VCO Design Tips

Chuck Clark AF8ZJuly 25, 2009

© Copyright by Charles F. Clark, Bluestem Consulting, Inc. 2009

Lesson's Phase Noise Model

L(f)=FkT/2Po[1+(fo/2QLf)2]

F=Active Device Noise Factor

k=Boltzmans Constant

T=Temperature

Po=Output Power

Fo=Oscillator Center Frequency

QL=loaded resonator quality factor

f=frequency offset from carrier

© Copyright by Charles F. Clark, Bluestem Consulting, Inc. 2009 Page 2

VCO Tips

Optimum Loaded to Unloaded Q

2/3 Ql/Qo 6 dB of resonator loss

This comes from solving Lessons eqn for the minimum of phase noise.

© Copyright by Charles F. Clark, Bluestem Consulting, Inc. 2009 Page 3

One Approach to Higher Q

© Copyright by Charles F. Clark, Bluestem Consulting, Inc. 2009 Page 4

Another

© Copyright by Charles F. Clark, Bluestem Consulting, Inc. 2009 Page 5

Q is purchased by the CUBIC FOOT!

© Copyright by Charles F. Clark, Bluestem Consulting, Inc. 2009 Page 6

Amplifier

Run a large DC current.

Servo the DC current

Small device capacitances

High FT Oscillator Transistor

NEC 85633 is excellent below 1200 MHz

NEC 68133 above

Infineon and NXP also make good parts

© Copyright by Charles F. Clark, Bluestem Consulting, Inc. 2009 Page 7

1/f Noise Feedback, Servo the current

© Copyright by Charles F. Clark, Bluestem Consulting, Inc. 2009 Page 8

Active Bias Circuit

PNP Transistor BC847

Beta =800

Low Noise

Suppresses gain of the RF transistor up to ~100 MHz.

Works the same as the feedback in a PLL

Can extend the noise improvement above the corner frequency of the PLL which is dictated by the reference frequency.

© Copyright by Charles F. Clark, Bluestem Consulting, Inc. 2009 Page 9

Phase Noise of PLL

10.00 100.0 1.000k 10.00k 100.0k 1.000M1.000 10.00M

-150.0

-130.0

-110.0

-90.00

-70.00

-50.00

-30.00

-170.0

-20.00

freq, Hz

PN

Tota

l

PN

_V

CO

_F

reeR

un

Loop Reduction of VCO Phase Noise

© Copyright by Charles F. Clark, Bluestem Consulting, Inc. 2009 Page 10

Its all about Loop Gain

© Copyright by Charles F. Clark, Bluestem Consulting, Inc. 2009 Page 11

Loop Transfer Function

B(s)=(Kd*F(s)*Ko)/(N*s)

Kd=Phase Detector gain constant

Ko/s= VCO gain, MHz/Volt

F(s)=loop filter response

N= loop divider ratio

s=Heaviside operator, 2*pi*f

© Copyright by Charles F. Clark, Bluestem Consulting, Inc. 2009 Page 12

Passive Filter

•Low Frequency Gain~1

•Other Passive filters

possible.

© Copyright by Charles F. Clark, Bluestem Consulting, Inc. 2009 Page 13

Loop Filter Gain

•Low Frequency Gain~∞

© Copyright by Charles F. Clark, Bluestem Consulting, Inc. 2009 Page 14

P has e Nois e vs R eferenc e F requenc y

-160.00-150.00-140.00-130.00-120.00-110.00-100.00

-90.00-80.00-70.00-60.00-50.00-40.00-30.00-20.00

1 10 100 1000 10000 100000 1E + 06 1E + 07

F requenc y

dB

c V C O

1 MHz R ef ActiveF ilter

100 kHz R efActive F ilter

© Copyright by Charles F. Clark, Bluestem Consulting, Inc. 2009 Page 15

Other Sources of Phase Noise

10.00 100.0 1.000k 10.00k 100.0k 1.000M1.000 10.00M

-150.0

-140.0

-130.0

-120.0

-110.0

-100.0

-90.00

-80.00

-70.00

-60.00

-50.00

-160.0

-40.00

freq, Hz

PN

To

tal

PN

_R

ef_

on

lyP

N_

PF

D_

on

lyP

N_

Lo

op

_D

iv_

on

lyP

N_

VC

O_

Fre

eR

un

Contributions to VCO Phase Noise

© Copyright by Charles F. Clark, Bluestem Consulting, Inc. 2009 Page 16

1 MHz Reference

10.00 100.0 1.000k 10.00k 100.0k 1.000M1.000 10.00M

-150.0

-140.0

-130.0

-120.0

-110.0

-100.0

-90.00

-80.00

-70.00

-60.00

-50.00

-160.0

-40.00

freq, Hz

PN

To

tal

PN

_R

ef_

on

lyP

N_

PF

D_

on

lyP

N_

Lo

op

_D

iv_

on

lyP

N_

VC

O_

Fre

eR

un

Contributions to VCO Phase Noise

© Copyright by Charles F. Clark, Bluestem Consulting, Inc. 2009 Page 17

Does the Reference Matter?

If other components limit the total noise….no.

If its noisy. Heck yes.

The application also matters.

© Copyright by Charles F. Clark, Bluestem Consulting, Inc. 2009 Page 18

Clean vs Poor Reference Osc

P has e Nois e vs R eferenc e S ourc e

-160.00-150.00-140.00-130.00-120.00-110.00-100.00

-90.00-80.00-70.00-60.00-50.00-40.00-30.00-20.00

1 10 100 1000 10000 100000 1E + 06 1E + 07

F requenc y

dB

c

Morion

E C S V C TC XO

© Copyright by Charles F. Clark, Bluestem Consulting, Inc. 2009 Page 19

Some Reference OscillatorsOscillator Phase Noise Comparsion

-170

-160

-150

-140

-130

-120

-110

-100

-90

-80

-70

-60

-50

1 10 100 1000 10000 100000 1000000 1000000

0

Offset Frequency

dB

c/H

z

Austron

TCXO

Morion MV85a

Crystek

10 MHz Colpitts

© Copyright by Charles F. Clark, Bluestem Consulting, Inc. 2009 Page 20

What is that Stuff???

-170

-160

-150

-140

-130

-120

-110

-100

-90

-80

-70

1 10 100 1000 10000 100000 100000

0

100000

00

IsoTemp OCXO

ECS TCXO

Qualcomm TCXO

© Copyright by Charles F. Clark, Bluestem Consulting, Inc. 2009 Page 21

Summary

Get all the Q you can.

Use a good oscillator transistor

Servo the current to reduce noise in the VCO below 100 MHz

Select the loop filter based on your application.

Select as high a reference frequency as possible

Find a clean reference.

© Copyright by Charles F. Clark, Bluestem Consulting, Inc. 2009 Page 22

References

Fundamentals of RF Circuit Design with Low Noise Oscillators. Jeremy Everard

Copyright © 2001 John Wiley & Sons Ltd

ISBNs: 0-471-49793-2 (Hardback); 0-470-84175-3 (Electronic)

Noise in High-Frequency Circuits and Oscillators, Schiek, Rolfes, Siweris, 2006 Joohn Wiley & Sons. ISBN

0-471-70607-6

The Design of Modern Microwave Oscillators for Wireless Applications, Rohde , Podar and Bock, 2005 John

Wiley and Sons. ISBN 0-471-72342-8

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ADS for simulation, by Agilent.

ADIsimPLL free from Analog Devices, www.ADI.com

http://www.analog.com/en/rfif-components/pll-synthesizersvcos/products/index.html

VCO data from Synergy Microwave, www.synergy.com DCMO80210-10

© Copyright by Charles F. Clark, Bluestem Consulting, Inc. 2009 Page 23