A 3.3 GHz Spread-Spectrum Clock Generator Supporting Discontinuous Frequency Modulations in 28 nm CMOS

Spread-spectrum clocking is an established approach to mitigate electromagnetic interference (EMI) of digital circuits, by intentionally sweeping the clock frequency. In this way, the energy of each clock harmonic is spread over a larger bandwidth, thereby reducing the peak of the interfering spectrum. This paper describes an highly flexible all-digital spread-spectrum clock generator (SSCG) realized with a standard-cells design flow. The developed circuit supports discontinuous frequency modulation profiles (with improved EMI reduction capability) and features reduced output jitter, due to delay interpolators and digital compensation of delay path asymmetries. The proposed SSCG is ideally suited for complex system on chips applications, having programmable spreading parameters, frequency synthesis capability and reduced recovery time to support local standby modes. The SSCG is implemented in bulk 28 nm CMOS technology, presents a maximum working frequency of 3.3 GHz and less than 3.2 ps(rms) output jitter. The measured peak level reduction of the clock power spectrum, at 1.0 GHz output frequency, is 27.0 dB with a 10% modulation depth. The power dissipation is 29.3 mW @ 3.3 GHz and the area occupation is 0.031 mm.