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www.thinkSRS.com Signal Enhancement Application Note #6 Many experimental techniques rely on the quantitative Noise Sources measurement of charged particles. Applications range from the very simple, such as the measurement of a DC anode An understanding of noise sources in a measurement is critical current by a picoammeter, to the complex, such as the to achieving signal-to-noise performance near theoretical measurement of atomic state lifetimes using time-resolved limits. The quality of a measurement may be substantially particle counting. degraded by a trivial error. For example, a poor choice of termination resistance for an electron multiplier may increase Often, the signal of interest is obscured by noise. The noise current noise by several orders of magnitude. may be fundamental to the process: discrete charges are governed by Poisson statistics which gives rise to shot noise. Shot Noise Or, the noise may be from more mundane sources, such as microphonics, thermal emf, or inductive pick-up. Light and electrical charge are quantized, and so the number of photons or electrons which pass a point during a period of This article will describe methods for making useful time are subject to statistical fluctuations. If the signal mean is measurements of weak signals, even in the presence of large M photons, the standard deviation (noise) will be M, hence interfering sources, emphasizing the electronic aspects of the the S/N=M/M=M. The mean (M) may be increased if the measurement. rate is higher or the integration time is longer. Short integration times, or small signal levels, will yield poor S/N The figure below details the elements of a typical values. The graph below shows the S/N which may be measurement. In this experiment the photo-ionization cross expected as a function of current level and integration time for section for a gas will be determined by passing a laser through a shot-noise limited signal. the gas and by measuring the number of ions which are created. Current Quanta (No./s) Background Signal 1A 18 Optics 1uS 10 Signal PMT Amp Detection