Re: Prototyping?

On Fri, 02 Nov 2007 10:12:56 -0700, Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:

John Larkin wrote:
On Thu, 1 Nov 2007 17:29:52 -0700, "Joel Koltner"
<JKolstad71HatesSpam@xxxxxxxxx> wrote:

"Joerg" <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote in message
news:kGtWi.2430$yV6.1485@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Agreed. I breadboard only when trying to use parts in really unorthodox ways
(happens a lot...) or when I need a one-off to control something and it
doesn't have to be pretty. Beats the usual 1-2 week wait for fab, stuffing
and all the Fedex in between.
OK... but say for something as "simple" as a UHF bandpass filter (say a
standard ham band one... 420-450MHz, assume you've decided you need a 5th
order Chebyshev filter to obtain the skirts you want), do you (or John) expect
you can design and layout a working PCB without either...

1) Performing simulation using one of the high-end CAD tools like Genesys,
ADS, or Microwave Office, which can use very good models of the capacitors and
inductors you're wanting to use as well as accounting for most parasitic
effects of pads and trace width variation.

LTSpice should be enough at 450 MHz. Or even just yank a prototype
filter from Williams' book and scale it. With surface-mount parts, 450
MHz isn't really different from audio.


Even 2.45GHz is done with discretes these days. What surprised me last
week (UHF filter design) was that nicely toleranced caps where only
$0.01 and inductors $0.05. That was almost as good as spotting a
favorite microbrew on sale.

We mostly do untuned wideband stuff, DC to as high as 4 GHz, which is
fine with surfmount parts on FR4. But if you want to do narrowband
filters, at some point you have to tune them, or go to saws or coaxial
ceramic resonators or something, since the discrete tolerances will
getcha.




2) Fully intending to perform a fair amount of tweaking (generally of
capacitor values, given the frequencies involved) once you actually build the
board.

Why tweak a prototype kluge, then move it to the real PCB, and tweak
it again? If you expect to have to tweak a few cap values to get it to
work, why waste time doing it twice? Besides, complex filters are
almost impossible to tweak experimentally, unless you're willing to
tolerate truly rotten passband response.

???

I certainly can't do so myself, and if someone else can I'd love to learn
their tricks! The fact that I've never read of such a method in amateur
publication (books, magazines, etc.) suggests to me that if probably isn't
doable... and the commercial guys just go with method #1 up there.

I don't think it will offend anybody to point out that amateurs are
amateurs.


Au contraire. I found that there still is a decent population among the
ham radio community who can design a nice GHz range filter and actually
make it work. Maybe only a few percent but IME the percentage of folks
coming out of university who could do that is nearly zero.

Our problem isn't to make one filter work. It's to make an entire
board work: bus or comm interface, power supplies, uP, firmware,
fpgas, dacs, adcs, filters, output amps... all in maybe 90 days. We
don't have the time to breadboard, or even play with eval boards,
except for small, rare situations. We certainly don't have time to
prototype any serious subsystem. So we lay out the real thing, fully
documented and formally released, and let Production build one or two
for us to test. That saves scads of time and hassle - they solder
better and cheaper than engineers - and if you're careful, the first
one is the real thing.

Intel has, or at least had, a similar philosophy: new chip ideas were
fabbed on the production lines, not in some backroom lab.

John


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