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.

•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.

Cosmic noise

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

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.

Internal Noise

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.

fTPn

fkTPn 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 outputnP

f

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.

VRLoadR

If the load is noiseless and is receiving the maximum noise power generated by our noisy resistor then the following is true:

R

V

R

V

R

V

R

VP nn

Load

n 4

)2( 2222

fRkTVn 4

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 fkTPn

Example

Determine the noise voltage produced

by a 1 M resistor at room

temperature (17 oC) over a 1 MHz

bandwidth.

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?

Example 2

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 currentq = charge of an electron = = direct diode current = frequency bandwidth of system

Shot noise

fqIi dcn 2

ni

dcIf

C19106.1

Find the shot noise current for a diode

with a forward bias of 1mA over a 100

kHz bandwidth.

Example

Addition of Noise due to several sources in series

Given two sources of thermal agitation,

The sum of their effect is given by

Noise Calculations

11 4 fRkTVn 22 4 fRkTVn

21

2

2

2

1, 44 fRkTfRkTVVV nntotn

)(4 21, RRfkTV totn

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 FigureSignal 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

n

s

P

P

powernoise

powersignalNS

/

In decibeln

s

P

PNS 10log10/

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 5V 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.

S/N

S/N

output

inputF

S/N

S/N log10 10 output

inputF dB

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

S/N

S/N log10 10 output

inputF

S/N 10log-S/N log10 1010 outputinputF

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 resistorfor their series combinationfor their parallel combination