Technical note 5

Ground – a path for signal currents?

Ground is usually considered the dedicated path for signal currents. This view is short-sighted:
1) The earth itself is not a good conductor – vertical monopole antennas use the earth ('ground') as the other 'pole'; an antenna matt must be applied to avoid ohmic loss in the resistance of the earth.
2) Accessible metal objects (housing, chassis, etc) of equipment connected to the mains must be grounded to the earth, to avoid electric shock (when the isolation of conductors carrying the mains voltage breaks down).
3) Electronic circuits need DC to operate. Ground is connected in most electronic equipment to the negative pole of the DC supply unit and then applied as DC supply line. Only DC flows via this ground.
4) Circuits communicate with other circuits via two conductors. For cost price reasons, one of the two signal lines of each circuit is usually integrated into a common ground system. This ground system is also applied as a (usually negative) supply rail, such as the groundplane on a printed wiring board.
To avoid resonances, all ground conductors must be tightly interconnected; no 'open' loops should exist.
The ground connections between the signal circuits and the ground system must be short and the loop area between the grounded and non-grounded signal conductor must be sufficiently small to avoid emission and reception of noise. Observe that, even when a screened signal cable is applied, part of the signal current flows via the ground system, increasing emissions. A good ground is not as good as often expected.