A Pure Data very simple 3 operator FM synth that works well with MIDI Theremin (version 2.3 and above) https://github.com/MrDham/OpenTheremin_V3_with_MIDI
This synth is developed in Pure Data language. for more information go to : www.puredata.info/.
It consist in 3 serialised FM operators (Modulator 2 --> Modulator 1 --> Generator), a filter, a VCA and a vibrato (mod wheel).
yml file gives configuration options to use it in Zynthian project https://zynthian.org/.
pitch bend range = 48 ! Update your MIDI theremin (version 2.3 and above) if it is only supporting 24 semitones max !