Ph.D. Candidate, Hong Kong University of Science and Technology
Meeting the regulatory body requirements of emitted radiations using on-chip circuit techniques
Research Background All electronic systems have inherent noise sources and a certain percentage of energy radiating from the system. In a differential signaling communication link, the distorted differential signal generates an unwanted common-mode (CM) component and certain spectral tones at the data-rate frequency and its harmonics. They are eventually radiated by the PCB parasitic antennas and degrade electromagnetic compatibility (EMC) performance. Regulatory bodies, for instance the Federal Communication Commission (FCC) and International Special Committee on Radio Interference (CISPR), prescribe a limit to control maximum radiation. Meeting these guidelines becomes more challenging at high speed as higher frequencies radiate more efficiently through small openings. Therefore, electromagnetic interference (EMI) is a challenging issue for packaging and PCB design at high speed. The EMI issue usually comes up late in the system, around the developmental stage, and requires a costly solution that is difficult to implement. Therefore, it is essential to counter the EMI issue during the product design phase instead of handling it at the end of product development. Research Overview The methodology for the CM noise analysis and prediction is proposed for the first time. The CM noise analysis and the CM noise level prediction of the source-series terminated (SST) driver, widely employed in industry, is presented. Finally, a perspective and novel circuit techniques on controlling the CM noise in the SST driver is presented. Research Results The research has elucidated the predominant CM noise source in the SST driver as the mismatched rise and fall time of an output signal produced by the non-linear parasitic impedance variations of the switching devices. The lowest CM noise level can be obtained at an optimal scaling factor and input swing amplitude, where there is a higher rise and fall time symmetry of the output signal. Results affirms that the most effective technique in suppressing the CM noise is controlling it at the source, i.e., the non-linear active circuit. A new technique to suppress the CM noise of a high-speed SST output driver from the signal source is presented. The process monitoring and control circuit (PMC) operates as a background monitoring scheme to detect problematic process corners, and operates to self-calibrate the output driver. The proposed technique results in lowering CM noise and able to meet FCC and CISPR requirements.