Microstrip Coupled VCOs for 40GHz and 43GHz OC768 Optical Transmission Derek K' Shaeffer, Ph'D' Stef

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Microstrip Coupled VCOs for 40-GHz and 43-GHz
OC-768 Optical TransmissionDerek K. Shaeffer,
Ph.D.Steffen Kudszus, Ph.D.
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Outline

• Introduction to Jitter in SONET Systems
• Review of Oscillator Phase Noise Theory
• VCO Architectural Considerations
• Simulation Results
• Experimental Results
• Summary Acknowledgments

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Introduction

• SONET OC-768 optical systems
• 40 Gb/s (43 Gb/s with FEC)
• Serial data
• VSR, 2km distances
• Timebase challenges
• Data jitter
• VCO phase noise
• Data-dependent jitter (DDJ)
• Duty-cycle errors (half-rate systems)
• Spec requires less than 2.5ps p-p _at_ 10-12 BER
• Thats 14 standard deviations!

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Integrated Optical Transponder
VCO in here
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Jitter in SONET Regenerators

• Two constraints on jitter
• High-Band Jitter above the clock recovery
  bandwidth must be limited to prevent RX data
  sampling errors
• Wide-Band Jitter above the cleanup PLL
  bandwidth must be limited to prevent FIFO data
  buffer overflow

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Jitter Measurement in SONET Systems

• Timing jitter is recovered from the transmit data
  stream
• Clock recovery loop eliminates jitter below its
  loop BW
• Lowpass filter eliminates high frequency
  components
• Jitter must not exceed specified peak-to-peak
  values over a 60-second measurement interval
• OC-768 Systems (following ITU G.8251)
• Wide-Band
• f120 kHz, f2320 MHz, Jp-p1.2 UI (30 ps)
• High-Band
• f116 MHz, f2320 MHz, Jp-p0.1 UI (2.5 ps)

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Relationship Between Jitter and Phase Noise

• Given an oscillator with a 1/Df2 phase noise
  power spectrum, and a measurement system clock
  recovery bandwidth f1
• Following G.8251, if the oscillator contributes
  25 of the jitter power budget, we have two phase
  noise requirements
• Wide-Band -96 dBc / Hz _at_ 1-MHz offset
• High-Band -89 dBc / Hz _at_ 1-MHz offset

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Review of Oscillator Phase Noise Theory

• Oscillators exhibit periodic moments of increased
  phase sensitivity to noise
• Characterized by a time-varying impulse response
• Ideally, resonator energy loss is refreshed
  during moments of low phase sensitivity when G is
  small

Hajimiri, JSSC, Feb 1998
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Phase Noise of Coupled Oscillators

• With two coupled oscillators (N2)
• Impulse sensitivity reduced by 6 dB
• Double the number of noise sources
• Net improvement is 3dB
• No better than spending twice the power
• Generally, improvement is 10log10(N)
• However, additional improvement beyond 3dB can be
  gained if coupling leads to better refresh pulse
  alignment

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A Prototype Pulsed Oscillator

• Colepitts oscillator generates refresh pulses
  coinciding with good moments of the resonator
  oscillation
• Applying Hajimiri phase noise theory to collector
  shot noise

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Minimizing Oscillator Jitter

• Minimize GRMS
• Optimize the timing of refresh pulses
• Use coupled, pulsed oscillators for this purpose
• Maximize resonator quality factor (Q)
• Careful selection of resonator type
• Use high-Q tuning elements, particularly
  varactors
• Spend the necessary current

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Effect of Loop Delay on Pulse Timing

• Reduction of loop gain and startup margin
• Off-resonance oscillation and reduction of
  oscillation amplitude
• Mistiming of refresh pulse

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Delay Compensation Using Coupled Oscillators

• Refresh pulse timing is corrected by
• The oscillator operates on-resonance if M is set
  properly

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Simulated Oscillation Waveforms
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Resonator Types

• Spiral
• Substrate eddy losses
• Challenge to model optimize
• Susceptible to noise injection by magnetic
  coupling
• Coplanar Waveguide
• Good shorted line
• Need two vias at the open end
• Shielded variety has Low L/um
• Unshielded variety is more susceptible to noise
  injection
• Microstripline
• Good open line
• Need one via at each end
• Higher L/um (Q 20 _at_ 40GHz)
• Easy to model and scale

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Resonator Considerations

• Shorted microstrip transmission line resonator
• Advantages
• Predictable with 2-D or 3-D field solvers
• Compact layout
• Immunity to substrate noise coupling (electric
  and magnetic)
• Scalability
• Reasonable quality factor (Q 15)
• Disadvantage
• Via losses at shorted end can significantly
  degrade Q
• MOS accumulation mode varactor
• Advantage
• Highest quality factor for a given capacitance
  adjustment range (Q30)
• Disadvantage
• Steep tuning slope

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Microstripline Test Measurements

• 2.5mm test line
• 4-mm thick Al over an M1/M2 shield
• Fit RLC simulation model at 40-GHz
• Q 15
• Loss 0.56 dB/mm

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Resonator Architecture

• Virtual ground at nodes X and Y replaces shorting
  via
• Active circuitry can be positioned at both ends
  in very close proximity to the lines
• Resistive elements damp out all but one
  oscillation mode

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Circuit Architecture

• Colepitts-like pulsed operation
• Capacitive coupling network sets the coupling
  coefficient independent of pulse network
  parasitics

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Quadrature VCO Block Diagram

• Two resonators, cross-coupled to produce in-phase
  and quadrature oscillations.
• For testing, only brought out the in-phase signal.

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Simulated Phase Noise Performance
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Experimental Prototype

• Implemented 40-GHz and 43-GHz oscillators
• Die area is 0.189 mm2 for 40-GHz version
• 120-GHz fT SiGe BiCMOS process
• Packaged in a custom ceramic package w/
  V-connectors
• Phase noise measurements taken on battery power
  w/ Vtune terminated to GND

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Tuning Characteristics
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Measured Phase Noise Performance
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Performance Summary
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Epilogue VCO in action in 41 MUX / CMU(ISSCC
2003 Paper 13.4, JSSC Dec 2003)
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20-GHz Clock Distribution

• Used on-chip transmission lines
• Common-base receivers terminate line and isolate
  load
• Can drive long line lengths with only modest
  current

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VCO Tuning Range
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CMU Phase Noise Measurements

• Jitter meets specifications with 7 dB margin
• Jitter could be further reduced by 1.8 dB by
  optimizing CMU bandwidth
• Optimum CMU bandwidth 15 MHz

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40 Gb/s Eye Diagram (231-1 PRBS)

• Used Agilent 86107A precision timebase and 83484A
  50-GHz plug-in
• Timebase jitter 150fs, RMS
• CMU random jitter generation (total) 125fs, RMS
• Data patterning jitter 3ps, pk-pk

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50 Gb/s Eye Diagram (Probed)

• Wafer probed through 3-ft of semi-rigid cable.
• Scope has about 1ps, RMS jitter
• Eye closure mostly due to cabling
• Small amount of eye asymmetry

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Summary and Acknowledgments

• Demonstrated 40-GHz and 43-GHz VCOs
• Both meet or exceed requirements for SONET OC-768
• Coupled oscillator architecture improves phase
  noise by about 11dB without additional power
  consumption
• Dual microstrip resonator eliminates via losses
  and minimizes layout parasitics
• 20-GHz versions applied in OC-768 transponder
  product
• Acknowledgments
• This work was performed at Big Bear Networks,
  Inc.
• Steffen Kudszus collaborated in this work
• Carlos Bowen for layout assistance
• Yuheng Lee for help with test package assembly
• Tad Labrie for testing assistance