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I have a waveform I need to filter that consists of 1Million discrete samples of what is predominantly a noisy sine wave that decends eventually into pure noise.
There are around 50 data points per sine cycle. Ideally I would like to band pass filter the signal without any attenuation.
What type of filter would be best?

Is it something like f(t) = sin(t*freq)*decay(t)? You said that it "decays" into noise.

If yes, and both frequency and this "decay" component are exactly known, you have linear problem. Try to use lsfitlinear() from lsfit subpackage.
Otherwise you have nonlinear problem; you have to parameterize both sin() and decay(), and to use lsfitcreatefg function to solve it.