Noise Reduction
Techniques
INC 336 Industrial Process Measurement
Assist. Prof. Pakorn Kaewtrakulpong, Ph.D.
INC, KMUTT
Intrinsic Noise Sources
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Thermal Noise or Johnson Noise
Shot Noise
Contact Noise
Popcorn Noise
Thermal Noise
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J.B. Johnson discovered in 1928.
From thermal agitation of electrons within a resistance.
Nyquist formed rms voltage for thermal noise as
Vt = 4kTBR
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k: Boltsmann constant, 1.38 x 10-23 J/K
T: temperature, K
B: equivalent noise bandwidth, Hz
R: resistance
Equivalent Noise Bandwidth
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For any network transfer function A(f), there is
an equivalent noise bandwidth of constant
magnitude of transmission A0 and bandwidth of
B=
„
∞
1
A0
2
∫
2
A( f ) df
0
Normally greater than filtering bandwidth
Shot Noise
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Vacuum tubes and semiconductors
Associated with current loss across a barrier.
W. Schottky (1918) shows rms current as
I sh = 2qI dc B
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q: electron charge, 1.6 x 10-19 C
Idc: average current, A
B: equivalent noise bandwidth, Hz
Contact Noise
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Imperfection of contact (usually in switch and relay) Æ
changes in conductance
1/f noise
I f = KI dc
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B
f
K: constant dependent on contact material
Idc: average current, A
f: frequency, Hz
B: equivalent noise bandwidth, Hz
Popcorn Noise
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Burst Noise
Due to manufacturer imperfection of
semiconductor devices especially at the junction
due to metallic impurity
Active Device Noise
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Noise Factor
Noise Power at Output of Actual Device
F=
Noise Power at Output of Ideal Device
F ≥1
F = 1; if actual device is an ideal device
F=
( S / N )i / p
( S / N )o / p
Power of Signal
S/N =
Power of Noise
Noise Figure, NF = 10 log F
Measurement of Noise Factor
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Two methods
Single frequency method
„ Noise diode or white noise method
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For both methods,
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the noise power at output of an ideal device is
videal , no = Avt
Single Frequency Method
Rs
vs
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Device or network with
voltage gain A
With generator turned off, measure
vno = ( Avt ) 2 + (device noise)2
RL
Single Frequency Method
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Next, generator is turned on and its magnitude is
increased until output power doubles (increases
by 3dB over that previously measured), then
2vno2 = ( Avs ) 2 + vno2
Avs = vno
vno2
F=
( Avt ) 2
⎛ vs ⎞
F =⎜ ⎟
⎝ vt ⎠
2
Single Frequency Method (cont.)
At 290K, vt = 1.6 × 10−20 BRs
vs2
F=
1.6 × 10−20 BRs
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Adv.
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Any value of RL may be used.
Disadv.
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Noise bandwidth of the device must be known.
Noise Diode or White Noise Method
I dc
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Noise
diode
Blocking
capacitor
Rs
Device or network with
voltage gain A
With no diode current, the rms noise voltage is
measured.
vno = ( Avt ) 2 + (device noise)2
RL
Noise Diode or White Noise Method
(cont.)
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Next, the diode current is increased until output
noise power doubles (increases by 3dB).
The shot noise generator is
ish = 3.2 × 10−19 I dc B
vsh = ish Rs
2vno2 = ( Avsh ) 2 + vno2
vno = Avsh = Aish Rs
Noise Diode or White Noise Method
(cont.)
( ish Rs ) 2
F=
vt2
F = 20 I dc Rs
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The noise factor is a function of Rs but in this
method the other influencing factor is the direct
current through the diode.
Adv
The method is frequency independent.
„ Both Rs and Idc are easily measured.
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Noise Factor in Cascade Circuit
Rs
Gain, G1
Noise
Factor, F1
Gain, G2
Noise
Factor, F2
Gain, G3
Noise
Factor, F3
Gain, GN
Noise
Factor, FN
FN − 1
F2 − 1 F3 − 1
+
+L +
F = F1 +
G1
G1G2
G1G2 L GN −1
RL
Basic Noise Reduction Techniques
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Definition of Interference
Grounding
Shielding
Interference
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1.
2.
3.
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1.
2.
3.
Sheingold (1980) classifies interference problem into three areas:
Problems generated locally by the materials used in the signal
path (e.g., unwanted thermocouples, ohmic contact of switches
and terminals)
Problems within a subsystem (e.g., grounds)
Problems originating in the outside world [e.g., electric,
magnetic, and RF (radio-frequency) interference]
Webster (1977) classifies interference into three types of
coupling
capacitive (electric fields)
inductive (magnetic fields)
resistive (ohmic voltages in ground conductors).
Grounding
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Ground = a reference connection (reference
potential)
Earth = a connection to earth
Point of interest: interference created by
resistive coupling in ground conductors
Grounding: Analog Circuits
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Interference voltages may develop on ground lines.
Rx = nonzero resistivity of the wire (in this case, 3 mΩ per 15 cm of
No. 18 copper wire)
Parallel Distribution of Power
Grounding: Analog Circuits (cont.)
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Radial or star distribution minimizes voltage drops in both hot
and ground wires.
Circuit 3 has no difference between the parallel and radial
distributions.
Radial or Star Distribution of Power
Grounding: Analog Circuits
Better solution: connect circuit 3 (higher current) closer to the power
supply, if possible.
Circuit 3 should be connected to an extra power supply, if available, to
avoid the resistance of a long wire while sharing the same power
supply.
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If the voltage drop on the power supply path does not affect the
operation of the circuits, a combination of parallel and radial
distribution could be used (star connection for the ground wire).
Grounding: Analog-Digital Circuits
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Digital signals → large current spikes along ground paths →
interference in analog circuits
When analog and digital circuits sharing one power supply, the
ground wire each must be different, with only one common
point. → to minimize common impedances between digital and
analog circuits.
One power supply, one common point
Grounding: Analog-Digital Circuits
(cont.)
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When using separate analog and digital power supplies,
each circuit is connected to its ground and both
grounds are tied to a single point.
Magnetic Field
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Large current Æ magnetic field
Magnetic field cuts a conductor Æ current is induced in
that conductor
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AC currents
DC currents via switches, relays, electronics, and brushes
Sources
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Rf signals e.g. digital signal of processor, CATV, broadband,
or baseband data communications cables (high frequency
noise) and
the high current signal of the power output stages Æ
produces significant magnetic fields within the electronic
chassis
Protection against Magnetic Field
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Separate sensitive input signal conditioning from
the other portions of the electronics.
Put small signal analog circuitry on a separate
card, covered by a magnetic shield, from the
computer and power electronics.
If not possible, group sensitive analog
processing components together and as far away
from sources of magnetic fields as possible.
(may be covered by a small magnetic shield box)
Protection against Magnetic Field
(cont.)
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Never run ac power line in the same raceway or
conduit
Shielding
Using ferromagnetic conduit for power lines or for
low-level analog signal
„ Spray some coatings to shield against high frequency
magnetic field.
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Shielding
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Magnetic absorptive
loss depends on
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Material
Thickness
Frequency of the
magnetic field
Electric Field
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Difference in
potentials Æ electric
field (free charges,
primarily electrons in
conductors respond to
this field)
Shielding
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The shield must be tied to an infinite
source/sink of charge.
Not only must the signal high be shielded,
but the signal common must be as well.
Do not use the shield as the signal common.
Otherwise, external electric fields can float
the ground up and down.
Ground Looping
Single Grounding
Grounded Devices
Isolation
Power Supply Grounding
Power Supply Grounding (cont.)