Vladimir Ivanov October 23, 2007

1. A Stabilization Technique forPhase-Locked Frequency SynthesizersTai-Cheng Lee and Behzad RazaviIEEE Journal of Solid-State Circuits, Vol. 38, June 2003 Vladimir Ivanov October 23, 2007

2. Outline • Integer-N PLL frequency synthesizer • Conventional architecture • Two proposals • Delay network • Synthesizer design • Simulations • Experimental results • Performance • Summary

3. Integer-N PLL frequency synthesizer • Phase-frequency Detector (PFD) compares phases and sends voltage pulses to CP • Charge Pump (CP) converts the voltage pulses into current pulses • Loop filter converts current pulses into a voltage level • Voltage-controlled Oscillator (VCO) produces frequency proportional to its control input

4. Conventional architecture • R1 provides the stabilizing zero • C2 lowers the ripple on Vcont • C1 determines the settling time • Tight tradeoff: settling time vs. ripple on Vcont • Goal: relax this tradeoff

5. Overview • To avoid overdamped settling, C2 ~ C1/10 • Therefore, C1 has to be large • Idea: stabilize by creating a zero without the resistor • Thus, C1 both defines the switching speed and suppresses the ripples • Approach: create a zero through a discrete-time delay • Achieves both fast settling and small ripple • Obviates the resistor in the loop filter => digital CMOS • “Amplifies” the value of the loop filter capacitor=> saves die area • Two proposals: delay before and delay after CP2

6. Proposal 1: delay before CP2 • CP1 drives C1 directly • CP2 injects charge in C1 after time ΔT • Transfer function: • Zero: • To have ωz low enough and desired loop behavior, ΔT ~ 500 ns

7. Proposal 1: delay before CP2 • Problems with Proposal 1 • The delay line has to: • provide very large ΔT and • accommodate a wide range of UP and DN pulsewidths • ΔT varies with process and temperature; therefore, the damping factor (and thus the stability) may be affected because • Proposal 2: place the delay line after CP2

8. Proposal 2: delay after CP2 • If loop settling time >> 1/fREF and C2>>C1: • C2 value “amplified” by 1/(1-) • ωz achieved without resistors • damping factor much less depended on process and temperature

9. Delay network

10. Synthesizer design

11. Comparison with conventional architecture • Loop filters: type A (delay-sampled) andtype B (conventional) • Gain: about 10 dB lower sidebands

12. Simulations • Simulation takes very long time due to: • Very different time scales • Large number of devices • Two models to speed up the design: • Linear discrete-time model (in Matlab):to compute optimal CP current, C1 and Cs • Transistor-level model: to study the nonidealities of PFD, CP, and VCO • Time contraction: fREF scaled up by 100;C2 and M scaled down by 100 • Divider realized as a simple behavioral model with ideal devices

13. Experimental results

14. Experimental results

15. Performance

16. Summary • Proposed PLL stabilization technique by creating a zero in the open-loop TF which: • Relaxes the tradeoff between the settling time and ripple on the VCO control voltage • Makes the resistor in the loop filter unnecessary • “Amplifies” the loop filter capacitor, saving die area

17. VCO design • Inductors: • 180μm x 180μm • ~ 14 nH with Q = 4 (100 fF) • Varactors: • 160 fF with tuning range ~ 12% • VCO phase noise: -120 dBc/Hz at 1 MHz