Two 50% cycle clocks with a 90% phase shift will give a 50% output when mixed by XOR gate.
To make a PWM signal, both clocks are put through an RC to delay them. The centre pot adjusts series resistance; reducing the series resistance increases the delay of one clock, increasing the series resistance reduces the delay of the other clock.
Reducing the phase difference between the two clocks from 90° towards 8° brings the two signals close to being in phase; the XOR result is a very short high. Increasing the phase difference from 90° towards 172° brings the two signals very close to being completely out of phase; XOR result is a very long high.
