Tim,

Microchip offers FilterLab.  Download it at:

http://www.microchip.com/1000/pline/tools/analog/software/flab/index.htm

TI/Burr-Brown offer FilterPro.  Download it at:

http://focus.ti.com/analog/docs/sampleutilities.tsp?path=templatedata/cm/utilities/data/filterpro&templateId=3&familyId=57&navigationId=9742

Sorry about the wrap!

The underlying problem is (as several people have tried to point out)
that because of the wide spread between your minimum frequency and your
maximum frequency, any FIXED low pass filter which will "round the
edges" of your minimum frequency will reduce the amplitude of your
maximum frequency signal tremendously.

The fix, if you want to use a fixed filter, is to modulate the pulse
width (on/off duty cycle) of a much higher frequency signal
"sinusoidally" at the frequency you want to appear at the output.

Dave

Timothy Box wrote:
> Can any body point me to a URL or is it a case of looking in an active filter book?

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