At 11:35 AM 5/12/2005 -0500, you wrote:
>Finally got around to implementing/trying some proportional filtering [
>N=N+(N-L)*K ] in one of my pieces of PIC code (which is essentially a
>frequency counter).  This thing is sweet!  I've experimented with K=1/4, 1/2,
>and 3/4.  For my app, 3/4 works well (but I still have a bit of averaging in
>there still, since I'm experimenting with combinations of averaging and
>proportional filtering).
>
>BTW, I chose these constants since they made for some simple assembly code.
>
>Curious about one thing though -- anyone have a correlation or calculation to
>compare software proportional filtering to an RC filter?  Or asked another
>way -- how do I determine the equivalent hardware filter time-constant for a
>proportional software filter?  I'm sure the update rate of the calculations
>will be a factor here.
>
>I don't really need this for anything specific, just think it will be an
>interesting tidbit of info.
>
>Cheers,
>-Neil.


This simulates a single-pole RC low pass filter with time constant
tau = Ts * (1/K - 1)  (where Ts is sample period)

So, if you want tau = 0.5 seconds and you are sampling 20 times a second
(Ts = 0.05) then you'd use K = 1/9 .

The output never quite reaches the input (with infinite precision math), so
it's an IIR filter. With a step change the error after t time is

% error  = 100% * exp(-t/tau)). So with 0.5 second tau, after 5 tau (2.5 
seconds)
you'd have an error of about 0.7%.



Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff


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