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3 Clocking Techniques for Reduction of Electromagnetic Interference

Whether it’s your home, workplace, or an industrial setup, electronic gadgets need to work in proximity to each other. And these devices may emit electromagnetic energy that affects the performance of the other gadgets and appliances. This is why you need electromagnetic interference shielding to protect all devices from unwanted radiations and comply with the standards laid down by industrial and government bodies. One of the primary sources of electromagnetic interference (EMI) is the clock tree. With a seamless clock tree design, your system operates flawlessly and without major problems associated with timing. Besides, you are also assured that your system meets all environmental compliance benchmarks such as FCC Class A and FCC Class B. Read on to learn about the three clocking techniques for EMI reduction.

  1. Signal Filtering

Did you know that harmful radiations may emanate from signal output traces and pins? These emissions are typically emitted from board traces as they are longer than the internal IC traces or clock device pins. If you use low-pass filters at data outputs and high-speed clocks, the frequency of the signal is effectively attenuated, particularly at high harmonics. You can choose RC-based low-pass filters which are chosen for their small board space requirement, simple design, and affordability. A low-pass filter is nothing but a single pole filter with an attenuation of 3dB at its cutoff frequency and 20dB per dec attenuation for frequencies more than that. Make sure that the cutoff frequency is double of the clock frequency. This is important to trim down the signal swing. Else, the signal swing might defy the logic threshold of the recipient digital circuits. RC-based filters are effective in cutting back on interference at high harmonics. You can choose more intricate filters like the second-order ones to reduce the force or effect of higher harmonics to a great extent. However, it is important to remember that these filters are costly as well as bulky.

  1. Shielding

With EMI shielding, you can enclose systems with a grounded conductive enclosure to eliminate interference and its radiations to the external environment. In some computing and consumer electronics, the use of such methods may prove a little expensive and unfeasible. Why? That is because of the physical limitations of such systems. This is the reason shielding engineers recommend the utilization of a solid ground for cutting back on EMI levels and for the maintenance of good signal integrity, particularly in high-speed signal trails. A small amount of radiation may be present even if you use a solid ground from the topmost surface of the trace. If you are using high-density boards, it may not be possible to assure a solid return path or ground for all signals without integrating additional ground layers, thus increasing the cost of the board.

  1. Spread-Spectrum Clocking

Circuit board designers employ appropriate transmission line design and slew rate control to cut back on unwanted EMI. But there may be a problem. Owing to multiple sources, the residual interference at core frequency and its harmonics might be higher than what you expect. In such situations, spread-spectrum clocking is one of the effective ways to reduce system-wide interference. Today’s shielding engineers and designers are aware of this technique to reduce the level of EMI or RFI radiations. If the system clock is dithered on purpose, the emitted energy is distributed over the spectrum, and so its emissions are reduced to a considerable extent. This helps your devices to meet the industrial or regulatory guidelines.

Choose the right electromagnetic interference shielding material and clocking techniques to reduce the harmful effects of EMI. However, you should choose spread-spectrum clocking only after conventional EMI reduction strategies have been applied such as grounding, shielding, use of ferrite beads, and design changes.

 

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