Solutions to EMI

Line Drivers with controllable Rise and Fall Times.  LINK--_V--
 

Applying a fast pulse to a transmission line, while observing the resulting reflections within the transmission line, one can deduce the nature of any faults or discontinuities, as well as, the distance to those faults--assuming one knows the velocity factor of the transmission line dielectric material (insulating material).
 

In the case of shielded cables, the shield is typically referenced or grounded at both ends. 

In the case of coax cables the shield acts as both the shield and the return current path for the signal carried on the center conductor. However, with shielded balanced differential pair (sometimes called "twisted pair")  the shield is just that, a shield. Often this shield can be grounded at either end; however, in the case of EMI suppression, grounding both ends is prudent. 

Though coax and shielded twisted pair are used for both audio and higher frequency signals, there can be differing effects on how the shield is treated. The most difficult case is that of Video, where both very low to high frequencies (~ 10 Hz to >4 MHz) are involved.

Audio run on coax is subject to "ground loop" noise, i.e., dissimilar ground potentials (especially 60 Hz) which can cause a noise current to flow in the shield, inducing that noise into the protected center conductor. As the frequency of interest increases and the coax gets longer the ground loop problem is diminished; mainly due to the increased inductive reactance (higher Z) of the coax shield.

When long runs are required and/or the environment is noisy, it is best to use shielded twisted pair; the reason being the differential pair's inherent rejection of common mode noise (CMR).

An extreme case might find a remedy in transformer coupling or even optical Isolation.
 

Properly terminated, differentially driven and received Shielded Twisted Pair offer the best performance.
 

Bypass & Decouple:
Bypass ALL active devices!
Remember that bypass capacitors are self resonant.
Connect capacitors using the shortest leads!

Voltage regulators offer the best decoupling
Caution: voltage regulators WILL generate parasitic oscillations if not properly bypassed!

ground plane:
Every signal should have its own ground return!

Shielding:
By definition: real shields have No current flowing thru them, and they are "Grounded," or referenced, at only one point!

Enclosures:
Leakage Apertures
Diffraction: Openings or Apertures in Enclosures Appear as Point Sources, and Radiate their Venomous Exhaling.

Area of the Apertures is less important than the longest dimension, e.g., Seams and Joints; greater than ~ 1/20 wavelength of the highest frequency of interest. They start looking like Slot Antennas. 

  Some Apertures:
   CRTs 
   Lamps/LEDs
   Switches 
   Connectors
.

A. Suppression of Emitter EMI

 Enclose noise sources in a shielded enclosure.

 Filter all leads leaving a noisy environment.

 Limit pulse rise & fall times.

 Relay coils should have surge damping.

 Shield and/or twist noisy leads.

 Ground both ends of shields used to suppress radiated interference (shield should be insulated).

 Twist low-level signal leads.

 Place low-level leads near chassis (especially if the circuit impedance is high).

 Twist and shield signal leads (coaxial cable may be used at high frequencies).

 Shielded cables used to protect low frequency, low-level signal leads should be grounded at 
 one end only (coaxial cable may be used at high frequencies with shield grounded at both ends).

 Insulate shield on signal leads.

 When low-level signal leads and noisy leads are in the same connector, separate them and 
 place the ground leads between them.

 Carry shield on signal leads through connectors on a separate pin.

 Avoid common ground leads between high-level and low-level equipment.

 Keep hardware grounds separate from circuit grounds.

 Keep ground leads as short as possible.

 Use conductive coatings in place of nonconductive coatings for protection of metallic surfaces.

 Separate noisy and quiet leads.

 Ground low frequency, low-level circuits at one point only (high frequencies and digital logic 
 are exceptions).

 Avoid questionable or accidental grounds.

 For very sensitive applications, operate source and load balanced to ground.

 Place sensitive equipment in shielded enclosures.

 Filter or decouple any leads entering enclosures containing sensitive equipment.

 Keep the length of sensitive leads as short as possible.

 Keep the length of leads extending beyond cable shields as short as possible. 

 Use low impedance power distribution lines.

 Avoid ground loops in low frequency, low-level circuits.

 Consider using the following devices for breaking ground loops: 
    Isolation transformers
    Common-mode chokes
    Optical couplers
    Differential amplifiers
    Guarded amplifiers
    Balanced circuits
    Hybrid ground

C. Receiver Noise Reduction

 Use only necessary bandwidth.

 Use frequency selective filters when applicable.

 Provide proper power supply decoupling.

 Bypass electrolytic capacitors with small high frequency capacitors.

 Separate signal, noisy, and hardware grounds.

 Use shielded enclosures.

 When using tubular capacitors, connect outside foil end to ground.

D. Controlling Digital System Emissions

 Minimize ground inductance by using a ground plane or ground grid.

 Locate bypass capacitors next to each IC in the system (use shortest leads possible).

 Use the smallest value decoupling capacitor that will do the job.

 Use a bulk decoupling capacitor to recharge the individual IC decoupling capacitors.

 Clock signal loop areas should be kept as close to zero as possible.

 All cables should be treated to minimize their common-mode current.

 All unused inputs on logic gates should be connected to either power or ground.

 I/O drivers should be located near where the cables leave the system.

 Use the lowest frequency clock, and slowest rise time that will do the job.

 Keep clock circuits and leads away from the I/O cables.