Suppressed carrier clock for reduction of electromagnetic radiated emission from high-speed digital system

In modern digital systems, most of electromagnetic radiated emission originates from high-speed digital clock signal. In order to reduce the electromagnetic radiated emission from the clock signal, spread spectrum clock had been used. It had been conventionally realized by modulating the period of a regular clock, using a phase locked loop (PLL) or a delay locked loop (DLL). However, neither of them can be implemented with simple circuit. To overcome this, suppressed carrier clock (SCC) has been recently proposed, which is implemented by intentionally skipping transitions of a regular clock. In this paper, we analyze the effects of modulation profile on the suppression of electromagnetic radiated emission. Numerical simulation results show that saw-tooth modulation is an optimal method in terms of performance and feasibility. We also propose a simple circuit for modulation and demodulation of the SCC with the saw-tooth modulation profile. Finally, we implemented the proposed circuit on an ALTERA chip, and it achieves 6 dB suppression of electromagnetic radiated emission.