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Originally Posted by Schmitty!
We had a similar issue, not with the FP motors, but with the Mabuchi motor.
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Schmitty,
Your video demonstrates the high impedance theory pretty well. The PWM cables act as nice little antennas and pick up the arcing of the brushes as you described. Many motors like the FP and Mabuchi have internal capacitors and/or series inductors to limit the EMI radiation. I have lobbied for teams to be able to add suppression and I will continue in the future. In most cases, when the RC is enabled, the output impedance of the RC is sufficient to reduce or eliminate the crosstalk. Although we didn't spend much time analyzing, wrapping the PWM cable in a ferrite core may have eliminated much of the disturbance and wouldn't have violated any of the robot electrical rules.
I am aware of one team adding capacitors across the PWM cables. I would never recommend this as the capacitance would almost certainly affect the rise time and pulse width making control less reliable and causing some unusual loading on the outputs of the RC.