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 circuit
- 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
30MHz
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 sunspot s 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 mankind such as spark-producing 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 circuit
- primary sources:
- 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
Thermal Noise- 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

II. MISCELLANEOUS NOISE
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 levels
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 transients produced from 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
I.
SIGNAL-TO-NOISE RATIO (S/N Ratio)
Relative measure of the desired signal power to the undesired power at the particular point in a system
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
Formula:
𝑆
𝑁
=
𝑃 𝑠
𝑃
𝑁
;
𝑆
𝑁
𝑑𝐵 = 10 log
𝑃 𝑠
𝑃
𝑁
In terms of voltages and resistance:
𝑆
𝑁
𝑑𝐵 = 10 log
𝑉 𝑛
𝑉 𝑠
2
𝑅 𝑖𝑛
2
𝑅 𝑜𝑢𝑡
If Rin =Rout
𝑆
𝑁
𝑑𝐵 = 20 log
𝑉 𝑠
𝑉 𝑛
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 microV?
II.
NOISE FACTOR (NF)/NOISE RATIO (NR)
AND NOISE FIGURE (F)
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
Formula:
𝐹 = 𝑖𝑛𝑝𝑢𝑡 𝑠𝑖𝑔𝑛𝑎𝑙 − 𝑡𝑜 − 𝑛𝑜𝑖𝑠𝑒 𝑟𝑎𝑡𝑖𝑜 𝑜𝑢𝑡𝑝𝑢𝑡 𝑠𝑖𝑔𝑛𝑎𝑙 − 𝑡𝑜 − 𝑛𝑜𝑖𝑠𝑒 𝑟𝑎𝑡𝑖𝑜
𝑁𝐹 = 10 log 𝑖𝑛𝑝𝑢𝑡 𝑠𝑖𝑔𝑛𝑎𝑙 − 𝑡𝑜 − 𝑛𝑜𝑖𝑠𝑒 𝑟𝑎𝑡𝑖𝑜 𝑜𝑢𝑡𝑝𝑢𝑡 𝑠𝑖𝑔𝑛𝑎𝑙 − 𝑡𝑜 − 𝑛𝑜𝑖𝑠𝑒 𝑟𝑎𝑡𝑖𝑜
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
III.
NOISE FACTOR IN TERMS OF Gain a. for ideal noiseless amplifier
𝑆
𝑁 𝑜𝑢𝑡 𝑜𝑢𝑡
=
𝐴
𝐴 𝑝 𝑝
𝑆
𝑁 𝑖 𝑖
=
𝑆 𝑖
𝑁 𝑖 b. for non-ideal amplifier
𝑆
𝑁 𝑜𝑢𝑡 𝑜𝑢𝑡
=
𝐴 𝑝
𝐴
𝑁 𝑖 𝑝
𝑆 𝑖
+ 𝑁 𝑑
=
𝑁 𝑖
𝑆 𝑖
+ 𝑁 𝑑
𝐴 𝑝
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 (N d
) = 6 x 10
-12
W

IV.
THE OVERALL S/N OF TANDEM CIRCUIT
A. The Friiss’ Formula
-use to compute the noise factor or noise temperature of a receiver composed of a number of cascaded stages
𝐹𝑇 = 𝐹
1
+
𝐹
2
− 1
𝐴
1
+
𝐹
3
− 1
𝐴
1
𝐴
2
+
𝐴
1
𝐹
𝑁
− 1
𝐴
2……
𝐴
𝑁
𝑆 𝑜𝑢𝑡
𝑁 𝑜𝑢𝑡
=
𝑆 𝑖
𝑁 𝑖
+ 𝑁𝐹
𝑇
B. With the same values:
𝑆
𝑁
𝑑𝐵 𝑜𝑣𝑒𝑟𝑎𝑙𝑙 =
𝑆
𝑁
𝑑𝐵 − 10 log 𝑀
C. Solving for the equivalent resistance
𝑅𝑒𝑞 = 𝑅1 +
𝑅2
𝐴
1
2
+
𝐴
1
2
𝑅𝑛
𝐴 𝑛−1
2
𝑁𝐹 = 1 +
𝑅 𝑒𝑞
𝑅 𝑎
′
; 𝑅 𝑒𝑞
′ = 𝑅 𝑒𝑞
+ 𝑅
Where : R a
= resistance of the antenna 𝑖
R i
= 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
V.
NOISE TEMPERATURE MEASUREMENT 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
Formula:
𝑇𝑒 = 𝑇 ( 𝐹 − 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 temperature?
SEATWORK
1.
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
2.
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
3.
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
Assignment No. 1
Direction: Provide the proper solution of each problem to obtain the given answer.
1.
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
2.
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
3.
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 rer

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 𝒅𝑩𝒎 = 𝟏𝟎 𝐥𝐨𝐠
𝑵 𝑾
𝟏𝟎 −𝟏𝟐 𝑾
= 𝟏𝟎 𝐥𝐨𝐠
𝑵
𝟏𝒎𝑾
− 𝟏𝟎𝒍𝒐𝒈
𝟏𝟎 −𝟏𝟐
𝟏𝒎𝑾 𝒅𝑩𝒎 = 𝒅𝑩𝒎 + 𝟗𝟎 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 interference.
For a 1-kHz test tone (pure tone) 𝑑𝐵𝑎 = 10 log
𝑁(𝑊)
10 −11.5
𝑊
= 10 log
𝑁
1𝑚𝑊
− 10 𝑙𝑜𝑔
10 −11.5
1𝑚𝑊 𝑑𝐵𝑎 = 𝑑𝐵𝑚 + 85
0 𝑑𝐵𝑎 = 10 −11.5
W = -85dBm
For a 3-kHz band of random ( white noise) 𝐝𝐁𝐚 = 𝐝𝐁𝐦 + 𝟖𝟐
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
Sample Problem
1.
15 dBa F1A weighted, equals = _________________
TRANSMISSION LEVEL POINT
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
DATA LEVEL 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:
1.
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 _____ dBa0.
2.
Reading a 58 dBrnC on your Western Electric 3A test set at a + 7 test point is equal to _____ dBrnc0.
Ass.
1.
A measurement of -75 dBm, C-message weighted, would be ____ dBrnc?
Ans: 15 dBrnc
2.
Your Western Electric 3A test set meter reading is -
23 dBm at a test point level of -8dB. This is _____ dBrnc0
Ans: 75 dBrnc0 groom