Single point ground above 1 MHz

A single-point ground connects each circuit to ground at one common point through its own wire. It is effective at low frequency (below 1 MHz) because the ground wire impedance is small and there are no loops.

Above 1 MHz it fails because:

1. The ground leads become long, and their inductance (X_L = 2πfL) becomes significant. For 1 m of wire L is about 1 μH, giving X_L = 6.3 ohm at 1 MHz and 63 ohm at 10 MHz, so the "ground" point is no longer at zero potential.
2. Any lead whose length reaches about λ/20 becomes a transmission line or an antenna, so the ground wires resonate and radiate or pick up noise.
3. Stray capacitance between the ground wires and chassis creates unintended multiple grounding paths, i.e. ground loops and common-impedance coupling.
4. The result is ground-potential differences, ground bounce, crosstalk, ringing and instability or malfunctioning of the circuit.

 C1 --L1--+
 C2 --L2--+--- point    each L = lead inductance;
 C3 --L3--+            Vg = L di/dt grows with f

Hence, for frequencies above about 10 MHz multi-point grounding with a ground plane (shortest paths, low inductance) is used; between 1 and 10 MHz a hybrid grounding system is used.