VCO Fundamentals John McNeill Worcester Polytechnic Institute mcneill@ece.wpi.edu

1. VCO Fundamentals John McNeill Worcester Polytechnic Institute mcneill@ece.wpi.edu

2. Overview • Functional Block Concept • Oscillator Review • Basic Performance Metrics • Methods of Tuning • Advanced Performance Metrics • Conclusion

3. Overview • Functional Block Concept • Applications • Specifications • Oscillator Review • Basic Performance Metrics • Methods of Tuning • Advanced Performance Metrics • Conclusion

4. Functional Block Concept • Input control voltage VTUNEdetermines frequency of output waveform

5. Applications: RF System • Downconvert band of interest to IF • VCO: Electrically tunable selection

6. Applications: Digital System • Clock synthesis (frequency multiplication) ÷ N • J. A. McNeill and D. R. Ricketts, “The Designer’s Guide to Jitter in Ring Oscillators.” Springer, 2009

7. Specifications • from data sheet showing specs

8. Overview • Functional Block Concept • Oscillator Review • Frequency Control • Amplitude Control • Types of Oscillators • Basic Performance Metrics • Methods of Tuning • Advanced Performance Metrics • Conclusion

9. Oscillator Review • Types of Oscillators • Multivibrator • Ring • Resonant • Feedback • Basic Factors in Oscillator Design • Frequency • Amplitude / Output Power • Startup

10. Multivibrator • Conceptual multivibrator oscillator • Also calledastable or relaxation oscillator • One energy storage element

11. Example: Multivibrator • Frequency: Controlled by charging current IREF , C, VREF thresholds • Amplitude: Controlled by thresholds, logic swing • Startup: Guaranteed; no stable state

12. Ring Oscillator • Frequency: Controlled by gate delay • Amplitude: Controlled by logic swing • Startup: Guaranteed; no stable state

13. Resonant Oscillator • Concept: Natural oscillation frequency of resonance • Energy flows back and forth between two storage modes

14. Resonant Oscillator (Ideal) • Example: swing (ideal) • Energy storage modes: potential, kinetic • Frequency: Controlled by length of pendulum • Amplitude: Controlled by initial position • Startup: Needs initial condition energy input

15. Resonant Oscillator (Real) • Problem: Loss of energy due to friction • Turns “organized” energy (potential, kinetic) into “disorganized” thermal energy (frictional heating) • Amplitude decays toward zero • Requires energy input to maintain amplitude • Amplitude controlled by “supervision”

16. LC Resonant Oscillator (Ideal) • Energy storage modes: Magnetic field (L current), Electric field (C voltage) • Frequency: Controlled by LC • Amplitude: Controlled by initial condition • Startup: Needs initial energy input (initial condition)

17. LC Resonant Oscillator (Real) • Problem: Loss of energy due to nonideal L, C • Model as resistor RLOSS;Q of resonator • E, M field energy lost to resistor heating • Amplitude decays toward zero

18. LC Resonant Oscillator (Real) • Problem: Loss of energy due to nonideal L, C • Requires energy input to maintain amplitude • Synthesize “negative resistance” • Cancel RLOSS with -RNEG

19. Negative Resistance • Use active device to synthesize V-I characteristic that “looks like” –RNEG • Example: amplifier with positive feedback • Feeds energy into resonator to counteract losses in RLOSS

20. Feedback Oscillator: Wien Bridge • Forward gain A=3 • Feedback network with transfer function b(f) • At fOSC, |b|=1/3 and b =0 • Thought experiment: break loop, inject sine wave, look at signal returned around feedback loop

21. Ab=1 • “Just right”waveform is self sustaining

22. Ab=0.99 • “Not enough”waveform decays to zero

23. Ab=1.01 • “Too much”waveform growsexponentialy

24. Feedback oscillator • Stable amplitude condition: Ab=1 EXACTLY • Frequency determined by feedback network Ab=1 condition • Need supervisory circuit to monitor amplitude • Startup: random noise; supervisory circuit begins with Ab>1

25. Resonant Oscillator (Real) • Stable amplitude condition: |RNEG| = RLOSS EXACTLY • Frequency determined by LC network • Startup: random noise; begin with |RNEG| > RLOSS • Amplitude grows; soft clip gives average |RNEG| = RLOSS |RNEG| < RLOSS |RNEG| = RLOSS |RNEG| > RLOSS

26. Clapp oscillator • L, C1-C2-C3 set oscillation frequency fOSC

27. Clapp oscillator • Circuit configuration • Equivalent circuit • MiniCircuits AN95-007, “Understanding Oscillator Concepts”

28. Clapp oscillator • Frequency:Determined by L, C1, C2, C3 • Amplitude: Grows until limited by gm soft clipping • Startup: Choose C1, C2 feedback for |RNEG | > RLOSS

29. Oscillator Summary • Typical performance of oscillator architectures: • BETTER • PHASE • NOISE • RESONANT • FEEDBACK • RING • MULTIVIBRATOR • kHz MHz GHz • FREQUENCY fOSC

30. Overview • Functional Block Concept • Oscillator Review • Basic Performance Metrics • Frequency Range • Tuning Range • Methods of Tuning • Advanced Performance Metrics • Conclusion

31. Basic Performance Metrics • from data sheet showing specs

32. Basic Performance Metrics • from data sheet showing specs

33. Basic Performance Metrics • Supply:DC operating power • Output • Sine:output power dBm into 50Ω • Square: compatible logic • Frequency Range • Tuning Voltage Range

34. Frequency Range • Output frequencyover tuning voltage range • Caution: Temperature sensitivity

35. Overview • Functional Block Concept • Oscillator Review • Basic Performance Metrics • Methods of Tuning • Advanced Performance Metrics • Conclusion

36. VCOs / Methods of Tuning • Require electrical control of some parameter determining frequency: • Multivibrator • Charge / dischargecurrent • Ring Oscillator • Gate delay • Resonant • Voltage control ofcapacitance in LC(varactor)

37. Example: Tuning Multivibrator • Frequency: Controlled by IREF , C, VREF thresholds • Use linear transconductanceGM to develop IREF from VTUNE + Very linear VTUNE – fOSC characteristic - But: poor phase noise; fOSC limited to MHz range

38. Tuning LC Resonator: Varactor • Q-V characteristic of pn junction • Use reverse bias diode for C in resonator

39. Example: Clapp oscillator • Tuning range fMIN, fMAXset by CTUNE maximum, minimum • Want C1, C2 > CTUNE for wider tuning range

40. Overview • Functional Block Concept • Oscillator Review • Basic Performance Metrics • Methods of Tuning • Advanced Performance Metrics • Tuning Sensitivity • Phase Noise • Supply Pushing • Load Pulling • Conclusion

41. Advanced Performance Metrics • Tuning Sensitivity (V-f linearity) • Phase Noise • Supply/Load Sensitivity

42. Tuning Sensitivity • from data sheet showing specs

43. Frequency Range • Change in slope [MHz/V]over tuning voltage range

44. Tuning Sensitivity • Why do you care? • PLL: Tuning sensitivity KO affects control parameters • Loop bandwidth wL (may not be critical) • Stability (critical!)

45. Varactor Tuning • Disadvantages of abrupt junction C-V characteristic (m=1/2) • Smaller tuning range • Inherently nonlinear VTUNE – fOSC characteristic

46. Hyperabrupt JunctionVaractor • Hyperabrupt junction C-V characteristic (m ≈ 2) + Larger tuning range; more linear VTUNE – fOSC - Disadvantage: Lower Q in resonator

47. Phase Noise • from data sheet showing specs

48. Phase Noise • Power spectrum “close in” to carrier

49. Phase Noise: RF System • Mixers convolve LO spectrum with RF • Phase noise “blurs” IF spectrum

50. Phase Noise: Digital System • Time domain jitter on synthesized output clock • Decreases timing margin for system using clock ÷ N