Noise I Abigail Firme

Introduction Characterization [1] Frequency Amplitude Mechanism responsible Types of Noise Interference Random Noise Johnson Noise Shot Noise Flicker Noise Random noise constant(ish) noise. Interference: instantaneous noise introduced into system from environment.

Johnson Noise “Thermal Noise” is caused by voltage generated by moving charge carriers. Also characterized as white noise with flat frequency spectrum. Sensitivity Limiting Factor. [1] Johnson noise, also called thermal noise, is caused by a voltage across charge carriers at equilibrium. Meaning that a resistor sitting on the table creates a noise voltage across its terminals. Since this can happen for any electrical conductor (resistive element), Johnson Noise is present in every electrical circuit. This noise floor is the limiting factor for instrument sensitivity in that it can drown out small signals. White noise, has the same noise power in each Hz of frequency. [4] [5]

Shot Noise Electronic Shot Noise rms Current Fluctuations [1] Usually small compared to Johnson Noise and 1/f noise. Optical Shot Noise Fluctuations in number of photoelectrons Probability follows Poisson distribution. [2] Electronic shot noise pertains to fluctuations in DC electric current. Fluctuations are due to the “flow” being made up of discrete charge carriers and not coherent “flow”. In a given interval, there will not be the same amount of photons collected. Talk about CCDs?? [6] [1]

Noise Reduction Johnson Noise Three options for noise reduction: Resistance, Temperature, and Bandwidth. Ideal resistance in superconductors: R = 0. Resistance or impedance reduction limited by equipment components. Temperature reduction is common when studying samples at low temperatures, but is not commercially feasible. Bandwidth reduction is most common. Averaging or filtering. Shot Noise Two options for noise reduction: Current and Bandwidth. Reduction of current through the system corresponding to high optical signal input. Bandwidth reduction Chemical electrodes with high impedance (as in chemical batteries), necessary use of semiconductors. Semiconductors produce noise via random generation and recombination of free charge carriers (quantum noise!). Using 10MOhm resistor and 20kHz bandwidth, room temp: V = ~17.4 microV, 263K: V = ~17.04 microV. Large temp change for significant noise voltage reduction (4.2K: ~ 2.2 microV). To decrease bandwidth, measure slowly, take more data points, and average over the results. Or, using a low-pass filter.

Photomultipliers Collect photons from low light and amplify the signal. Kept at low temperatures to eliminate thermal noise. Remaining noise is shot noise. [3] Background Light Signal Light Photons meet the photocathode which converts them to electrons. The electrons are amplified by the dynodes and the total signal is received by the anode. Background light noise can be decreased by light boxes. [2]

Amplifier Noise Amplifies total signal plus noise, but introduce additional noise in the process. [1] Low noise amplifiers choose optimal Ic Based on in Based on Rs Amplifiers can be subject to temperature changes that increase outputted noise. e_n: input noise voltage, i_n: noise current, I_C collector current, R_s: source resistance [1] [7]

References [1] Paul Horowitz, Winfield Hill, ‘The Art of Electronics’, 1989, p. 286-290. [2] Robert L. McClain, John C. Wright, ‘Description of the Role of Shot Noise in Spectroscopic Absorption and Emission Measurements with Photodiode and Photomultiplier Tube Detectors: Information for an Instrumental Analysis Course’, American Chemical Society, 2014, p. 1455 – 1457. [3] Hamastu Photonics K. K., ‘Photomultiplier Tubes: Basics and Applications’, Third Edition, 2007, p. 129 – 133. [4] http://www.bitlanders.com/blogs/free-electrons-in-metals-and-related-terms/253854 [5] http://www.rfcafe.com/references/electrical/noise-power.htm [6] https://www.codrey.com/dc-circuits/conventional-current-vs-electron-current/ [7] https://gpio.com/2017/02/16/what-is-a-low-noise-amplifier/ Electronic shot noise pertains to fluctuations in DC electric current. Fluctuations are due to the “flow” being made up of discrete charge carriers and not coherent “flow”. Talk about CCDs??