UNTTED STATES EPARTMENT OF OMMERCE

'UBLICATION NBS TECHNICAL NOTE 604

U.S.

APARTMENT

OF

COMMERCE

National

Bureau

G of

9 andards

A*.

Efficient Numerical

and Analog Modeling

of Flicker Noise Processes
NATIONAL BUREAU OF STANDARDS

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) o IS 1*TZ

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UNITED STATES DEPARTMENT OF COMMERCE

Maurice H. Stans, Secretary

^NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director

NBS TECHNICAL NOTE 604

ISSUED JUNE 1971

Nat. Bur. Stand. (U.S.), Tech. Note 6D4, 22 pages (June 1971) CODEN: NBTNA

Efficient Numerical and Analog Modeling

of Flicker Noise Processes

J. A. Barnes and Stephen Jarvis, Jr.

Time and Frequency Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

*' W >

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide

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TABLE OF CONTENTS

Page

1. Introduction 1

2. The Filter Cascade 2

3. Efficient Generation of Approximate Flicker

Noise Numbers 10

4. Acknowledgment 13

5. References 18

in
LIST OF FIGURES

Figure Page

1 Basic R-C filter with voltage transfer function 3 given by eq (1 ).

2 Filter magnitude error £, eq (17), relative to 6 A|s"2 | for A = 0.43, T./1

) = 0. 04 seconds, and

8 = — over frequency range (1 Hz, 4 kHz).

3 Realization of impedance z

x

(CO), eq (19), with R = 8 20 kO and T2

'

' = 0. 04 seconds, which approximates

fractional capacitor of order one -half over frequency

range (1 Hz, 4 kHz).

4 Operational amplifier system with transfer function 9 given by eq (20).

5 Fortran program for computing N approximate 14

flicker numbers by recursion relations, eq (36).

("N = 1024" has been specified. )

6 Sample of 1024 approximate flicker numbers 15

generated from program of figure 5.

7 Square root of Allan variance CT(t) for data sample 16

of figure 6.

8 Impulse response function y of digital filter. 17

IV
Efficient Numerical and Analog Modeling

of Flicker Noise Processes

J. A. Barnes and Stephen Jarvis, Jr.

It is shown that by cascading a few simple resistor - capacitor filters, a filter can be constructed which generates

from a white noise source a noise signal whose spectral

density is very nearly flicker, |f | , over several decades

of frequency f. Using difference equations modeling this

filter, recursion relations are obtained which permit very

efficient digital computer generation of flicker noise time

series over a similar spectral range. These analog and

digital filters may also be viewed as efficient approximations

to integrators of order one -half.

Keywords: Analog noise simulation; computer noise simulation;

digital filters; flicker noise; fractional integration; recursive

digital filters.

1. INTRODUCTION

One of the most pervasive noise processes observed in electronic

systems is flicker noise, whose spectral density varies as |f | over many decades of frequency, often beyond the low frequency limit to which the spectral density can reasonably be measured. (See, for

example [l] - [6] and their bibliographies. ) While its occurrence is very

widespread, its cause, or causes, is as yet uncertain, despite the

attention of many investigators.

In order to study flicker noise and to simulate the behavior of

systems with flicker noise, several authors have presented mathematical

models which generate a discrete [7], [8] or continuous [9] - [14] flicker

noise from a white noise process. Because of the intrinsically long
correlation time associated with the flicker noise process, the models

have required large computer memories, or extensive computation, or

both. The purpose of this paper is to present an algorithm which permits

one to generate, in a very efficient manner with negligible computer

memory requirements, a sequence of numbers with a very nearly flicker

noise spectral density over several decades of frequency. It is derived

from a cascade of simple resistor -capacitor filters which, when realized,

also permits one to generate from a white noise source an analog noise

signal which has a spectral density which is very nearly flicker over

several decades of frequency.

2. THE FILTER CASCADE

Consider the filter shown in figure 1. Its voltage transfer function

is:

g(C0) = YM = i+jq>T 8 s ^' x(co) 1 + jwOi + x a ) [L >

where Tx = R: C (2)

t3 =R2 C (3)

and C0= 2 77f. (4)

Equation (1) is valid provided the loading on the output, y, of the filter

is negligible. This assumption, thus, might require the filter to be

followed by an isolation amplifier in practice. As it will be seen,

however, it is possible to construct a complete flicker filter with only one amplifier and still not compromise the validity of eq (1).

For low frequencies (<jO(Ti + t2 ) << l ), one finds that g(C0) « 1; and

T2 for high frequencies (to t2 >>1), one finds g(C0) « ; . If one were to

T*i + Ts have a sequence of such filters such that for the i-th filter

A

{) = (Pf^K i= 1.2....N, ( 5 ) where jS is some constant such that (8 < 1,
>>

u

rf —||-AA/Wv\^

^

o

•H

4->U

a

4->

be

nj -u

i—

i

o

>

CT1

'55

g? W tuD

0)u

bo h
and

T2

(i) = (/3) 1 T8

<°> (6)

and if these filters were cascaded with appropriate isolation amplifiers

between successive stages (to eliminate loading effects), the overall

voltage transfer function would become:

N[ 1+ iC0T2

( °V 1 G(co)=n —

"" a p

. . i=lL l +jco(tx(o) + rJo) )^

(7)

One may consider a set of n sections of this filter cascade

corresponding to i = j + 1, j + 2, . • . j

4- n, where j is arbitrary.

This subset of filters would change the (magnitude) transfer function by

the factor

/ r (o) \n

T

(o) +T (o)

kTj, + T3

in the frequency interval

where

CO. < CO

J < V

CO = J- =

j T

(i)

1

e> T

(o) *

(8)

If this filter is to approximate a filter whose transfer function varies

as

'G*(CO)| 2 =A8 |cor

where A is a constant, then

0»<« ) *

G*(co.

COJin

CO.

ry

= £

-nry T (o) 2

lT W+T«

2n

(9)

(10)

Thus,

18 =

T2

(o)

(o) _,_ , (o) + T2 (ID
- 1 in eq (8) and, thus, eq (11) becomes

A more precise approximation to a flicker filter is obtained by increasing

/8 (i.e., the ratio of the t's given in eq (12)). However, the frequency

N

range of the complete filter is proportional to P . Thus, to improve

the precision without reducing the frequency range both $ and the

number of sections, N, must be increased.

It is of value to take as a practical example the particular values:

N = 4 (13)

Ti

(o) + Ta(o) = 3Ts(o) (u)

and use the variable

jO)T2

(l)

. (15)

The filter's transfer function then becomes (from eq (7))

1

— p This transfer function should be expected to have approximately an s behavior over a relative frequency range of (8 = 9 , nearly four

decades of frequency. In fact, the magnitude error

i e= |G(W)| - A| s"

2

|, (17)

l is within ± -g dB relative to A | s ' over the range 0. 2 5 ^ |s | ^ 1000, a relative frequency range of 4000 where A = 0. 43. The location of the

approximated frequency band depends on T . For the value T = 0. 04

2 3 seconds, the approximating band is (1 Hz, 4 kHz); the error is shown in

figure 2. One may forget, for the time being, the heuristic arguments which

led to generating eq (16) as an approximation to the function

1 G*(CO) = As" 2

. (18)

Equation (16) simply gives a mathematical function which approximates

eq (18) rather well over a substantial frequency range. Thus, one could

consider an impedance, zi (CO), defined by

zi(to) = RoG(CO), (19)

where the constant Ro has the dimensions of ohms. The exact realization of zi(C0) may be obtained by a partial fractions expansion of eq (16)

and one obtains the impedance shown in figure 3 for R = 20kO and

T2 1 = 0. 04 seconds. (The exact realization of an impedance given by

eq (18) would be much more difficult. ) Thus, figure 3 is an approximation to a "fractional capacitor" [ 15] - [17] of order one -half.

This impedance can be used in a straightforward fashion in connection with a single operational amplifier to realize a transfer function

given by eq (16). It is no longer necessary to consider the filter cascade

with its isolation amplifiers mentioned above. This new, active filter

is realized as follows:

An operational amplifier is a device whose transfer function is approximated by a large, real, negative number, -k, over a large

frequency range which is assumed to include the frequency domain of

interest. If one connects an operational amplifier with two impedances

zx(co) and Z2(co) as shown in figure 4, the overall transfer function

becomes

eoM - k

When

then

e.(C0) 1 + (1 + k) z a /zi

k» [5i

Z2

(20)

e. (60) zs
X

a.

sO

a.

a.

CO

a

IT)

Z*89*

^"zos M/W—

I

on stz-z —WWM

0>l 500 '9 —VXAAAH

on oz AAAA/H

a

X i—i SI o ffi CM o , | •iH

II 4-1O CD O cci bo

ri M-l PI

4-J to

cd

CD

4-> >n

03 U OI—

1

o cr

*—

0) cr

1 ft u

Mh

CD ft

rt *-i CD 3 o

>

•(H o

i—

i

,fl «4-l N £ flj 0) rC u CO

1 S T3 <u rt £ fl T3 O o

<Dr

o

CD

to 13

• w

xh U

MH o O

O

a d

ii U

J-> „ O

ni _h -u N v— C\] H

•H e o

r—

1

nJ

ni •V ft

CD s «S Ph nl U

a;u

bo
CDo

01>* ,

in o

r\i

u

CD cr1 <+H <D

ft ^

J2

rrt

G

CD .—i >

(Ti (3 bo O

(3

-u>

n

Rt

H

Jh 4J

CDft

up

CDu

bo
Thus, if one chooses z 2 to be a pure resistance of value Ro, the overall transfer function is just G(Cd) given by eq (16). Such a device has,

in fact, been built and tested at the National Bureau of Standards.

It is interesting to note that if one interchanges the impedances

zi and Z2 in figure 4, one obtains a prewhitening filter for flicker noise

in the frequency range from 1Hz to 4 kHz.

3. EFFICIENT GENERATION OF APPROXIMATE

FLICKER NOISE NUMBERS

From the impulse response function

Mt) = ^= (22)

corresponding to the transfer function

one can generate a flicker signal eo(t) from a white noise source e.(t)

by the convolution integral [18]:

e,(T)

e (t) =r.

dT ^T- (23)

Discrete flicker numbers can be approximated by a discrete analog

convolution, setting t = n 6t: n m

(eo) m=y h(t )(e.) , (24)

Z_, m i m-n

n=l

where the (e.) are uncorrelated random deviates. To obtain a flicker in

spectral density over a relative frequency range r, the memory length

M must be very large and so also the number of remembered deviates

(e.) for each output (eo) . This is not a very efficient use of

computer memory space nor of computer time.

A very efficient flicker number generator can be constructed from

the filter cascade introduced in section 2. The differential equation for

10
the network of figure 1 is:

y(t) + y(t)(Ri + R2 )C = R2Cx(t) + x(t). (25)

This can be approximated by the difference equation for the discrete

values y = y(n fit): n

= (1 6t

) | Ts Yn+1 [ Ta + T2

; yn TX + t2 ~n+l ' Ta + t2 ~n'

, 6t - T5 x . . + x (26)

with, as before, T. = R.C. Let

and

y =

R

fit Ti + T2

'

Tl + T3

The recursion relation for y is then

(27)

(28)

yn+ l = (1 " yK + Rx

n+ 1

" (R - yK> (29)

for an unloaded filter. As in section 2, we shall again consider four

such filter sections, with parameters y and R , i = 1, 2, 3, 4, inputs x and outputs y . Cascading the filters, the output of filter i is

the input of filter i + 1:

y

« = x

(2 > n n

(2) (3)

y ^ - x

(4) n n

(30)

Note that this is equivalent to assuming the existence of isolation amplifiers between sections of the analog filter. That is, eq (2 9) gives the

exact filter section response (no loading effects) and its output is exactly

the input to the next stage (eq (30)).

11
Then, 'with the earlier definitions:

(i)

R

(l) = R = 1/3 , and

=

(i)

(i) , ,(i)

= (1/3)*

'1 + T"2

In this situation the higher -numbered filters have the shorter time

constants. It seems reasonable to let the shortest time constant, Ta

'

be equal to the inverse of the Nyquist frequency, f NQ (2)(6t)

Then

and

fit =

l- ^

,(i) = I(I\ 9 "21

( i= 1.4) •

(31)

(32)

(33)

( 4)

! >

(34)

(35)

The resulting recursion relations for the four -filter cascade become:

x

(a) (D = /4|73\ (D +

l n+l yn+i \4374/ yn 3 n+i

(3) (3)_ /485\ W + I x

0) X

n+i yn+i~ \486/ Yn

+

3 n+i

1457 \ (1) 4374/ X

n

161 \ (3) 186 / n

C W =y (3 > = /«) y

(3) + Ix (3) - fll) x

(l n+i Yn+i I 54/ Y

n

+

3 n+i 154/ x

n

(s)

(4) ,1 (4) 1 (4) 3 n+i b n

4) /5\ (.,

(36)

When the x

n are uncorrelated random deviates, and the Ix , y J

I o I are taken to be zero initially (to minimize the transient), the above recursion equations yield a set of numbers, y

4

, which approximate flicker

noise over a range of about 1000 periods of 6t. (The addition of a fifth,

lower frequency, or i = 0, stage should extend this range to nearly 10, 000

periods.) The recursion is neatly accomplished on a digital computer

12
with a random number generator and storage locations for the eight

,. ,. „. . , ., , (s) (3) (4) (1) (2) distinct coefficients plus r

the seven numbers x , x , x , y , y , n n n n n y

(3)

, y

U>

- n n Figure 5 shows a Fortran program written for the generation of 1024

numbers which approximate a flicker noise sample using eqs (36).

Figure 6 shows a sample of N = 1024 numbers obtained using this program.

This block of data was subjected to a time -domain statistical analysis

developed by Allan [19], [20]. Figure 7 shows the Allan variance, (7(t),

for independent samples. For pure flicker noise of arbitrary length,

a(T) has no T -dependence. The deviations of the plot from a straight

horizontal are probably due to the small sample size.

Figure 8 is a plot of the actual impulse response function of the

digital filter described in the Fortran program of figure 5.

4. ACKNOWLEDGMENT

The authors wish to thank Mr. D. W. Allan of the National Bureau

of Standards for his assistance in the statistical analysis of the data.

13
PROGRAM FLICKER

DIMENSION V(5, 2)

C SET INITIAL VALUES V(I, 1) TO ZERO MEAN VALUES

DO 1 1=1, 5

1 V(I,1) = 0. C GENERATE AND PRINT N FLICKER NUMBERS

N = 1024

DO 2 1=1, N

C SELECT RANDOM V(l, 2) UNIFORMLY DISTRIBUTED ON (-|, |)

V(l, 2) = RANF (-1) -0. 5 C SOLVE RECURSION RELATIONS

V(2, 2) = . 999771*V(2, 1) + . 333333*V(1, 2) - . 333105*V(1, 1)

V(3, 2) = . 997942*V(3, 1) + . 333333*V(2, 2) - . 331276*V(2, 1)

V(4, 2) = . 981481 *V(4, 1) + . 333333*V(3, 2) - . 314815*V(3, 1)

V(5, 2) = . 833333*V(5, 1) + . 333333*V(4, 2) - . 166667*V(4, 1) PRINT 11, V(5, 2)

11 FORMAT (2XF12.6)

C RESET V

DO 3 J=l, 5

3 V(J, 1)=V(J,2)

2 CONTINUE

CALL EXIT

END

Figure 5. Fortran program for computing N approximate

flicker numbers by recursion relations, eq (36).

("N = 1024" has been specified. )

14
001 x

( ^

15
©

:zz o :

Q

C

= =e: = —e —0.

4—1 -

o

1—

I

I

T3

fc- b

<DO

C

o

o

u

01

bo

*

16
H|CU I \ / * / ° /o /

A) O

pd/ / / / / / / / " c c/o/o/o /

o° 9

o' / _ ooo

o'

<y 0// /

/° /

/-

•r-l coOc

vm

o>

enao

a,

en

cu

Jh

CDen 1—1 r

CD 1 O—1

9SUOdS3"JJ

17
5. REFERENCES

[l] van der Ziel, A., Noise, (Prentice -Hall, Englewood Cliffs, NJ,

1954).

[2] Atkinson, W. R., Fey, L., and Newman, J., "Spectrum analysis

of extremely low frequency variations of quartz oscillators, " Proc. IEEE (Corres.), 51, No. 2, p. 379, February 1963.

[3] van der Ziel, A., Fluctuation phenomena in semiconductors, (Academic Press, New York, NY, 1959).

[4] McWhorter, A. L. , "l/f noise and related surface effects in

germanium, " Tech. Report 80 (MIT Lincoln Lab., Lexington, MA,

May 1955).

[5] Mandelbrot, B. B., and Wallis, J. R. , "Some long -run properties

of geophysical records, " Water Resources Research, 5, pp. 321-

340, April 1969.

[6] Derksen, H. E., and Verveen, A. A., "Fluctuations of resting

neural membrane potential, " Science, 151, pp. 1388-1399,

March 18, 1966.

[7] Barnes, J. A., and Allan, D. W. , "A statistical model of flicker

noise," Proc. IEEE, 54, No. 2, pp. 176-178, February 1966.

[8] Mandelbrot, B. B., and Wallis, J. R., "Computer experiments

with fractional Gaussian noises, " Water Resources Research, 5, pp. 228-267, February 1969.

[9] Halford, D., "A general mechanical model for |f| spectral

density random noise with special reference to flicker noise 1/ |f [, " Proc. IEEE, 56, No. 3, pp. 251-258, March 1968.

[10] Muller, O., and Pest, J., "Thermal feedback in power semiconductor devices, " IEEE Trans. Electron Devices, ED-17, No. 9, pp. 770-782, September 1970.

[11] Mandelbrot, B. B. , and Van Ness, J. W. , "Fractional Br ownian

motions, fractional noises and applications, " SIAM Review, 1_0, pp. 422-437, October 1968.

18
[12] Mandelbrot, B. B., "Some noises with 1/f spectrum, a bridge

between direct current and white noise, " IEEE Trans. Information

Theory, IT -13, No. 2, pp. 2 89-2 98, April 1967.

[13] van Vlict, K. M., and van der Ziel, A, "On the noise generated

by diffusion mechanisms, " Physica, 24, pp. 415-421, 1958.

[14] MacFarlane, G. G., "A theory of contact noise in semiconductors, " Proc. Phys. Soc. (London), 63B, pp. 807-814, October 1950.

[15] Carlson, G. E., and Halijak, C. A., "Simulation of the fractional

derivative operator \

rs and the fractional integral operator \/l/s, " Kansas State University Bulletin, 4_5, pp. 1-22, July 1961.

[16] Carlson, G. E., andHalijak, C. A., "Approximation of fractional

capacitors (1/s) ' n by a regular Newton process, " IEEE Trans.

Circuit Theory, CT-11 , No. 2, pp. 210-213, June 1964.

[17] Roy, S.C.D., and Shenoi, B. A., "Distributed and lumped RC

realization of a constant argument impedance, " J. Franklin

Institute, 282, pp. 318-32 9, November 1966.

[18] Middleton, D. , Introduction to Statistical Communication Theory,

(McGraw-Hill, New York, N.Y., I960).

[19] Allan, D. W. , "Statistics of atomic frequency standards, " Proc.

IEEE, 54, No. 2, pp. 221-230, February 1966.

[20] Barnes, J. A., et al. , "Characterization of frequency stability,"

NBS Tech. Note 394, October 1970; (to appear in IEEE Trans.

Instrumentation and Measurement, IM-20, May 1971).

19

FORM NBS-I14A (1.71) 2. Gov't Accession No.

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

PUBLICATION OR REPORT NO. NBS TN 604

3. Recipient's Accession No.

4. TITLE AND SUBTITLE Efficient Numerical and Analog Modeling of Flicker Noise

Processes

5. Publication Date June 1971

6. Performing Organization Code

7. AUTHOR(S)

J. A. Barnes and Stephen Jarvis, Jr.

8. Performing Organization

10. Project/Task/Work Unit No 2730900

9. PERFORMING ORGANIZATION NAME AND ADDRESS NATIONAL BUREAU OF STANDARDS DEPARTMENT OF COMMERCE

WASHINGTON, D. C. 20234

11. Contract/Grant No.

12. Sponsoring Organization Name and Address 13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here.)

It is shown that by cascading a few simple resistor -capacitor filers, a filter can be

constructed which generates from a white noise source a noise signal whose spectral

density is very nearly flicker, ff | , over several decades of frequency f. Using

difference equations modeling this filter, recursion relations are obtained which

permit very efficient digital computer generation of flicker noise time -series over

a similar spectral range. These analog and digital filters may also be viewed as

efficient approximations to integrators of order one -half

.

17. KEY WORDS (Alphabetical order, separated by semicolons) Analog noise simulation; computer noise simulation; digital filters; flicker noise;

fractional integration; recursive digital filters.

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fxl UNLIMITED.

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TO NTIS.

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UNCLASSIFIED

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$5.00; foreign, $6.25*. • Engineering and Instrumentation

Reporting results of interest chiefly to the engineer

and the applied scientist. This section includes many

of the new developments in instrumentation resulting from the Bureau's work in physical measurement,

data processing, and development of test methods.

It will also cover some of the work in acoustics, applied mechanics, building research, and cryogenic

engineering. Issued quarterly. Annual subscription:

Domestic, $5.00; foreign, $6.25*.

TECHNICAL NEWS BULLETIN

The best single source of information concerning the Bureau's research, developmental, cooperative and

publication activities, this monthly publication is designed for the industry-oriented individual whose

daily work involves intimate contact with science and

technology

for engineers, chemists, physicists, re- search managers, product-development managers, and company executives. Annual subscription: Domestic,

$3.00; foreign, $4.00*.

• Difference in price i3 due to extra cost of foreign mailing.

Applied Mathematics Series. Mathematical tables, manuals, and studies. Building Science Series. Research results, test methods, and performance criteria of building materials, components, systems, and structures. Handbooks. Recommended codes of engineering

and industrial practice (including safety codes) de- veloped in cooperation with interested industries, professional organizations, and regulatory bodies. Special Publications. Proceedings of NBS conferences, bibliographies, annual reports, wall charts, pamphlets, etc. Monographs. Major contributions to the technical

literature on various subjects related to the Bureau's

scientific and technical activities. National Standard Reference Data Series. NSRDS provides quantitative data on the physical and chemical properties of materials, compiled from

the world's literature and critically evaluated. Product Standards. Provide requirements for sizes, types, quality and methods for testing various industrial products. These standards are developed cooperatively with interested Government and industry groups and provide the basis for common understanding of product characteristics for both buyers and sellers. Their use is voluntary.

Technical Notes. This series consists of communications and reports (covering both other agency and

NBS-sponsored work) of limited or transitory interest. Federal Information Processing Standards Publications. This series is the official publication within

the Federal Government for information on standards adopted and promulgated under the Public Law

89-306, and Bureau of the Budget Circular A-86

entitled, Standardization of Data Elements and Codes

in Data Systems.

Consumer Information Series. Practical information, based on NBS research and experience, covering

areas of interest to the consumer. Easily understandable language and illustrations provide useful back- ground knowledge for shopping in today's technological marketplace.

NBS Special Publication 305, Supplement 1, Publications of the NBS, 1968-1969. When ordering, include Catalog No. C13.10:305. Price $4.50;

foreign, $5.75.

Order NBS publications from: Superintendent of Documents

Government Printing Office Washington, D.C. 20402
U.S. DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20230

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