1654 IEEE JOURNAL OF SOLID STATE CIRCUITS VOL 36 NO 11 NOVEMBER 2001 Rotary Traveling Wave Oscillator Arrays A New Clock Technology John Wood Terence C Edwards Member IEEE and Steve Lipa Student Member IEEE Abstract Rotary traveling wave oscillators RTWOs represent a new transmission line approach to gigahertz rate clock generation Using the inherently stable LC characteristics of on chip VLSI interconnect the clock distribution network becomes a low impedance distributed oscillator The RTWO operates by creating a rotating traveling wave within a closed loop differential transmission line Distributed CMOS inverters serve as both transmission line amplifiers and latches to power the oscillation and ensure rotational lock Load capacitance is absorbed into the transmission line constants whereby energy is recirculated giving an adiabatic quality Unusually for an LC oscillator multiphase 360 square waves are produced directly RTWO structures are compact and can be wired together to form rotary oscillator arrays ROAs to distribute a phase locked clock over a large chip The principle is scalable to very high clock frequencies Issues related to interconnect and field coupling dominate the design process for RTWOs Taking precautions to avoid unwanted signal couplings the rise and fall times of 20 ps suggested by simulation may be realized at low power consumption Experimental results of the 0 25 m CMOS test chip with 950 MHz and 3 4 GHz rings are presented indicating 5 5 ps jitter and 34 dB power supply rejection ratio PSRR Design errors in the test chip precluded meaningful rise and fall time measurements Index Terms Clocks MOSFET oscillators phase locked oscillators phased arrays synchronization timing circuits transmission line resonators traveling wave amplifiers Researchers have therefore looked to alternative oscillator mechanisms for better phase stability and lower power consumption Previous transmission line systems such as salphasic distribution 6 distributed amplifiers 7 and adiabatic LC resonant clocks 8 provide only a sinusoidal or semisinusoidal clock making fast edge rates difficult to achieve This paper introduces the rotary traveling wave oscillator RTWO a differential LC transmission line oscillator which produces gigahertz rate multiphase 360 square waves with low jitter Extension of the RTWO to rotary oscillator arrays ROAs offers a scalable architecture with the potential for low power low skew clock generation over an arbitrary chip area without resorting to clock domains Simulations predict rise and fall times of 20 ps on a 0 25 m process and a of the integrated maximum frequency limited only by the circuit technology used Experiments show that although the RTWO operates differentially careful attention is required to guard against magnetic field couplings between the clock conductors and other structures if the potential performance of these oscillators is to be realized II CONCEPT OF THE ROTARY CLOCK OSCILLATOR I INTRODUCTION A Fundamentals and Structures C LOCKING at gigahertz rates requires generators with low skew and low jitter to avoid synchronous timing failures The notion of a clocking surface becomes untenable at gigahertz rates 1 frequently mandating that large VLSI chips are subdivided into multiple clock domains and or utilize skew tolerant multiphase circuit design techniques 2 Techniques such as distributed phase locked loops PLLs 3 and delay locked loops DLLs 4 can control systematic skew to within 20 ps but are complex introduce random skew i e jitter and have area penalties H tree distribution systems while simple are difficult to balance and can use upwards of 30 of a chip s total power budget 5 All these systems are inherently single phase induce large amounts of simultaneous switching noise and can be highly susceptible to this noise Manuscript received March 20 2001 revised June 28 2001 This work was supported by Multigig Ltd and also supported in part by the National Science Foundation under Award EIA 31332 J Wood is with MultiGig Ltd Northampton NN8 1RF U K e mail john wood multigig com T C Edwards is with Engalco Huntington YO32 9NY U K e mail enquiries engalco com S Lipa is with the Microelectronics Systems Laboratory North Carolina State University Raleigh NC 27695 USA Publisher Item Identifier S 0018 9200 01 08220 8 The basic ROA architecture is shown in Fig 1 A representative multigigahertz rotary clock layout has 25 interconnected RTWO rings placed onto a 7 7 array grid Each ring consists of a differential line driven by shunt connected antiparallel inverters distributed around the ring This arrangement produces a single clock edge in each ring which sweeps around the ring at a frequency dependent on the electrical length of the ring Pulses are synchronized between rings by hard wiring which forces phase lock Fig 2 illustrates the theory behind the individual RTWO Fig 2 a depicts an open loop of differential transmission line exhibiting LC characteristics connected to a battery through an ideal switch When the switch is closed a voltage wave begins to travel counterclockwise around the loop Fig 2 b shows a similar loop with the voltage source replaced by a cross connection of the inner and outer conductors to cause a signal inversion If there were no losses a wave could travel on this ring indefinitely providing a full clock cycle every other rotation of the ring the M bius effect In real applications multiple antiparallel inverter pairs are added to the line to overcome losses and give rotation lock Rings are simple closed loops and oscillation occurs spontaneously upon any noise event Unbiased startup can occur in 0018 9200 01 10 00 2001 IEEE WOOD et al ROTARY TRAVELING WAVE OSCILLATOR ARRAYS 1655 Fig 3 Waveforms of line voltage and line current for the 3 4 GHz clock simulation example B Waveforms Fig 3 shows simulated waveforms of a 3 4 GHz RTWO taken at an arbitrary position on the ring The design has the following characteristics for reference Fig 1 phase Basic rotary clock architecture The signs denote points with same Fig 2 Idealized theory underlying the RTWO a Open loop of differential conductors to a battery via a switch b Similar loop but with the voltage source replaced by the inner and outer conductors cross connected either rotational sense usually in the direction of lowest loss Deterministic rotation biasing mechanisms are possible e g directional coupler technology or gate displacement 9 Once a wave becomes
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