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A-[Shield]-should have No Currents Flowing thru it!
As with Bypassing, Decoupling, and the Groundplane, Shielding is also a subject that gets little coverage in the classroom; but is emphasized in many Application Notes, and used in many real designs!

Again, the legacy is that many Design Engineers and Technicians ignore it at their peril!

 A Shield is a blocking or containing barrier against electrostatic fields, and to a lesser degree, magnetic fields.
In a circuit or system, there are many discrete signals which are directed to specific destinations only. Often these signals can interfere with other unintended signal destinations or devices. Shielding can prevent or diminish this effect.
As with the Groundplane, a Shield can be constructed of one or more conductive homogenous planars. However, the shield departs from the Groundplane by its need to NOT contain conducted or induced electrical currents.
Shielding can be anything from using a coaxial or shielded cable, to a sealed conductive chamber for circuit isolation. Shielding serves a reciprocal purpose: it protects the circuit it is shielding from outside noise or unwanted signals; and conversely, it contains its own signals and thus protects the outside world from interference of its own making. 

Shielding is mostly used to block electrostatic or "E" fields (Faraday shield). However, if ferrous metal (tempered Mu Metal works best for magnetic fields) is used, then both electrostatic and some level of magnetic shielding is accomplished. This is especially useful where open frame transformers or unshielded coils are used and would otherwise exchange signals by mutual inductance. 

Fortunately, the Magnetic component of the (interfering) signal diminishes as the cube of the distance. That is, for any high frequency signal the dominant component beyond ~1/6 wavelength is Electrostatic. This is referred to as Near Field & Far Field radiation. See Fig.

Ideal Shields have No Current Flowing thru them: 
Ideally they are "Grounded," or Referenced, at only One Point!
One important requirement for a shield to be effective, is that there must be no currents flowing through the shield itself. This is best accomplished by connecting the reference or common, to only one point on the shield, thus preventing any current flow. The reason for this is that any current flowing in the shield material itself can produce secondary fields on the other side of the shielding material and thereby reduce the effectiveness of the shield. An example of this might be a shielded cable, where the shield has a different potential at each end, with the resulting current flow in the shield, inducing unwanted noise into the center or shielded conductors, as well as, to the outside world.  (In this situation one might find a remedy by disconnecting one or the other ends of the cable's shield. However, this may not prove satisfactory in certain environments, and may require a "Guard" potential, or better still: Optical Isolation.)
Active Shielding
There is an active form of shielding where fields of counter EMF (equal but opposite) are generated to cancel out the offending fields. A good and simple example of this is the AC power transformer, where a "shorted turn" is used to generate a nulling field. 

The shorted turn, is a seamless band of copper that wraps the transformer core in one direction. When cut by the rising and collapsing magnetic flux -- caused by the transformer action -- the shorted turn acts as a very low impedance, high current secondary winding, and generates a counter EMF, and because this winding is shorted, it generates a rising and collapsing magnetic field of opposite polarity thereby nulling the original stray magnetic flux. In some cases of severe common mode noise, the shield can be made to carry an equal but opposite noise current to counter the interfering noise. However, this is not for the faint-of-heart: any slight change of the mechanical or electrical parameters, and the canceling noise becomes the noise.

Some power transformers use a Faraday shield between primary and secondary ... however, this is not a shorted turn.  The current flow in a shorted turn is very high! The shorted turn would have to link only the leakage flux to be workable ... in other words, the turn is around the outside of the magnetic core. 

It will soon become apparent that Shielding is a very popular strategy in EMC applications.
RF Emitter inside a Shielded Enclosure
        Example of "Ideal" Shielding: no leakage
Shield Reflecting
Shield Reflecting, with 
some "Parasitic" Radiation
Leakage of Apertures in 
Shielded Enclosure: Diffraction
Apertures act as "Point Sources"
Driven Shield Radiating
Currents in Shield create "Antenna Effect"
The area of an Aperture is less important than its length—as in seams or joints. The rule of thumb is the maximum dimension of an aperture should not be greater than 1/20 the wavelength of the highest frequency of interest. Such apertures can act as "Slot Antennas."
In dealing with a Radiation Source --be it an Antenna or a Circuit Board: 

The dominant energy in the Far Field is Electric.

The dominant energy in the Near Field  is Magnetic.

In any high frequency signal the dominant radiation component under ~1/6 wavelength from the source, is Electromagnetic; this is referred to as Near Field radiation.

Any signal beyond ~1/6 wavelength, is Electrostatic, and is referred to as Far Field radiation; sometimes called the Plane Wave.

Near Field & Far Field Radiation
Well shielded container made from soldered copper clad printed circuit board material. 
Shielded Audio Cable
Isolated copper island under IC 
acting as a shield 
Examples of Shielding
Graphics Card with Shielded TV Tuner 
Shielded PC power supply
 Real World Applications of Shielding
Conductive paint
Custom gasket
Solderable & snap-in metal shields
 "O" Ring
Conductive ATV
 Connector gaskets
 Compressible gasket

Additional Shielding Aids
Conductive paint, conductive gasket making silicone (ATV), and assorted conductive gaskets.

Circuit Board Layout
 CAD Drawing
  Signal Input
Signal Output
Power Input
  Shield Side
Note the single grounding (reference) point.
1) Shielding can be anything from using a coaxial or shielded cable, to a sealed conductive chamber for circuit isolation. 

2) Shielding serves a reciprocal purpose: it protects the circuit it is shielding, as well as, protects the outside world. 

3) There must be no currents flowing through the shield itself. 

4) To prevent current flow, connection to common should be at only one point on the shield.

5) The maximum dimension of an aperture should not be greater than 1/20 wavelength.


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