Various Noise and Noise

Reduction Techniques

Prepared by: Pritesh B.Gohil (1005)

Guided By: Sanjeev Gupta

Pritesh.gohil@ymail.com

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