Design Techniques for Radiation Hardened Phase Locked Loops
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Design Techniques for Radiation Hardened Phase Locked Loops Anantha Nag Nemmani, Martin Vandepas, Kerem Ok, Kartikeya Mayaram and Un-Ku Moon Oregon State University Electrical Engineering and Computer Science 2005 MAPLD International Conference ◊ Ronald Reagan Building and International Trade Center Washington, D. C. ◊ September 7 -9, 2005 Effects Total Ionization Dose (TID) Single Event Effects (SEE) Effect on Phase Locked Loops (PLLs) SEE on PLLs • VCO tuning curves shift – Change in nominal frequency and VCO gain • Change in chargepump current • Loop parameters changed as a result • Loop transfer function altered • Accumulation of charge in gate oxide – Vth NMOS ↓ & Vth PMOS ↑ Digital PLL Tuning curves of Lee/Kim Ring oscillator before and after radiation. • Transient error currents • Modelled as sum of exponential current sources SEE on last stage of frequency divider • Digitally controlled analog oscillator (DCAO) • Time-to-digital converter (TDC) • Digital loop filter Analysis • Lee/Kim delay cell • Fine/Coarse tuning Digital Loop Filter Time-to-digital converter Digitally controlled analog oscillator • Fine tuning current DAC • 6 fine bits + 6 coarse bits • • Exponential delay chain Latches Sign detection Pseudo-thermometor encoder Operation of a TDC • Type – II loop filter Analysis Verification DCAO resolution 1. DCAO: SEE on control node Openloop gain For optimum performance Phase margin - ΦM Incremental phase accumulated 2. Phase detector: 3. Loop filter: Control word CPPLL Analogy Proportional Integral gain Radiation Hardening Self-Calibration TDC Output Response to frequency step at the input. Simulink Agrees with mathematical model Normalizing gain Redundancy & Majority Voting Layout Single Event Hardening • Loop parameters – f. LSB & d. T – Process dependent – Gn, α & β – Digital constants • Loop dynamics are dependent only dependent on: • Self calibration – Calibrate Kdig to the desired value Time-to-digital converter Frequency Divider MAPLD 2005/228 Accumulator