Report

VCO Fundamentals John McNeill Worcester Polytechnic Institute [email protected] Overview • • • • • • Functional Block Concept Oscillator Review Basic Performance Metrics Methods of Tuning Advanced Performance Metrics Conclusion 2 Overview • Functional Block Concept – Applications – Specifications • Oscillator Review • Basic Performance Metrics • Methods of Tuning • Advanced Performance Metrics • Conclusion 3 Functional Block Concept • Input control voltage VTUNE determines frequency of output waveform 4 Applications: RF System • Downconvert band of interest to IF • VCO: Electrically tunable selection 5 Applications: Digital System ÷N • Clock synthesis (frequency multiplication) J. A. McNeill and D. R. Ricketts, “The Designer’s Guide to Jitter in Ring Oscillators.” Springer, 2009 6 Specifications • from data sheet showing specs 7 Overview • Functional Block Concept • Oscillator Review – Frequency Control – Amplitude Control – Types of Oscillators • Basic Performance Metrics • Methods of Tuning • Advanced Performance Metrics • Conclusion 8 Oscillator Review • Types of Oscillators – Multivibrator – Ring – Resonant – Feedback • Basic Factors in Oscillator Design – Frequency – Amplitude / Output Power – Startup 9 Multivibrator • Conceptual multivibrator oscillator – Also called astable or relaxation oscillator • One energy storage element 10 Example: Multivibrator • Frequency: Controlled by charging current IREF , C, VREF thresholds • Amplitude: Controlled by thresholds, logic swing • Startup: Guaranteed; no stable state 11 Ring Oscillator • Frequency: Controlled by gate delay • Amplitude: Controlled by logic swing • Startup: Guaranteed; no stable state 12 Resonant Oscillator • Concept: Natural oscillation frequency of resonance • Energy flows back and forth between two storage modes f OSC 13 1 2 LC 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 14 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” 15 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) 16 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 17 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 18 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 19 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 20 Ab=1 • “Just right” waveform is self sustaining 21 Ab=0.99 • “Not enough” waveform decays to zero 22 Ab=1.01 • “Too much” waveform grows exponentialy 23 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 24 Resonant Oscillator (Real) |RNEG| < RLOSS • • • • |RNEG| = RLOSS |RNEG| > RLOSS 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 25 Clapp oscillator f OSC C eq 1 2 LC eq 1 1 1 1 C C C 1 2 3 • L, C1-C2-C3 set oscillation frequency fOSC 26 Clapp oscillator • Circuit configuration • Equivalent circuit MiniCircuits AN95-007, “Understanding Oscillator Concepts” Clapp oscillator Z eq 1 j C1 1 j C 2 • Frequency: Determined by L, C1, C2, C3 • Amplitude: Grows until limited by gm soft clipping • Startup: Choose C1, C2 feedback for | RNEG | > RLOSS gm C 1C 2 2 Oscillator Summary • Typical performance of oscillator architectures: BETTER PHASE NOISE RESONANT FEEDBACK RING MULTIVIBRATOR kHz MHz GHz FREQUENCY fOSC 29 Overview • Functional Block Concept • Oscillator Review • Basic Performance Metrics – Frequency Range – Tuning Range • Methods of Tuning • Advanced Performance Metrics • Conclusion 30 Basic Performance Metrics • from data sheet showing specs 31 Basic Performance Metrics • from data sheet showing specs 32 Basic Performance Metrics • Supply: DC operating power • Output – Sine: output power dBm into 50Ω – Square: compatible logic • Frequency Range • Tuning Voltage Range 33 Frequency Range • Output frequency over tuning voltage range • Caution: Temperature sensitivity 34 Overview • • • • • • Functional Block Concept Oscillator Review Basic Performance Metrics Methods of Tuning Advanced Performance Metrics Conclusion 35 VCOs / Methods of Tuning • Require electrical control of some parameter determining frequency: • Multivibrator – Charge / discharge current • Ring Oscillator – Gate delay • Resonant – Voltage control of capacitance in LC (varactor) 36 Example: Tuning Multivibrator I REF • Frequency: Controlled by IREF , C, VREF thresholds f OSC • Use linear transconductance GM to develop IREF from VTUNE I REF G M V TUNE 4 CV REF f OSC G M V TUNE 4 CV REF + Very linear VTUNE – fOSC characteristic - But: poor phase noise; fOSC limited to MHz range 37 Tuning LC Resonator: Varactor Q Cj dQ dV R Cj C j0 V m R 1 V bi • Q-V characteristic of pn junction • Use reverse bias diode for C inresonator 38 Example: Clapp oscillator f OSC 1 2 LC TUNE 1 C TUNE C1 C TUNE C2 • Tuning range fMIN, fMAX set by CTUNE maximum, minimum • Want C1, C2 > CTUNE for wider tuning range 39 Overview • • • • • Functional Block Concept Oscillator Review Basic Performance Metrics Methods of Tuning Advanced Performance Metrics – Tuning Sensitivity – Phase Noise – Supply Pushing – Load Pulling • Conclusion 40 Advanced Performance Metrics • Tuning Sensitivity (V-f linearity) • Phase Noise • Supply/Load Sensitivity 41 Tuning Sensitivity • from data sheet showing specs 42 Frequency Range • Change in slope [MHz/V] over tuning voltage range 43 Tuning Sensitivity K d i o 1 s Z KO s I s L K d K O Z I • Why do you care? – PLL: Tuning sensitivity KO affects control parameters – Loop bandwidth L (may not be critical) – Stability (critical!) 44 Varactor Tuning Cj C j0 V m TUNE 1 V bi f OSC f OSC 1 2 LC 1 2 LC j0 V m 2 TUNE V bi m 1 2 • Disadvantages ofabrupt junction C-V characteristic (m=1/2) – Smaller tuning range – Inherently nonlinear VTUNE – fOSC characteristic 45 Hyperabrupt Junction Varactor Cj C j0 V m TUNE 1 V bi f OSC f OSC 1 2 LC 1 2 LC j0 V m 2 TUNE V bi m 1 2 m 2 • Hyperabrupt junction C-V characteristic (m ≈ 2) + Larger tuning range; more linear VTUNE – fOSC - Disadvantage: Lower Q in resonator 46 Phase Noise • from data sheet showing specs 47 Phase Noise • Power spectrum “close in” to carrier 48 Phase Noise: RF System • Mixers convolve LO spectrum with RF • Phase noise “blurs” IF spectrum 49 Phase Noise: Digital System ÷N • Time domain jitter on synthesized output clock • Decreases timing margin for system using clock 50 Shape of Phase Noise Spectrum • LC filters noise into narrow band near fundamental • High Q resonator preferred to minimize noise 51 Phase Noise: Intuitive view • Sine wave + white noise; Filter; limit; Result: 52 Phase Noise: Intuitive view • Sine wave + white noise; Filter; limit; Result: 53 Phase Noise Description • • • • • Symmetric; look at single sided representation Normalized to carrier: dBc At different offset frequencies from carrier White frequency noise: phase noise with -20dB/decade slope Other noise processes change slope; 1/f noise gives -30dB/decade 54 Phase Noise Specification • • • Symmetric; look at single sided Normalized to carrier: dBc At different offset frequencies from carrier 55 Sources of Phase Noise White noise in VTUNE signal path Noise of active devices Thermal noise: Losses in resonator, series R of varactor 56 Supply / Load Sensitivity • Ideally tuning voltage is the only way to change output frequency – In reality other factors involved – Mechanism depends on specifics of circuit • Power supply dependence: Supply Pushing • Impedance mismatch at output: Load Pulling 57 Supply Pushing • Change in fOSC due to change in supply voltage • Clapp oscillator: supply affects transistor bias condition, internal signal amplitudes 58 Load Pulling • Change in fOSC due to impedance mismatch at output • Clapp oscillator; reflection couples through transistor parasitic to LC resonator 59 Overview • • • • • • Functional Block Concept Oscillator Review Basic Performance Metrics Methods of Tuning Advanced Performance Metrics Conclusion 60 Summary: VCO Fundamentals • First order behavior – Tuning voltage VTUNE controls output frequency – Specify by min/max range of fOSC, VTUNE • Performance limitations – Linearity of tuning characteristic – Spectral purity: phase noise, harmonics – Supply, load dependence • Different VCO architectures trade frequency range, tuning linearity, phase noise performance 61 Questions? 62