UNTTED STATES EPARTMENT OF OMMERCE 'UBLICATION NBS TECHNICAL NOTE 604 U.S. Efficient Numerical APARTMENT OF COMMERCE and Analog Modeling National Bureau G of 9 andards of Flicker Noise Processes A*. — NATIONAL BUREAU OF STANDARDS 1 The National Bureau of Standards was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measure- ment system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology, the Center for Computer Sciences and Technology, and the Office for Information Programs. 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The Office consists of the following organizational units: Office of Standard Reference Data—Office of Technical Information and Publications—Library—Office of Public Information—Office of International Relations. 1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washing- ton, D.C. 20234. - Part of the Center for Radiation Research. 3 Located at Boulder, Colorado 80302. o ) UNITED STATES DEPARTMENT OF COMMERCE IS 1*TZ Maurice H. Stans, Secretary ^NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director I 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 a means for making available scientific data that are of transient or limited interest. Technical Notes may be listed or referred to in the open literature. Washington, D.C. 20402 For sale by the Superintendent of Documents, U.S. Government Printing Office, (Order by SD Catalog No. C13.46:604), Price 35 cents. Stock Number 0303-0861 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 1 for = 0.43, T./ ) = 0. 04 seconds, and A|s"2 | A 8 = — over frequency range (1 Hz, 4 kHz). 3 Realization of impedance z (CO), eq with = x (19), 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: i+jq>T 8 g(C0) = YM = [L s ^' > x(co) 1 + jwOi + x a ) where T x = 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