Peter O. Brackett said:
John:
[snip]
*If* you can get the experts to say more than "No" and "You are daft."
John
[snip]
Jeeeshhhh....
For what you pay for the information on this NG! Be thankful for small
gifts!
BTW... if you had to research and cull out all of the information provided
on this particular
thread by yourself you are probably looking at several months of sole effort
work, providing
you don't come to wrong conclusions!
If you attended a *good* school where the professors actually knew and
taught the stuff
you learned on this thread you would likely have paid up to $20,000 and
taken 12 months
to learn it. Of course you would first have to had gone to the effort to
find a *good* school
where the instructors actually know this particular stuff! There aren't
many!
If you hired an expert consultant, say like myself, my one time retainer and
per diem fees
for this kind of information would have cost you at least a couple of
thousand dollars. Of
course first you would have to go to the effort of finding a qualified and
knowledgeable
consultant with an appropriate track record.
And gee... here you got all this "free" information on lossless LC
ladders...
I know that on the Internet, "information wants to be free"!
But do you really have to "trash" the experts who don't really have to read
questions
and post answers?
For example, the only pay back I get from posting to this NG is the
occasional call usually
from a lurker who actually becomes a client and pays for my services.
I feed my family from "billable hours". Jeesh... my time is valuable, I
don't hang out here
simply to entertain and educate freeloaders!
Especially when they offer only diatribes in return.
"Cool your jets" John.
Regards,
--
Peter
Freelance Professional Consultant
Signal Processing and Analog Electronics
Indialantic By-the-Sea, FL.
Peter, I for one would like to offer you my thanks for your lucid, and IMHO
excellent, posts on this subject. I certainly have learned from your posts,
(which I have archived) and have done some research (unnecessary but I like
to be thorough) to confirm your statements regarding LC ladder filter
sensitivity. I have written and tested a variety of digital filters, and the
LC ladder-like structures certainly do work best; I have also just recently
completed a redesign for a client, in which I replaced an infinite-gain BPF
(Q=10) with a negative-K sallen-key BPF. The original design missed the
target centre-frequency by about 15% due to finite GBW, and component
tolerance (GBW variation was the worst) caused tremendous changes in
pass-band gain, rendering the overall circuit inoperable.
I didnt even have to figure out the component sensitivity formulae, as its
all been published before, and a little bit of research showed the original
topology was the worst possible choice.....SPICE Monte-Carlo simulations
confirmed this (in order to model GBW i used a laplace block with a
first-order model, and adjusted Aol and Fo manually - changes in Aol made
very little difference (as expected), but changes in Fo were quite dramatic;
the 1st-order model behaved very like the opamp models, but was easier to
modify).
Correcting the original design for nominal GBW didnt help much, although
using $$ 0.1% discretes tamed the design, except for GBW variations....which
a minimum GBW of 20MHz fixed nicely. In this case cost was the driver, so a
lower sensitivity design (higher pass-band gain, lower Q) allowed me to use
a cheaper opamp, and slightly fewer discretes (1% R, 2% C), yet attain much
better performance (over time, temperature and tolerance - the three "T's" )
than the expensive reworked original design.
It would appear that many "designers" dont take the three T's into account
at all, and are thus blissfully unaware of sensitivity analysis.
cheers
Terry