All known FM tricks are used in the FM Aid, you can play with the frequency ratios between carrier and moduator, control the modulation index by the FM potentiometer and set the modulator's amplitude. The output signal is being patched back to the modulator's input what results in feedback FM. As the modulator input is normalized to the carrier input you can use the module with one oscillator only, in this case there won´t be and frequency differences but still interesting sounds resembling a wave folder.
Usually the FM Aid expects a sawtooth wave at the carrier input that will generate trangle, sine and square by a curve converter. But you can use any audio signal at the carrier's input and get very complex sounds.
Technically speaking FM AID doesn't use Frequency Modulation but Phase Modulation that produces a brighter sound.
modulator input, carrier input, CV input
waveform outputs: triangle, sine, sawtooth, square
3U Eurorack module, 8HP wide, 30mm deep
Current draw 45mA @ +12V and 45mA @ -12V