Filtering

Low pass filter - ideally attenuates high frequency modulations while allowing high frequency modulations to pass unchanged

High pass filter -

Band pass filter


http://mail.google.com/mail/?ui=2&ik=f7d0bbec44&view=att&th=11c8c0105d528182&attid=0.1&disp=vgp&realattid=f_flfmf9cz0


If the noise has zero mean then one way to low pass filter is to average over nearby signals

Causal filter
A recursive filter
xlp(t) = a*x(t) +(1-a)*x(t-1)

Get 

xhp = x - xlp


Sampling -- You must sample at least twice as fast as the highest frequency you want to measure (Nyquist)

Want to filter out frequencies higher than your Nyquist frequency (half sampling rate) in order to prevent aliasing

aliasing demo
http://www.michaelbach.de/ot/mot_wagonWheel/index.html