Undesirable coupling is the unintended connections between two circuits, that may result in undesirable mixed-frequency signal at the load.
This connection is mainly due to:
1. stray inductance: where the long run of the two wiring in close proximity to each other and in the same conduit results in magnetic coupling between the two circuits (can be simulated as a transformer with poor magnetic coupling).
2. stray capacitance: where the parallel wiring and insulation between them result in electrostatic coupling between the circuits (can be simulated as a low-capacitance capacitor).
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These two circuits simulate the an industrial case where a high-voltage AC power circuit (the upper circuit) is coupled with low-voltage DC control circuit (the bottom circuit). And the result of this coupling can be clearly shown at the load where a mixed-frequency signal (AC+DC) is present across the load instead of a single DC signal.
Undesirable coupling can't be completely eliminated, but the effect of coupling can be reduced by reducing the effect of stray inductance and stray capacitance, as follow:
1. Reducing the stray inductance by using two-conductor wiring instead of single-conductor wiring, where the resulting inductance in both conductors will be equal and opposite, thus cancelling each other out.
2. Reducing the effect of stray capacitance by wrapping the DC wiring with grounded metal shield (braid shield or metal foil), which serve as a Faraday Cage.
Also decoupling capacitor can be connected in parallel with the load to serve as low-pass filter and block high-frequency noise.
