Clock dithering for electromagnetic compliance using spread spectrum phase modulation

As more electronic systems operate at higher frequencies and bandwidths, they tend to emit more electromagnetic interference (EMI). Due to lack of shielding, portable systems such as notebook computers have come under increasing pressure to comply with strict EMI emission regulations, such as FCC and CISPR. The data bus and the clock are major EMI sources because they conduct high currents and form large loops. This paper focuses on spreading the frequency-spectra of the clock and data signals to reduce their peak EMI emissions for EM compatibility. In a similar work, noise is intentionally injected onto a VCO control voltage in a clock PLL to achieve the effect of frequency modulation. In this case, the phase of the dithered clock varies unpredictably so mixing of both dithered and non-dithered clocks within the same system is avoided. In the proposed scheme, the phase difference between two arbitrary edges is limited within half a period. Hence, it is more practical in real system applications. Since it is implemented with a delay-locked loop (DLL), there are no PLL problems such as accumulated phase error, difficulties in designing stable loop filters, or area costs.