AN10005 Spread Spectrum Clock Oscillators

With increasing processor speeds and data rates in today’s electronic systems, electromagnetic interference (EMI) has become a major challenge for designers. EMI generated in one device can impede the operations of other electronic devices in its proximity. The clock is often the largest contributor of EMI in a system. The frequency spectrum of square-wave clocks consists of a fundamental tone as well as a collection of higher harmonics. Filtering, shielding, and good PCB layout practices can limit EMI in a system, but they add cost and consume valuable board space. An alternate approach is to reduce the noise generated by the clock. By modulating the clock frequency slowly over time, the peak spectral energy in both the fundamental and harmonic frequencies is reduced. This reduction is useful because regulatory governmental agencies, such as the FCC or CE, use the peak power within a specific bandwidth, typically 100 kHz to quantify radiated EMI.

 

 
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