[sdiy] Sallen-Key Bessel Low Pass - behaviour and RC values

[Neil Johnson]

Hi Tom,

> So much "mojo" is just sloppy
> engineering that not struggling to get technical perfection isn't a crime,
> and might even be a benefit (if you get lucky).

"Technical perfection" is not engineering, that is science.

Engineering = Science + Economics

So, just because a solution is not perfect does not necessarily mean
it is sloppy.  Take your example of filter component values.  If you
want technical perfection then you'll most likely need to invest in
custom resistor values.  And then don't forget to compensate for
component value tolerance, component aging, temperature coefficients,
etc.  Best to use metal foil resistors as they exhibit almost zero
excess noise.  And use vacuum capacitors to avoid any problems due to
less-than-perfect dielectric properties.  Watch out for op-amp
imperfections too.  The AD797 is a very low noise op-amp.  Tricky to
tame, and bloody expensive, but we don't want any slop, right?

Or would you rather apply some trade-offs to design a product you can
make and sell in the kinds of quantities that your target market could
sustain?

Of course, sloppy engineering does exist.  But often times, unless you
know the complete design history, it is easy to ascribe something as
"sloppy" when actually it is the result of careful trade-offs.  Or
even simply making the best use of the available components at the
time.

Neil
--
http://www.njohnson.co.uk