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

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

2. 2 Outline Introduction to Jitter in SONET Systems Review of Oscillator Phase Noise Theory VCO Architectural Considerations Simulation Results Experimental Results Summary & Acknowledgments

3. 3 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 @ 10-12 BER That’s 14 standard deviations! Introduction

4. 4 Integrated Optical Transponder

5. 5 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

6. 6 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” f1=20 kHz, f2=320 MHz, Jp-p=1.2 UI (30 ps) “High-Band” f1=16 MHz, f2=320 MHz, Jp-p=0.1 UI (2.5 ps)

7. 7 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 @ 1-MHz offset High-Band: -89 dBc / Hz @ 1-MHz offset

8. 8 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

9. 9 Phase Noise of Coupled Oscillators With two coupled oscillators (N=2) 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 10*log10(N) However, additional improvement beyond 3dB can be gained if coupling leads to better refresh pulse alignment

10. 10 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…

11. 11 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

12. 12 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

13. 13 Delay Compensation Using Coupled Oscillators Refresh pulse timing is corrected by: The oscillator operates on-resonance if M is set properly

14. 14 Simulated Oscillation Waveforms

15. 15 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 @ 40GHz) Easy to model and scale

16. 16 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 (Q~30) Disadvantage Steep tuning slope

17. 17 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

18. 18 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

19. 19 Circuit Architecture Colepitts-like pulsed operation Capacitive coupling network sets the coupling coefficient independent of pulse network parasitics

20. 20 Quadrature VCO Block Diagram Two resonators, cross-coupled to produce in-phase and quadrature oscillations. For testing, only brought out the in-phase signal.

21. 21 Simulated Phase Noise Performance

22. 22 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

23. 23 Tuning Characteristics

24. 24 Measured Phase Noise Performance

25. 25 Performance Summary

26. 26 Epilogue – VCO in action in 4:1 MUX / CMU(ISSCC 2003 Paper 13.4, JSSC Dec 2003)

27. 27 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

28. 28 VCO Tuning Range

29. 29 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

30. 30 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

31. 31 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

32. 32 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