Report Number: CSL-TR-94-629
Institution: Stanford University, Computer Systems Laboratory
Title: Precise Delay Generation Using Coupled Oscillators
Author: Maneatis, John George
Date: June 1994
Abstract: This thesis describes a new class of delay generation structures which can produce precise delays with sub- gate delay resolution. These structures are based on coupled ring oscillators which oscillate at the same frequency. One such structure, called an array oscillator, consists of a linear array of ring oscillators. A unique coupling arrangement forces the outputs of the ring oscillators to be uniformly offset in phase by a precise fraction of a buffer delay. This arrangement enables the array oscillator to achieve a delay resolution equal to a buffer delay divided by the number of rings. Another structure, called a delay line oscillator, consists of a series of delay stages, each based on a single coupled ring oscillator. These delay stages uniformly span the delay interval to which they are phase locked. Each delay stage is capable of generating a phase shift that varies over a positive and negative range. These characteristics allow the structure to precisely subdivide delays into arbitrarily small intervals. The buffer stages used in the ring oscillators must have high supply noise rejection to avoid losing precision to output jitter. This thesis presents several types of buffer stage designs for achieving high supply noise rejection and low supply voltage operation. These include a differential buffer stage design based on a source coupled pair using load elements with symmetric I-V characteristics and a single-ended buffer stage design based on a diode clamped common source device. The thesis also discusses techniques for achieving low jitter phase-locked loop performance which is important to achieving high precision. Based on the concepts developed in this thesis, an experimental differential array oscillator delay generator was designed and fabricated in a 1.2-um N- well CMOS technology. The delay generator achieved a delay resolution of 43ps while operating at 331MHz with peak delay error of 47ps.