A study of the interference potential of spread spectrum clock generation techniques

A technique to reduce the radiated emissions of digital clock signals and associated harmonics has previously been presented (Hardin et al. 1994). This technique, referred to as spread spectrum clock generation, or SSCG, intentionally broadbands a normally narrowband signal by frequency modulating the trapezoidal clock signal with a unique modulating waveform. As a result of this modulation, the amplitudes of the harmonics of the clock signal can be reduced by as much as 2-22 dB, depending on the frequency of measurement and the frequency deviation. Since the decrease in the amplitudes of the clock harmonics is accomplished by increasing the bandwidth occupied by the harmonics, a number of SSCG clock harmonics may correspond in frequency to an intentionally transmitted signal, such as radio or television. This raises the question of whether SSCG clock generation techniques result in an increase in the interference potential to wideband broadcast receivers as compared to existing clock generation techniques. An additional question which arises is whether existing electromagnetic interference (EMI) measurement procedures are appropriate for products utilizing this technology. The paper presents experimental studies of the interference characteristics of SSCG to broadcast receivers, in particular to television receivers. The results indicate that SSCG presents no greater interference potential to television receivers and in the case of the television audio quality, creates significantly less interference than standard clock generation techniques and that existing EMI measurement procedures are appropriate.