Aleenanne - Electrical : Electrical Circuits

SHIELDING

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Bypassing, Decoupling, S H I E L D I N G and Groundplane

`Layout`	`= Stability`	`= E M C`	`= Performance`
`Bypassing`	`= Shunting`	`= Diversion`	`= Stabilization`
`Decoupling`	`= Isolation`	`= Separation`
`Shielding`	`= Blocking`	`= Impeding`	`= Protection`
`Groundplane`	`= Return`	`= Sinking`	`= Referencing`

A \|/ Groundplane\|/serves as the Return Path for All Signal Currents

An "Ideal" [ S h i e l d ] has No Currents Flowing Through It

G r o u n d p l a n e

A ground plane is a special and very important component of any circuit. In essence, it is the return path for all signals including the power distribution. The ground plane can be thought of as homogeneous for the DC power only. In all other situations it is strictly inhomogeneous. All this means is, that all grounds are not the same. As various circuits use the ground plane for their signal and power return paths, currents--conducted and induced--are caused to flow throughout the ground plane, and potiently can affect any or all other circuits, and can cause real problems.

There are only two ways to model the ground plane in a complex signal environment: and nobody knows what they are!

One can start to understand the function and design (and FM) of ground planes if one does the following:

1) Draw a map of all signals in a circuit, their inputs, outputs, paths, and their various connections to and from the ground plane;

2) Then model the inductances, capacitances, parallel and series resistances while noting the power distribution paths and returns and their respective noise content;

3) And don't forget all bypassing devices and their contributions to the model;

4) Since the ground plane is mostly inductive, note must be taken of any other inductances in proximity to the ground plane, such as transformers, chokes, tuned circuits, etc., and their contributing fields at all relevant frequencies.

D E M O of Return Currents in a G R O U N D P L A N E

Demonstrates Return Currents in a Groundplane

Click_Itfor Anim

Demonstrates Return Currents in a Groundplane


Internal Vcc & Ground Layers	Seperation

M u l t i--l a y e r Printed Circuit Boards

The use of multi-layer printed circuit boards allow the use of multiple ground planes, as well as buried (under the signal layers) Vcc and Grd. layers. These layers are sandwiched together and act as a very efficient distributed bypass capacitor. A variation on this is to have the Vcc and Grd. layers as the outer or intermediate layers, thus shielding the buried signal layers; or some combination thereof.

G R O U N D P L A N E--verses--S H I E L D I N G

DEMO: The Demo is of a resonant Groundplane where a second Groundplane (or Shield) in brought into proximity, and the effect on its resonate frequency.

Click_It for Anim

Second Groundplane

Resonant Groundplane

This "Resonant Groundplane" demo is a simple example of what can be a very complex problem; i.e., a non-homogeneous groundplane (many apertures) with many different return currents from many different devices running at various rates; having rise and fall times from a few nanoseconds, to fractions of a nanosecond--all sharing the Groundplane as their return paths.

This can gives rise to device oscillations and instabilities, generating NOISE and Crosstalk, etc. This effect is due to the imperfect "A.C. grounding of active devices, and the many parasitic reactances that inhabit the circuit & board topology. This complex environment could, I suppose, be analyzed and the "tuned circuits" identified... --you get the idea.

To make the point that the use of GOOD Groundplane Design is important, especially in a mixed environment (analog/digital), an experiment that I call a "resonant groundplane" was designed.

In the example, a 4" X 6" copper clad board was used, and a narrow (1/8") strip of the copper cladding was peeled almost the length of the board, such that, electrically it looked like the letter "U." At the edge of the board where the peeling began, a capacitor was soldered, bridging the planes on both sides of the narrow GAP. Then both sides are driven (the capacitor is shunting) with a loosely coupled signal sweep generator and adjusted until the resonance is found. The display presentation is of a swept network having a definite resonance. Then a separate homogeneous groundplane brought into close proximity, which causes the resonant frequency to increase in frequency, as well as, the "Q" of the resonance to diminish.

The implication in all this is that a GOOD groundplane design has as few APERTURES as is practicable. And where apertures are unavoidable, than shielding or a secondary parasitic groundplane can ameliorate the problems.

Shielding

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 at the cube of the distance. That is, any high frequency signal the dominant component beyond ~ 1/2 wavelength is Electrostatic. This is referred to as Near Field & Far Field radiation.

When is a Shield a Shield?

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, at only one point on the shield, thus preventing any flow of current. 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 reducing the effectiveness of the shield. An extreme case of this might be a shielded cable, whose shield has a different potential at each end, and the resulting current flow in the shield, inducing unwanted noise into the center or shielded conductors. (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 cancelling noise becomes the noise noise!