Executive Summary
This application note will discuss some tips to reduce electromagnetic noise and interference, when designing two-layer printed circuit boards (PCBs). This document will present instances when a PCB is susceptible to electromagnetic noise and interference and will provide solutions for particular instances when designing a PCB. Examples of PCB designs will be given to help clarify.
I. Introduction
In order to maximize the results of a PCB design, certain design aspects need to be taken into consideration. There are a couple of advantages to PCBs over a typical proto-board. One main advantage is that when using a PCB to implement a design, the size of the circuitry is greatly reduced, and in most cases, especially today, the smaller the circuitry the better. Another advantage is that by simply incorporating a design on a PCB, which would replace a proto-board, the circuitry becomes less vulnerable to external noise sources. However, even though less vulnerable, the consumer must still take into consideration that electromagnetic noise and interference still will exist. Electromagnetic noise and interference will disturb the performance of circuitry. To reduce these burdens, useful PCB design methods will be discussed. First, the different aspects of noise will be discussed to give an understanding for what noise is with respect to circuits. Then, design methods for reducing ground noise will be implemented for laying out a PCB in software in addition to the concern of noise due to the power system layout of a PCB; design techniques will be mentioned to minimize noise. Finally, the importance of component placement with respect to other components, and the effect this placement has on noise, will be discussed along with helpful techniques to avoid problems with noise. After understanding the affects of electromagnetic noise and interference with PCBs, a PCB will be capable of maximum performance for the consumer.
II. PCB Software
To understand that some of the topics discussed concerning the layout of the PCB, it will be assumed that the reader has some background in using PCB software and laying out the circuitry. In order to facilitate the understanding of this document, examples will be given, which will be relating to the discussions of electromagnetic interference and noise and which I could implement using PCB software. The software that I have used is Express PCB. To learn more about this software, please go to the following link: http://www.expresspcb.com/ExpressPCBHtm/Download.htm. This link will point the reader to an online resource, and will also provide PCB design tips. Additionally, this site permits that the software be downloaded free of charge. For further understanding of this software, there is a “Quick Start Guide” in the help tab in Express PCB for beginning users.
III. Electromagnetic Noise and Interference
Electromagnetic interference (EMI) and noise is an annoyance that circuit designers are discovering to be a bigger problem on a day to day basis. Here, we will look at some of the theory of electromagnetic noise and interference. It is important to understand that noise, with respect to circuitry, simply is an inductance. An inductance is a property of an electric circuit by which an electromotive force is induced in it by a variation of current in the circuit itself, or in a neighboring circuit; induced in it as a result of a changing magnetic flux. (1) An electromotive force is a force that produces an electric current. (1) Therefore, noise is essentially inductance. Many times in this
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document I will discuss the issue of this inductance, which is referring to electromagnetic noise. There are many forms of inductance that can occur in circuitry. I will discuss three forms of inductance that I have taken into consideration when designing the PCB layout. The three forms of inductance are: internal self inductance, external self inductance, and mutual inductance. (2)
Internal self inductance deals with the dimensions of the conductor. That is, this inductance is “proportional to the length of a conductor and also depends on cross-sectional geometry.”(2) Here, the concern is that when laying out a PCB, you want to minimize the length of the traces. “External self inductance depends on the circuit geometry;” (2) like mentioned earlier, inductance is induced as a result as a changing magnetic flux. The concern for PCB layout is that the circuitry should be set up on the PCB board in such a way that a magnetic field will not result and therefore not result in an inductance / noise. One such way to avoid inducing such a magnetic field is to minimize the loop area of the circuitry (this will be discussed later with more specific examples). The third form of inductance of concern when laying out a PCB is mutual inductance, which is “controlled primarily by minimizing the length of critical leads which carry large transient currents (clock leads, and line or bus drivers).”(2)
If you minimize these forms of inductance in the circuitry, noise will also be reduced. These forms of inductance, if produced, are directly proportional to the noise that is generated in the circuitry. We have defined the three forms of inductances that should be taken into consideration when designing PCBs, the next sections will discuss more specifically some examples of how to reduce problems with electromagnetic noise and interference (inductance). |
