An enclosure that has high conductivity and completely surrounds a piece of equipment forms an excellent shield against RFI radiation, provided it is grounded A useful rule of thumb, however, is that below 2 to 3 MHz interference from one component to the next in a system is primarily electromagnetic (i. e, the coupling is by magnetic and electric fields), whereas above this frequency radiated (r-f) energy is the primary carrier of interference This means that low-frequency shielding should be mostly composed of ferromagnetic materials to eliminate magnetic fields and high-frequency shielding should have high conductivity since magnetic fields are not significant Often, if magnetic materials, such as steel, are used at high frequencies, the ohmic resistance they have (compared to materials such as silver) causes potential differences and subsequent electric fields to be set up in shields around sensitive circuits, thereby nullifying some of the effectiveness of the shields In the gray area (a few megahertzs) composite shielding, such as copper-coated steel, is often used. These shields should be used wherever the adjacent equipment or components are sensitive to interference (e. g, coaxial shields applied on input leads and shields placed over any gaps in the case if either enclosed or external circuits are r-f radiation-sensitive).

Though shielding is effective in the elimination of interference, additional suppression is sometimes added by filtering inputs, outputs, and power connections to elimi­
nate noise riding in on those lines. This can be accom­plished by placing feed-through capacitors or other filters on lines entering the shielded enclosure where the affected equipment is located and by using diode—capacitor isola­tion networks on power-supply leads where they enter the instrument circuit-board area within each individual piece of equipment This ensures decoupling of equipment

The power source for instrumentation equipment should be free from spikes, jitter, and poor regulation. Without proper filtering and regulation, any transients that occur will couple through power transformers in equipment (through mterwinding capacitance) and cause difficulty m sensitive circuits This problem is usually eliminated by a power-line filter which may be no more than a 0.02-juF, 600-volt capacitor connected from each side of the line to ground.

In summary, interference can, and usually does, enter equipment wherever there is an unprotected entryway It is up to the equipment installer to make sure that no signals other than those desired by the circuit designer enter the equipment To accomplish this task, he must be aware of the many techniques available for suppression of interfer­ence.