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Bob (and o-list):
> But your filter seems to become unstable with small values of Length.
As you probably saw from the letter I blind-carbon-copied to you, the
problem is due to the adjusted angular cutoff frequency. At a length of
about 3.1075, the formula for Wc* blows up because it tries to calculate
tan(pi/2).
And of course at values less than 3.1075, the result of the tangent is
negative, which makes things just as bad.
A positive way to look at this is, a length of 6 in this critically-damped
filter is equivalent to a length of about 2 in XAverage or T3 -- all will
have a 3 dB cutoff wavelength of 6. So a 6-bar critically damped filter
is similar to a 2-bar moving average, which is probably as small as one
would need.
-Alex
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