NOISE AND NOISE REDUCTION
1. NOISE SOURCES
• Disturbance tends to obscure a desired signal
• Internal generated (intrinsic) or external pick-up (extrinsic)
• Interference is noise (electrical, mechanical, acoustic, electrochemical, . . .)
• Distinguish by frequency spectrum and amplitude distribution:
1. White noise: flat frequency distribution
2. Pink noise (contact noise): inversely proportional to frequency
(1/f)
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2. INTRINSIC NOISE
• Present in all electronic measuring systems
• Set a lower limit on measurement
a. Thermal noise:
• Produced by the motion of free electrons due to temperature in a
resistance
• Noise power is constant per unit of bandwidth across the usable
electronic spectrum (white noise).
• Noise Power:
Pn = kTB
K= Boltzmann’s constant, 1.38x10-23 J/K
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• Noise Voltage:
V
( n )2
Pn = 2
R
then Vn = 4kTBR
• Noise Current:
in =
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4kTB
R
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b. Shot noise
• Caused by the flow of current across a potential barrier (ex. PN
junction)
• Same power per unit of bandwidth (white noise)
• Given in term of current
ish = 2eI DCB
ish=rms shot noise current
e=electron charge, 1.6x10-19C
• Noise power of a transistor
Pn = Gk ( Te + Tin )B
G=transistor gain
Te=effective noise temperature, K
Tin=effective input noise temperature, K
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c. Contact noise
• Generated mechanisms
• Causes are not well understood
1. Depends on quantity of current and types of resistors
2. Contact (flicker, pink) noise found in transistors
d. Combining noise sources
• Apply superposition theorem to find total noise
• Series noise (rms):
Vt = V12 + V22 + V32 +
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+ Vn2
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3. EXTRINSIC NOISE
• Noise from another circuit on the same PCB (cross talk) or from the
environment
• Need to be suppressed to reduce interference
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Conductive coupling:
• Requires 2 or more conductive paths to the noise source.
• Noise induced into the circuit when ground reference points at different
potentials (signal return and ground return in different wires).
• Hypothetical model:
Others:
EMI: Electric field interference, Magnetic field interference
Power line interference
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4. NOISE MEASURING
• Taking bandwidth and type of noise into account
• Noise energy increases with the bandwidth of the measurement system.
• Noise measurement and calculation depend on noise type and spectrum of the
noise (white noise or flicker noise).
a. SNR
• Ratio of signal compared to noise in power or voltage (S/N=dB)
• Noise floor: level of noise stay fixed below signal level.
• Improve S/N:
1. Increase signal strength
2. Decrease noise level
3. Limit noise bandwidth
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b. Noise factor
• Specify the added contribution of an amplifier to the S/N due to noise
generated within the amplifier itself.
• Most contributed in the 1st stage of the amplifier (high amplification).
• Measure the quality of the amplifier using in the circuit.
• Noise factor F (dimensionless)
F=
Si / Ni
So / N o
S N
or F = i x o
So N i
F=
No
Ap N i
Ni=input noise power deliver to the amp.
No=output noise power delivered by the amp.
Ap=amp. power gain
FdB = 10 log F
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L2
D1
M6
M5
Vin
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M1
M2
M3
M4
L1
Frequency
tuning
D2
Vout
M9
M10
M7
M8
Quality
factor
tuning
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c. Equivalent noise temperature
• Alternative to noise factor of an amp.
• The added noise as an increase in temperature (noise increasing).
• This temperature is equivalent noise temperature of the amplifier:
Teq = 290(F − 1)
• The amplifier is equivalent to the source resistance delivering the
noise at that temperature
`
Vn =
4kTBR
• Equipment to measure noise:
1. True rms meter
2. Oscilloscope
3. Spectrum analyzer
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5. NOISE REDUCTION
Unwanted alteration to the signal:
- Drift (PS, temp.)
- Internal generated random noise (thermal, shot noise)
- External noise (capacitive, inductive pick up)
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1. Screening techniques
• Screening reduces capacitive and inductive pick-up
• Enclose cables, amplifiers, etc.., in metal which is earthed
• Voltages picked up are short-circuited to earth.
• When earth connecting is made more than one, earth loops can create
large noise voltages.
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• An earth loop:
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2. Suppressors
• Suppress electrical nose caused by sparking (motors, relays,..).
• Noise transferred by electromagnetic radiation or along the power supply
cable (noise source and equipment sharing the same power supply).
• Capacitors, Chokes or Varistors. Applied as close to the noise source as
possible.
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3. Capacitors
• Reservoirs to smooth out voltage variations,
• Conduct high frequency transient to earth.
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4. Chokes (beads)
• Smooth out rapid variations of current,
• Put in series with power supply leads to act as barriers to noise.
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5. Varistors
• Connected across power supply leads to short-circuit transient highvoltage “spike”.
• Semiconductors (high resistance at voltages up to a stated maximum)
• Excess maximum voltage, resistance drops to a low value.
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6. Filters:
LPF
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BPF
HPF
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7. Grounds, Shielding and connecting wires
• Low-level signals of <100mV are more susceptible to errors
induced by noise.
• Three simple rules:
1. Keep the connecting wires as short as possible;
2. Keep signal wires away from noise sources;
3. Use a wire shield and proper ground.
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a. Ground and ground loops
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b. Shields
• Long wires act as antennae and will pick up stray signals
• A piece of metal foil or wire braid wrapped around the signal wires
and connected to ground.
• A shield ground loop is prevented by grounding the shield at only
one point.
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c. Connecting Wires
• Single cable is efficient only for short connections and involving only a
few wires.
• Flat cable: multiple conductors arranged in parallel strips, (groups of 4, 9,
or 25).
1. Short connections between adjacent electrical boards
2. Applications with large signals (1V or more).
• Twisted pairs widely used to interconnect transducers and equipment.
1. The intertwining of the wires offers some immunity to noise.
2. Cables containing several twisted pairs are available.
3. Shielded twisted pair wraps the twisted pair within the metallic foil
shield. Shielded twisted pairs are one of the best choices for most
applications.
• Coaxial cable is the choice for high-frequency signals.
• Optical cable transmits low-level signal over long distances. Optical cable
is virtually noise free from magnetic fields and harsh environments.
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