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Automatic Gain Controlled Ring Oscillator

IP.com Disclosure Number: IPCOM000240773D
Publication Date: 2015-Feb-27
Document File: 7 page(s) / 357K

Publishing Venue

The IP.com Prior Art Database

Abstract

An automatic gain controlled differential ring oscillator is proposed. A ring oscillator latch inverters drive strength is automatically optimized with the help of an oscillation detection circuit. The ring oscillator can create oscillations at all process, voltage and temperature with lower (optimum) drive strength of latch inverters, reduced power consumption, and lower phase noise.

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Automatic Gain Controlled Ring Oscillator

Abstract

An automatic gain controlled differential ring oscillator is proposed. A ring oscillator latch inverters drive strength is automatically optimized with the help of an oscillation detection circuit. The ring oscillator can create oscillations at all process, voltage and temperature with lower (optimum) drive strength of latch inverters, reduced power consumption, and lower phase noise.

Introduction

Ring oscillators are widely used in PLL’s due to the advantage of lower power consumption, smaller chip area and wide output frequency tuning range.                                                                                                                   To meet the need of clock signal with better common mode noise rejection, easier design of voltage controlled oscillator (VCO) buffer and multiple even number of output phases in various applications, ring oscillators with differential structures become more preferable than the single-ended ones.

A ring oscillator is a chain of delay elements forming a loop. The differential ring oscillator unit cell consist of forward inverter path or inverter stage and back to back inverter path or latch stage.

                              

Figure 1: Differential ring oscillator unit stage

Figure 1 shows the differential ring oscillator unit stage. The latch stage drives the output of the oscillator stage to opposite states and ensures oscillations in the differential ring oscillator.

Figure 2: Two stage differential ring oscillator

Figure 2 shows a two stage differential ring oscillator.  In the two stage differential ring oscillator, the total number of inverters in the forward path is 4 (even) and the total number of inverters in the latch path is 3 (odd).  The latch stage inverters drive the output of the oscillator stage to opposite states and ensure oscillations in the differential ring oscillator.  The optimum drive strength of the latch stage inverters is required for the differential ring oscillator. If the drive strength of the latch stage inverters is lower, then the oscillator cannot create oscillations and if the drive strength of the latch stage inverters is higher than it will consume more power and the phase noise of the oscillator will be higher.  Thus, a ring oscillator design is needed that ensures oscillations with optimum drive strength of latch inverters.  

Across PVT, the oscillator gain varies. In best case (ff_fet) the oscillator gain will be higher, which corresponds to higher power consumption and more phase noise so oscillator latch strength should be lower in best case to reduce power consumption and lower phase noise. The proposed ring oscillator ensures oscillations at ring oscillator output with optimum drive strength of latch inverters for reduced power consumption and lower...