theswellylife:
So I am building a contactless heart monitor, using a parallax x-band motion detector.
Maybe I'm missing something, but according to the Parallax X-Band Motion Detector User Guide, its output is binary, not analog. That would suggest sampling it by using edge detection on a digital input and timing the transition intervals to derive frequency might be the best approach. Also, on basic principles of Doppler radar, it's not clear to me how the output frequency is necessarily the heart rate measurement you're looking for hence your filter requirements derivation probably isn't right.
I'd suggest an iterative approach where you set up the Arduino (or a digital scope) to collect a sample of the sensor output, import that to Matlab, and work out your signal processing/detection scheme in that environment prior to trying to implement it on the Arduino.