Antenna System Design Tutorial | Signal To Noise Ratio | Decibel

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  Designing Antenna Systems for MaximumSensitivity Communications Link Quality The design of an antenna system involves deciding how much antenna gain is needed and specifying amplifiers andreceivers for a specific signal-to-noise ratio. For a given power density, the gain of the antenna determines the power into thereceiver. Our job is to choose the antenna gain for acceptable performance. If the antenna gain is too little, then the signal willbe noisy. If it is too large, the signal will be distorted due to saturation of the receiver.When you listen to a radio and the signal is weak you hear noise. This audio noise is caused by electromagnetic noise fromthe random fluctuation of electrons. Some of this noise comes from outside the antenna system (external noise) and somefrom inside the receiving system (internal noise). The quality of the received signal depends on the ratio of signal power tonoise power, called signal-to-noise ratio, or SNR.In this tutorial we will learn how to calculate compression points, SNR, sensitivity, and minimum discernible signal levels. Thiswill enable us to predict receiving system performance and ultimately design the system for maximum sensitivity.In a communications link, if the signal is too noisy we could increase the transmitter power, the transmit or receive antennagains, or decrease the noise added by the receive system. In other words, the signal-to-noise ratio out of the receiver can beimproved by increasing the signal power at the input to the receiver or by decreasing the noise added by the receive system.First we will learn how to quantify the noise added by the system and then how to calculate the signal-to-noise ratio.In the diagram above, for a given power density, the gain of the receive antenna determines the signal power into thereceiver. We need to choose the receive antenna gain for acceptable performance. If the gain is too little, then the signal willbe noisy. If it is too large, the signal will be distorted due to saturation of the receiver. Definition:    Amplifier gain  is the ratio of the component's or system's signal output power to its signal input power. G [dB] = 10 log (S / S) = S[dBm] - S[dBm] Compression Point The 1dB compression point tells us the largest signal the receiver, amplifier, or system can handle without significantdistortion. The 1 dB compression point refers to the input or output that results in 1dB of gain compression. This compressioncauses saturation distortion. Example:  An amplifier, whose input and output power are shown in the diagram below, normally has 30 dB of gain. When it isat the 1 dB compression point it only has 29 dB of gain and its output is distorted. oioi Antenna System Design Tutorial de 711/12/2012 10:56 a.m.  The compression point can be referenced to the input or the output. For receivers or systems, the maximum output is usuallyspecified. The variables CP and CP will be used to refer to input and output referenced compression points, respectively. Inthe previous exampleCP = -30 dBmCP = -1 dBm Example:  What is the input compression point for this amplifier? Solution:  19 dB is the amplifier gain when it is in 1 dB of gain compression and thereforeCP = CP[dBm] - 19dB = -14dBm An antenna system has a receiver, transmission line, and usually one or more amplifiers. The receiver and amplifiers all havetheir separate compression points. Only the compression point of one of the components will end up being the determiningfactor for the compression point of the system. Example:  What is the system 1 dB compression point (referenced to the input) for the antenna system shown below? Solution:  In this case, as the power level into the system is increased the first component to go into compression is the ioioio Antenna System Design Tutorial de 711/12/2012 10:56 a.m.  receiver. The input, when the receiver is at its 1 dB compression point, isP = 0dBm - 20dB = -20dBmTherefore the system compression point (referenced to its input) is -20 dBm. Example:  What is the system 1 dB compression point (referenced to the input) for the antenna system shown below? Solution:  This time the transmission line has 8 dB of attenuation. We need to determine which component saturates first, theamplifier or the receiver. When the amplifier output is at its compression point of 5 dBm, the receiver input is 8 dB less, or -3dBm. So the preamp compresses before the receiver and the system 1 dB compression point (referenced to the input) isCP = 5dBm - 19dB = -14dBm Noise Figure We first studied compression points in order to determine the maximum signal that a receiving system can handle. Next weexamine noise in order to understand the minimum signal power that a system can detect. The noise produced by somethingis generally proportional to its absolute temperature. Absolute temperature is measured in units of Kelvin.  Converting betweenKelvin, Celsius, and Fahrenheit is through the following formulas:K = 273 + CC = (F - 32)(5 / 9)The noise delivered from a resistor to a matched load is equal to:N = kTBwhere N is in watts, T is the temperature of the resistor in Kelvin, B is the bandwidth in Hertz, and k=1.38x10 is Boltzman'sconstant. Exercise:  What is the noise power delivered to this receiver? iiooooii-23 Antenna System Design Tutorial de 711/12/2012 10:56 a.m.  Definition:  The noise temperature  of an antenna is the temperature T, such that the noise coming out of the terminals of theantenna is kTB. The antenna noise temperature is a function of the temperature of the matter in the direction the antenna ispointed in, its antenna pattern, the frequency, and the radiation efficiency of the antenna. At microwave frequencies, for antennas whose takeoff angle is approximately zero degrees elevation, the half of theantenna's field of view includes the earth and the other half the sky. The temperature of the earth is about 290 Kelvin, but itonly occupies half of the beamwidth. The result is that the antenna noise temperature T is approximately 150K. Earth stationantennas see lower temperatures because they look up into the cold sky. HF antennas have a high antenna noisetemperature due to HF atmospheric noise generated mostly by lightning storms around the world. Definition:  The noise figure  is a way of describing the amount of noise generated in a receiver, amp, transmission line,antenna system, or other component. The noise figure of a component or system is defined as the signal-to-noise ratio at theinput divided by the signal-to-noise ratio at the output, with the input noise equal to the noise available from a matchedresistance at a temperature of T=290 Kelvin.F = (S / N)/(S / N) where N = kTBF[dB] = 10 log FThe term noise factor   is often used to describe the linear form of noise figure. In other words the ratio of input to output SNRis described as the noise factor and noise figure is the noise factor measured in dB.The higher the noise figure (or noise temperature) of a component or system, the more noise it adds to the signal. For thisreason, it is desirable to have low noise figure (or noise temperature) components and systems. Noise figures (or noisetemperatures) of receivers and amplifiers are specified in manufacturers' datasheets. Noise figures can also be measuredwith a noise figure meter. A component generates noise internally, which degrades the SNR of the signal passing through itby adding noise to the signal. A component can be modeled as a noise-free amplifier of gain G, with noise added to the signalin a summing junction preceding the perfect amp, as shown below. aaa0iiooi0 Antenna System Design Tutorial de 711/12/2012 10:56 a.m.
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