Signals and noise

1. BIRLA INSTITUTE OF
TECHNOLOGY

2. SIGNALS AND NOISE
BY
RAVIKANT KUMAR
MCA/10040/2012

3. THE SIGNAL TO NOISE RATIO:
(S/N)
The signal to noise ratio is a representative
marker it that is used in describing the
quality of an analytical method or the
performance of an instrument.
For a dc signal, S/N = mean / standard deviation =
x/s where s is the standard deviation of the
measured signal strength and x is the mean of the
measurement -x/s is the reciprocal of the relative
standard deviation (RSD)
S/N=I/RSD

4. THE SIGNAL TO NOISE RATIO
• The purpose of this
Mathcad document is to
allow the student to gain
a familiarity with the
concepts of signal-to-
noise ratios and to
explore the advantages
of ensemble averaging
and digital filtering
analytical signals.
Simulated noisy signals
are used to guide the
student through a series
of individual exercises.

5. SIGNAL TO NOISE
ENHANCEMENT
For some measurements only minimal efforts are
required for maintaining a good signal to noise ratio
because the signals are relatively strong and the
requirements for precision and accuracy are low.
Examples:
Weight determinations made in synthesis and color
comparisons made in chemical content
determinations

6. SIGNAL SMOOTHING
ALGORITHMS
• The signal-to-noise ratio (SNR) of a signal can
be enhanced by either hardware or software
techniques. The wide use of personal
computers in chemical instrumentation and
their inherent programming flexibility make
software signal smoothing (or filtering)
techniques especially attractive. Some of the
more common signal smoothing algorithms
described below.

7. SIGNAL SMOOTHING
ALGORITHMS
• The simpler software technique
for smoothing signals
consisting of equidistant points
is the moving average. An
array of raw (noisy) data
[y1, y2, …, yN] can be converted
to a new array of smoothed
data. The "smoothed point"
(yk)s is the average of an odd
number of consecutive 2n+1
(n=1, 2, 3, ..) points of the raw
data yk-n, yk-n+1, …, yk-
1, yk, yk+1, …, yk+n-1, yk+n, i.e.

8. ENSEMBLE AVERAGE
• In ensemble average successive sets of data are
collected and summed point by point. Therefore, a
prerequisite for the application of this method is the
ability to reproduce the signal as many times as
possible starting always from the same data point,
contrary to the previous two algorithms which
operate exclusively on a single data set.

12. FILTERING
• Although amplitude and the phase relationship of input and output
signals can be used to discriminate between meaningful signals and
noise, frequency is the property most commonly used.
• White noise can be reduced by narrowing the range of measured
frequencies, environmental noise can be eliminated by selecting the
proper frequency.
• Three kinds of electronic filters are used to select the band of measured
frequencies:
Low Pass Filters
High Pass Filters
Band Pass Filters

13. LOW PASS FILTER SCHEMATIC
AND GRAPH

14. HIGH PASS FILTER SCHEMATIC
AND GRAPH

15. BAND PASS FILTER SCHEMATIC
AND GRAPHS

16. BOXCAR AVERAGING...

17. ENSEMBLE AVERAGING...

18. MOVING AVERAGE SMOOTH...

19. TYPES OF NOISE
• Chemical: This noise arises from uncontrollable variables in
the chemistry of the system such as variation in temperature,
pressure, humidity, light and chemical fumes present in the
room.
• Instrumental : Noise that arises due to the instrumentation
itself. It could come from any of the following components-
source, input transducer all signal processing elements, and
the output transducer. This noise has many types and can
arise from several sources.

20. CATEGORIES OF
INSTRUMENTAL NOISE
• Thermal or Johnson
• Shot Noise
• Flicker Noise
• Environmental Noise

21. THERMAL NOISE...
• Noise that originates from the thermally induced
motions in charge carriers is known as thermal
noise. It exists even in the absence of current flow.
• Since thermal noise is independent of the absolute
values of frequencies, it is also known as "white
noise."

22. THERMAL NOISE (CONT)
• V is the average voltage
due to thermal noise, k is
the Boltzmann
constant, T is the
absolute temperature. R
is the resistance of the
electronic device, and Af
is the bandwidth of
measurement
frequencies

23. SHOT NOISE
• Shot noise refers to the random fluctuations of
the electric current in an electrical
conductor, which are caused by the fact that the
current is carried by discrete charges
(electrons).
• The strength of this noise increases for growing
magnitude of the average current flowing
through the conductor. Shot noise is to be
distinguished from current fluctuations in
equilibrium, which happen without any applied
voltage and without any average current
flowing. These equilibrium current fluctuations
are known as Johnson-Nyquist noise.

24. SHOT NOISE (CONT)
• The sub-Poissonian
shot-noise power, S, of a
metallic resistor as a
function of its
length, L, as predicted
by theory. Indicated are
the elastic mean-free
path, l, the electron-
electron scattering
length, lee, and the
electron-phonon
scattering length lep.

25. FLICKER NOISE...
• Its magnitude is inversely proportional to
frequency of signal
• Can be significant at frequencies lower than
100 Hz
• Causes long term drift in de
amplifiers, meters, and galvanometers
• Can be reduced significantly by using wire-
wound or metallic film resistors rather than
composition type

26. FLICKER NOISE (CONT)
• Flicker Noise is associated with crystal surface
defects in semiconductors and is also found in
vacuum tubes.
• The noise power is proportional to the bias
current, and, unlike thermal and shot noise,
flicker noise decreases with frequency.

27. FLICKER NOISE (CONT)
• Change in electrical flicker noise power in hot
carrier semiconductors can be explained by
fluctuations in the intensity of impurity
scattering, which contradicts the Hooge-
Kleinpenning-Vandamme hypothesis, which
relates flicker conduction noise to lattice
scattering.
• It has been shown that such noise can be
caused by fluctuations in the effective number
of neutral scattering centers within the
semiconductor volume. This source modulates
carrier mobility, i.e., mobility fluctuations are a
secondary effect.

28. ENVIRONMENTAL NOISE
• Environmental noise is due to a composite of noises
from different sources in the environment surrounding
the instrument.
• Much environmental noise occurs because each
conductor in an instrument is potentially an antenna
capable of picking up electromagnetic radiation and
converting it to an electrical signal.
• There are numerous sources of electromagnetic
radiation in the environment including ac power
lines, radio and TV stations, gasoline engine ignition
systems, arcing switches, brushes in electrical
motors, lightening, and ionospheric disturbances.

29. ENVIRONMENTAL NOISE (CONT)

30. TYPES OF HARDWARE
• Grounding- this allows electromagnetic
radiation to be absorbed by the shield thus
avoiding noise generation in the instrument
circuit -important when using high-impedance
transducers (i.e. glass electrodes)
• Shielding- shielding consists of surrounding a
circuit, or some of the wires in a circuit with a
conducting material that is attached to earth
ground

31. GROUNDING
• The grounding of audio equipment is there for one primary purpose: to
keep you alive. If something goes horribly wrong inside one of those
devices and winds up connecting the 120 V AC from the wall to the box
(chassis) itself, and you come along and touch the front panel while
standing in a pool of water, YOU are the path to ground. This is bad.
• So, the manufacturers put a third pin on their AC cables which is
connected to the chassis on the equipment end, and the third pin in the
wall socket. This third pin in the wall socket is called the ground bus and
is connected to the electrical breaker box somewhere in the facility. All of
the ground busses connect to a primary ground point somewhere in the
building.
• This is the point at which the building makes contact with the earth
through a spike or piling called the grounding electrode. The wires which
connect these grounds together MUST be heavy-gauge (and therefore
very low impedance) in order to ensure that they have a MUCH lower
impedance than you when you and it are a parallel connection to ground.
The lower this impedance, the less current will flow through you if
something goes wrong.

32. Conductor Out From
Low Dynamic Range
(< 60 dB)
Med Dynamic Range
(60 to 80 dB)
High Dynamic Range
(> 80 dB)
Low EMI High EMI Low EMI High EMI
Ground Electrode 6 2 00 00 0000
Master Bus 10 8 6 4 0
Local Bus 14 12 12* 12* 10*
Maximum resistance for any cable (W) 0.5 0.1 0.01 0.001 0.0001
* Do not share ground conductors - run individual branch grounds. In all cases the ground conductor must not be smaller than the neutral
conductor of the panel it services.
GROUNDING (CONT)

33. DIFFERENCE AMPLIFIERS
• Any noise generated in the transducer circuit is
particularly critical because it usually appears in an
amplified form in the instrument read out. To attenuate
this type of noise, most instruments employ a difference
amplifier for the first stage of amplification.
• Common mode noise in the transducer circuit generally
appears in the phase at both the inverting and non-
inverting inputs of the amplifier and is largely subtracted
out by the circuit so that the noise at its output is
diminished substantially.

34. LOCK-IN AMPLIFIERS:
•Permit recovery of signals even
when S/N is unity or less
•Generally requires a reference
signal at same frequency and
phase (must have fixed phase
relationship) as signal to be

35. ANALOG FILTERING
• Any noise generated in the transducer circuit is
particularly critical because it usually appears in
an amplified form in the instrument read out. To
attenuate this type of noise, most instruments
employ a difference amplifier for the first stage
of amplification.
• Common mode noise in the transducer circuit
generally appears in the phase at both the
inverting and noninverting inputs of the
amplifier and is largely subtracted out by the
circuit so that the noise at its output is
diminished substantially.

36. DIGITAL FILTERING:
Moving-window boxcar method is
a kind of linear filtering where it
assumed that there is an
approximate linear relationship
among points being sampled -
more complex polynomial
relationships derive a center
point for each window

37. MODULATION
• Amplifier drift and flicker noise often interfere with the amplification of
a low frequency or dc signal and 1/f noise is often much larger than
noises that predominate at larger frequencies therefore modulators
are used to convert these to a higher frequency where 1/f is less
troublesome
• The modulated signal is amplified then filtered with a high-pass filter
to remove the amplifier 1/f noise
• The signal is then demodulated and filtered with a low-pass filter in
order to provide an amplified dc signal to the readout device
• Noise is a concern because source intensity and detector sensitivity
are low which result 'in a small electrical signal from transducer

38. REFERENCES
• http://www.cas.org
• http://www.chemcenter/org
• http://www.sciencemag.org
• http://www.chemcenter/org
• http://www.kerouac.pharm.uky.edu/asrg/wave/
wavehp.html
• http://www.anachem.umu.se/jumpstation.htm
• http://hplc.chem.vt.edu/