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Common Printed Circuit Board Design Problems
It is important that software analysis tools address the following major problem areas of PCB design.
The fast clock speeds and rapid edge rates needed in many PCB designs require proper management of board level design, layout and signal interconnects
in order to minimize EMI. High speed switching has the capability of producing electromagnetic waves that generate resonance, power/ground bounce,
simultaneous switching noise, reflections, and coupling between traces and power/ground planes. The effects of improper ground plane design can hinder
the performance of high-speed clocks and synchronous busses. Most major sources of emissions appear due to signal related parameters such as the clock
or pulse repetition frequency, signal edge rate, and signal ringing due to impedance mismatches. The greater the performance characteristics of
semiconductor components in a circuit design, the more electromagnetic interference that may occur. Therefore, the design of a PCB must eliminate any
antennas that are capable of radiating electromagnetic energy. Loops of signal and the corresponding ground return lines carrying high frequency signals must be minimized.
Typical PCB Design Problems
The synchronous activity of a large number of clocked devices causes the current switching events to occur simultaneously. The PCB traces connected
to integrated circuit (IC) input/output (I/O) pins can form effective antennas that radiate noise, coupling it to the external cables.
The synchronous nature of a circuit design can cause glitches and emissions from the power supplies that provide voltage to the electronics and clocked circuits.
Oscillator circuits can also become a source of EMI when the oscillator swings rail to rail, producing harmonics due to the squaring of the output sine wave.
Single layer boards are much more likely to cause EMI problems than multi-layer boards. Two layer and four layer boards generally have a minimal of radiation in comparison,
if properly designed. A common problem at the system level is the radiation due to cables interconnecting the PCB with external support equipment or peripherals such as displays,
external processors, and keyboards. In many cases, there may be only one ground wire between the PCB and external peripherals. The inductive ground conductor provides the return
for all of the RF energy carried to the external equipment through the multiple wiring. Any impedance in the ground wire will prevent some of the RF energy from returning to the
PCB through the normal ground path. This radiated RF energy may be coupled back through undesirable paths including some of the external equipment. The emissions may cause EMI
in other parts of the system, as well as to EMI/EMC testing facilities.
Differential mode noise is potentially generated by every signal in electronic and electrical equipment. It travels down its trace to the receiving device and
then back along the return path, resulting in a differential voltage between the two conductors. A circuit design should be based on minimizing the magnitude of this current,
its frequency content, and the signal rise and fall times. PCB loops formed by the signal and its return can be reduced through proper design techniques utilizing more
returns and by using signal/return twisted pair wiring.
Common mode noise or common impedance noise results from an electrical current or signal traveling down the signal and
return lines simultaneously. For this type of noise, there is no differential voltage between the signal and its return. In this case, the impedance that is common to the
signal and the return paths causes the source noise voltage. Common mode noise is often present in cables due to the PCB signal connections and returns forming a common impedance.
This type of noise may be reduced through the use of proper PCB design techniques by reducing the common node impedance or by placing a ferrite bead around the cable.
Crosstalk is another type of interference caused by noise or undesired signals being coupled from one source to another. Crosstalk can propagate into connecting cables and
greatly increase the chances of EMI.
The PCB design techniques for addressing susceptibility are somewhat similar to those for emissions. Loop areas of greater size will tend to receive larger signal levels.
They are also capable of radiating EMI at higher levels. Ground bounce, or common impedance coupling of circuits, can cause circuit operational failure and the associated EMI.
For example, if a ground path has high impedance, it may cause a driving circuit’s reference voltage to shift, resulting in its input to the microcomputer to be outside of
the device’s required switching range for normal operation.