Contains reports on two research projects

Adler, Richard B.

Bess, L.

Cruz, J. B., Jr.

Gross, J.

Hilibrand, J.

Ingersoll, J. G.

Kosowsky, D. I.

Lull, R. E.

McWhorter, A. L.

Wiesner, Jerome B.

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XI. SEMICONDUCTOR NOISEProf. R. B. Adler J. B. Cruz, Jr. D. I. KosowskyProf. J. B. Wiesner J. Gross R. E. LullL. Bess* J. Hilibrand A. L. McWhorterJ. G. IngersollA. MODIFICATION OF NOISE ANALYZERStudies were made on compressing the frequency scale at the higher end of theplot of the noise spectrum analyzer (1, 2) in order to shorten the time for each test run.The present local oscillator of the analyzer uses a motor-driven signal generator(GR-805C) with a resulting frequency scale that is almost linear.Although a logarithmic frequency scale is most desirable, the possibility of usingstandard capacitor characteristics in either LC or RC oscillators was also investi-gated. However, the resulting frequency scales were not sufficiently compressed.To realize a fairly logarithmic frequency scale, a special inductance, capacitance,or resistance has to be used. It was found that a logarithmic potentiometer in parallelwith a constant resistance generates the desired function of R with respect to shaftrotation (or time) in an RC oscillator.A Wien bridge oscillator with special R-tuning is therefore being constructed toreplace the present GR-805C generator. It will be mechanically driven, and it isexpected to sweep the noise spectrum logarithmically from 1 kc/sec to 60 kc/sec, andalmost linearly from 20 cps to 1 kc/sec.J. B. Cruz, Jr.References1. Quarterly Progress Report, Research Laboratory of Electronics, M.I.T., Jan. 15,1954, pp. 47-48.2. Quarterly Progress Report, Research Laboratory of Electronics, M.I.T., July 15,1954, pp. 65-67.B. EQUIPMENT FOR NOISE AMPLITUDE PROBABILITY DISTRIBUTIONMEASUREMENTSEquipment for measuring amplitude probability densities is being constructed. Ablock diagram of the system is shown in Fig. XI-1. There are essentially two indepen-dent systems here: the probabiloscope (1), which yields the amplitude density in photo-graphic form directly; and the moments-measurement system.Essentials of the probabiloscope are shown in Fig. XI-2. A very high contrast filmis used so that (idealizing slightly) the film is either fully exposed or unexposed. The*Lincoln Laboratory-71-HIGH-FREQUENCYSWEEPPHOTOGRAPHPROBABILOSCOPE OF AMPLITUDEDISTRIBUTIONP(x)FUNCTIONGENERATORSy-x2  INTEGRATOR Cxyx4INTEGRATOR x4Fig. XI-1Amplitude measurement system.NONOISEINCREASINGDENSITYdOPTICALWEDGEHIGHCONTRASTFILMFig. XI-2Essentials of the probabiloscope.NLIGHT-SPOTPHOTOMULTIPLIER FEEDBACK SIGNALTUBE TO VERTICALPLATESAMPLIFIERFig. XI-3Essentials of the function generator.0 0 q O CHOPPER / AMPLIFER t -I METERSHORT TIMEINTEGRATOR CONSTANTT=I SECOND VOLTAGE TOVOLTAGE COILSFig. XI-4Essentials of the integrator.-72-NOISEVOLTAGEx(t)L ,t)(XI. SEMICONDUCTOR NOISE)percentage of incident light transmitted by the optical wedge drops exponentially towardthe left. Considering a small area on the film, the integrated light transmitted to it isa linear function of the time the noise voltage spends in that particular range dx at theamplitude level x0 and an exponential function of the distance from the right edge of thefilm d. On developing the film (by suitable techniques for increasing the contrast) acurve will be obtained that will be the desired amplitude distribution plotted to a loga-rithmic scale. Normalizing the area under the curve and replotting on a linear scalewill yield the desired amplitude distribution.The moments of the noise voltage will be used as criteria for the departure fromgaussian of the amplitude density. The essentials of the function generators (2) usedto obtain the moments are shown in Fig. XI-3. The noise voltage is applied to thehorizontal deflection plates of a cathode-ray tube. A mask cut to the desired functionalrelation shields a part of the face of the tube. The output from the photomultiplier tubeis amplified and applied to the vertical plates so that the spot is kept just below the faceof the mask. This feedback signal is then the desired function of the input.It is necessary to use long integration times, especially on the higher order moments,if we wish to use the system at low frequencies. For a 95 per cent probability of +5 percent accuracy of the fourth moment, an input 1/f frequency spectrum, and a low-frequencycutoff of 0. 2 cps, a 30-hour integration period has been found necessary. An electro-mechanical arrangement designed for very long integration times is shown in Fig. XI-4.The high-frequency limit of the function generators will probably be 40 kc/secbecause of the phosphor persistence of the cathode-ray tube used. The low-frequencylimit will be imposed by the more stringent of: (a) the length of integration time, or(b) the low-frequency response of the noise preamplifier.J. HilibrandReferences1. E. R. Kretzmer, Bell System Tech. J. 31, 751-763 (1952).2. A. B. Macnee, Sc. D. Thesis, Department of Electrical Engineering, M.I.T. (1948).-73-

Semiconductor Noise

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Semiconductor Noise

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