Custom Search
|
|
|
||
 MIL-HDBK-419A
The single-point ground accomplishes each of the three functions of signal circuit grounding mentioned at the
beginning of this chapter. That is, a signal reference plane is established in each unit or piece of equipment and
these individual reference planes are connected together and to the earth electrode subsystem. An important
advantage of the single-point configuration is that it helps control conductively-coupled interference. As
illustrated in Figure 5-15, closed paths for noise currents in the signal ground network are avoided, and the
interference voltage,
in the facility ground system is not conductively coupled into the signal circuits via
the signal ground network. Therefore, the single-point signal ground network minimizes the effects of lower
frequency noise currents which may be flowing in the facility ground.
Single-point grounds, however, also become transmission lines at higher frequencies with earth being the other
side of the line. In addition, every piece of equipment bonded to this transmission line will act as a tuned stub.
In the presence of digital signals (square waves) the tuned circuits will ring at the specific frequencies to which
they are resonant. Since single-point grounds behave as transmission lines at rf frequencies, they will have
different impedances as a function of frequency, i.e., they may appear as inductors, capacitors, tuned circuits,
insulators or pure resistance, and therefore become extremely poor grounds. In a large installation, another
major disadvantage of the single-point ground configuration is the requirement for long conductors. The long
conductors (1/8
at the highest frequency of concern) prevent the realization of a satisfactory reference for
Figure 5-15. Use of Single-Point Ground Configuration to Minimize Effect of Facility Ground Currents
5-23
|
 
|
|
|
||
