VCO 1 & 2 accidentally sync

[stephenm]

When VCO 1 and 2 are tuned very close, like a beat every several seconds, they will often sync.  It happens at the point when the two sawtooth resets coincide.

Anyone else seeing this problem?

- Stephen

[DrJustice]

That's phase locking which occurs when VCO 1 and 2 are very close in frequency. VCO 1 will phase lock to VCO 2 until a certain difference in phase and/or frequency is achieved, so when they're tuned very close and would normally make a very slow beating they will periodically snap into lock.

[stephenm]

And it's a defect.  It usually results from insufficient power supply bypassing, such that the reset pulse from one osc is coupled into the other via the power supply, or it's due to poor PCB routing, where the output of one osc is routed too close to a sensitive part of the other, namely its integrator.

[DrJustice]

Well, I wouldn't call it a "defect", but a "non optimal quirk". Those are found in various parts of many synths; phase locking, leakage, unintended non linearities etc., due to the physical design, as you obviously know BTW, it's only VCO 1 that phase locks to VCO 2 - no other phase locking occurs between oscillator pairs.