An analytical method for determining the optimal modulating waveform for dithered clock generation

A new method for determining the optimal modulating waveform for spread spectrum clock generation (SSCG) is proposed to minimize the peak amplitude of the harmonic spectra measured in electromagnetic interference (EMI) tests. The optimal waveform can be easily obtained as a solution of a first-order differential equation that includes the impulse response of the receiver filter used for the spectrum measurement. To verify the validity of the proposed method, modulating waveforms and the resultant harmonic spectra are calculated for various modulating frequencies and resolution bandwidths. Results show that the optimal modulating waveform becomes similar to a simple triangular waveform if the modulating frequency is much lower or much higher than the resolution bandwidth. It is also found that dithering is most effective when the modulating frequency is equal to or slightly higher than the resolution bandwidth. The proposed method is applicable to and especially effective for dithered clock systems with a programmable modulating waveform.