Various noise sources and its reduction techniques
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Transcript of Various noise sources and its reduction techniques
Various Noise and Noise
Reduction Techniques
Prepared by: Pritesh B.Gohil (1005)
Guided By: Sanjeev Gupta
Introduction
• Biggest environmental factor
• Overall performance of circuit is dependent on noisecharacteristics.
• Noise can be random, repetitive or occurringcontinuous.
• May affect current or voltage and may occur at anyfrequency from DC to very high frequencies.
• Fetches extra amount of attention in instrumentationand measurements system.
• Overall accuracy highly affected.
Noise Sources
• Noise problem can generate either in the outer world
or it can be caused and communicated within the
system or it can be the local problem with particular
circuit or connection.
• Classified in 3 category
1. Internal Noise
2. External Noise
3. Local Noise
Internal Noise Sources
• Internal to the system and generated internally into components.
1. White Noise:
It is uniform noise over entire frequency spectrum and has Gaussianamplitude distribution. There are two mechanisms causing white noise.
a. Shot Noise (Shottky Noise):
It is noise current caused by fact current flow is not a continuous processbut is due to the movement of individual electrons , which are discretecharged particles.
The RMS value of shot noise over a chosen frequency range is given by
s= 2qIbf ; Ib,q
b. Thermal Noise (Johnson’s Noise):
Due to random motion of thermally charged particles in any resistive path.
Independent of current flow.
The charge will occur in random manner at the two ends giving rise to anoise voltage, which increases with temperature.
RMS value over chosen bandwidth is
Vt= 4KTRF
2. Flicker Noise (1/F noise or Pink noise):
The flicker noise occurs when current in the circuit is controlled bylocalized barrier, whereas shot noise is due to random way electronssurmount the barrier. Flicker noise is due to fluctuations in theeffectiveness of the barrier.
Superimposed on shot noise at very low range of frequency.
Increase when the frequency decreases.
RMS value given by
V = K ln (fc / f1)
Fc is corner frequency which separates the dominant white andpink noise region
Noise in Time Domain
6
white noise
1/f noise
3. Popcorn Noise (Burst Noise):
Caused by imperfection in the semiconductor production process or
materials.
Ex. Low frequency burst bias current change is produced by faults in the
surface of processed wafers. This is normally called popcorn noise.
4. Barkhausen Noise:
Introduced in system which include magnetic sensor.
This occur due to the finite size of domains in the ferromagnetic material
and the random manner in which directions of orientation of such
domains are changed during magnetization.
5. Contact Noise:
Because of breakdown of contact in
current path.
Because of adjacent material like
carbon resistors.
Reduced by improved design of component.
External Noise Sources
• Effect of atmosphere or electrical system.
1. Switching Current & Voltage:
High current load, which is switched ON/OFF.
SMPS generate signal noise between 1KHz to 10KHz
2. Power lines interference:
In a high current flowing line parallel to the signal line
Fluorescent lighting system produces noise at 50Hz, 100 Hz or
120 Hz depending upon the local main frequency.
3. Sparking and radiation:
Anything which cause spark and arching
Ex. Relays, switch arching, motor with commutators.
Noise in range 30 MHz to GHz gets radiated in form of
electromagnetic wave. The varying electric and magnetic field
produce noise in the other system. Ex. TV, Radio, High freq.
Carrying line.
Can also radiate at 100KHz. Ex open conductor and unshielded
lines.
4. Environmental and Atmospheric Noise:
Spurious radio waves causes by lightening discharge in
thunderstorms. (in form of impulse and spread over all radio
spectrum, less severe in freq above 30MHz.)
Solar noise
LOCAL NOISE SOUIRCES
• Mainly associated with interconnection of circuits, interfacing of
boards, transmission cables and power supply effects.
1. Coupling Noise:
a. Capacitive coupling:
Associated with varying electrostatic field, or difference in
potential, between two conductor coupled by stray capacitance.
Ex. 2 pin connector.
b. Magnetic Coupling:
Example of mutual inductance
2. Crosstalk:
When one signal line pickup the signal from the signal line
running parallel to it.
Because of E-field coupling.
3. Power Supply Induced Noise:
Caused by use of transformer
4. Interfacing and Cable Noise:
Generated when
Cable or the interconnection wires build up a potential along the
length and also get exposed to the external fields.
Improper ground loop
Noise Reduction Techniques
1. Internal noise reduction techniques:
Can not removed but preventive measurement can be
taken
Noise is uniformly distributed to entire frequency range.
So b/w of analog section of system should be limited that
really needed
Source resistor and bias current should be as small as other
consideration permits.
For VHF range operation RF transistors should be used
which are remarkably low noise.
2. External noise reduction techniques:
a). Faraday shield:
Earthed shield put between the source and the victim of
noise. Restricts E-field propagation.
b). ESD protection:
Zener diode for excess current protection
Connecting two zener back to back.
c). Enclosure Shield:
Power line and high voltage should either be kept away
from signal line or both should be properly shielded.
Plastic enclosure shield is best to protect from radiation
noise.
Ground and return line in a system
3. Local noise reduction techniques:
• Many local induced noise can be solved by proper
grounding, shielding and filtering.
a). Grounding
• Fig. illustrate the difference between ground and return line.
• Its not power or signal current return path.
• Should be only one connection between return line and
ground.
Grounding in Analog circuits
Grounding in mixed signal system
•Analog and digital circuit in mixed signal system use
different power supply.
•Digital signal are fast changing waveform
•If common ground then, fast changing digital signal will
modulate the analog return current.
•Interference and creation of noise voltage.
b). Shielding
• Metal box around the conductor
• To reduce electric field interference.
Capacitive and inductive coupling
For effective shielding, proper grounding techniques should be
adopted.
c). Filtering
• Radio frequency interference
Affect the performance of the system
RFI signal coupled into amplifier through,
• Filtering power supply
•Filtering amplifier input
•Filtering amplifier output
References
Slideshare.com
Noise source and reduction techniques by Raman K. Attri
PC Based Instrumentation By N. Mathivanan
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YOU