designer knows what the worst cases are and how the system will behave under those circumstances. And, I probably don't go so far as to be looking for a FMECA very often at all. However, getting another -3dB out of a filter stop band that's already at -60dB just because you just learned how to do it might be an unwarranted exercise. [No, I'm not saying this example is never justified! Just often not.] I once managed a large project and the leader of one software team went to a software conference, spoke with one of the world's technical leaders, came home and trashed 6 months of work because he had found a better way. Funny, his part of the project didn't complete on time - and therefore didn't complete at all. So much for "better". Fred
Is there a point to theoretical understanding?
Started by ●March 4, 2004
Reply by ●March 8, 20042004-03-08
Reply by ●March 8, 20042004-03-08
On Tue, 9 Mar 2004 09:55:19 -0800, "Terry Given" <the_domes@xtra.co.nz> wrote: [snip]> >When I re-designed the system, I threw out the low-level mux, and kept Q a >fair bit lower, achieving overall gain/attenuation with cascaded sections. I >looked at producing a theoretical expression for BPF H(s) incorporating GBW >(i am a pedant, so enjoy this sort of nonsense),Which definition are you? ... Main Entry: ped�ant Pronunciation: 'pe-d&nt Function: noun Etymology: Middle French, from Italian pedante 1 obsolete : a male schoolteacher 2 a : one who makes a show of knowledge b : one who is unimaginative or who unduly emphasizes minutiae in the presentation or use of knowledge c : a formalist or precisionist in teaching> but in the end simulated it >in spice cos it was faster (using first a VCVS, then a laplace block). >trivial to show using spice that Aol alone had bugger all effect, but GBW >was v. important. comparisons with model using opamp showed good agreement. >GBW *and* excess phase matter a lot. See "Gyrator*.pdf" on the S.E.D/Schematics page of my website. [snip]> >Why are analogue designers smarter than digital designers? how hard is it to >count to 1...... > >Digital is a small subset of Analog ;-) ...Jim Thompson -- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | | http://www.analog-innovations.com | 1962 | Will you still need me, will you still feed me, when I'm sixty-four?
Reply by ●March 9, 20042004-03-09
"John Larkin" <jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote in message news:vb7k40l9131ug8lkmvd81rof57vq3j09uu@4ax.com...> On Thu, 04 Mar 2004 10:38:27 -0800, Chris Carlen > <crcarle@BOGUS.sandia.gov> wrote: > > > >This has me a a little bit concerned. I could spend a few months > >designing a system, by exercising all my theoretical knowledge. But > >this essentially amounts to reinventing the wheel, if the tools are > >available to do it for me, and there is no need for a highly optimized > >solution. Meanwhile the guy down the hall produces a result in two > >weeks, and tells his boss "look I make a custom DSP digital filtering > >machine" when he can't even spell c-o-n-v-o-l-u-t-i-o-n. I wonder if > >someone like myself who really *wants* to start from scratch just > >belongs in academia or someplace where getting off on intellectual > >tangents is the point rather than an impedement. Fortunately, Sandia is > >close enough to that that I don't get into any trouble here if I take my > >time. > > > > Who do you think the boss prefers: the guy who did it in two weeks, or > the guy who wants to study the problem for nine months or so?Im finishing off the artwork for a job now that fits in nicely with this thread. the customer paid a "digital" design house $100,000 or so to design a fairly complex analogue circuit, using lots of photodiodes, muxes and filters. It was quite late, and didnt work very well at all. The "designer" never thought about SNR etc, and didnt realise that muxing 120 photodiodes (50uA or so) (spread over about 1 ft^2) with HC logic is a bit iffy; and then feeding the result into a high-Q (20) filter is a good way to detect mux switching (all every E,H field in the near vicinity), but is kind of crap for the purpose at hand. The great bit about theory though was the filter design - he "cookbooked" an infinite gain positive-negative feedback BP filter. but didnt read the pages about opamp GBW effects - even though he used an 8MHz GBW opamp, the mid-band gain was high enough that GBW detuned the filter by about 10%, knocking the hell out of the center-frequency gain. plus of course the BPF filter described above starts with a resistive divider - in his case divide-by-50, which seemed a bit stupid really. When I re-designed the system, I threw out the low-level mux, and kept Q a fair bit lower, achieving overall gain/attenuation with cascaded sections. I looked at producing a theoretical expression for BPF H(s) incorporating GBW (i am a pedant, so enjoy this sort of nonsense), but in the end simulated it in spice cos it was faster (using first a VCVS, then a laplace block). trivial to show using spice that Aol alone had bugger all effect, but GBW was v. important. comparisons with model using opamp showed good agreement. When I went to design the filter, the FIRST thing I did was a literature search (own library, 120 mins) to see what topologies I could use, and it was easy to see that his choice was the worst one. a negative-gain sallen-key BPF was much better; a few hours with spice proved that conclusively. Then when I went to design the filter, I used the "cookbook" expression, for R1=R2+R, Rf = kR, C1=C2=C. the expressions were nice and simple, but I wondered if I could do better. re-forming the raw equations, then setting R1C1 = R2C2, I got a more complex set of design equations, but managed to squeeze a fair bit more pass-band gain, while keeping Q fairly low. Finally, i re-scaled the theoretical center-frequency to compensate for GBW, AND checked the effect of GBW variations over time, temperature, device & sock colour, to ensure all was well. the upshot - a cheaper product, that actually works, for a lower cost. All due to theoretical understanding. Why are analogue designers smarter than digital designers? how hard is it to count to 1......
