FIU CHM 4130 - CHAPTER 5_Xiao_Signal_To_Noise_2018 (58 pages)

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CHAPTER 5_Xiao_Signal_To_Noise_2018



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Chapter 5 Signals and Noise Signal to Noise Ratio Types of Noise Signal to Noise Ratio Enhancement Signal Averaging Filtering Signal and Noise The signal is what you are measuring that is the result of the presence of your analyte Noise is extraneous information that can interfere with or alter the signal It can not be completely eliminated but hopefully reduced Noise is considered random indeterminate SIGNAL with NOISE SIGNAL with NOISE averaged out The reason for a Signal to Noise Ratio The average strength of the noise N is constant and independent of the magnitude of the signal Signal vs Noise Why is a Signal to Noise Ratio Signal to noise ratio S N Parameter describing quality of data often referred to as figure of merit What is a Signal to Noise Ratio Signal mean X Noise standard deviation s This is mathematically the inverse of the RSD or we can say that S 1 N RSD S N 4 3 1 Very little confidence in ability to determine peaks S N 43 1 Signal and Noise 1 What are signal and noise 2 Why is using a signal to noise ratio 3 The relationship between signal to noise ratio and RSD S N Objective A S N ra o of 3 is usually the minimum that is acceptable Increase the signal to noise ra o More signal vs noise means a lower STDEV More precise measurement Lower STDEV means a be er LOD Lower limits of detec on Reduce as much of the noise as possible by carefully controlling condi ons Temperature power supply varia ons etc etc etc Noise Types of Noise in Chemical Analysis 1 Chemical Noise 2 instrumental noise Types of Noise 1 Chemical Noise Undesired chemical reac ons in the System being analyzed Examples Temperature Position of chemical equilibria Pressure Humidity Moisture content of sample Vibra ons Stratification of powdered solids Light intensity Photosensitive materials Fluctua ons uncontrolled variables Types of Noise 2 Instrumental Noise Noise from instrumental components a complex composite several types sources A ects all types of instruments Can oSen be controlled physically e g temp or electronically hardware and soSware Types of Noise 2 Instrumental Noise Energy Source Sample Analytical Signal Input Transducer Detector Signal Processor Output Transducer Readout Types of Noise 2 Instrumental Noise Energy Source Sample Analytical Signal Input Transducer Detector Signal Processor Output Transducer Readout Types of Noise 2 Instrumental Noise Energy Source Sample Analytical Signal Input Transducer Detector Signal Processor Output Transducer Readout Types of Noise 2 Instrumental Noise Energy Source Sample Analytical Signal Input Transducer Detector Signal Processor Output Transducer Readout Instrumental Noise a Thermal Johnson Noise Electronic noise Thermal agita on of electrons in load resistor Due to di erent veloci es and movement of electrons in electrical components root mean square noise voltage resistance of the resis8ve element rms temperature frequency bandwidth V 4kTR f Dependent on both temperature and frequency bandwidths not frequency Can be reduced by reducing temperature of electrical components narrowing bandwidth and minimizing the resistance Disappear only at absolute zero Independent of physical size of the resistor Considered white noise occurs at all frequencies Thermal noise in the resistance of the signal source is the fundamental limit on achievable signal sensi vity b Shot Noise Occurs when there is a poten al barrier wherever electrons or charged par cles cross junc ons di erent materials such as PN junc on diode a transistor vacuum tube etc root mean square current uctua8on irms 2Ie f average direct current frequency bandwidth Considered white noise occurs at all frequencies Due to the sta s cal varia on of the ow of electrons current across some junc ons Some of the electrons jump across the junc on right away Some of the electrons take their me jumping across the junc on Shot noise in a current measurement can be minimized only reducing bandwidth Note that if the ac ve device provides ampli ca on the noise also gets ampli ed along with the signal c Flicker Noise 1 f one over f Noise Associate with crystal surface defects in semiconductors the oxide coa ng on the cathode in vacuum tubes Frequency dependent Magnitude is inversely propor onal to frequency Signi cant at frequencies less than 100 Hz Not a white noise referred to as pink noise Results in long term dri in electronic components such as dc ampli ers light sources voltmeters and current meters Can be reduced by using resisters that are metallic wire wound one d Environmental Noise Room should be cold A composite of di erent forms of noise that arises from the surroundings Unlimited possible sources Can o en be eliminated by elimina ng the source Other noise sources can not be eliminated Methods of elimina ng it Moving the instrument somewhere else Isola ng condi oning the instruments power source Controlling temperature in the room Control expansion contrac on of components in instrument Elimina ng interferences Stray light from open windows panels on instrument Turning o radios TV s other instruments Environmental noise spectrum 1kHz 500 kHz 3 Hz 60 Hz Noise 1 Types of noise 2 Instrumental noise a Thermal Johnson Noise b Shot Noise c Flicker Noise d Environmental Noise Improve S N ratio Hardware design components such as filters choppers and modulators lock in amplifiers Remove or attenuate the noise a b c d Shielding and grounding Analog filtering Modulation and demodulation Lock in amplifiers 2 Software various computer procedures Extract signal from noisy data a Ensemble Averaging b Boxcar Averaging c Moving Averaging Improving S N hardware Hardware Shielding and Grounding Reduce the noise that arises from environmentally generated electromagne c radia on Faraday cage a two way street 1 Any noise with an electronic component that exists outside the cage is completely cancelled within that space 2 Any noise created inside the cage is prevented from escaping to the outside world An example No shielding and grounding With shielding With shielding and grounding Improving S N hardware Analog Filtering Although amplitude and the phase rela onship of input and output signals can be used to discriminate between meaningful signals and noise frequency is the property most commonly used White noise can be reduced by narrowing the range of measured frequencies environmental noise can be eliminated by selec ng the proper frequency Three kinds of electronic lters are used to select the band of measured


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