VCO Fundamentals John McNeill - PowerPoint Presentation

Slide1

VCO Fundamentals

John McNeill

Worcester Polytechnic Institute

mcneill@ece.wpi.eduSlide2

Overview

Functional

Block Concept

Oscillator ReviewBasic Performance MetricsMethods of TuningAdvanced Performance MetricsConclusion

2Slide3

Overview

Functional

Block Concept

ApplicationsSpecificationsOscillator ReviewBasic Performance MetricsMethods of Tuning

Advanced Performance Metrics

Conclusion

3Slide4

Functional Block Concept

Input control voltage V

TUNE

determines frequency of output waveform4Slide5

Applications: RF System

Downconvert

band of interest to IF

VCO: Electrically tunable selection5Slide6

Applications: Digital System

Clock synthesis (frequency multiplication)

÷ N

6

J. A. McNeill

and D. R. Ricketts

, “The Designer’s Guide to Jitter in Ring Oscillators.” Springer, 2009 Slide7

from data sheet showing specs

Specifications

7Slide8

Overview

Functional

Block Concept

Oscillator ReviewFrequency ControlAmplitude ControlTypes of OscillatorsBasic Performance Metrics

Methods of Tuning

Advanced Performance Metrics

Conclusion

8Slide9

Oscillator Review

Types of Oscillators

Multivibrator

RingResonantFeedbackBasic Factors in Oscillator Design

Frequency

Amplitude / Output Power

Startup

9Slide10

Multivibrator

Conceptual

multivibrator

oscillatorAlso called astable or relaxation oscillator

One energy storage element

10Slide11

Example: Multivibrator

Frequency: Controlled by charging current

I

REF , C, VREF thresholdsAmplitude: Controlled by thresholds, logic swing

Startup: Guaranteed; no stable state

11Slide12

Ring Oscillator

Frequency: Controlled by gate delay

Amplitude: Controlled by logic swing

Startup: Guaranteed; no stable state

12Slide13

Resonant Oscillator

Concept: Natural oscillation frequency of resonance

E

nergy flows back and forth between two storage modes13Slide14

Resonant Oscillator (Ideal)

Example: swing (ideal)

Energy storage modes: potential, kinetic

Frequency: Controlled by length of pendulumAmplitude: Controlled by initial positionStartup: Needs initial condition energy input14Slide15

Resonant Oscillator (Real)

Problem: Loss of energy due to friction

Turns “organized” energy (potential, kinetic) into “disorganized” thermal energy (frictional heating)

Amplitude decays toward zeroRequires energy input to maintain amplitudeAmplitude controlled by “supervision”

15Slide16

LC Resonant Oscillator (Ideal)

Energy storage modes:

Magnetic field (L current), Electric field (C voltage)

Frequency: Controlled by LCAmplitude: Controlled by initial conditionStartup: Needs initial energy input (initial condition)

16Slide17

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

17Slide18

LC Resonant Oscillator (Real)

Problem: Loss of energy due to

nonideal

L, CRequires energy input to maintain amplitudeSynthesize “negative resistance”Cancel R

LOSS

with -R

NEG

18Slide19

Negative Resistance

Use active device to synthesize V-I characteristic that “looks like” –R

NEG

Example: amplifier with positive feedbackFeeds energy into resonator to counteract losses in R

LOSS

19Slide20

Feedback Oscillator: Wien Bridge

Forward gain A=3

Feedback network with transfer function

b(f)At f

OSC

, |

b

|=1/3 and



b

=0

Thought experiment:

break loop, inject sine wave, look at signal returned around feedback loop

20Slide21

A

b

=1

“Just right”waveform is self sustaining

21Slide22

A

b

=0.99

“Not enough”waveform decays to zero

22Slide23

A

b

=1.01

“Too much”waveform growsexponentialy

23Slide24

Feedback oscillator

Stable amplitude c

ondition:

Ab=1 EXACTLYFrequency determined by feedback network A

b

=1 condition

Need supervisory circuit to monitor amplitudeStartup: random noise; supervisory circuit begins with

A

b

>1

24Slide25

Resonant Oscillator (Real)

Stable amplitude condition: |R

NEG

| = RLOSS EXACTLYFrequency determined by LC networkStartup: random noise; begin with |RNEG

| > R

LOSS

Amplitude grows; soft clip gives average |R

NEG

| = R

LOSS

25

|R

NEG

| < R

LOSS

|R

NEG

| = R

LOSS

|R

NEG

| > R

LOSSSlide26

Clapp oscillator

L, C1-C2-C3 set oscillation frequency

f

OSC

26Slide27

Clapp oscillator

Circuit configuration

Equivalent circuit

MiniCircuits

AN95-007, “Understanding Oscillator Concepts” Slide28

Clapp oscillator

Frequency:

Determined by L, C1, C2, C3Amplitude: Grows until limited by g

m

soft clipping

Startup: Choose

C1, C2 feedback

for |

R

NEG

| > R

LOSSSlide29

Oscillator Summary

Typical performance of oscillator architectures:

29

kHz MHz GHz FREQUENCY

f

OSC

BETTER

PHASE

NOISE

RESONANT

RING

MULTIVIBRATOR

FEEDBACKSlide30

Overview

Functional

Block Concept

Oscillator ReviewBasic Performance MetricsFrequency RangeTuning RangeMethods of Tuning

Advanced Performance Metrics

Conclusion

30Slide31

from data sheet showing specs

Basic Performance Metrics

31Slide32

from data sheet showing specs

Basic Performance Metrics

32Slide33

Basic Performance Metrics

Supply:

DC operating powerOutputSine: output power dBm

into 50Ω

Square: compatible

logic

Frequency Range

Tuning Voltage Range

33Slide34

Frequency Range

Output frequency

over tuning voltage rangeCaution: Temperature sensitivity34Slide35

Overview

Functional

Block Concept

Oscillator ReviewBasic Performance MetricsMethods of TuningAdvanced Performance MetricsConclusion

35Slide36

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

)

36Slide37

Example: Tuning

Multivibrator

Frequency: Controlled by IREF , C, V

REF

thresholds

Use linear

transconductance

G

M

to develop

I

REF

from

V

TUNE

+ Very linear

V

TUNE

–

f

OSC

characteristic

- But: poor phase noise;

f

OSC

limited to MHz range

37Slide38

Tuning LC Resonator:

Varactor

Q-V characteristic of

pn junctionUse reverse bias diode for C in resonator

38Slide39

Example: Clapp oscillator

Tuning

range

fMIN, fMAX

set by C

TUNE

maximum, minimum

Want C

1

, C

2

> C

TUNE

for wider tuning range

39Slide40

Overview

Functional

Block Concept

Oscillator ReviewBasic Performance MetricsMethods of TuningAdvanced Performance MetricsTuning Sensitivity

Phase Noise

Supply Pushing

Load Pulling

Conclusion

40Slide41

Advanced Performance Metrics

Tuning Sensitivity (V-

f

linearity)Phase NoiseSupply/Load Sensitivity41Slide42

from data sheet showing specs

Tuning Sensitivity

42Slide43

Frequency Range

Change in slope [MHz/V]

over tuning voltage range43Slide44

Tuning Sensitivity

Why do you care?

PLL: Tuning sensitivity K

O

affects control parameters

Loop bandwidth

w

L

(may not be critical)

Stability (critical!)

44Slide45

Varactor

Tuning

Disadvantages of abrupt junction C-V characteristic (

m=1/2)Smaller tuning rangeInherently nonlinear V

TUNE

–

f

OSC

characteristic

45Slide46

Hyperabrupt

Junction

VaractorHyperabrupt junction C-V characteristic (m ≈ 2)

+ Larger tuning range; more linear

V

TUNE

–

f

OSC

- Disadvantage: Lower Q in resonator

46Slide47

from data sheet showing specs

Phase Noise

47Slide48

Phase Noise

Power s

pectrum “close in” to carrier

48Slide49

Phase Noise: RF System

Mixers convolve LO spectrum with RF

Phase noise “blurs” IF spectrum

49Slide50

Phase Noise: Digital System

Time domain jitter on

synthesized

output clockDecreases timing margin for system using clock

÷ N

50Slide51

Shape

of

Phase Noise Spectrum

LC filters noise into narrow band near fundamentalHigh Q resonator preferred to minimize noise51Slide52

Phase Noise: Intuitive view

Sine wave + white noise;

Filter; limit; Result:

52Slide53

Phase Noise: Intuitive view

Sine wave + white noise;

Filter; limit; Result:

53Slide54

Phase Noise Description

Symmetric; look at single sided representation

Normalized to carrier:

dBcAt different offset frequencies from carrierWhite frequency noise: phase noise with -20dB/decade slope

Other noise processes change slope; 1/f noise gives

-30dB/decade

54Slide55

Phase Noise Specification

Symmetric; look at single sided

Normalized to carrier:

dBcAt different offset frequencies from carrier

55Slide56

Sources of Phase Noise

Noise of active devices

56

Thermal noise:

Losses in resonator,

series R of

varactor

White noise in V

TUNE

signal pathSlide57

Supply / Load Sensitivity

Ideally tuning voltage is the only way to change output frequency

In reality other factors involved

Mechanism depends on specifics of circuitPower supply dependence: Supply PushingImpedance mismatch at output: Load Pulling

57Slide58

Supply Pushing

Change in

fOSC due to change in supply voltage

Clapp oscillator: supply affects transistor bias condition, internal signal amplitudes

58Slide59

Load Pulling

Change in

fOSC due to impedance mismatch

at

output

Clapp oscillator; reflection couples through transistor parasitic to LC resonator

59Slide60

Overview

Functional

Block Concept

Oscillator ReviewBasic Performance MetricsMethods of TuningAdvanced Performance MetricsConclusion

60Slide61

Summary: VCO Fundamentals

First order behavior

Tuning voltage V

TUNE controls output frequencySpecify 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

61Slide62

Questions?

62