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EMC Component Group: EMC Clock to Control EMI Introductory Theory of Operation.  
 
 
 

Theory of Operation
EMC Clock to Control EMI 

Introduction:

Practically every electric device and equipment generate unintended electromagnetic radiation, which will get worse as technology advances, especially ever faster electrical signals required in PC, digital devices and equipment.  The electromagnetic radiation, if not control, will cause Electromagnetic Interference (commonly referred as EMI) not only to digital and analog devices and equipment, but also to critical communications and medical devices.  To control EMI (also known as EMC – Electromagnetic Compatibility) and to preserve the limited spectrum or airspace, the FCC and the European Community (under CE Mark EMC Directive) require manufacturers to comply with appropriate radiation emissions standards.  Other international communities such as Asia and Australia are also following the same suite.  With the recent advance in semiconductor and computing technology (faster clock speeds, lower voltage/power, etc.), coupled with the explosion of the wireless technologies and interests, it is easy to understand why EMC is becoming one of the most important standard requirements in the new-technology world.  As EMC becomes a standard design requirement for companies around the world, fresh ideas and effective methods must be explored to meet the radiation requirements and at the same time control costs.  It is EMC Components mission to bring the innovative EMC solutions that first allow manufacturers to meet EMC regulations, and second, substantially reduce the cost of EMI control over the product life cycle.  One of the most exciting new solutions is spread spectrum clock generator.  Spread spectrum modulates the signal and spreads the energy over a wider frequency band. It has been successfully shown that by using spread spectrum, radiation is lowered from 3 to 20 dB, depending on the frequencies and the degree of the modulation. 



Theory:

Spread spectrum is, more or less, frequency modulating the clock signal with a unique waveform. In the frequency domain this is equivalent to reducing the peak by distributing the energy of each fundamental and harmonic over a wider range. The spread-spectrum method must be controlled and slow compared to clock rate to guarantee that the change in the clock rate is transparent to the system. Essentially, spread spectrum is a modulation method wherethe modulation is measured as a percentage. For example, a 0.5 percent modulation means that a 100 – MHz clock is modulated between 99.5 and 100.5 MHz. This is called a center 0.5 percent modulation, since the 100-MHz fundamental frequency remains the center frequency.  Designer must keep in mind that both cycle-to-cycle and peak-to-peak jitter must remain within the system’s specifications. 

Another important factor is the modulation frequency, which is usually in the kilohertz range. This is basically a measure of the rate at which the frequency is swept between 99.5 and 100.5. The linear sweep is predictable and most prevalent. The spread-spectrum method must guarantee also that the minimum clock period is not violated. The clock is usually swept between 99.5 and 100 MHz to avoid exceeding the maximum  frequency of the system, a method called downspreading. In this case the clock frequency deviation is measured as a negative percentage; here the spread is –0.5 percent.

In the past shielding and filtering were the most prevalent method used to control EMI.  The problem is that unless the radiation is controlled at the source, it will get on every other circuits on the board and will get beyond the shields via cables as both common and differential modes.  This is why shielding and filtering can get very expensive.  As frequency increases, the effects of transmission line and ground impedance will amplify the radiation, and at the same time wavelength decreases, making shielding and filtering less effective. 

EMC clock oscillators solve the radiation problems at the source, effectively eliminate all the high frequency effects and problems exist in the traditional methods.

Overall, the spread spectrum method has allowed system performance to increase without compromising EMI. Using EMC Clock Oscillator will not only reduce packaging, shielding, and design-cycle costs, but also expedite time-to-market of products otherwise may fail regulatory EMI limits and standards.  Furthermore, using EMC Clock Oscillator will simplify complicated circuits and associated design rules into a single standard clock oscillator.

   
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