EMI suppression involves grounding, shielding, and filtering.
1. Grounding
An ideal ground plane is a zero-potential, zero impedance body that can be used as a reference for all signals in associated circuitry, and to which any undesired current can be transferred for the elimination of its effects.
The multiple-point grounding minimizes ground lead lengths. The ground plane might be a ground wire that is carried throughout the system or a large conductive body.
Bonding
The physical implementation for grounding is done through bonding of a low-impedance path between two metal surfaces to make a structure homogeneous with respect to the flow of electrical currents, thus avoiding the development of potentials between the metallic parts, since such potentials may result in EMI.
- provide protection from electrical shock
- power circuit current return paths
- antenna ground plane connections
- minimize the potential difference between the devices
- can carry large fault current
- direct bond is a metal-tometal contact between the elements connected
- indirect bond is a contact through the use of conductive jumpers
Bond Quality
The dc resistance Rdc = length of the bond / (conductivity * cross-sectional area)
The ac resistance Rac = length of the bond / (conductivity * width of the bond * skin depth)
Bonding effectiveness can be expressed as the difference (in dB) between the induced voltages on an equipment case with and without the bond straps.
2. Shielding
The purpose of shielding is to confine radiated energy to a specific region or to prevent radiated energy from entering a specific region. Shields may be in the form of partitions and boxes as well as in the form of cable and connector shields.
Shield types:
- solid
- nonsolid (e.g., screen)
- braid, as is used on cables.
Shielding Effectiveness SE = 10*log(10) * (incident power density / transmitted power density)
- incident power density is the power density at a measuring point before a shield is installed and the
- transmitted power is the power density at the same point after the shield is in place
- electric field strength SE = 20*log(10) * (Ei / Et)
- magnetic field strength SE = 20*log(10) * (Hi / Ht)
3. Filtering
An electrical filter is a network of lumped or distributed constant resistors, inductors, and capacitors that offers comparatively little opposition to certain frequencies, while blocking the passage of other frequencies. Filters are used to substantially reduce the levels of conducted interference.
Insertion Loss IL = 20*log(10) * (V1 / V2)
- V1 is the output voltage of a signal source with the filter in the circuit
- V2 is the output voltage of the signal source without the use of the filter
Low-pass filters IL = 10*log(10) * (1 + F^2) dB
- F = PI*f*R*C for capacitive filter (f = frequency)
- F = PI*f*L/R for inductive filter (f = frequency)
Lumped System is an electrical circuit with passive elements (e.g. R, L, C) that are constant.
For example, the current at a capacitor with capacity C is i(t) = C * (dv(t) / dt)
A lumped element size is much smaller than the wavelength of the applied voltages and currents. In this case wave propagation effects may be neglected.
Distributed System is an electrical circuit with passive elements (e.g. R, L, C) where the inductance, capacity and resistance are not constant but functions of time and space length. This leads to partial derivatives of i(t,x) and v(t,x) in t (time) and x (position).