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Why study noise?
WYPR-FM 88.1 MHz
Transmitted power 15,500 W
Received power
~0.00000000000001 W
~19 orders ofmagnitude
Noisesources and effects
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Why study noise?
Noise is one of the principle limiting factors inthe performance of communication systems.
Electrical noise is any undesired voltages or
currents that end up appearing at the receiveroutput. An example is static that is commonly
encountered on broadcast
AM radio.
In a practical system noise is always present.
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Types of noise
Noise is divided into two broad categories:
External noise is noise introduced in the
transmission channel.
Internal noise is noise introduced inside the
receiver itself.
Information or
intelligence
(audio, video,
data, etc.)
Recovered
information
orIntelligence
Free space (radio), wire,
fiber-optic cable, etc.
Transmitter(TX)
Communicationschannel or medium
Receiver(RX)
External
noise
Internal
noise
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External noise
Industrial noise is caused by human made
electrical sources (motors,
generators, ignition)
Atmospheric noise is due to naturally
occurring disturbances in earthsatmosphere such as lightening.
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External noise
Extraterrestrial noise (solar noise)is electrical
noise due to solar and cosmic activity.
Dominates at higher frequencies and can be aserious problem in satellite communications.
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Internal noise
Internal noise is noise introduced inside the
receiver.
Three types of internal noise we will consider Thermal noise
Semiconductor noise
Intermodulation distortion
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Thermal noise
Thermal noise is caused by random motion offree electrons and vibrating ions in a
conductor due to heat.
Noise Power in watts is directly proportionalto Bandwidth in Hz, and the temperature in
degrees Kelvin.Thermal noise voltage as a function of timeRandom electron motion due to heat
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Thermal noise
Thermal noise is also termed white noise or
Gaussian noise.
Just as white light contains all frequencies, white
noise contains an equal weighting of allfrequencies.
Frequency (Hz)
Frequency spectrum of thermal noise
Thermal noise voltage as a function of time
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N KTB!N= noise power (watts)
B =bandwidth (hertz)
K= Boltzmanns proportionality constant
(1.38 10-23 Joules per kelvin)
T= absolute temperature (kelvin)
10 log 10 log 10 log0.001 0.001
dBm
KTB KT N B! !
@ 290k 174 dBm 10 log
dBmN B!
Thermal noise
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2
224
4
N
N
N
V
V N KTB V RKTB
R R
! ! ! p !
2V
P VIR
! !
V IR!
The noise voltage produced by a resistorR over a bandwidthB can be calculated
Thermal noise voltage
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Thermal noise voltage
Thermal noise design considerations; Since
thermal noise is proportional to resistance,temperature, and bandwidth, receiver designsthat reduce these values will have superiorperformance.
23
4
where rms noise voltage
oltzman's constant (1.38 10 J/K)
temperature, K ( C 27
3)bandwidth, Hz
resistance,
n
n
v kTBR
v
k
T
B
R
!
!
! v
! r !
! ;
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Example Problem 1
The bandwidth of a receiver with a 75-; input resistance is 6
MHz. If the temperature is 29C, what is the input thermal noise
voltage?
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Semiconductor noise
The other major category of internal noise
originates from semiconductor devices such as
diodes and transistors.
Semiconductor noise is comprised of
Shot noise
Transit-time noise
Flicker noise
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Shot noise
Shot noise, the largest contributor to transistor
noise, is due to the random paths of the
current carriers flowing in semiconductors.
circuit symbol for a diode
diode
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Transit-time noise
Transit-time noise occurs at high frequencies
when transit time of charge carriers crossing
the semiconductors junction approaches the
signals period.
This type noise increases rapidly when
operating above the devices high-frequency
cutoff. Time required to crosspn-junction close to period (T)
of the signal.
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Flicker noise
Flicker noise results from minute variations in
resistance in semiconductor material.
Flicker noise is inversely proportional to
frequency and sometimes referred to as 1/fnoise or pink noise.
Flicker noise voltage as a function of time
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Flicker noise
Flicker noise is also found in resistors and
conductors.
Typeofresistor Noise voltagerange(QV)
Carbon-composition 0.1-3.0
Carbon-film 0.05-0.3
Metal-film 0.02-0.2
Wire-wound 0.01-0.2
Flicker noise for various types of resistors
metal-film
carbon-
composition wire-wound
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Semiconductor noise
Total noise voltage of semiconductor devices
varies with frequency with different types of
noise predominating in different regions.
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Measuring noise
To quantify the effect of noise on a signal, we
use the signal-to-noise (S/N) ratio or SNR.
A strong signal and weak noise results in a high
S/N ratio.
A weak signal and strong noise results in a low S/N
ratio.
signal without noise 20 d S/N ratio 10 d S/N ratio
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Signal-to-noise ratio
Signal to noise ratio can be expressed in terms
of voltage or power. signal voltage (rms)noise voltage (rms)
or here signal po er (W)
noise po er (W)
s
ns s
sn n
n
V
VS V S P
PN V N P
P
! !
! ! ! !
+
signal
(Vs orPs)
noise
(Vn
orPn)
signal plus noise
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Signal-to-noise (decibels)
Signal to noise ratio is most commonly
reported in decibels
/ ratio (d ) using voltage: d 20log
/ ratio (d ) using power: d 10log
s
n
s
n
V
S NV
PS N
P
!
!
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Noise ratio (NR)
We often need to quantify how much noise a
device adds to a signal as it passes through
the device.
One measure is the noise ratio (NR) which is
simply the ratio of input S/N to output S/N.
amplifier
output signalinput signal
/ inputNR
/ output
S N
S N!
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Noise figure (NF)
When the noise ratio (NR) is expressed in
decibels, its called the noise figure (NF).
Since the output S/N ratio will be less than theinput, the NR > 1 and NF > 0.
For an ideal device, NR = 1.0 and NF = 0 dB.
In practice, NF less than 2 dB is excellent.
amplifier
output signalinput signal
10log [dB]!
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Noise Ratio (NR) and Noise Figure (NF)
NF = 10 log (NR)
NF (dB) = (SNR)input (dB) (SNR)output (dB)
output
input
SNR
SNRNR !
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Example Problem 2
A transistor amplifier has a measured S/N power ratio of
10,000 at its input and 5,624 at its output.
(a) Calculate the NR.
(b) Calculate the NF.
amplifier
output signalinput signal
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Noise in cascaded stages
The total noise performance of a cascade ofamplifiers depends upon the noise ratio andpower gain of each stage.
The total noise performance of multiple stagesis given by Friis formula
whereAi the power gain andNR
inoise ratio of the i th stage.
signal input output
A1, NR1 A2, NR2 A3, NR3
2 31
1 1 2 1 2 1
NR 1 NR 1 NR 1NR NR n
n A A A A A A
! L
L
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Example Problem 3
The gain of the three stages of an amplifier are 8.45 dB, 10.79
dB, and 20 dB. The noise figures associated with these stages
are 2.04 dB, 3.0 dB, and 9.29 dB. What is the overall NR and NF
for this cascade of amplifiers?
signal input output
A1 =8.45 d
NF1 =2.04 d
A2 =10.79 d
NF2 =3.0 d
A3 =20.0 d
NF3 =9.29 d
NF
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Implications of Friis formula
The total noise performance of a receiver is
invariably determined by the very first stage.
Beyond the first and second stage, noise is nolonger a problem.
signal input output
A1, NR1A2, NR2 A3, NR3
2 31
1 1 2 1 2 1
NR 1 NR 1 NR 1NR NR n
n A A A A A A
! L
L
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Noise Temperature
N
N KTB T KB
! p !
T= environmental temperature (290 Kelvin)
N= noise power (watts)
K= Boltzmanns constant (1.38 10-23 J/K)
B = total noise factor (hertz)
1
e
T T F!
Te= equivalent noise temperature
T= environmental temperature (290 Kelvin)
F = noise factor (dimensionless)
1e
TF
T!
1F u
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Hartley-Shannon Theorem:
Significance of SNR
Hartley-Shannon Theorem (also called
Shannons Limit) states that the maximumdata rate for a communications channel is
determined by a channels bandwidth and
SNR.
A SNR of zero dB means that noise power
equals the signal power.
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Noise Effects on Communications
Data
May be satisfactory in the presence of whitenoise but impulse noise will destroy a data signal
BER (Bit Error Rate) is used as a performancemeasure in digital systems
Voice
White noise (continuous disturbance) can bebothersome to humans but impulse noise can beacceptable for speech communications
SNR (Signal-to-Noise Ratio) is used as aperformance measure in analog systems