Delay-based Spread Spectrum Clock Generator Subramaniam Venkatraman Matthew Leslie University of California, Berkeley EE 241 Final Presentation May 9 th 2005

Motivation In digital systems, the most amount of electromagnetic interference (EMI) is caused by the high-speed digital clock Strict FCC regulations on maximal permitted EMI Traditional solutions: shielding the chip, filtering the clock signal Solution: Spread the spectrum of the clock over a small frequency range within the timing margin (usually +/- 1%) so peak emission is attenuated Often accomplished by modulating the frequency of the VCO in a PLL [Keith B. Hardin, Spread Spectrum Clock Generation for the Reduction of Radiated Emissions]

Delay Cell Array Based Approach SSCG usually implemented with 50KHz modulation waveform to prevent interference with FM systems PLL is required to have a large loop bandwidth hence making it more susceptible to noise Leads to large jitter in clock which is unacceptable Instead can be implemented with a variable delay elements which introduce appropriate delays and shift the effective frequency Leads to smaller random jitter, simpler implementation and reduction in area The idea has been reported but uses 200 delay stages and shows unnatural clock waveform with maximum energy in the 4 th harmonic 0.35µ CMOS technology used to compare with reported results from Jonghoon Kim et al.

Digital Delay Array Each delay cell comprised of delay element and a positive latch: EN DQ DQ DQ … Delay Cell #1Delay Cell #2Delay Cell #N Result: Edge-to-edge jitter varied in deterministic fashion.

Digital Delay Array Element Characteristics: Delay varies linearly with Lx. All elements have identical ‘off’ delay. Keeps t pLH and t pHL the same. Identical fanout independent of delay t pHL and t pLH W/L x

Comparison of Clock Attenuation Reported results show: Large power in even clock harmonics Percentage spread not constant with frequency Our results show: Dominant power in odd harmonics Percentage spread constant with frequency [Jonghoon Kim; 2004 IEEE international symposium on electromagnetic compatibility]

Improved Design For a 1GHz system with 50KHz modulating waveform, number of stages in earlier design would be 20,000 Removes the direct dependency of number of stages on the ratio between clock frequency and modulation frequency which is major limiting factor in previous design Triangular modulation waveform applied to delay elements would lead to clock pulses with equal period and therefore constant frequency Create modulation waveform needed by integrating the triangular wave Use this modified waveform to control the current through a current-starved differential delay element Non linearity in improved design can be corrected by changing input waveform

Improved Design Jitter is dictated by two characteristics of the delay element:  the power supply rejection ratio (PSRR), represented by the numerator  the maximum slope at the switching-point of each delay element, represented by the denominator. Differential stage provides high PSRR Latch based design provides reduced rise and fall time

Comparison with MATLAB Results Triangular modulation causes non ideal frequency spread but the error caused is 1.7dB and simplifies implementation Our implementation shows greater spread than designed for due to non- linearity of analog delay element

Comparison of Results ReportedOur simulation of reported work Our design Power consumption 120mW132mW8.6mW Area3400µm*90µm3290*90(µm) 2 13*90(µm) 2 Jitter with clean vdd -7.09ps9.45ps Jitter with 50mV p-p supply noise ps29.76ps Attenuation9dB on 4 th harmonic 7dB on fundamental 8dB on fundamental

Conclusions & Future Work Delay cell based SSCG implemented shows low power and simple circuit implementation 8dB of clock attenuation on fundamental Approach requires constant number of delay stages with increasing clock frequency unlike previous reported result Modulation frequency not hard-coded in the circuit The linearity of the delay stage needs to be improved either using circuit techniques or by compensating initial input signal Jitter can be reduced by trading off with number of stages An actual implementation would be needed to test for actual jitter caused and clock attenuation achieved