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.