The DDFs of yore used switched capacitor filters to create a very narrow bandwidth signal to measure phase.   The Teensy 3.2 has plenty of horsepower to do the same.  There are many ways of doing this and I hoped to use the Teensy Audio Library.    The library offers three filters: FIR, biquad and state variable.  16 concatenated biquad sections may work.  By using what seems to be a high Q value of 5 - 25 the raw heading readings  settle down nicely.   After fiddling with the other filters the biquad stands out as the best.  The Teensy Audio Library runs only at a 48ksps rate for high fidelity.  I would probably be better off sampling at something like  8 ksps.  If I roll my own I may end up doing a DFT for the Doppler Tone and second harmonic bins.  I can read the phase directly from the Doppler bin which eliminates the need for the zero crossing measurement  and bandpass filter/comparitor.   I would love to ditch the comparitor IC and parts.    

The rotational rate/Doppler frequency needs to be variable.  With the biquad I can restart the filter after changing the center frequency.  Likewise I can easily change the filter Q which should be very useful.  Going to an FIR filter requires a new set of coefficients for each frequency.  Some DDFs offer a limited set of frequencies which is good enough.  State variable filters might be an easier option.