8/2/2019 665 Ring Oscillators

1/13

Amplifier A

OSCILLATORS

Fundamentals. Linear Aspects: Oscillation conditions.

Frequency

Selective

Network

o sx

+

+

ox

fx

)s(A)s(1)s(A

A1A

x

xA

s

of ===

)s(L1)s(A)s(1)s(D ==

1)j()j(A)j(L ooo == Barkhausen Criteria

Note that for the circuit to oscillate at one frequency the oscillation

criterion should be satisfied at one frequency only; otherwise the

resulting waveform will not be a simple sinusoid.

8/2/2019 665 Ring Oscillators

2/13

Ring Oscillators The main difficulty for using submicron CMOS ring

oscillators in wireless communication systems is theirrelatively poor phase noise response.

Precaution is required to achieve as low phase noise aspossible from CMOS ring oscillators.

Dominant noise sources in IC environment arecommon-mode signals in nature (e.g. power supplynoise, substrate-coupled noise).

Fully differential design is a must!

Courtesy of Fikret Dulger

8/2/2019 665 Ring Oscillators

3/13

Ring Oscillators

++-

180

H(j)

(a)

+++

360

H(j)

(b)

+++

0

H(j)

(c)

Various views of oscillatory feedback system.

Vout

CLM1

RD

VDD

One-pole feedback Two-pole feedback system

CLM1

RD

Vout

CLM2

RD

VDD

E F

CLM1

RD

VoutCLM2

RD

VDD

E F -1

Ideal

Two-pole feedback system plus an inverter

8/2/2019 665 Ring Oscillators

4/13

( )3

0

3

0

1

+

=

s

AsH

Three-stage ring oscillator

VoutCLM3

RD

VDD

G

CLM1

RD

CLM2

RD

E F

03 =osco

60tan 1 =

o

osc

( )( ) ( )

( )( ) 3

0

3

0

3

0

3

0

3

0

3

0

3

0

1

11

1As

A

s

As

A

sVsV

in

out

++ =

++

+

=

1

1

32

0

3

0 =

+ osc

A

20 =A

8/2/2019 665 Ring Oscillators

5/13

Modifications to the current-starved VCO to set minimum and maximum frequencies.

R

M5 M7VinVCO M1M8

M2

M3

M4

VDD

M9M6

VDD

First stage of VCO

Current starved VCO.

M5VinVCO M1

M2

M3

M4

VDD

M6

VDD VDD VDD

ID4

ID4

Osc out

Single ended Inverters

(delay elements)

8/2/2019 665 Ring Oscillators

6/13

(a)

(b)

+ -

- +

X1

Y1

+ -

- +

X2

Y2

+ -

- +

X3

Y3

+ -

- +

X4

Y4

(a) Five-stage single-ended ring oscillator, (b) four-stage differential ring oscillator

XY

VDD

M1

R1 R1

M2

ISS

M3

R1 R1

M4

ISS

M5

R1 R1

M6

ISS

t

VDDVX

VY VDD + R1 ISS

8/2/2019 665 Ring Oscillators

7/13

Fully-Differential Ring Oscillator

Vo1

(in phase) Vo2

(quadrature)

8/2/2019 665 Ring Oscillators

8/13

One potential delay element

Vin-

Vdd

frequency

controlIb

Vbias

Vbias

Vout

Vin+

8/2/2019 665 Ring Oscillators

9/13

Replica Biasing needed for the DelayElements

Vref

Vdd

Vdd

Vbias

( off-chip )

( to delay

elements )

frequency

controlIb

8/2/2019 665 Ring Oscillators

10/13

Frequency Control Frequency of oscillation, ( fosc), is controlled through the

bias current of a delay element ( Ib). Maximum peak-to-peak voltage, ( Vpp), is controlled

through the replica biasing.

Total output capacitance of a delay element, ( Cout

),directly affects the frequency of oscillation.

outpp

bosc

CV

If

.

8/2/2019 665 Ring Oscillators

11/13

Chip Photomicrograph

8/2/2019 665 Ring Oscillators

12/13

Test Results

8/2/2019 665 Ring Oscillators

13/13

Source coupled voltage-controlled oscillators (also known as source coupled multivibrators).

VinVCO

VDD

ID

Output Output

M6M5

M1 M2

M3 M4

C

VinVCO

VDD

Output Output

M6M5

M1 M2

M3 M4

C

ID

a) NMOS b) CMOS

M1 (off) M2 (on)C

ID IDID

X Y

Simplified schematic of source coupledoscillator, M1 is on and M2 is off.

t t time

Output

Y

X

M2 onM1 off

M1 onM2 off

M2 onM1 off

M1 onM2 off VDD-VTHN

VDD-2VTHNVDD-VTHN

VDD-2VTHN

VDD-3VTHN

Voltage waveforms for the NMOS source coupled VCO.

Coupled Multivibrators