Common-Mode Noise
It is noise current that flows in the same direction in all conductors of a cable bundle and returns through earth. Because it behaves differently from the useful signal's go-and-return pair, it cannot be suppressed with twisted pairs and differential inputs; the targeted measures are ferrites and correct earthing.
Differential mode noise enters through the outgoing conductor and leaves through the return conductor. In common mode, however, the current travels in the same direction in all conductors and closes the circuit through earth. In inverters the source of this current is the current that rapid voltage changes drive through the parasitic capacitances between the motor and cable and earth.
- Why it matters: common-mode current entering the receiver circuit can shift the reference point and cause a valid signal to be read incorrectly; the result is intermittent response.
- Measures: a common-mode choke or ferrite ring (all conductors passed through together), short earthing of large cross-section, and 360° termination of the motor cable screen at both ends.
- What does not help: using only a twisted pair or placing a capacitor between conductors; these target the differential mode.
Correct diagnosis starts with distinguishing which mode is dominant; trying a ferrite is a practical test that shows this quickly.
The context in which this term is used: Wireless Crane Remote Control Selection Guide