RingOscillator_eSim

Design and simulation files for 3-stage CMOS ring oscillator using eSim The study of ring oscillators is fundamental to understanding basic timing elements in digital and mixed-signal circuits. Abstract: This document explores the design, simulation, and analysis of a basic 3-stage CMOS ring oscillator, emphasizing its structure, operation, and real-world applications. Reference Circuit Details: A ring oscillator typically consists of an odd number of inverter stages (here, three CMOS inverters), each powered by a standard VDD and GND rail. The core structure creates a feedback loop by connecting the output of the last inverter back to the input of the first, ensuring that the signal continuously toggles without a steady-state solution, thus creating self-sustained oscillations. Each inverter consists of a PMOS and NMOS pair arranged such that when one transistor is on, the other is off, achieving logical inversion and minimal static power consumption. Reference Circuit Diagram: The circuit diagram for such an oscillator displays three inverter blocks connected sequentially, with the output of the final block feeding back into the first inverter’s input, forming a closed loop. The typical schematic features source terminals of PMOS transistors tied to VDD, NMOS sources to GND, gates connected together as the inverter input, and drains joined to form the output nodes, with a voltage probe placed at one output. Reference Circuit Waveform: When simulated, the ring oscillator output reveals a classic square wave oscillation at each stage, with each inverter output phase-shifted by one-third of the period. The waveform’s period is determined by propagation delay in a single inverter, and the frequency can be measured from the time between rising or falling edges in the simulation plot. Simulations: Using eSim, the design was first completed and checked for errors, then transient analysis was performed. The results confirmed the expected oscillatory behavior: the output node voltage toggles cleanly between high and low, forming a square wave with a period closely aligning with theoretical predictions. Additional simulation results, such as the FFT spectrum, reveal prominent energy peaks at the fundamental frequency and odd harmonics, confirming the periodic, square-wave nature of the output. Conclusion: The experiment demonstrates that a ring oscillator, by virtue of its feedback structure and inverter propagation delays, operates as a simple, robust clock generator ideal for on-chip test structures and timing references. It illustrates foundational concepts in digital logic, circuit design, and practical oscillation. Acknowledgement: The successful completion of this project was made possible by the eSim platform developed by the FOSSEE team at IIT Bombay and by the helpful online resources and tutorials provided by the open hardware community. Special thanks are extended to project mentors, classmates, and the open-source contributors whose resources, netlists, and documentation greatly facilitated the understanding and implementation of ring oscillator circuits. REFERENCES : K. Watanabe, H. Yamauchi, “Design of High-Frequency Ring Oscillators in 130nm CMOS,” IEEE Transactions on Circuits and Systems, vol. 63, no. 7, pp. 1024-1030, 2016. R. Gupta, P. Sharma, “Low-Power Ring Oscillator Design for VLSI Applications,” Proceedings of IEEE ICCIC, pp. 45-49, 2019.