J
John Larkin
Guest
On Fri, 19 Sep 2014 21:23:48 +0100, "Kevin Aylward"
<ExtractkevinRemove@kevinaylward.co.uk> wrote:
The other place simulation fails is when the sim would have to run for
days or years to produce an accurate result. Like a 2-minute event
simulated at 10 fs time steps. Or something that #$%^& LT Spice
refuses to converge on.
Don't get me wrong, some things, like filter design, are crazy to do
by simulation or experiment. Ditto things that involve real physics
fundamentals.
The diode recovery thing is not suited to math analysis, because we
don't know stuff like doping profiles of diodes that we can buy, and
Spice doesn't handle diffusion anyhow. We may as well use a behavioral
model based on part measurement, limited to modeling the behavior that
happens to matter right now. The textbooks tell us what sorts of
general behaviors we can expect in diodes.
--
John Larkin Highland Technology, Inc
jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
<ExtractkevinRemove@kevinaylward.co.uk> wrote:
"John Larkin" wrote in message
news:120p1a1o20l3o1q4htcib1k38v8s1isno5@4ax.com...
On Fri, 19 Sep 2014 19:45:56 +0100, "Kevin Aylward"
ExtractkevinRemove@kevinaylward.co.uk> wrote:
"Phil Hobbs" wrote in message news:54138195.8000608@electrooptical.net...
Granted, in your situation.
More often I'm trying to solve novel problems. A long time ago, it was
making clean BPSK SS UHF cheaply from a cheap crystal, at micropower,
fast-settling, with a lot of other constraints. There simply isn't an
equation that outputs a novel topology.
Quite so. OTOH calculating the fundamental limits as a function of the
crystal Q and transistor noise can be pretty illuminating. A few years
ago
when I was building stabilized lasers for downhole applications, I had to
go into a lot of that stuff, and learned a lot. (Leeson's equation for
oscillator noise is sort of the electronic analogue of the Schawlow-Townes
minimum line width of a laser.)
Leesons equation is only qualitatively, useful. Its F is unknown, so its
useless to actually calculate LF PN noise from.
If you can't calculate how good it _could_ be, how do you know when you're
done? It's a pity to declare victory and leave, when there's another 20
dB
available with affordable devices.
In fact, you can't calculate PN manually in practice, for real systems. The
equations are way too intractable. The only realistic way to design for low
phase noise is to use a phase noise simulation tools, e.g. Cadence RF or
Agilent.
I have paper on my site by A. Demir
(http://www.kevinaylward.co.uk/ee/phasenoise/phasenoise.html) which
irrefutably shows that techniques such as the Hajimiri-Lee ISF approach is
toilet paper.
The idea of using pen and paper to try and calculate closed form solutions
to todays problems is dead. It is a fact that the percentage of systems
with
closed form solutions compared to systems without, is the limit as x->0.
Its a paradigm shift to understand that using simulation tools to get the
answers is the best way and only way. I can an answer and gain real
understanding of a problem in minutes compared to pissing about with a
pencil and pad for months.
There is still, I think, an academic-leftover prejudice for equations.
Yes. Its job security. What would they do if they did not produce equations
10 lines long.
It took me a while to eliminate my own prejudice. I once spent an inordinate
amount of time learning all sorts of ways to solve differentia equations.
In a practical, complex, nonlinear system, the math may be
enlightening but not practical for quantitative solutions.
Enlightening sometimes, often one cant see the forest because the trees are
in the way. The equations are usually unmanageable. Middlebrook appreciated
and stressed this, illustrated by his reduced element theorem. He didn't go
far enough.
Simulation can be enlightening, too.
Very enlightening. Regarding phase noise, bench work simply can not compete
with the tests you can do in the virtual world. e.g. the sims spit out the
value of each contributing noise noise value, in order.
But then, some physical systems aren't well suited to simulation either, so
people still breadboard.
Yes. Simulation only works when the individual models accurately reflect
reality, and correct theory expressed in a numerical solvable equations
exist. The reverse/forward recovery diode thread shows that there are still
areas lacking.
The other place simulation fails is when the sim would have to run for
days or years to produce an accurate result. Like a 2-minute event
simulated at 10 fs time steps. Or something that #$%^& LT Spice
refuses to converge on.
Don't get me wrong, some things, like filter design, are crazy to do
by simulation or experiment. Ditto things that involve real physics
fundamentals.
The diode recovery thing is not suited to math analysis, because we
don't know stuff like doping profiles of diodes that we can buy, and
Spice doesn't handle diffusion anyhow. We may as well use a behavioral
model based on part measurement, limited to modeling the behavior that
happens to matter right now. The textbooks tell us what sorts of
general behaviors we can expect in diodes.
--
John Larkin Highland Technology, Inc
jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com