Regarding TPU research... I just wrote up some wave heterodyning modeling software in Pure Data... this lets you model PLL (phase-locked) square waves of even and odd multiples, with separate phase control and PWM on each wave! With built-in software scope and audio output! This shows us how a phase locked HEXFET output might look (without the magnetics)!
I might add this also gives you the audio output of your wave combinations on your computer speakers.
FeynDyne v1.0 D1
http://www.feynmanslab.com/images/post2/screenshot1_cropped2.png
To use, download and install PureData Extended from http://puredata.info/community/projects/software/pd-extended . Then just download and open the PD file I have provided, and you can experiment with heterodyning square waves before you wire the MOSFETs.
FeynDyne v1.0 D1 - Wave HeteroDyning Modellng Software in PureData , for experimental physics
http://www.feynmanslab.com/docs/
http://www.feynmanslab.com/docs/feynman_osc_tpu2_multi_D1.pd
Cheers,
Feynman