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VCO Fundamentals

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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 9 Overview • Functional Block Concept – Applications – Specifications • Oscillator Review • Basic Performance Metrics • Methods of Tuning • Advanced Performance Metrics • Conclusion 10 Functional Block Concept • Input control voltage V TUNE determines frequency of output waveform 11 Applications: RF System • Downconvert band of interest to IF • VCO: Electrically tunable selection 12 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 13 Specifications • from data sheet showing specs 14 Overview • Functional Block Concept • Oscillator Review – Frequency Control – Amplitude Control – Types of Oscillators • Basic Performance Metrics • Methods of Tuning • Advanced Performance Metrics • Conclusion 15 Oscillator Review • Types of Oscillators – Multivibrator – Ring – Resonant – Feedback • Basic Factors in Oscillator Design – Frequency – Amplitude / Output Power – Startup 16 Multivibrator • Conceptual multivibrator oscillator – Also called astable or relaxation oscillator • One energy storage element 17 Example: Multivibrator • Frequency: Controlled by charging current IREF , C, V REF thresholds • Amplitude: Controlled by thresholds, logic swing • Startup: Guaranteed; no stable state 18 Ring Oscillator • Frequency: Controlled by gate delay • Amplitude: Controlled by logic swing • Startup: Guaranteed; no stable state 19 Resonant Oscillator • Concept: Natural oscillation frequency of resonance • Energy flows back and forth between two storage modes 1 f = OSC 2π LC 20 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 21 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” 22 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) 23 LC Resonant Oscillator (Real) • Problem: Loss of energy due to nonideal L, C – Model as resistor R LOSS ; Q of resonator • E, M field energy lost to resistor heating • Amplitude decays toward zero 24 LC Resonant Oscillator (Real) • Problem: Loss of energy due to nonideal L, C • Requires energy input to maintain amplitude • Synthesize “negative resistance” • Cancel R LOSS with -RNEG 25 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 R LOSS 26 Feedback Oscillator: Wien Bridge • Forward gain A=3 • Feedback network with transfer function βββ(f) • At fOSC , | βββ|=1/3 and ∠βββ =0 • Thought experiment: break loop, inject sine wave, look at signal returned around feedback loop 27 Aβββ=1 • “Just right” waveform is self sustaining 28 Aβββ=0.99 • “Not enough” waveform decays to zero 29 Aβββ=1.01 • “Too much” waveform grows exponentially 30 Feedback oscillator • Stable amplitude condition: Aβββ=1 EXACTLY • Frequency determined by feedback network Aβββ=1 condition • Need supervisory circuit to monitor amplitude • Startup: random noise; supervisory circuit begins with Aβββ>1 31 Resonant Oscillator (Real) |R NEG | < R LOSS |R NEG | = R LOSS |R NEG | > R LOSS • Stable amplitude condition: |R NEG | = R LOSS EXACTLY • Frequency determined by LC network • Startup: random noise; begin with |R NEG | > R LOSS • Amplitude grows; soft clip gives average |R NEG | = R LOSS 32 Clapp oscillator 1 f = OSC 2 π LC eq 1 C = eq 1 1 1 + + C1 C2 C3 • L, C1-C2-C3 set oscillation frequency fOSC 33 Clapp oscillator • Circuit configuration • Equivalent circuit MiniCircuitsAN95-007, “Understanding Oscillator Concepts” Clapp oscillator 1 1 gm Zeq = + − 2 jωC1 jωC2 ω C1C2 • Frequency: Determined by L, C1, C2, C3 • Amplitude: Grows until limited by g m soft clipping • Startup: Choose C1, C2 feedback for | RNEG | > R LOSS Oscillator Summary • Typical performance of oscillator architectures: BETTER RESONANT PHASE NOISE FEEDBACK RING MULTIVIBRATOR kHz MHz GHz FREQUENCY fOSC 36 Overview • Functional Block Concept • Oscillator Review • Basic Performance Metrics – Frequency Range – Tuning Range • Methods of Tuning • Advanced Performance Metrics • Conclusion 37 Basic Performance Metrics • from data sheet showing specs 38 Basic Performance Metrics • from data sheet showing specs 39 Basic Performance Metrics • Supply: DC operating power • Output – Sine: output power dBm into 50 Ω – Square: compatible logic • Frequency Range • Tuning Voltage Range 40 Frequency Range • Output frequency over tuning voltage range • Caution: Temperature sensitivity 41 Overview • Functional Block Concept • Oscillator Review • Basic Performance Metrics • Methods of Tuning • Advanced Performance Metrics • Conclusion 42 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) 43 Example: Tuning Multivibrator I • Frequency: Controlled by f = REF OSC 4 CV REF IREF , C, V REF thresholds • Use linear transconductance IREF = GM VTUNE GM to develop IREF from VTUNE G f = M V OSC 4CV TUNE + Very linear VTUNE – fOSC characteristic REF - But: poor phase noise; fOSC limited to MHz range 44 Tuning LC Resonator: Varactor Q dQ C = j dV C j 0 R C = j m V 1 R + Vbi • Q-V characteristic of pn junction • Use reverse bias diode for C in resonator 45 Example: Clapp oscillator 1 C C f = 1+ TUNE + TUNE OSC 2 π LC TUNE C1 C2 • Tuning range fMIN , fMAX set by C TUNE maximum, minimum • Want C 1, C 2 > C TUNE for wider tuning range 46 Overview • Functional Block Concept • Oscillator Review • Basic Performance Metrics • Methods of Tuning • Advanced Performance Metrics – Tuning Sensitivity – Phase Noise – Supply Pushing – Load Pulling • Conclusion 47 Advanced Performance Metrics • Tuning Sensitivity (V-f linearity) • Phase Noise • Supply/Load Sensitivity 48 Tuning Sensitivity • from data sheet showing specs 49 Frequency Range • Change in slope [MHz/V] over tuning voltage range 50 Tuning Sensitivity K K τ ω ≈ d O Z 1 L + sτ Z KO τI Kd (θi − θ o ) sτ I s • Why do you care? – PLL: Tuning sensitivity K O affects control parameters – Loop bandwidth ωωωL (may not be critical) – Stability (critical!) 51 Varactor Tuning C 0 C = j j m VTUNE 1 + Vbi 1 f = OSC 2π LC m 2 1 V f ≈ TUNE m =1 2 OSC 2 π LC j 0 Vbi • Disadvantages of abrupt junction C-V characteristic (m=1/2) – Smaller tuning range – Inherently nonlinear VTUNE – fOSC characteristic 52 Hyperabrupt Junction Varactor C 0 C = j j m VTUNE 1 + Vbi 1 f = OSC 2π LC m 2 1 V f ≈ TUNE m =1 2 2 OSC 2 m → π LC j 0 Vbi • Hyperabrupt junction C-V characteristic (m ≈ 2) + Larger tuning range; more linear VTUNE – fOSC - Disadvantage: Lower Q in resonator 53 Phase Noise • from data sheet showing specs 54 Phase Noise • Power spectrum “close in” to carrier 55 Phase Noise: RF System • Mixers convolve LO spectrum with RF • Phase noise “blurs” IF spectrum 56 Phase Noise: Digital System ÷ N • Time domain jitter on synthesized output clock • Decreases timing margin for system using clock 57 Shape of Phase Noise Spectrum • LC filters noise into narrow band near fundamental • High Q resonator preferred to minimize noise 58 Phase Noise: Intuitive view • Sine wave + white noise; Filter; limit; Result: 59 Phase Noise: Intuitive view • Sine wave + white noise; Filter; limit; Result: 60 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 61 Phase Noise Specification • Symmetric; look at single sided • Normalized to carrier: dBc • At different offset frequencies from carrier 62 Sources of Phase Noise White noise in VTUNE signal path Noise of active devices Thermal noise: Losses in resonator, series R of varactor 63 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 64 Supply Pushing • Change in fOSC due to change in supply voltage • Clapp oscillator: supply affects transistor bias condition, internal signal amplitudes 65 Load Pulling • Change in fOSC due to impedance mismatch at output • Clapp oscillator; reflection couples through transistor parasitic to LC resonator 66 Overview • Functional Block Concept • Oscillator Review • Basic Performance Metrics • Methods of Tuning • Advanced Performance Metrics • Conclusion 67 Summary: VCO Fundamentals • First order behavior – Tuning voltage V TUNE controls output frequency – Specify by min/max range of fOSC , V TUNE • 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 68 Questions? Thank you to our presenter John McNeill and our sponsor Mini-Circuits 69.
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