At the equipment level, the individual signal reference planes for the various networks must be connected
together to prevent personnel shock hazards (see Chapters 5 and 9) and to provide as near as possible, the same
signal reference for all networks. Thus, the signal reference plane may extend over large distances within a
facility. The assumption that this large reference plane provides zero impedance paths is not valid; the series
inductance and resistance of the conductors forming the signal reference plane and the shunt capacitance to
nearby conductive objects must be considered. Currents flowing in the signal reference plane will develop
voltages across this impedance and will produce electric and magnetic fields around the conductors.
6.2 COUPLING MECHANISMS.
Coupling is defined as the means by which a magnetic or electric field produced by one circuit induces a voltage
or current in another circuit. Interference coupling is the stray or unintentional coupling between circuits
which produces an error in the response of one of the circuits. The possible sources of spurious signals and the
mechanisms by which this interference is coupled into a susceptible circuit must be understood in order to guard
against interference pickup by sensitive signal circuits.
The techniques for reducing pickup depend on the type of interference present. Interference is broadly
classified by its coupling means; i.e., as either being conductive or free-space. Conductive coupling occurs
when the interfering and the interfered-with circuits are physically connected with a conductor and share a
common impedance. Free-space coupling occurs when a circuit or source generates an electromagnetic field
that is either radiated and then received by a susceptible circuit or that is inductively or capacitively coupled
(near-field) to a susceptible circuit.
6.2.1 Conductive Coupling.
Power lines entering a facility provide good conductive coupling paths from interference sources external to the
facility. This interference can easily be conducted into a particular unit or piece of equipment via the power
lines entering the equipment. Also, interference can conductively couple between various circuits inside the
equipment on the common dc power lines. If one dc power supply is utilized with several circuits operating over
various signal voltage and frequency ranges, the operation of one circuit may adversely affect the operation of
other circuits. For example, if both the preamplifier and the power amplifier sections of an audio amplifier are
supplied from a single dc power supply, variations in the relatively large current drawn by the power amplifier
can appear as supply voltage variations at the preamplifier. These variations can be large compared to the
operating signal levels in the preamplifier; the unwanted variations are amplified along with the desired signals
and may produce distortion in the output of the amplifier.
Another set of paths for conductive coupling of interference is offered by the signal lines. In general, signal
lines enter each facility and each unit or piece of equipment; i.e., such signal lines are usually necessary for
interfacing electronic circuits. Interference can be conductively coupled into facilities, equipment, and circuits
as readily by signal lines as by power lines.