NOISE

ELECTRONIC COMMUNICATION SYSTEMSSecond Edition

Roy BlakeNOISENoiseIt originates both in channel and communication systems.

Consist of undesired, usually random, variations that interfere with the desired signals and inhibit communication.It cant be avoided completely but its effects can be reduced by various means, such as reducing the signal bandwidth, increasing the signal power, and using low-noise amplifier for weak signals.

3Classification of NoiseExternal NoiseInternal Noise

External noiseStatic noise like thunderstorms, automobile ignitions systems, domestic and industrial equipment, light dimmers, vacuum cleaners to computers, can also cause objectionable noise.Suns and stars

Internal Noise Noise generated by electronic equipment4External NoiseEquipment NoiseNoise is generated by equipment that produces sparks.Any fast rise time voltage or current can also generate noise.Has a broad frequency spectrum but its energy is not distributed over the frequency range.Generally more severe at lower frequenciesAtmospheric NoiseOften called static noise.Can propagate for long distances

External NoiseMost of the energy of lightning is found at relatively low-frequency to several megahertz.Nothing can be done to reduce static noise at the source.Noise blanking is a technique to reduce static noiseSpace NoiseThe main contributor of this noise is the sunStars can also contribute space noiseStatic NoiseCircuits are available to reduce its effect by taking advantage of the fact that this noise has a very high peak-to-average power ratio; that is, the noise occurs in short intense burst with relatively long periods of time between burst. It is often possible to improve communication by simply disabling the receiver for the duration of the burst

Space noiseCan be serious problem on satellite reception, which become impossible when the satellite is in line between the sun and the receiving antenna.Affects higher frequencies (VHF and above)Cannot affect lower frequency because it can penetrate the ionosphere.6Internal NoiseThermal NoiseProduced by random motion of electrons in a conductor due to heat.Found everywhere in electronic circuitry.Power density of thermal noise is constant with frequency.Sometimes called white, johnson, gaussian noise7Internal NoiseEquation is based on the assumption that power transfer is maximum.That is, the source and the load are assumed to match.Filter is used to reduced noise.Reduce and temperature or the bandwidth

Cryogenics is a technique use to reduce temperature by applying liquid nitrogen on the equipmentBandwidth reductionDoes not depend on material used or the amount of current passing through it.8Internal NoiseExampleA receiver has a noise power bandwidth of 10kHz. A resistor that matches the receiver input impedance is connected across its antenna terminals. What is the noise power contributed by the resistor in the receiver bandwidth, if the resistor has a temperature of 27OC?4.14x10-17W

9Internal NoiseDraw the circuitDerive the formulaV is in RMS value10Internal NoiseExampleA 300 resistor is connected across the 300 antenna input of a television receiver. The bandwidth of the receiver is 6MHz, and the resistor is at room temperature (20OC). Find the noise power and the noise voltage applied to the receiver input.24.2x10-15 W5.4uV11Internal NoiseShot NoiseDue to random variations of current flow in active devices such as tubes, transistors and semiconductor diodes.Describes the random arrival of electrons arriving at the anode of a vacuum tube, like individual pellets of shot from a shotgun.It is usually represented by a current sourcePower spectrum resembles that of thermal noise, having equal energy in every hertz of bandwidth.12Internal NoiseInternal NoiseExampleA diode noise generator is required to produce 10uV of noise in a receiver with an input impedance of 75, resistive, and a noise power bandwidth of 200kHz. What must the current through the diode be?276mA14Internal NoisePartition NoiseSimilar to shot noise, but it only occurs in a devices where a single current separates into two or more paths.Excess Noise1/f noiseIt is believed to be caused by variations in carrier densityUsed for testing and setting up audio systems.Partition NoiseBJT emitter current is the sum of the collector and base currents.No equation for calculating it but the manufacturers provide noise figure on their data sheets.Not a problem in FET because there is no gate current.

Excess NoiseAlso called flicker noise and pink noise(because it affects low-frequency)Insignificant above 1kHzFound in tubes, but greatly affects semiconductors and in carbon resistor.

15Internal NoiseTransit-Time NoiseOccurs when the time take by the charge carriers to cross a junction is comparable to its period of the signal.Some of the carriers may diffuse back across the junction, causing a fluctuating current that constitutes noise.Significant in microwave devices.Addition of Noise from Different SourcesAddition of Noise from Different SourcesExampleThe circuit in Figure shows two resistor in series at two different temperatures. Find the total noise voltage and noise power produced at the load, over a bandwidth of 100kHzR1 = 1000, 300K & R2 = 200, 400 K0.506x10-15W18Signal to Noise RatioThe main reason for studying and calculating noise power or voltage is the effect that noise has on the desired signal.It is the ratio of signal to noise power that is important than the noise power aloneIt is often difficult to measure

It is not really the amount of noise that concerns us but rather the amount of noise compared to the level of the desired signalUsually expressed in decibelsMost important specification of any communication system

It may be possible to measure the noise power by turning of the signal, but it is not possible to turn off noise in order to measure the signal power alone. Instead of S/N, it is sometimes called (S+N)/N, often found in receiver specs.19Signal to Noise RatioExampleA receiver produces a noise power o 200mW with no signal. The output level increases to 5W when a signal is applied. Calculate (S+N)/N as a power ratio and in dB.

2514dB20Noise FactorNoise Factor unitlessNoise Figure Decibel 21Noise FigureExampleThe signal power at the input to an amplifier is 100uW and the noise power 1uW. At the output side, the signal power is 1W and the noise power is 30mW. What is the amplifier noise figure and factor? How much noise does the amplifier contributes?3

22Noise FigureExampleThe signal at the input of an amplifier has S/N of 42 dB. If the amplifier has a noise figure of 6dB, what is the S/N at the output (in dB).36dB23Equivalent Noise TemperatureIs another way of specifying the noise performance of a device.