Download Lecture 2: Noise - Essex County College

Lecture 1: Introductory Topics
Prof. Park
ELC 222
Essex County College
Prof. Park
ELC 222
1
Modulation
• Modulation is the process of putting
information onto a high-frequency carrier
for transmission.
• The low-frequency information is called
the intelligence.
• The high-frequency medium is called the
carrier.
• The demodulation is the reverse process
of modulation.
Prof. Park
ELC 222
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Mathematical Representation of
Sine Wave
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v = Vp sin(t + )
Where v = instantaneous value
Vp = peak value
 = angular velocity = 2f
 = phase angle
AM: Amplitude Modulation
FM: Frequency Modulation
PM: Phase Modulation
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ELC 222
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Electrical Noise
• Electrical noise: Any undesired voltages or
currents that ultimately end up appearing
in a circuit.
• Static: Electrical noise that may occur in
the output of a receiver.
• External Noise: Noise introduced by the
transmitting medium.
• Internal Noise: Noise introduced by the
receiver.
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ELC 222
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External Noise
• Human-Made Noise: Noise produced by sparkproducing system such as engine ignition
systems, fluorescent lights, commutators in
electric motors, and power lines.
• Atmospheric Noise: Noise caused by naturally
occurring disturbances in the earth’s
atmosphere.
• Space Noise: Noise produced outside the
earth’s atmosphere.
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ELC 222
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Internal Noise
• Thermal Noise: Noise caused by thermal
interaction between free electrons and vibrating
ions in a conductor.
• Shot Noise: Noise introduced by carriers in the
pn junctions of semiconductors
• Excess Noise: Noise occurring at frequencies
below 1khz, varying in amplitude inversely
proportional to the frequence
• Transit-Time Noise: Noise produced in
semiconductors when the transit time of the
carriers crossing a junction is close to the
signal’s period.
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ELC 222
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Thermal Noise
• Thermal Noise: Noise caused by thermal
interaction between free electrons and
vibrating ions in a conductor.
• Johnson Noise: Another name for thermal
noise, first studied by J. B. Johnson in
1928.
• White Noise: Another name for thermal
noise because its frequency content is
uniform across the spectrum.
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ELC 222
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Thermal Noise
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Pn = kTf
k = Boltzmann’s constant (1.3810-23 J/K)
T = Resistor temperature in kelvin (K)
f = Frequency bandwidth of the system
The rms noise voltage en has a maximum at
en  4kTfR
Prof. Park
ELC 222
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Example 1-4
Determine the noise voltage produced by a 1Mohm resistor
at room temperature (17C) over 1MHz bandwidth.
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ELC 222
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A communication system block diagram
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ELC 222
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Noise effect on a receiver’s first and second
amplifier stages
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ELC 222
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Resistance noise generator
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ELC 222
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Device noise versus frequency
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ELC 222
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Signal-To-Noise Ratio
• Signal-To-Noise Ratio: Relative measure of
desired signal power to noise power
• Noise Figure (NF): A figure describing how
noisy a device is in decibels
• Noise ratio (NR): A figure describing how noisy
a device is as a ratio having no units
NF  10 log 10
Prof. Park
Si / N i
 10 log 10 NR
So / N o
ELC 222
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Example 1-6
• A transistor amplifier has measured S/R of
10 at its input and 5 at its output.
– A) Calculate the NR
– B) Calculate the NF
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ELC 222
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Noise Due to Amplifiers in cascade
• Friiss’s formula
• NR = NR
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ELC 222
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Information and Bandwidth
• Hartley’s Law:
information  bandwidth  time of transmission
• Fourier Analysis: Method of representing
complex repetitive waveforms by sinusoidal
components
• Fast Fourier Transform (FFT): A technique for
converting time-varying information to its
frequency component
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ELC 222
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AM vs. FM
AM
FM
Low Limit
535 kHz
88 MHz
High Limit
1605 kHz
108 MHz
Channel BW
10 kHz
200 kHz
Baseband BW
5 kHz
15 kHz
Max. Stations
107
100
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ELC 222
Analog TV
6 MHz
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Example 1-11
• Determine the resonant frequency for the circuit
below. Calculate its impedance at f = 12 kHz.
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ELC 222
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Example 1-12
• Determine the resonant frequency for the circuit
when R1 = 20, R2 = 1, L = 1mH, C = 0.4µF,
and ein = 50 mV. Calculate eout at fr and at f = 12
kHz.
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ELC 222
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Example 1-13
• A filter circuit has a response as below.
Determine (a) bandwidth, (b) Q, (c) L if C =
0.001µF, and (d) total circuit resistance.
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ELC 222
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Example 1-14
• A parallel LC tank circuit is made up of an
inductor of 3mH and a winding of 2. The
capacitance is 0.47µF. Determine (a) fr, (b) Q,
(c) Zmax, and (d) BW.
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ELC 222
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