notes2 noise

I. Electrical Noise – any undesirable electrical energy that falls
within the passband of the signal
Classification of Noises
A. Uncorrelated Noise
- present regardless of whether there is a signal
present or not
A.1. External Noise – generated outside the device or
- primary sources are:
Atmospheric Noise – naturally occurring
electrical disturbances that originates
within the Earth’s atmosphere that is
commonly called static electricity
- the sputtering, crackling heard from the
speaker when no signal is present
- sources are lightning
- the magnitude of the energy is inversely
proportional to its frequency wherein it
becomes insignificant at frequencies below
Extraterrestrial Noise – consists of electrical
signal that originate from outside the
Earth’s atmosphere called deep-space noise
- originates from the Milky Way, other
galaxies and the sun
- subdivided into two categories:
- Solar Noise generated directly from the
sun’s heat ; a quite condition is when there
is a relatively constant radiation intensity
exists; a high intensity is when a sporadic
disturbances caused by sunspots activity
and solar flare-ups that follows a cyclic
pattern that repeats every 11 years
- Cosmic Noise – distributed throughout the
galaxies wherein the sources are located
much farther away than the sun and the
intensity are much smaller; this is often
called black-body noise
Man-made Noise – noise that is produced by
mechanism such as commutators in electric
lights motors, automobile ignition systems,
ac power generating and switching
equipment, and fluorescent ; sometimes
called industrial noise
A.2. Internal Noise – generated within the device or
- primary sources:
Noise -consist of the random
fluctuation of the electric current in
an electronic conductor
- due to the shot effect caused by random
variation in the arrival of electrons or
hole at the output electrode of an
amplifying device
Transit-Time Noise- caused when the time taken
by the electron travel from the emitter
collector of a transistor becomes
comparable to the signal being amplified
- its greatest effect is at higher frequencies
“high-frequency noise
ThermalNoise- Also called White Noise because
its frequency content is spread out
throughout the usable spectrum
- Also referred to as Gaussian Noise
- Primary source is the rapid and random
motion of charge carriers inside the
resistive component when heater
Flicker Noise -noise found at low audio
frequencies in transistors.
-negligible above 500Hz
-otherwise known as “low-frequency
noise”. Modulation noise, excess noise,
1/f noise, pink noise
B. Correlated Noise
– form of internal noise that is correlated (mutually
related) to the signals and cannot be present in a
circuit unless there is a signal (no signal, no noise)
- Produced by nonlinear amplification and includes
harmonic and intermodulation distortion that are
non-linear distortion that creates unwanted
frequencies that interfere with the signal and
degrade performance
- Different characteristic
Harmonic/Amplitude Distortion – occurs when
unwanted harmonics of the signal is produced
that are integer multiple of the original signal
referred to as first harmonic/fundamental
frequency, two times the original signal
frequency is second harmonic
Intermodulation Distortion – generation of
unwanted sum and difference frequencies
produced when two or more signals mix in a
nonlinear device referred to as cross products
that is produced when harmonics as well as
fundamental frequencies mix in nonlinear
A. Partition Noise
- Occurs whenever current is divided between two
electrodes and result to random fluctuation
B. Burst Noise
- Another low frequency noise found in transistor
that appears as series of bursts at two or more
- Also called “pop-corn noise”
C. Avalanche Noise
- Large noise spikes present in the avalanche
current due to oscillation that results in the
avalanching action
III. Impulse Noise
- characterized by high-amplitude peaks of short
duration in the total noise spectrum consists of
sudden burst of irregularly shape pulses generally
between few microsecond to milliseconds
- Include
electromechanical switches (relays and solenoids),
electrical motors, appliances, electric lights, power
lines, automotive ignition system, poor quality
solder joints, and lightning
IV. Interference
- An external noise when information signal from
one source produces frequencies that falls
outside their allocated bandwidth and interfere
with information signal from other source
Relative measure of the desired signal power to
the undesired power at the particular point in a
The higher the value the better the system
For telephone services - S/N = 30dB
For video transmission – S/N = 45dB
For data transmission – S/N = 15 dB



 = 10 log

Sample Problem:
What is the signal-to-noise ratio in (dB) of an amplifier
with a signal voltage of 30mV and a noise voltage of 45
Noise Factor – used to specify how noisy a device is.
- figures or merits used to indicate how much the
signal-to-noise ratio deteriorates as a signal
passes through a circuit or series of circuits.
- ratio of the input S/N ratio to the output S/N ratio
usually determined at the standard noise
temperature 290K (17 degC)
- unit less or absolute value
Noise Figure- dB value of noise factor
  −  −  
  −  −  
 = 10 log
  −  −  
  −  −  
Note: For an ideal noiseless device: the input S/N is equal to
the output S/N thus NF=1 and F=0dB
Sample Problem:
1. What is the input equivalent signal-to-noise ratio for
a receiver with a noise figure of 15 dB and an output
signal-to-noise ratio of 60db
a. for ideal noiseless amplifier



b. for non-ideal amplifier


In terms of voltages and resistance:

 = 10 log


If Rin =Rout

 = 20 log

Sample Problem
1. For non –ideal amplifier and the following
parameters, determine
a. Input S/N ratio (dB)
b. Output S/N ratio (dB)
c. Noise factor and noise figure
Given: input signal power = 2 x 10-10 W
input noise power = 2 x 10-18 W
Power gain = 1, 000,000
Internal noise (Nd) = 6 x 10-12 W
A. The Friiss’ Formula
-use to compute the noise factor or noise temperature
of a receiver composed of a number of cascaded
2 − 1 3 − 1
 − 1
 = 1 +
1 2 1 2……
An amplifier operating over the frequency range of
455kHz to 460kHz has 200kΩ input resistor. What is
the RMS noise voltage at the input to the amplifier if
the ambient temperature is 17degC.
Ans: 4 microV
A mixer with input resistance of 300Ω has equivalent
noise resistance of 100Ω. What is the noise voltage
at the input when the effective noise bandwidth is
25khz, the source voltage is 2µV and with internal
resistance of 20Ω?
Ans: 0.281microV
A three stage amplifier in cascade has the following
parameters: Stage 1 has the gain of 5 and noise
factor 2, Stage 2 has a gain of 8 and noise factor 4,
Stage 3 has a gain of 15 and a noise factor of 6. Find
the overall noise factor of the system.
Ans: 2.725


B. With the same values:


 − 10 log 
C. Solving for the equivalent resistance

 = 1 +
1 −1 2
 = 1 +

Assignment No. 1
;  =  + 
Where : Ra = resistance of the antenna
Ri = input resistance
Sample Problem:
1. A 3-stage amplifier has stages with the
following specification: Stage 1: power
gain = 10. Noise factor=2; stage 2: power
gain=20, noise factor=4; Stage 3: power
gain= 30, noise factor=5. Calculate the
total noise factor of the system assuming
matched condition
a. Equivalent Noise Temperature (Te)
-not the actual noise temperature is not the actual
operating temperature of the device.
-for a noiseless receiver Te=0
Direction: Provide the proper solution of each problem to
obtain the given answer.
 =  (  − 1)
Where : T = environmental temperature ( ref value is 290K)
F = noise factor
Sample Problem:
Given a noise factor of 10, what is the equivalent noise
A three stage amplifier has the following gains and
noise figures (as ratios) for each of the stage : Power
gain and noise figure of 10 and 3 respectively for
stage 1, 20 and 4 for stage 2, 30 and 5 for stage 3.
Find the total gain.Ans: 38 dB
The (S+N)/N ratio of the output of an amplifier can
be measured by measuring the output voltage with
and without an input signal. When this is done for a
certain amplifier, it is found that the output is 2V
with the output signal switched on and 15mV with it
switched off. What is the (S+N)/N?Ans: 133.33
A single-stage amplifier has a 200khz bandwidth and
a voltage gain of 100 at room temperature. Assume
that the external noise is negligible and that 1 mV
signal is applied to the amplifier’s input. Calculate
the output noise voltage if the amplifier has a 5dB
noise figure and the input noise is generated by a 2kilo ohm resistor.Ans. 458 micro ohm
4. An amplifier operating over a 5-MHz bandwidth has a
100-Ω input resistance. It is operating at 27degC, has
a voltage gain of 200 and has an input signal of
5µVrms. Determine the rms output noise signal
The Noise Units
a. dBrn ( dB above reference noise)
- unit of a 144 weighting network where the old 144
telephone handset was the device used
Sample Problem

 =   − =  

15 dBa F1A weighted, equals = _________________
 =  + 
b. dBrn 3-kHz flat
- the dBrn 3-kHz flat noise reading taken when a
filter that has a flat response form 30Hz to
30KHz. It was found to be typically 1.5dB higher
than dBrnC readings for equal noise level
c. dBa (dB above adjusted noise)
- this is the unit of the F1A weighting network
using the improved F1A telephone handset. A 1
kHz test tone at -85dBm produced negligible
interference. A 3-kHz band of random (white
noise) at -82 dBm produced negligible
For a 1-kHz test tone (pure tone)

 = 10 log −11.5 = 10 log
− 10 

Defined as the optimum level of a test tone on a
channel at some point in a communication
system. It is the ratio of signal power or noise
power at a certain point to the power of the
same signal or noise at a reference point
A parameter equivalent to TLP except that TLP is
for voice circuits whereas DLP is used as a
reference for data transmissions. DLP is always
13 dB below the voice level for the same point.
Sample Problem:
 =  + 85
You are measuring noise at +3dB level point, using
the Lenkurt 601A, FA1 weighting network, and a flat
meter. The meter reading is -57dBm. This is _____
Reading a 58 dBrnC on your Western Electric 3A test
set at a + 7 test point is equal to _____ dBrnc0.
A measurement of -75 dBm, C-message weighted,
would be ____ dBrnc?
Ans: 15 dBrnc
Your Western Electric 3A test set meter reading is 23 dBm at a test point level of -8dB. This is _____
Ans: 75 dBrnc0 groom
0  = 10−11.5 W = -85dBm
For a 3-kHz band of random ( white noise)
 =  + 
Type equation here.
d. dBrnc (dB above reference noise, C-message weighted )
- This is the unit of the C-message weighting
network using a more sufficient 500 telephone
handset. It is similar to dBrn using a 1pW of
power reference
e. pWp ( picowatts, psophometrically weighted)
- this unit used in psophometric noise weighting
(European). This noise weighting assumes a
prefect receiver thus its weighting curve
corresponds to the frequency response of the
human ear only. 1.0 pWp is equivalent to an
800Hz tone at -90 dBm
Related flashcards
Create Flashcards