Triangular Modulation Using Switched-Capacitor Scheme for Spread-Spectrum Clocking

A triangular modulation profile using switched-capacitor scheme for spread spectrum clock generator (SSCG) is presented in this paper. The working principle consists of modulating the oscillator´s constant clock frequency with a triangular signal. This will spread the energy of each harmonics into a certain frequency band, thus reducing the peak amplitude of electromagnetic interference (EMI) at harmonic frequencies. This work aims to implement triangular modulation with a constraint of 20pF total capacitor for cost efficiency. The device models of a 1um high-voltage CMOS process is applied for Hspice simulation. The modulating signal which has a frequency of 257 Hz and amplitude of 150 mVpp is used to modulate a 65 kHz oscillator. The frequency deviation is maintained to be within ± 4 kHz. Simulation shows an EMI reduction of 13.8 dB. This work is applied in the oscillator circuit of a PWM controller IC. The power consumption is 2.16mW. Theoretical analysis of traditional modulations and the proposed scheme are also formulized. The hand calculation of the modulation index using the proposed formula has only a maximum error of 3.1 percent compared to simulation.