Those Bessel things are cool. You can set FM deviation really close by disappearing the carrier or some sidebands.
PLLs are fun. My latest one triggers an LC oscillator from an external input, and phase-locks it, at its natural frequency, to an OCXO within a microsecond or so. It still hurts my head to think about that. I did the analog stuff and delegated the FPGA and the hard math.
A good OCXO has a few picoseconds of RMS jitter, timing out a 1-second delay. A cheap XO has many nanoseconds.
Bang-bang PLLs are interesting too, with a d-flop as the phase detector. I invented that in my youth too, for an analog time-slot telephone system. They also hurt my head.