> I need to design a filter with the following spec.
> Im not sure about the cutoff but it needs to maintain the following
> figures up to 70kHz:
>
> amplitude flatness to within 0.01%
> phase flatness to within 0.01%
>
> and be around 40dB down by 700kHz.

70 to 700 = 1 decade.
You get 20 dB per decade per pole so you want at least two poles.
You can get 2 poles with RC alone but almost certainly not to meet your
spec.

While you can do this with LC the normal method would be to use an active
filter UNLESS there are good reasons to go passive. Using an active filter
you would probably use 4 poles to relieve design constraints. Some versions
can be built with a dual opamp.

I think you'll find that the phase and gain flatness are a real challenge.
Chebychev is definitely out. Butterworth is maximally flat but Bessel has
best phase flatness.

There are any number of free opamp design programs around - most IC
manufacturers provide them including Linear Technology, NatSemi, Burr Brown
(now TI), Microchip and more.

Here's Microchip's offering

        http://www.microchip.com/1010/pline/tools/analog/software/flab/

Don't pay money for any such software - lots of free versions around.



        Russell McMahon

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