Today I am patching a combo I finally can achieve: a Shepard Tone, the sonic equivalent of a barbershop pole.
To do it entirely on an analog modular synth, I have the mighty Doepfer A-143-1 quad EG/LFO, with each chained to the next, and separately patched to the CV inputs on an Intellijel Quad VCA. Four individual triangle VCOs are sent to the VCA inputs, so they fade in and out with the cycle of the quadrature LFO. The same signal coming from the Doepfer channels is fed simultaneously to the 1v/8va input of each VCO
matching that channel on the VCA.
The effect happens as each VCO is changed in pitch according to the LFO at the same time it is being faded in with the VCA by the same LFO shape. As the next LFO fires it fades the next VCO the same way, one after the other like an ouroboros.
With each quadrant of the LFO represented (+/- sign/cosign), the continuous sound you hear is a constant rising tone. Put through a bandpass filter to fine-tune the audible range, the result is an audio illusion: the pitch of the sound seems to always be going forever upward.