Jitter and timing measurement with CMOS circuits · Compare to oscilloscope measurements more...

Jitter and timing measurement withCMOS circuits

K. A. JenkinsIBM T.J. Watson Research Center

with significant help from Anup Jose, Don Beisser, Ken Shepard, and David Heidel

Timing jitter is increasingly important as computer clock rates increase; jitter limits accuracy of timing measurements

Contemporary circuits demand jitter of a few ps or less

1. Is there some fundamental limit below which jitter in CMOS gates can not be reduced? (eg. Thermal noise)

Experimental determination of jitter limit in 90 nm, 1.2V CMOS inverter delay chain:

If each inverter adds jitter independently to a propagating signal, the variances add, , eg 2=mj2 and jitter will be proportional to the square root of the number of inverters. If added jitter at each stage is correlated, then standard deviations add, eg =mj net is linearly proportional to the number of stages.

reference

full chain

64 144 256 400 576 784

input

P1

P3

P3

P3

P2

Two experiments: 1) unloaded inverters (FO1)2) heavily loaded inverters (FO11)

Pulsegenerator

oscilloscope

Experimental determination of jitter limit

reference

selected length

full chain

Sample oscilloscope traces

01002003004005006007008000

1

2

3

4

5

1.3 V

1.2V

1.1 V

1.0 Vunloaded delay chain

rms jitter (ps)

stage number

Raw jitter data

Measured jitter includes‘scope jitter (addedin quadrature)

0 100 200 300 400 500 600 700 8000

1

2

3

4

5

1.1V

1.3 V

1.2 V

1.0 V

unloaded delay chain

rms

jitte

r (p

s)

stage number

0 100 200 300 400 500 600 700 8000

5

10

15

20

25

30

35

1.3 V

1.2V

1.0 V1.0 V

1.1 V

loaded delay chain (FO11)

rms

jitte

r (p

s)

stage number

scope jitter subtracted

Unloaded chains (FO1) loaded chains (FO11)

0 20 40 60 80 100 120 140 160 1800

5

10

15

20

25

30

35

1.3 V

1.2 V

1.1 V

1.0 Vloaded delay chain (FO11)

0.11 ps/ns

rms

jitte

r (ps

)

delay (ns)

0 5 10 15 20 250.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0.12ps/ns

1.3V

1.2 V

1.1 V

1.0 Vunloaded delay chain

rms

jitte

r (ps

)

delay (ns)

Jitter per delay (instead of per stage)

jitter/delay~1E4 (0.12 ps/ns)

Traced to residual power supply noise of about 80 to 100 V (rms)

0.7 0.8 0.9 1.0 1.1 1.2 1.30.00

0.05

0.10

0.15

0.20

0.25

0.30

unloaded delay chainjit

ter/

dela

y (p

s/ns

)

VDD

(V)

Effect of power supply level

2. Onchip measurement of jitter and skew

Existing methods:

Time to digital converters:

many latches driven by buffered signal and clock; arrival time is digitized

jitter is obtained by reading latch states repeatedly;

2. Measurement onchip is an attractive way to measure small amounts of jitter (and skew)

C

D

QC

D

QC

D

QC

D

Q

signal

counter counter counter counter

ref clock

…128 stages

ReferenceRestle et al, ISSCC 2004

Previous work: latched delay line (“Skitter”); resolution ~ stage delay

1 1 0 0Binary code indicates arrival time of signal after clock

C

D

Q C

D

Q C

D

Q C

D

Q

t1t1t1t1

t2t2t2t2

signal

ref clock

Dudek et al, JSSC 35, 2000

counter counter counter counter

Previous work: vernier delay line:10 ps resolution

1 1 0 0

A simpler onchip circuit with ps accuracy:

Single latch with delayed clocksmall arealow powercontinuous operation (no startup noise issues)device matching not important

Full disclosure: proof of principle only not ready to drop into product chip not tested in ‘hostile’ environment

C

D

o

o

signal

ref clock latch

signal delay chain

clock variable delay chain o

r.o counter

statemachine

clock counter

ref counter

latch counter

Vcntlenable ro

bypass

22

144

This circuit: single latch circuit:

5

C

D

o

signal

ref clock latch

signal delay chain


statemachine

latch counter

Vcntlenable ro

bypass

22

144

Voltage controlled delay element varies the arrival of clock to latch varying clock with respect to data integrates jitter distribution to obtain

cumulative distribution function

This circuit: single latch circuit:

5

D

C

tclock

jitter distribution cumulative distribution function

Changing delay of clock with respect to data generates cumulative distribution function (time integral of jitter spectrum)

tclock


14

tclock


cumulative distribution function

tclock

jitter distribution

Recover jitter by differentiating

C

D

o

o

signal

ref clock latch

signal delay chain


r.o counter

statemachine

clock counter

ref counter

latch counter

Vcntlenable ro

bypass

22

144 5

Implementation in 0.13 m, 1.2 V CMOS test chip

Delay elements: invertersLatch: differential edgetriggered (Montanaro, 1996)

192m x 22 m

delay chain and latch

delay chain, latch, and counter

Delay chains and latch: 3200 m2

Counter: 3200 m2

reference

clock(a) tracking, or longterm, jitter

(b) period jitter

jitter

jitter

definitions

C

D

o

o

signal

ref clock latch

signal delay chain


r.o counter

statemachine

clock counter

ref counter

latch counter

Vcntlenable ro

bypass

22

144 5

Normal operation: compare signal to reference clock

C

D

o

o

signal

ref clock latch

signal delay chain


r.o counter

statemachine

clock counter

ref counter

latch counter

Vcntlenable r.o.

bypass

22

144 5

Calibration of clock delay: convert chain to ring oscillator

1.10 1.15 1.20 1.25 1.30

0.66

0.68

0.70

0.72

0.74

0.76

0.78

0.80

0.82

0.84

7/27/2004 08:15:57

1.3 Polynomial Fit of CAL2_B

Y =1.333170.49495 X

calibration data

dela

y (n

s)

vdd_var(V)

Delay calibration:Sample measurement

about 0.5 ps/mV

calibrate

1.10 1.15 1.20 1.25 1.30

0.66

0.68

0.70

0.72

0.74

0.76

0.78

0.80

0.82

0.84

7/27/2004 08:15:57


Y =1.333170.49495 X

calibration data

dela

y (n

s)

vdd_var(V)

1.14 1.16 1.18 1.20 1.22 1.24

0

5000000

10000000

15000000

20000000

25000000 channeltochannel (HP8131)

de

riva

tive

Vdd_ var(V)

Measured CDF

Sample measurement

about 0.5 ps/mV

apply signal and ref to macro

1.14 1.16 1.18 1.20 1.22 1.24

0

5000000

10000000

15000000

20000000

25000000 channeltochannel (HP8131)

deri

vativ

e

Vdd_ var(V)

Extracted jitter distribution

Sample measurement

about 0.5 ps/mV

Delay calibration:

differentiate and apply calibration

1.10 1.15 1.20 1.25 1.30

0.66

0.68

0.70

0.72

0.74

0.76

0.78

0.80

0.82

0.84

7/27/2004 08:15:57


Y =1.333170.49495 X

calibration data

dela

y (n

s)

vdd_var(V)

jitte

r sp

ect

rum

time (10 ps/div)

Compare to oscilloscope measurements

(Jitter generated externally)

8131 cyclecycle jitter

sigma=12.5 psd

eri

vativ

e

time (10 ps/div)

Sigma=4.2 ps

Sigma=10.6 ps

8131 channeltochannel

sigma=3.2 ps

de

riva

tive

time (10 ps/div)

(jitter generated externally)

Compare to oscilloscope measurements more complicated spectrum

Sigma=4.2 ps

(phase modulated pulse generator)


Sigma=4.2 ps

820 810 800 790 780 770 7600.0

0.2

0.4

0.6

0.8

1.0

raw signal

ratio

of

cou

nts

time (ps)

measured CDF



Sigma=4.2 ps

820 810 800 790 780 770 7600.0

0.2

0.4

0.6

0.8

1.0

raw signal

ratio

of

cou

nts

time (ps)

9.8 ps


10.3 ps

de

riva

tive

time (5 ps/div)

measured jitter

C

D

o

o

ref clock latch

signal delay chain


r.o counter

statemachine

clock counter

ref counter

latch counter

Vcntlenable ro

bypass

22

144 5

Resolution test: compare reference to itself

Resolution measurement:

Effect of operating frequency all circuit blocks work at >2.5 GHz

1.230 1.235 1.240 1.245 1.250 1.255 1.2600

200

500 MHz clock frequency

1 ps

de

riva

tive

Vdd_var(V)

1.230 1.235 1.240 1.245 1.250 1.255 1.2600

200

400

600

800

bypass mode

50 MHz clock frequency

de

riva

tive

Vdd_var(V)

Noise increases due to layout oversight (i/o power supply connectedto delay chain supply)

50 MHz

500 MHz

0.4 ps corresponds to ~ 850 GHz bandwidth

How is this possible in a technology of about 70 GHz cutoff frequency?

1/fT

Resolution bandwidth determined by uncertainty in switching time, not rise time

Size and power

Combined reference chain, delay chain latch, and i/o test point power:

@1.2 V, f=200 MHz, P ~1.2 mW

Delay chains and latch: 3200 m2

Counter: 3200 m2

Measurement time

t=(counts)(steps)(clock period)+ overhead

eg, 25 time steps, 10000 counts, 1 GHz clock t=800 s.

reference

clock(a) tracking, or longterm, jitter

(b) period jitter

jitter

jitter

definitions

C

D

o

o

signal

ref clock latch

signal delay chain


r.o counter

statemachine

clock counter

ref counter

latch counter

Vcntlenable ro

bypass

Modification for cycle to cycle jitter:delay chain difference equal to clock period

Will test this in Brahma 9SF run

C

Dclocksignal latch

signal delay chains

variable delay chain

clock / r.o counterstatemachine

latch counter

Vcntlenable ro

bypassOCD

OCD OCD

OCD

length select

one period delayfixed delay

period jitter: insert additional delay equal to one period

Comparison of oscilloscope measurement with onchip measurement

CMOS 9SF (0.09 m) pad cage demonstration: Apply known signal to macroGood reproduction of applied signal

fclock

=540 MHz

=5.4 ps

20 ps/div

fclock

=540 MHz

=1.2 ps

20 ps/div

=1.8 ps

=5.7 ps

=1.7 ps

=1.7 ps540 MHz falling edge

CMOS 9SF pad cage demonstration

fclock

=540 MHz

20 ps/div

fclock

=1080 MHz

=1.03 ps

20 ps/div

=1.8 ps

=1.7 ps

Comparison of oscilloscope measurement with onchip measurement

=1.8 ps

540 MHz falling edge

1080 MHz rising edge

Resolution~ 1ps

Effect of Vdd noise on resolution

Quiet supply

300 mV Gaussian noise with 10 MHz BW applied to delay chain voltage; probe decoupling of 109 nF to ground reduces the actual noise on the circuit

Note that the prototype used simple inverters which are not noise immune!

fclock

=1080 MHz

=2.3 ps

20 ps/div

fclock

=1080 MHz

=1.03 ps

20 ps/div

1080 MHz rising edge

C

signal

ref clocklatch

signal delay chain

o

multiple measurement points: measure jitter and skew (with single variable chain) bypass calibrates each test point

signal delay chain

signal delay chain

reference variable delay chain

signal

signal

bypass

6 8 10 12 14 16 18 20 22 24 26 28 300.00E+000

1.00E+008

2.00E+008

3.00E+008

4.00E+008

5.00E+008

6.00E+008

6 8 10 12 14 16 18 20 22 24 26 28 300.00E+000

1.00E+008

2.00E+008

3.00E+008

4.00E+008

5.00E+008

6.00E+008

skew=16 ps

test point 2test point 1

fclock

=2 GHz

CD

F

time (ps)

Skew measurement on 2 GHz experimentalresonant clock circuit; 0.18m technologyCourtesy of Ken Shepard, Columbia Univ.

Used differential buffers to avoid power supply noise effect

Which technology is best?

Similar resolution has been seen with 3 technologies:

0.18 m, 1.5 V0.13 m, 1.2 V0.09 m, 1.2 V

Tradeoffs:

Scaled technologies have potentially better resolution due to faster transition times,But lower power supply operation makes them more susceptible to noise adding jitter

VDD

VDD

D D_

CLK

CLK CLK

S_R_

Q Q_

Montanaro. J., et al. ‘ A 160MHz, 32b, 0.5W CMOS RISC microprocessor’, IEEE J. SolidState Circuits, 1996, 31, (11), pp. 17031714.

Differential latch senseamp latch

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Jitter and timing measurement with CMOS circuits · Compare to oscilloscope measurements more...

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