PCB _printed circuit board_ Layout and EMI _electromagnetic i

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PCB (printed circuit board) Layout and EMI (electromagnetic interference)
By John Artiuch Created 09/20/2008 - 10:44

Pcb cad EMI PCB PCB layout printed circuit board To have a smart PCB layout that controls or eliminates any electromagnetic interference (EMI)and noise is the most important aspects of a good reliable design. This is especially important in multi layer PCB designs. A noisy system can lead to unpredictable logic and phase-locked loop failures that reduce overall reliability, as well there is potential that the device will not pass government standards for EMI. The main sources of noise in embedded systems come from switching power supplies, common mode current, noisy ground islands, high frequency switching signals, oscillators, phase-locked loop circuits, crosstalk between signals, the transmission line effect, and from hardware sources like disk drives. For example, when two high frequency signal lines are close together they can couple due to their capacitances and mutual inductance. Signal lines can effectively act like antennas emitting electromagnetic waves. Decoupling capacitors of the power and ground planes can be used to short circuit or filter noise, however this only works to a certain frequency. High frequencies are a lot trickier to deal with and require more advanced filters to stop noise from affecting adjacent circuits. A good PCB layout which considers current paths, return loops as well the physical location of certain components relative to one another can dramatically reduce noise and radiation. The best place to start is looking at the noise generated in the power supply lines. This noise comes from current spikes caused by switching logic or instantaneous current requests from devices down the line. The problem gets worse as the frequency of the switching increases. Decoupling capacitors between power and ground do help in terms of smoothing out the voltage spikes but this will not significantly eliminate EMI. In some cases the addition of an inductor behind the decoupling capacitor can help eliminate some of the current instability. The inductor has high resistance at high frequencies impeding some of the noise. To avoid excessive impedance a resistor in parallel can limit the inductors effects. The inductor should be placed as close as possible to the area of the circuit that is generating the noise. Next all the traces should be kept as short as possible cutting down on the effective area of the loops that act as antennas. One way to achieve this is by simply placing IC’s that interact at higher frequencies right next to one another, making it possible for shorter traces. Also, signal lines can run parallel to the return lines or ground lines cutting down on the surface area of the loop. All unused areas can be plated with copper and connected to ground at several points, adding extra shielding. Another trick is based around the oscillator in the system. This is usually the area of the circuit where the highest frequencies are present, sometimes a good thing to do is to add a resistor in series with the oscillator output to slow down the rise and fall edges of the signal. To keep phase-locked loops clean and unaffected by noise, the best solution is to isolate the loops from the actual power supply. This can be accomplished by
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PCB (printed circuit board) Layout and EMI (electromagnetic interfer...

adding a low drop out voltage regulator to the circuit. The regulator adds to component costs however phase-locked loops tend to be the most sensitive to noise and it maybe worthwhile protecting them to insure overall system reliability. Finally, impedance matching and proper signal termination are required to cut back on crosstalk and transmission line effects. Laying out a good circuit board with the proper component selection can save you a lot of time and money in the design process. The finished product will certainly be more reliable with less chance of undesirable or random behavior. It is not only good practice but crucial to take the above factors into consideration when designing embedded systems. There are CAD programs that can optimize for EMI however generally speaking it is best to do it yourself right from the beginning. Trademarks

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