Title: Microstrip Coupled VCOs for 40GHz and 43GHz OC768 Optical Transmission Derek K' Shaeffer, Ph'D' Stef
1Microstrip Coupled VCOs for 40-GHz and 43-GHz
OC-768 Optical TransmissionDerek K. Shaeffer,
Ph.D.Steffen Kudszus, Ph.D.
2Outline
- Introduction to Jitter in SONET Systems
- Review of Oscillator Phase Noise Theory
- VCO Architectural Considerations
- Simulation Results
- Experimental Results
- Summary Acknowledgments
3Introduction
- 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!
4Integrated Optical Transponder
VCO in here
5Jitter 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
6Jitter 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)
7Relationship 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
8Review 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
9Phase 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
10A 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
11Minimizing 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
12Effect 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
13Delay Compensation Using Coupled Oscillators
- Refresh pulse timing is corrected by
- The oscillator operates on-resonance if M is set
properly
14Simulated Oscillation Waveforms
15Resonator 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
16Resonator 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
17Microstripline 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
18Resonator 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
19Circuit Architecture
- Colepitts-like pulsed operation
- Capacitive coupling network sets the coupling
coefficient independent of pulse network
parasitics
20Quadrature VCO Block Diagram
- Two resonators, cross-coupled to produce in-phase
and quadrature oscillations. - For testing, only brought out the in-phase signal.
21Simulated Phase Noise Performance
22Experimental 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
23Tuning Characteristics
24Measured Phase Noise Performance
25Performance Summary
26Epilogue VCO in action in 41 MUX / CMU(ISSCC
2003 Paper 13.4, JSSC Dec 2003)
2720-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
28VCO Tuning Range
29CMU 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
3040 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
3150 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
32Summary 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