Noise Lecture 6
Definition • Sources of noise • Noise Calculations
Definition: • Electrical noise may be said to be the introduction of any unwanted energy, which tend to interfere with the proper reception and reproduction of transmitted signals.
Sources of noise • External • Atmospheric • Industrial • Extraterrestrial • Solar noise • Cosmic noise • Internal
Atmospheric • Atmospheric noise also known as static • It is caused by naturally occurring disturbances in the earth’s atmosphere SOURCES • lightening discharges, • thunderstorms and other natural electric disturbances.
Nature and Form • It comes in the form of amplitude modulated impulses. • Such impulse processes are random and spread over the whole of the RF spectrum used for broadcasting. • It consists of spurious radio signals with many frequency components.
It is propagated in the same way as ordinary radio waves of the same frequency. • Any radio station will therefore receive static from thunderstorms both local and distant. • It affects radio more than it affects television. The reason, field strength is inversely proportional to frequency.
At 30MHz and above atmospheric noise is less severe for two reasons: • Higher frequencies are limited to line of sight propagation • Very little of this noise is generated in the VHF range and above.
Industrial • Noise made by man easily outstrips any other between the frequencies of 1 to 600 MHz. • This includes such things as car and aircraft ignition, electric motors, switching equipment, leakage from high voltage lines etc.
Extraterrestrial Solar noise • This is the noise that originates from the sun. • The sun radiates a broad spectrum of frequencies, including those, which are used for broadcasting.
The sun is an active star and is constantly changing • It undergoes cycles of peak activity from which electrical disturbances erupt. • The cycle is about 11 years long.
Cosmic noise • Distant stars also radiate noise in much the same way as the sun. • The noise received from them is called black body noise. • Noise also comes from distant galaxies in much the same way as they come from the milky way.
Extraterrestrial noise is observable at frequencies in the range from about 8MHz to 1.43GHz. Apart from man made noise it is strongest component over the range of 20 to 120MHz. Not much of it below 20MHz penetrates below the ionosphere
Internal Noise This is the noise generated by any of the active or passive devices found in the receiver. This type of noise is random and difficult to treat on an individual basis but can be described statistically. Random noise power is proportional to the bandwidth over which it is measured.
Gaussian Noise This is the cumulative effect of all random noise generated both external and internal to the communication system and averaged over a period of time. This includes all frequencies. For electronic circuits this more specifically called white noise, Johnson noise or Thermal noise.
Thermal Noise The noise generated by the agitation and interaction of electrons is called thermal noise. The internal kinetic energy of a particle can be expressed through its temperature. The kinetic energy of a body is zero at a temperature of absolute zero. The noise generated by a resistor, for example, is proportional to its absolute temperature as well as the bandwidth over which the noise is to be measured.
where k = Boltzmann’s constant J/K (joules per Kelvin) T = absolute temperature in Kelvin, K = 273 + oC = frequency bandwidth of system = maximum noise power output
Any ordinary resistor not connected to a voltage source will have a voltage associated with it in such a case the resistor may be represented diagrammatically as shown. R RLoad V
If the load is noiseless and is receiving the maximum noise power generated by our noisy resistor then the following is true:
Observations For maximum power transfer: V =Vn/2 That is t the voltage across the load is half the voltage of the noise generating resistor. Also
Example Determine the noise voltage produced by a 1 M resistor at room temperature (17 oC) over a 1 MHz bandwidth.
Example 2 An amplifier operating over the frequency range from 18 to 20 MHz has a 10 kilo ohm input resistor. What is the rms noise voltage at the input to the amplifier if the ambient temperature is 27 oC?
Shot noise • In a transistor the major contributor to noise is called shot noise. The formula for shot noise in a diode is given as: • = rms shot noise current • q = charge of an electron = • = direct diode current • = frequency bandwidth of system
Example Find the shot noise current for a diode with a forward bias of 1mA over a 100 kHz bandwidth.
Noise Calculations • Addition of Noise due to several sources in series • Given two sources of thermal agitation, • The sum of their effect is given by
Example Calculate the noise voltage at the input of a television RF amplifier using a device that has 200 ohm equivalent noise resistance and a 300 ohm input resistor. The bandwidth of the amplifier is 6 MHz and the temperature is 17oC.
Noise Figure Signal to noise Ratio • Two main reasons why we calculate equivalent noise of a device • to compare two devices in order to evaluate their performance • to compare the signal and the noise at the same point to ensure that noise is not excessive The measure for this calculation is the signal to noise ratio. This is a relative measure of the desired signal power to the noise power
Signal to noise Ratio In decibel
Example An amplifier operating over a 4 MHz bandwidth has a 100-ohm input resistance. It is operating at 27 oC, has voltage gain of 200 and has an input signal of 5mV rms. Determine the rms output signals (desired and noise) disregarding any external sources of noise. Calculate the signal to noise ratio at the output.
Noise Figure This term is used to describe how noisy a device is. It is a ratio of the signal to ratio at the input to the signal to noise ratio at the output. dB
Example A transistor amplifier has a measured S/N power of 10 at its input and 5 at its output. Calculate the noise figure. Show that the equation can be written as
Example • Two resistors, 5 kohm and 20 kohm, are at 27oC. Calculate the thermal noise power and voltage for a 10 kHz bandwidth • for each resistor • for their series combination • for their parallel combination