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Some circuit designs are fabricated on tiny silicon wafers and others consist of various components connected by cables. However, the circuits that are often the center of an EMC engineer's attention are those that are laid out on fiberglass epoxy boards.
Printed circuit boards similar to the one illustrated in Figure 1 can be found in nearly all electronic systems. Circuit components with metal pins are connected by copper traces. Pin-in-hole components are held to the board by their pins, which extend through the board and are soldered to the traces on the opposite side. Single-layer boards have all of the traces routed on one side of the board.
Double-layer boards have traces on both sides. Many boards have several layers of copper traces separated by layers of fiberglass epoxy or a similar dielectric. These are referred to as multi-layer boards. The number of layers is usually even. Four-layer boards are very common in low-cost products. Boards with dozens of layers are sometimes used to connect densely populated boards with high component pin counts. Multi-layer boards usually have entire layers with solid copper planes dedicated to the distribution of power to the components on the board.
These planes are usually named after the component pins they are connected to. For example, a copper plane connecting all of the V CC components pins to the power supply is often called a V CC plane. The placement of components and the routing of traces usually play a crucial role in determining the electromagnetic compatibility of products employing printed circuit boards.
Well laid out boards will not radiate significantly on their own and they do a good job of minimizing currents and fields that might couple noise to cables or other objects off the board. They also are configured to minimize opportunities for external currents or fields to couple interfering signals on to the board.
Most board designers employ a list of guidelines to help place components and route traces. For example, a typical guideline might be "minimize the length of all traces carrying a digital clock signal. Suppose you're laying out a high-speed multi-layer printed circuit board and you need to route a trace carrying a high-frequency signal from a digital component to an analog amplifier.
You want to minimize the chance of having an electromagnetic compatibility EMC problem, so you search the web for EMC design guidelines and you find three guidelines that seem to pertain to your situation:. You envision the three possible routing strategies illustrated in Figure 2.
The first routing strategy routes the trace directly between the two components, but leaves the plane between them solid. The second routing strategy gaps the plane, but routes the trace over the gap. The third routing strategy routes the trace around the gap. Each of these alternatives violates one of the guidelines. Which is the best choice? Is each alternative equally good because it satisfies 2 of 3 guidelines?
Are they all bad because they all violate at least one guideline? These are the types of questions that circuit board designers are faced with every day.
Making the right choice can be the difference between a board that meets all requirements and a board that has severe radiated emissions or susceptibility problems. In this case, one of the choices is much better than the other two. However before we reveal the correct answer, let's develop a strategy for evaluating printed circuit board layouts.
With a proper strategy, the correct answer to this quiz question should become apparent. In this tutorial, we will explore 4 steps that every EMC engineer should apply when laying out a printed circuit board or reviewing an existing board design.
These steps are:. By taking the steps outlined above first, component placement and trace routing decisions will become clearer. It should also be much more apparent which design guidelines are most important and which are not important at all for a specific design.
A typical circuit board may have dozens, hundreds or even thousands of circuits. Each circuit is a potential source of energy that might eventually be coupled unintentionally to other circuits or devices. Each circuit is also a potential victim of unintentionally coupled noise. However, some circuits are much more likely than others to be a noise source and other circuits are much more likely to be victims. EMC engineers and board designers should be able to recognize those circuits that are potentially good sources and those that are potentially most susceptible.
Circuits of particular interest are discussed below. Synchronous digital circuits employ a system clock that must be sent to every active component on or off the board that needs to interpret the digital signal.
Clock signals are constantly switching and have narrow band harmonics. They are often among the most energetic signals on a printed circuit board.
For this reason, it is not uncommon to see narrow band radiated emission peaks at harmonics of the clock frequency, as illustrated in Figure 3. Figure 3: Radiated emissions from a product with a MHz clock. In this figure, the radiated emissions are clearly dominated by harmonics of the MHz clock. The noise floor from — MHz is the thermal noise of the spectrum analyzer used to make the measurement corrected to reflect the antenna factor.
In order to make this product compliant with the FCC or CISPR Class B radiated emission specification, the clock source amplitude would have to be decreased, the unintentional "antenna" made less efficient, or the source-antenna coupling path attenuated.
Most of the traces on a digital printed circuit board are carrying digital information rather than clock signals. Digital signals are not as periodic as clock signals, and their random nature results in noise that is more broadband.
Digital signals that toggle more often can result in radiation similar to clock signals. An example of this would be the least significant bit on a microprocessor address bus, since stepping through consecutive addresses can cause this signal to toggle at the clock frequency.
The exact form and strength of the radiation from digital signals depends on many factors including the software running and the encoding scheme employed. Generally, data signals are a less troublesome source than clock signals; however high-speed data can still produce significant amounts of noise.
Switch-mode power supplies and DC-DC converters generate different voltages by switching the current into a transformer on and off rapidly. Typical switching frequencies are in the 10 - kHz range. The spikes of current generated by this switching can couple noise to the power output and other devices on the board. Although this noise signal is relatively periodic i. The small hump in the noise floor around MHz in Figure 3 is due to power switching noise. In this product, the switching noise is negligible relative to the clock noise.
However in other products the power switching noise can dominate, since only the upper harmonics of the switching noise fall in the frequency range where radiated emission are measured. Power switching noise can always be reduced by slowing down the transition time of the switching circuit.
However, this reduces the efficiency of the power supply, so alternate methods are preferred. Possible solutions are discussed in the Conducted EMI tutorial. Analog signals can be broadband or narrowband, high frequency or low frequency. If your board employs analog signals, it is a good idea to be familiar with what these signals look like in both the time and frequency domains. Narrowband, high-frequency analog signals can be particularly difficult to work with.
Fortunately, since analog signals tend to be sensitive to low levels of noise, signal integrity concerns usually dictate that they are laid out in a manner that will minimize radiated emissions. Generally speaking, DC power and low-speed digital signals do not have enough power at radiated emission frequencies to be troublesome. Nevertheless, these traces are often the source of the most difficult radiated emissions problems. This is because the unintentional high frequency voltages and currents on these traces can be as great as or greater than the voltages and currents on high-speed traces.
Figure 4: Near magnetic field above a packaged integrated circuit. Figure 4 shows a map of the near magnetic field above a dynamic random access memory module commonly used in personal computers.
The near magnetic field provides an indication of the currents flowing in the lead frame of the component package. The frequency of the measurement is the third harmonic of the clock frequency.
Note that more current is being drawn from the DC power supply pins than is being drawn from the signal pins. Figure 5 shows a similar plot of the near magnetic fields above a microprocessor implemented in a field programmable gate array FPGA. In this figure, we see that the currents injected onto some of the low-speed address lines are nearly as strong as the currents in the clock signal.
How do high-frequency currents and voltages appear on low-frequency data lines? There are several ways that this can happen. Most have to do with the design and layout of the integrated circuits ICs connected to these traces. Some ICs do a good job of containing their internally generated noise and others do not.
A poor design can put high-frequency voltage fluctuations on every input and output trace connected to the IC. Good designs can be relatively quiet. When laying out a printed circuit board with an unfamiliar IC that is clocked internally at a high frequency, it is a good idea to treat every pin on that IC as if it were a high-frequency source with the same characteristics as the internal clock. Otherwise, the power or low-speed digital traces could be the most significant sources of radiated emissions.
Perhaps the most important distinction between digital circuit designers and EMC engineers is that EMC and signal integrity engineers pay close attention to the currents flowing in a circuit as well as the voltages. This is a very important point. Most poor designs are the direct result of neglecting to consider where the signal currents were likely to flow.
Although this has already been discussed in a previous section, the subject of current path identification is so crucial to good printed circuit board design that it's worth reviewing the main concepts here. First and foremost,. The same amount of current that flows out one side of a source must be drawn in from the other side. At low kHz and lower frequencies, impedance is dominated by resistance, so current takes the path s of least resistance. At high MHz and higher frequencies, impedance is dominated by the inductance term so the current takes the path of least inductance.
At high frequencies depending on the length of the interconnects and the current carried by the conductors, the interconnects tend to act as antennas, resulting in EMI. These EMI radiations interfere with other devices present in the vicinity. There are international standards that limit the level of emissions. Thus, it is highly important to measure electromagnetic radiation and control these radiations. Whether it is within the set standards? EMC ensures that the system must perform as intended under the defined safety measures. EMI focuses on the testing requirements and interference between the neighboring equipment.
Enter your mobile number or email address below and we'll send you a link to download the free Kindle App. Then you can start reading Kindle books on your smartphone, tablet, or computer - no Kindle device required. To get the free app, enter your mobile phone number. This book provides the knowledge and good design practice for the design or test engineer to take the necessary measures to improve EMC performance and therefore the chance of achieving compliance, early on in the design process. For the suppliers, not only will their products have the competitive edge because they have known EMC performance, but they will be prepared should EMC compliance become mandatory in the future.
This book provides the knowledge and good design practice for the design or test engineer to take the necessary measures to improve EMC performance and therefore the chance of achieving compliance, early on in the design process. For the suppliers, not only will their products have the competitive edge because they have known EMC performance, but they will be prepared should EMC compliance become mandatory in the future. For consumers it is a distinct advantage to know how a component will behave within a system with regard to EMC. Shows how to achieve EMC compliance early on in the design process Provides the knowledge to trace system EMC performance problems Follows best design practices. We are always looking for ways to improve customer experience on Elsevier. We would like to ask you for a moment of your time to fill in a short questionnaire, at the end of your visit. If you decide to participate, a new browser tab will open so you can complete the survey after you have completed your visit to this website.
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Some circuit designs are fabricated on tiny silicon wafers and others consist of various components connected by cables. However, the circuits that are often the center of an EMC engineer's attention are those that are laid out on fiberglass epoxy boards. Printed circuit boards similar to the one illustrated in Figure 1 can be found in nearly all electronic systems.
Enter your mobile number or email address below and we'll send you a link to download the free Kindle App. Then you can start reading Kindle books on your smartphone, tablet, or computer - no Kindle device required. To get the free app, enter your mobile phone number.
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