High speed digital design fundamentals - Signal integrity issues - ElectroMagnetic Interference (EMI)

Signal Integrity Issues 

ElectroMagnetic Interference (EMI)

    Basic physics around electronics states that flow of electrons in a wire creates magnetic field around the wire. Similarly if a conductor is placed in the varying magnetic field, currents flows in the wire in the direction as to oppose the cause

    Hence a single trace always radiate. If the conductors are differential then the magnetic field of both are cancelled by each other and net radiation will be reduced considerably. Hence it is always normal to see that why differential conductors can be operated at very high speeds. Another fact to support the above statement is that since the current is flowing in the differential wires, the magnetic field of one conductor creates the current in the other conductor in the opposite direction. Since the currents too were flowing in opposite direction, this effect helps in pushing electrons in the required direction themselves. Hence relatively less voltage and hence less power is required.

Current always flows in the path of least inductance. Hence it is very important to make sure that the current flows in the path what the circuit designer/PCB designer have assumed. This greatly reduces the loop area and hence EMI.

To summarize, Signal integrity issues can be parried by following the methods below:

• Avoid slots in plane
• Avoid unnecessary loops 
• Provide very low inducatnce path underneath the signal using places
• Have alternate ground pins in the connectors when mating with other cards
• Have bypass capacitors spread across the board  (Helps in Reducing loop areas)
• Keep unrelated planes and unrelated traces seperate from each other
• Strip line performs better than micro strip lines in EMI
• Use good high speed design techniques everywhere in your design
• Avoid/Minimize stubs and extra trace lenghts