Spread spectrum clock generator and associated method

Claim 1

A clock circuit for generating a clock output signal with reduced amplitude electromagnetic interference (EMI) spectral components, said clock circuit comprising: oscillator means for generating a reference frequency signal; and spread spectrum clock generating means cooperating with said oscillator means for generating a spread spectrum clock output signal having a fundamental frequency and reduced amplitude EMI spectral components at harmonics of the fundamental frequency, said spread spectrum clock generating means comprising clock pulse generating means for generating a series of generally rectangularly shaped electrical clock pulses; and spread spectrum modulating means cooperating with said clock pulse generating means for modulating same to broaden and flatten amplitudes of impulse-shaped EMI spectral components which would otherwise be produced by said clock pulse generating means, said spread spectrum modulating means comprising frequency modulating means for frequency modulating said clock pulse generating means, said frequency modulating means comprising profile modulating means for modulating the clock pulse generating means with a periodic waveform having a percentage of frequency deviation profile as a function of percentage of a period (% Period) of the periodic waveform defined in a second quadrant by F 5 which is defined as a function generally as follows: 100% (0.45(% Period/25) 3 +0.55(% Period/25)); wherein for a first quadrant between -25% to 0% Period the profile is equal to -F 5 (-% Period); wherein for a third quadrant between 25% to 50% Period the profile is equal to F 5 (50-% Period); and wherein for a fourth quadrant the profile is equal to -F 5 (% Period-50).