The Best Paper Award
Low Pass Filter-Less Pulse Width Controlled PLL Using Time-to-Soft Thermometer Code Converter
Toru Nakura , Kunihiro Asada
（英文論文誌C　平成24年2月号掲載）
Toru Nakura Kunihiro Asada        
　A Phase Locked Loop (PLL) is one of the most fundamental building blocks of LSIs, and almost all LSIs are equipped with one. Low jitter is a strong requirement for the PLL design, but low jitter requires a large silicon area for the low pass filter, which results in high cost on advanced CMOS processes. Also in advanced CMOS processes, the power supply voltage is reducing while transistor switching speeds are increasing. For analog circuit design, the reduced supply voltage means degraded voltage-domain resolution while the high-speed transistors provide improved time-domain resolution. We are therefore facing a new paradigm in which the time-domain resolution of a digital signal edge transition is superior to the voltage-domain resolution of analog signals.
　This paper presents a technique that achieves a small area and low jitter PLL by accommodating the paradigm shift by using a pulse width controlled oscillator instead of a voltage controlled oscillator. In the pulse width controlled oscillator, a pulse width is converted into a digital signal. Analog to digital signal conversion suffers inevitably from quantization noise. However, this circuit uses a soft-thermometer code that uses a 1 bit analog signal at the ZERO/ONE boundary of the conventional thermometer code and use of this scheme makes it possible to achieve quantization noise free digitization and a low jitter PLL. In addition, this circuit does not use a sensitive voltage adjustment for the voltage controlled oscillator, so the area consuming low pass filter can be eliminated, with the result that a small area PLL can be realized.
　This paper describes a high performance and small area PLL, achieved by making use of the paradigm shift from voltage to time resolution, and so spotlights a guideline for analog circuit design in advanced CMOS technologies.

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