Re: Prototyping?

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.


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.

We just finished a 30-day eval of the Nuhertz filter design software,
and we ordered the LC filter version today, for $2K. It's got a few
bugs, but they expect to fix them soon. It's pretty spiffy; it knows a
zillion filter forms I'd never heard of (inverse Chebychev, hourglass,
Legendre) and can do cool stuff like systhesize filters from finite-Q,
standard-value inductors.

We're trying to design a 32 MHz 7-pole lowpass for a DDS-based
arbitrary waveform generator, with demanding frequency response *and*
step response, and it's really hard to do by hand.


Trying to meet a customer schedule deadline, I've seen tens of thousands of
dollars spent to do a quick-turn of a many-layer PCB containing relatively
simply circuitry such as RF switches, splitters, etc... but going to, say,
3GHz. And I've seen engineers find out within no more than 15 minutes of
receiving a stuffed PCB that their design doesn't meet some prescribed
isolation or frequency response flatness requirement. I cringe, because I
could have predicted before spending that sort of dough that there was no
better than a 50/50 chance that it would have worked anyway. Now, from a
business perspective at some point perhaps you can't increase those odds so
you might as well spend the money anyway (a 50% chance of something working
for, say, $25k may be better than a 0% chance of something working for free
and your missing a $60k customer payment...), but what I've taken away from
such experience is that one is better off going through *more* prototyping
cycles *early on* in the project so that you hopefully don't have to fight
such Pyrrhic battles in the first place.

Better yet is to not prototype at all, ever, and design everything as
if you are serious about selling the first one. If you assume the
first few iterations will not work, you'll always be right.

John

.

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