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Transcript of Electronic Instrumentation & Control Systems
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ELECTRONIC INSTRUMENTATION &CONTROL SYSTEMS
(WLE-306)
Presented by:
Mr. Shahnawaz Uddin
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Unit-4
MISCELLANEOUS INSTRUMENTS
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Amplitude Distortion Distortionis the alteration of the original shape (or other characteristic)
of a signal, waveform, or other form of information
Distortion is usually unwanted and in practice, many methods are
employed to minimize it
In signal processing, a noise-free system can be characterized by a
transfer function, such that the output y(t) can be written as a function of
the input x(t) as: y(t) = F(x(t))
When the transfer function comprises only a gain (A) and delay (T), thenthe output is undistorted
Distortion occurs when the transfer function F is more complicated than
this, e.g., if F is a linear function of frequency (for instance a filter whose
gain and/or delay varies with frequency), then the signal will experiencelinear distortion
The linear distortion will not change the shape of a single sinuosoid, but
will usually change the shape of a multi-tone signal
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Amplitude distortionis distortion occurring in a system, subsystem,
or device when the output amplitude is not a linear function of the
input amplitude
For example, in case of a transistor, output is a linear function of
input only for a fixed portion of the transfer characteristic, i.e., Ic = Ib
When output is not in this portion, two forms of amplitude distortion
might arise:
(i) Harmonic Distortion, & (ii) Intermodulation Distortion(i) Harmonic distortion:
The creation of harmonics of the fundamental frequency of a
sinusoidal wave to a system
(ii) Intermodulation distortion: This form of distortion occurs when two sinusoidal waves of
frequencies f1and f2are present at the input, resulting in the creation
of several other frequency components, whose frequencies include
(f1+ f2), (f1- f2), (2f1- f2), (2f2f1), and in general (mf1 nf2) for
integer m and n
Amplitude Distortion (-contd.)
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Generally the strength of the unwanted output falls rapidly as m and n
increase
Amplitude distortion is measured with the system operating understeady-state conditions with a sinusoidal input signal
When other frequencies are present, the term "ampl i tude"refers to
the amplitude of fundamental frequency component only
It can be shown mathematically (Fourier Series Analysis) that any
complex waveform is made up of a fundamental frequency (f0)component and its harmonics (2f0, 3f0, 4f0, )
It is often desired to measure the amplitude of fundamental or each
harmonic individually, and can be performed by instruments called
wave analyzers Wave analyzers are also referred to as frequency selective
voltmeters, carrier frequency voltmeters, orselective level
voltmeters
Some wave analyzers have the facility of automatic frequency
control, in which the tuning automatically locks to the signal
Amplitude Distortion (-contd.)
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This makes it possible to measure the amplitude of signals that are
drifting in frequency by amounts that would carry them outside the
widest pass-band available
Harmonic distortion analyzers measure the total harmonic content in
the waveforms
Harmonic distortion can be quantitatively measured very accurately with
harmonic distortion analyzer, generally called a distortion analyzer
The total harmonic distortion (THD) is given by
where, D2, D3, D4, represent 2nd, 3rd, 4th, harmonics
The harmonic distortion analyzer measures the total harmonic distortion
without individually the amplitude & frequency of each component
These analyzers can be used along with a frequency generator or a source
of white (or pseudo-random) noise to measure the frequency response of
amplifiers, filters, etc.
Amplitude Distortion (-contd.)
...DDDD 242
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2
2
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Fig. (4.1) Graph of a Waveform and the distorted versions of the same waveform
Amplitude Distortion (-contd.)
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Basic Wave Analyzer
A basic wave analyzer is shown in fig. (9.1a), and consists of a
primary detector (a simple LC circuit)
This LC circuit is adjusted for resonance at the frequency of the
particular harmonic component to be measured
The intermediate stage is a full wave rectifier, to obtain the
average value of the input signal
The indicating device is a simple dc voltmeter that is calibrated
to read the peak value of the sinusoidal input voltage
Since, the LC circuit is tuned to a single frequency, it passes
only the frequency to which it is tuned and rejects all other
frequencies
A number of tuned filters, connected to the indicating device
through a selector switch, would be required for a Wave
Analyzer
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Basic Wave Analyzer (-contd.)
Fig. (9.1a) Basic Wave Analyzer
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Basic Wave Analyzer (-contd.)
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Frequency Selective Wave Analyzer
Wave analyzer (fig. 9.1b) consists of a very narrow pass-band
filter section which can be tuned to a particular frequency within
the audible frequency range (20 Hz -20 kHz) The complex wave to be analyzed is passed through an adjustable
attenuator, which serves as a range multiplier and permits a large
range of signal amplitudes to be analyzed without loading the
amplifier
The driver amplifier applies the attenuated input signal to a high-Q
active filter (a low pass filter, which allows the selected frequency
to pass and reject all others)
The magnitude of this selected frequency is indicated by the meter
and the filter section identifies the frequency of the component
The filter circuit consists of a cascaded RC resonant circuits and
amplifiers
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The capacitors are varied for range changing (i.e., coarse tuning)
& the potentiometer is used to change the frequency within the
selected pass-band (i.e., fine tuning), hence, this wave analyzer is
also called a frequency selective voltmeter
The selected signal output from the final amplifier stage is applied
to the meter circuit & to an un-tuned buffer amplifier
The main function of the buffer amplifier is to drive output devices,such as recorders or electronics counters
The meter has several voltage ranges as well as decibel scales
marked on it
It is driven by an average reading rectifier type detector The bandwidth of the instrument is very narrow, typically about 1%
of the selective band given in response characteristics (fig. 9.2)
Frequency Selective Wave Analyzer (-contd.)
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Frequency Selective Wave Analyzer (-contd.)
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Frequency Selective Wave Analyzer (-contd.)
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H t d W A l
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Heterodyne Wave Analyzer
The wave analyzers are useful for measurement in the audio
frequency range only, i.e., for measurements in the RF range and
above (MHz range), an ordinary wave analyzer cant be used
Hence, special types of wave analyzers working on the principle of
heterodyning (mixing) are used, which are known as Heterodyne
wave analyzers
In Heterodyne wave analyzer, the input signal to be analyzed is
heterodyned with the signal from the internal tunable local oscillatorin the mixer stage to produce a higher IF frequency
By tuning the local oscillator frequency, various signal frequency
components can be shifted within the pass-band of the IF amplifier
The output of the IF amplifier is rectified and applied to the metercircuit
An instrument that involves the principle of heterodyning is the
Heterodyning tuned voltmeter (shown in fig. 9.3)
The input signal is heterodyned to the known IF by means of a
tunable local oscillator 15
H t d W A l ( td )
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The amplitude of the unknown component is indicated by the
VTVM (Vacuum Tube Voltmeter) or output meter
The frequency of the component is identified by the local oscillatorfrequency, i.e., the local oscillator frequency is varied so that all
the components can be identified
The fixed frequency amplifier is a multistage amplifier, which can
be designed conveniently because of its frequency characteristics With the use of a suitable attenuator, a wide range of voltage
amplitudes can be covered
Their disadvantage is the occurrence of spurious cross-modulation
products, setting a lower limit to the amplitude that can bemeasured
Heterodyne Wave Analyzer (-contd.)
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Two types of frequency-selective amplifiers find use in Heterodyne
wave analyzers
The first type employs a crystal filter (band-pass arrangement),having a center frequency of 50 kHz; another type uses a resonant
circuit in which the effective Q has been made high and is controlled
by negative feedback
When a knowledge of the individual amplitudes of the component
frequency is desired, a heterodyne wave analyzer is used
A modified heterodyne wave analyzer is shown in fig. 9.4
In this analyzer, the attenuator provides the required input signal for
heterodyning in the first mixer stage, with the signal from a local
oscillator having a frequency of 30-48 MHz The first mixer stage produces an output which is the difference of
the local oscillator frequency and the input signal, to produce an IF
signal of 30 MHz
Heterodyne Wave Analyzer (-contd.)
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H t d W A l ( td )
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This IF frequency is uniformly amplified by the IF amplifier
This amplified IF signal is fed to the second mixer stage, where it
is again heterodyned to produce a difference frequency or IF ofzero frequency
The selected component is then passed to the meter amplifier and
detector circuit through an active filter having a controlled band-
width The meter detector output can then be read off on a db-calibrated
scale, or may be applied to a secondary device such as a recorder
This wave analyzer is operated in the RF range of 10 kHz -18 MHz
with 18 overlapping bands selected by the frequency range controlof the local oscillator
The bandwidth, which is controlled by the active filter, can be
selected at 200 Hz, 1 kHz, and 3 kHz
Heterodyne Wave Analyzer (-contd.)
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H t d W A l ( td )
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Heterodyne Wave Analyzer (-contd.)
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Heterod ne Wa e Anal er ( contd )
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Heterodyne Wave Analyzer (-contd.)
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H i Di t ti A l
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Harmonic Distortion AnalyzerFundamental Suppression Type:
Distortion analyzer measures the total harmonic power present in the
test wave rather than the distortion caused by each component The simplest method to suppress the fundamental frequency by
means of a high pass filter whose cut-off frequency is a little above
the fundamental frequency
Thus, the high pass filter allows only the harmonics to pass and the
total harmonic distortion (THD) can then be measured
The most commonly used harmonic distortion analyzers based on
fundamental suppression are as follow:
(i) Employing a Resonance Bridge, (ii) Wien's Bridge Method
(iii) Bridged T -Network Method
(i) Employing a Resonance Bridge:
The bridge, shown in fig. (9.5), is balanced for the fundamental
frequency, i.e., L & C are tuned to the fundamental frequency21
Harmonic Distortion Analyzer ( contd )
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The bridge is unbalanced for theharmonics, i.e., only harmonic
power will be available at the output terminal and can be measured
If the fundamental frequency is changed, the bridge must bebalanced again by varying L & C
If L & C are fixed components, then this method is suitable only when
the test wave has a fixed frequency
Indicators can be thermocouples or square law VTVMs (VacuumTube Volte Meters), which indicate the rms value of all harmonics
When a continuous adjustment of the fundamental frequency is
desired, a Wien bridge arrangement is used (shown in fig. 9.6)
(ii) Wien's Bridge Method:
The bridge is balanced for the fundamental frequency, therefore,
fundamental energy is dissipated in the bridge circuit elements
Only the harmonic components reach the output terminals
Harmonic Distortion Analyzer (-contd.)
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Harmonic Distortion Analyzer ( contd )
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The harmonic distortion output can then be measured witha meter
For balance at the fundamental frequency:
C1= C
2= C, R
1= R
2= R, R
3= 2R
4
(iii) Bridged T -Network Method:
As shown in fig. (9.7), L & C's are tuned to the fundamental
frequency, and Ris adjusted to bypass fundamental frequency
The tank circuit being tuned to the fundamental frequency, thefundamental energy will circulate in the tank and is bypassed by the
resistance
Only harmonic components will reach the output terminals and the
distorted output can be measured by the meter
The Q of the resonant circuit must be at least 3-5
One method of using a bridge T-network is given in fig. (9.8)
The switch S is first connected to point A so that the attenuator is
excluded and the bridge T-network is adjusted for full suppression of
the fundamental frequency, i.e., minimum output
Harmonic Distortion Analyzer (-contd.)
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Harmonic Distortion Analyzer ( contd )
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Minimum output indicates that the bridged T-network is tuned to the
fundamental frequency & fundamental frequency is fully suppressed
The switch is next connected to terminal B, i.e. the bridged T-network
is excluded
Attenuation is adjusted until the same reading is obtained on the
meter
The attenuator reading indicates the total rms distortion
Note:
Distortion measurement can also be obtained by means of a wave
analyzer; knowing the amplitude & frequency of each component; the
harmonic distortion can be calculated
However, distortion meters based on fundamental suppression aresimpler to design and less expensive than wave analyzers
The disadvantage with the harmonic distortion analyzers is that they
give only the total distortion and not the amplitude of individual
distortion components
Harmonic Distortion Analyzer (-contd.)
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Harmonic Distortion Analyzer ( contd )
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Harmonic Distortion Analyzer (-contd.)
Fig. (9.5) Resonance Bridge
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Harmonic Distortion Analyzer ( contd )
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Harmonic Distortion Analyzer (-contd.)
Fig. (9.6) Wiens Bridge Method 26
Harmonic Distortion Analyzer ( contd )
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Harmonic Distortion Analyzer (-contd.)
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Harmonic Distortion Analyzer ( contd )
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Harmonic Distortion Analyzer (-contd.)
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Spectrum Analyzer
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Spectrum Analyzer The most common way of observing signals is to display them on an
oscilloscope, with time on the x-axis (i.e., amplitude of the signal
versus time)
It is also useful to display signals in the frequency domain; theinstrument providing this frequency domain view is the spectrum
analyzer
A spectrum analyzer provides a calibrated graphical display on its
CRT, with frequency on the horizontal axis and amplitude (voltage)on the vertical axis
Displayed as vertical lines against these coordinates are sinusoidal
components of which the input signal is composed
The height represents the absolute magnitude, and the horizontal
location represents the frequency
These instruments provide a display of the frequency spectrum over
given frequency band
Spectrum analyzers use either (i) a parallel filter bank, or(ii) a
swept frequency technique 29
Spectrum Analyzer ( contd )
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(i) Spectrum Analyzer using Parallel Filter Bank:
In a parallel filter bank analyzer, the frequency range is covered by a
series of filters whose central frequencies and bandwidths are so
selected that they overlap each other (as shown in Fig. 9.9a)
Typically, an audio analyzer will have 32 of these filters, each covering
one third of an octave
For wide band narrow resolution analysis, particularly at RF or
microwave signals, the swept technique is preferred
(ii) Spectrum Analyzer using Swept Receiver Design:
As shown in fig. (9.9b), the sawtooth generator provides the sawtooth
voltage which drives the horizontal axis element of the scope and this
sawtooth voltage is frequency controlled element of the voltage tunedoscillator
As the oscillator sweeps from fminto fmaxof its frequency band at a linear
recurring rate, it beats with the frequency component of the input signal
& produces an IF, whenever a frequency component is met during its
sweep
Spectrum Analyzer (-contd.)
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Spectrum Analyzer (-contd )
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The IF corresponding to the frequency component is amplified and
detected if necessary, and then applied to the vertical plates of the
CRO, producing a display of amplitude versus frequency
One of the principal applications of spectrum analyzers has been in
the study of the RF spectrum produced in microwave instruments
In a microwave instrument, the horizontal axis can display a wide
range (2-3 GHz) for a broad survey and a narrow range (30 kHz) as
well for a highly magnified view of any small portion of the spectrum
Signals at microwave frequency separated by only a few kHz can be
seen individually
The basic block diagram of an RF spectrum analyzer (fig. 9.13)
covers the range 500 kHz to 1 GHz, which is representative of asuper-heterodyne type
The input signal is fed into a mixer which is driven by a local oscillator
(which is linearly tunable electrically over the range 2-3 GHz)
Spectrum Analyzer (-contd.)
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Spectrum Analyzer (-contd )
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The mixer provides two signals at its output that are proportional in
amplitude to the input signal but of frequencies which are the sum
and difference of the input signal & local oscillator frequency
The IF amplifier is tuned to a narrow band around 2 GHz, since the
local oscillator is tuned over the range of 2-3 GHz, only the inputs
that are separated from the local oscillator frequency by 2 GHz will be
converted to IF frequency band, pass through the IF frequency
amplifier, get rectified & produce a vertical deflection on the CRT From this, it is observed that as the sawtooth signal sweeps, the local
oscillator also sweeps linearly from 2-3 GHz
The tuning of the spectrum analyzer is a swept receiver, which
sweeps linearly from 0 to 1 GHz
The sawtooth scanning signal is also applied to the horizontal plates
of the CRT to form the frequency axis
Spectrum analyzers are widely used in radars, oceanography, and
bio-medical fields
Spectrum Analyzer (-contd.)
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Spectrum Analyzer (-contd )
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Spectrum Analyzer (-contd.)
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Basic Spectrum Analyzer Using Swept Receiver Design
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Basic Spectrum Analyzer Using Swept Receiver Design
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Basic Spectrum Analyzer Using Swept Receiver Design
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Basic Spectrum Analyzer Using Swept Receiver Design
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Basic Spectrum Analyzer Using Swept Receiver Design
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Basic Spectrum Analyzer Using Swept Receiver Design
Fig. (9.12) Test Waveform as seen on X-axis (time) & Z-axis (frequency)
Fig. (9.13) RF Spectrum Analyzer 36
Q-METER
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Q METER
The overall efficiency of coils and capacitors intended for RF
applications is best evaluated using the Q-value
The Q-meter is an instrument designed to measure some electrical
properties of coils and capacitors
The principle of Q-meter is based on series resonance; the voltage
drop across the coil or capacitor is Q-times the applied voltage
(where Q is the ratio of reactance to resistance, XL/R)
If a fixed voltage is applied to the circuit, a voltmeter across thecapacitor can be calibrated to read Q directly
At resonance XL= XCand EL= I XL, EC= I XC, E = IR
Therefore,
From the above equation, if E is kept constant, the voltage across the
capacitor can be measured by a voltmeter calibrated to read directly
in terms of Q
E
E
R
X
R
XQ CCL
37
Q-METER (-contd )
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A practical Q-meter circuit is shown in fig.(10.7)
The wide range oscillator, with frequency range from 50 kHz to 50 MHz,
delivers a current to the shunt resistance (Rsh) having a value of 0.02
Rsh introduces almost no resistance into the tank circuit and therefore,
represents a voltage source of magnitude e with a small internal
resistance
The voltage across the capacitor is measured by an electronic voltmeter
corresponding to ECand calibrated directly to read Q The circuit is tuned to resonance by varying C until the electronic
voltmeter reads the maximum value
The resonance output voltage E, corresponding to EC , is E = Q x e
That is, Q = E/e
Since, e is known, the electronic voltmeter can be calibrated to read Q
directly
The inductance of the coil can be determined by connecting it to the test
terminals of the instrument
Q METER ( contd.)
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Q-METER (-contd.)
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The circuit is tuned to resonance by varying either the capacitance or the
oscillator frequency
If the capacitance is varied, the oscillator frequency is set to a given
frequency & resonance is obtained If the capacitance is preset to a desired value, the oscillator frequency is
varied until resonance occurs
The inductance of the coil can be calculated from known values of the
resonant frequency & resonating capacitor (C)
The Q indicated is not the actual Q, because the losses of the resonating
capacitor, voltmeter and inserted resistance are all included in the
measuring circuit The actual Q of the measured coil is somewhat greater than the
indicated Q
This difference is negligible except where the resistance of the coil is
relatively small compared to the inserted resistance Rsh
Q METER ( contd.)
C)f2(
1Lor,
LC2
1f,XX
2CL
39
Q-METER (-contd.)
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Q METER ( contd.)
Fig. (10.7) Circuit Diagram of a Q-meter
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Q-METER (-contd.)
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Factors Causing Error during Q-measurement:
(1)At high frequencies the electronic voltmeter may suffer from losses
due to the transit time effect
The effect of Rshis to introduce an additional resistance in the tank
circuit, as shown in fig. (10.8)
To make the Qobsvalue as close as possible to Qact, Rshshould bemade as small as possible (Rshvalue of 0.02-0.04 introduces
negligible error)
(2)Another source of error, and probably the most important one, is the
distributed capacitance or self capacitance of the measuring circuit
Q METER ( contd.)
)R
R1(QQ,Hence
RR1
RRR
QQ
RR
LQand
R
LQ
shobsact
shsh
obs
act
sh
obsact
41
Q-METER (-contd.)
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Q METER ( contd.)
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Q-METER (-contd.)
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The presence of distributed or stray capacitances modifies the actual
Q and the inductance of the coil
At the resonant frequency, at which the self capacitance and inductance
of the coil are equal, the circuit impedance is purely resistive; thischaracteristic can be used to measure the distributed capacitance
One of the simplest methods of determining the distributed capacitance
(Cs) of a coil involves the plotting of a graph of 1/f2against C (in pF) as
shown in fig. (10.9a)
The frequency of the oscillator in the Q meter is varied and the
corresponding value of C for resonance is noted
The straight line produced to intercept the x-axis gives the value of Cs
Q METER ( contd.)
s2
s
2
2
s
2
2
CCthen,0f
1
If
)CC(L4f
1or,
)CC(L2
1fand
L4Slope,therefore,
4
SlopeL
43
Q-METER (-contd.)
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The value of unknown can also be determined from the above
equation
Another method of determining the stray or distributed capacitance
(Cs) of a coil involves making two measurements at different
frequencies
The capacitor C of the Q-meter is calibrated to indicate the
capacitance value
The test coil is connected to the Q-meter terminals as shown infig.(10.9b)
The tuning capacitor is set to a high value position (to its maximum)
and the circuit is resonated by varying the oscillator frequency
Suppose the meter indicates resonance & the oscillator frequency isfound to be f1& the capacitance value to be C1
The oscillator frequency of the Q-meter is now increased to twice the
original frequency, i.e., f2= 2f1, and the capacitor is varied until
resonance occurs at C2
Q METER ( contd.)
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Q-METER (-contd.)
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The resonant frequency of an LC circuit is given by
Therefore, for the initial resonance condition, the total capacitance of the
circuit is (C1+ Cs)and the resonant frequency is given by
After the oscillator and the tuning capacitor are varied for the new value
of resonance, the capacitance is (C2+ Cs), therefore,
But f2= 2f1, therefore,
Hence, C1+ Cs= 4 (C2+ Cs)
The distributed capacitance can be calculated using the above equation
Q ( co td )
LC2
1f
)CC(L2
1f
s1
1
)CC(L2
1f
s2
2
)CC(L2
12
)CC(L2
1
s1s2
3
C4CC 21s
45
Q-METER (-contd.)
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Q METER ( contd.)
46
Examples
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ExamplesEx. 10.1: The self capacitance of a coil is measured by using the
outlined in the previous section. The first measurement is at f1=1 MHz
& C1=500 pF. The second measurement is at f2=2 MHz & C2=110 pF.
Find the distributed capacitance. Also calculate the value L.(Ans. 20 pF, 48.712 H)
Ex. 10.2: Calculate the value of the self capacitance when the following
measurements are performed: f1=2 MHz & C1=500 pF
f2=6 MHz & C2=50 pF
(Ans. 6.25 pF)
Problem-1:The distributed capacitance was found to be 20 pF by use
of a Q-meter. The first resonance occurred at C1=300 pF & f1was
half the second resonance frequency. Determine the value of f2at the
second resonance (given L=40 H) (Ans. 2.8 MHz)47
Electroencephalogram (EEG)
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Electroencephalogram (EEG)
An electroencephalogram (EEG) is a test that measures and records
the electrical activity of the brain
Special sensors (electrodes) are attached to your head and hookedby wires to a computer
The computer records your brain's electrical activity on the screen or
on paper as wavy lines
Certain conditions, such as seizures, can be seen by the changes in
the normal pattern of the brain's electrical activity
EEG may be done to:
Diagnose epilepsy and see what type of seizures are occurring
Check for problems with loss of consciousness or dementia
Find out if a person who is in a coma is brain-dead
Study sleep disorders, such as narcolepsy
Watch brain activity while a person is receiving general
anesthesia during brain surgery48
EEG (-contd.)
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Help find out if a person has a physical problem (problems in the
brain, spinal cord, or nervous system) or a mental health problem
How EEG is Done? The EEG record is read by a doctor who is specially trained to
diagnose and treat disorders affecting the nervous system
(neurologist)
You will be asked to lie on your back on a bed or table or relax in achair with your eyes closed
The EEG technologist will attach 10 to 20 flat metal discs (electrodes)
to different places on your head, using a sticky electrolyte paste or
jelly to hold the electrodes in place (A cap with fixed electrodes may
be placed on your head instead of individual electrodes) The electrodes are hooked by wires to an EEG machine that records
the brain activity drawn by a row of pens on a moving piece of paper
or as an image on the computer screen
EEG ( contd.)
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EEG (-contd.)
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You may be asked to breathe deeply and rapidly (hyperventilate), usually
20 breaths a minute for 3 minutes
You may be asked to look at a bright, flashing light called a strobe
(photic or stroboscopic stimulation)
Results: There are several types of brain waves:
Alpha Waves have a frequency of 8 to 12 cycles per second. Alpha
waves are present only in the waking state when your eyes are closed
but you are mentally alert. Alpha waves go away when your eyes areopen or you are concentrating.
Beta Waves have a frequency of 13 to 30 cycles per second. These
waves are normally found when you are alert or have taken high doses
of certain medicines, such as benzodiazepines.
Delta Waves have a frequency of less than 3 cycles per second. Thesewaves are normally found only when you are asleep or in young children.
Theta Waves have a frequency of 4 to 7 cycles per second. These
waves are seen in drowsiness or arousal in older children and adults; it
can also be seen in meditation
( )
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Fig. (1)The cerebrum contains the frontal, parietal, temporal and occipital lobes51
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Fig. (2)The 1020 electrode system for measuring the EEG
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Fig. (3)A man undergoing an EEG, wearing a cap equipped with electrodes
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Fig. 4(a) Four types of EEG waves
Fig. 4(b) When the eyes are
opened, alpha waves disappear
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Electroencephalogram (EEG)
Normal In adults who are awake, the EEG shows mostly alpha waves and betawaves.The two sides of the brain show similar patterns of electrical activity.There are no abnormal bursts of electrical activity and no slow brain
waves on the EEG tracing.If flashing lights (photic stimulation) are used during the test, one area
of the brain (the occipital region) may have a brief response after eachflash of light, but the brain waves are normal.
Abnormal The two sides of the brain show different patterns of electricalactivity. This may mean a problem in one area or side of the brain is
present.The EEG shows sudden bursts of electrical activity (spikes) or sudden
slowing of brain waves in the brain. These changes may be caused by
a brain tumor, infection, injury, stroke, or epilepsy.55
El h l (EEG)
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Electroencephalogram (EEG)
Abnormal The EEG records changes in the brain waves that may not be injust one area of the brain. A problem affecting the entire brain-
such as drug intoxication, infections (encephalitis), or metabolic
disorders (such as diabetic ketoacidosis) that change the chemical
balance in the body, including the brain-may cause these kinds of
changes.The EEG shows delta waves or too many theta waves in adultswho are awake. These results may mean brain injury or a brain
illness is present. Some medicines can also cause this.The EEG shows no electrical activity in the brain (a "flat" or
"straight-line" EEG). This means that brain function has stopped,
which is usually caused by lack of oxygen or blood flow inside
the brain. This may happen when a person has been in a coma. In
some cases, severe drug-induced sedation can cause a flat EEG.56
EEG (-contd.)
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What factors may affect the EEG Test?
Reasons why the results may not be helpful include:
(i) Moving too much
(ii) Taking some medicines, such as those used to treat seizures
(antiepileptic medicines) or sedatives, tranquilizers, and barbiturates
(iii) Being unconscious from severe drug poisoning or a very low body
temperature (hypothermia)(iv) Having hair that is dirty, oily, or covered with hairspray or other hair
preparations. This can cause a problem with the placement of the
electrodes.
( )
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Electrocardiography
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Electrocardiography
An electrocardiogram (ECG or EKG) is an electrical recording of the
heart activity over time and is used in the investigation of heart
diseaseBritish physiologist Augustus D. Waller was the pioneer of
electrocardiography and in 1887 published the first human
electrocardiogram
In 1903 Dutch physiologist, Willem Einthoven, transformed thiscurious physiologic phenomenon into an indispensable clinical
recording device that is still used today
ECG is a surface measurement of the electrical potential generated
by electrical activity in cardiac tissue
The human heart can be considered as a large muscle whose
beating is simply a muscular contraction which develops a potential
to be measured in the form of ECG58
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Fig. (1)59
Electrocardiography (-contd.)
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Three Leeds of ECG:
The differential potential is
measured between the right and left
arm, between the right arm and the
left leg and between left arm and left
leg
These three measurements are
referred to as leads I, II, IIIrespectively
The signal from the body is being
amplified because the signals from
the body are small and weak,
ranging from 0.5 mV to 5.0 mV
Signals are filtered to remove the
noise, then after digital conversion
through ADC the digital signal is
sent to computer
g p y ( )
Fig. (2)
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Electrocardiography (-contd.)
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Fig. (3)Block diagram of an electrocardiograph. The normal locations for
surface electrodes are right arm (RA), right leg (RL), left arm (LA), and left
leg (LL). Physicians usually attach several electrodes on the chest of the
patients as well.
Resistors
and switch
Amp ADC
Signal
processorMonitor
PrinterStorage
LA
LL
RA
RL
Electrocardiography ( contd.)
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Fig. (4) Schematic representation of normal ECG
Electrocardiography ( contd.)
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Types of ECG Recordings
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yp g
Bipolar Leads recordvoltage between electrodesplaced on wrists & legs(right leg is grounded)
Lead Irecords between
right arm & left arm
Lead II: right arm & left leg
Lead III: left arm & left leg
Fig. (5)63
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Fig. (6)Einthovens triangle. Lead I is from RA to LA, lead II is from RA to
LL, and lead III is from LA to LL.
0IIIIII
64
Causes of Cardiac
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Cycle
3 distinct waves are
produced during cardiac
cycle
P wave caused by atrial
depolarization
QRS complex caused by
ventricular depolarization T wave results from
ventricular repolarization
Fig. (7)65
Elements of the ECG
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P wave: (Depolarization of both atria)
Relationship between P and QRS helps distinguish various cardiac
arrhythmiasShape and duration of P may indicate atrial enlargement
PR interval:(from onset of P wave to onset of QRS)
Normal duration = 0.120.2 sec
Represents atria to ventricular conduction time (through His
bundle)
Prolonged PR interval may indicate a 1st degree heart block
QRS complex:(Ventricular depolarization)Larger than P wave because of greater muscle mass of ventricles
Normal duration = 0.08 - 0.12 sec
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Its duration, amplitude, and morphology are useful in diagnosing
cardiac arrhythmia, ventricular hypertrophy, Myocardial Infarction
(MI), electrolyte derangement, etc.
Q wave greater than 1/3 the height of the R wave, greater than
0.04 sec are abnormal and may represent MI
ST segment:
Connects the QRS complex and T wave
Duration of 0.08-0.12 secT wave:
Represents repolarization or recovery of ventricles
Interval from beginning of QRS to apex of T is referred to as the
absolute refractory period
QT Interval:
Measured from beginning of QRS to the end of the T wave
Normal QT is usually about 0.40 sec
QT interval varies based on heart rate67
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Fig. (8)
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Ultrasound System
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Ultrasound is one of the most widely used modalities in medical imaging,
which is regularly used in cardiology, obstetrics, gynaecology, abdominal
imaging, etc.
Mostly, it is used in non-invasive techniques, although an invasive
technique like intra-vascular imaging is also possible
Ultrasound systems are signal processing intensive with various imaging
modalities and different processing requirements in each modality, digital
signal processors (DSP) are finding increasing use in such systems The advent of low power system-on-chip (SoC) with DSP and RISC
processors is providing portable and low cost systems without
compromising the image quality necessary for clinical applications
The term ultrasound refers to frequencies that are greater than 20 kHz,
which is commonly accepted to be the upper frequency limit the humanear can hear
Typically, ultrasound systems operate in the 2 MHz to 20 MHz frequency
range, although some systems are approaching 40 MHz for harmonic
imaging 69
Ultrasound System: Basic Funct ional i ty
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Ultrasound System: Basic Funct ional i ty
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Fig.(1 ) shows the basic functionality of an ultrasound system, which
demonstrates how transducers focus sound waves along scan lines
in the region of interest
In principle, the ultrasound system focuses sound waves along a
given scan line so that the waves constructively add together at the
desired focal point
As the sound waves propagate towards the focal point, they reflect
off on any object they encounter along their propagation path
Once all of the sound waves along the given scan line have been
measured, the ultrasound system focuses along a new scan line until
all of the scan lines in the desired region of interest have been
measured
To focus the sound waves towards a particular focal point, a set of
transducer elements are energized with a set of time-delayed pulses
to produce a set of sound waves that propagate through the region of
interest, which is typically the desired organ and the surrounding
tissue 71
Ultrasound System: Basic Funct ional i ty
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This process of using multiple sound waves to steer and focus a
beam of sound is commonly referred to as beam-forming
Once the transducers have generated their respective soundwaves, they become sensors that detect any reflected sound
waves that are created when the transmitted sound waves
encounter a change in tissue density within the region of interest
By properly time delaying the pulses to each active transducer, the
resulting time-delayed sound waves meet at the desired focal
point that resides at a pre-computed depth along a known scan
line
The amplitude of the reflected sound waves forms the basis for the
ultrasound image at this focal point location Envelope detection is used to detect the peaks in the received
signal and then log compression is used to reduce the dynamic
range of the received signals for efficient display and can be
analysed by the doctor or technician72
Ultrasound System: System Components
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Ultrasound System: System Components
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The beam-former control unit, as shown in Fig. (2), is responsible for
synchronizing the generation of the sound waves and the reflected
wave measurements The controller knows the region of interest in terms of width and
depth and gets translated into a desired number of scan lines and a
desired number of focal points per scan line
The beam-former controller begins with the first scan line and excites
an array of piezo-electric transducers with a sequence of high-voltage
pulses (of the order 100 V & 2 A) via transmit amplifiers
The pulses go through a Tx/Rx switch, which prevents the high-
voltage pulses from damaging the receive electronics
Note that these high-voltage pulses have been properly time delayedso that the resulting sound waves can be focused along the desired
scan line to produce a narrowly focused beam at the desired focal
point
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The beam-former controller determines which transducer elements to
energize at a given time and the proper time delay value for each
element to properly steer the sound waves towards the desired focalpoint
As the sound waves propagate toward the desired focal point, they
migrate through materials with different densities; with each change
in density, the sound wave has a slight change in direction &
produces a reflected sound wave
Some of the reflected sound waves propagate back to the transducer
& form the input to the piezo-electric elements in the transducer
The resulting low voltage signals are scaled using a variable
controlled amplifier (VCA) before being sampled by ADCs The VCA is configured so that the gain profile being applied to the
received signal is a function of the sample time since the signal
strength decreases with time (e.g., it has travelled through more
tissue)75
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The number of VCA and ADC combinations determines the number
of active channels used for beam-forming
It is usual to run the ADC sampling rate 4 times or higher than thetransducer centre frequency
Once the received signals reach the Rx beam-former, the signals are
scaled and appropriately delayed to permit a coherent summation of
the signals
This new signal represents the beam-formed signal for one or more
focal points along a particular specific scan line
Once the data is beam-formed, depending on the imaging modes,
various processings are carried out, e.g., it is common to run the
beam-formed data through various filtering operation to reduce outband noise
In B (Brightness) mode, demodulation followed by envelope detection
and log compression is the most common practice
76
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Several 2D noise reduction and image enhancement functions are
also performed in this mode
In spectral mode, a windowed Fast Fourier Transform (FFT) isperformed on the demodulated signal & displayed separately
It is also common to present the data on a speaker after
separation of forward and reverse flow
In these systems, a repeated set of pulse is sent through thetransducer
In between the pulses, the received signal is recorded
There is an alternate mode where a continuous pulse sets are
transmitted, which are known as continuous wave (CW) systems
These systems are used where a more accurate measurement of
velocity information is desired using Doppler techniques
The disadvantage of this system is that it loses the ability to
localize the velocity information 77
Ultrasound System: System Components
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In these systems, a separate set of transducers are used for
transmission and reception
Due to large immediate reflection from the surface of thetransducer, the dynamic range requirement becomes very high to
use ADC to digitize the reflected ultrasound signal and maintain
enough signal to noise (SNR) for estimating the velocity
information
Therefore, an analog beam-forming is usually used for CW
systems followed by analog demodulation
Such systems can then use lower sampling rate (usually in kHz
range) ADCs with higher dynamic range
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Ultrasound System: Imaging Modes
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A-mode (Amplitude) Imaging:
It displays the amplitude of a sampled voltage signal for a single
sound wave as a function of time This mode is considered 1D and used to measure the distance
between two objects by dividing the speed of sound by half of the
measured time between the peaks in the A-mode plot, which
represents the two objects in question
This mode is no longer used in ultrasound systems
B-mode (Brightness) Imaging:
It is the same as A-mode, except that brightness is used to represent
the amplitude of the sampled signal
B-mode imaging is performed by sweeping the transmitted soundwave over the plane to produce a 2D image
Typically, multiple sets of pulses are generated to produce sound
waves for each scan line, each set of pulses are intended for a
unique focal point along the scan line 80
CW (C ti W ) D l
Ultrasound System: Imaging Modes
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CW (Continuous Wave) Doppler:
In this mode, a sound wave at a single frequency is continuously
transmitted from one piezo-electric element and a second piezo-
electric element is used to continuously record the reflected soundwave
By continuously recording the received signal, there is no aliasing in
the received signal
Using this signal, the blood flow in veins can be estimated using theDoppler frequency
However, since the sensor is continuously receiving data from
various depths, the velocity location cannot be determined
PW (Pulse Wave) Doppler:
For this several pulses are transmitted along each scan line and the
Doppler frequency is estimated from the relative time between the
received signals
Since pulses are used for the signaling, the velocity location can also
be determined81
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Color Doppler:
For this, the PW Doppler is used to create a color image that is super-
imposed on top of B-mode image
A color code is used to denote the direction and magnitude of the flow,
e.g., red typically denotes flow towards the transducer and blue denotes
flow away from it
A darker color usually denotes a larger magnitude while a lighter color
denotes a smaller magnitudePower Doppler:
In this, instead of estimating the actual velocity of the motion, the
strength or the power of the motion is estimated and displayed
It is useful to display small motion and there is no directional information
in this measurement
Spectral Doppler:
Itshows the spectrum of the measured velocity in a time varying manner
Both PW & CW Doppler systems are capable of showing spectral
Doppler
82
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M-mode:
This display refers to scanning a single line in the object and then
displaying the resulting amplitudes successively, which shows themovement of a structure such as a heart
Because of its high pulse frequency (up to 1000 pulses per second),
this is useful in assessing rates and motion and is still used
extensively in cardiac and fetal cardiac imaging
Harmonic Imaging:
It is a new modality where the B-mode imaging is performed on the
second (or possibly other) harmonics of the imaging
Due to the usual high frequency of the harmonic, these images have
higher resolution than conventional imaging, however, due to higherloss, the depth of imaging is limited
Some modern ultrasound systems switch between harmonic and
conventional imaging based on depth of scanning83
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This system imposes stringent linearity requirements on the signal
chain components
Elasticity/Strain Imaging: It is a new modality where some measures of elasticity (like Youngs
modulus) of the tissue (usually under compression) is estimated and
displayed as an image
These types of imaging have been shown to be able to distinguish
between normal and malignant tissues
This is currently a very active area of research both on clinical
applications and in real-time system implementation
84
Basic Ultrasound Machine
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Basic Ultrasound Machine Components:
Central Processing Unit (CPU)
Transducer probe
Transducer Pulse Controls
Display
Keyboard/Cursor
Disk Storage
Printers
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Wh t i EEG?
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What is an EEG? An electroencephalogram is a measure of the brain's
voltage fluctuations as detected from the electrodes.
It is an approximation of the cumulative electrical
activity of neurons.
Background 1875 - Richard Caton discovered electrical
properties of exposed cerebralhemispheres of rabbits and monkeys.
1924 - German Psychiatrist Hans Bergerdiscovered alpha waves in humans and
coined the term electroencephalogram 1950s - Walter Grey Walter developed
EEG topography - mapping electricalactivity of the brain.
H B i
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Human Brain
Frontal LobesPersonality, emotions, problem solving.
Parietal lobesCognition, spatial relationships andmathematical abilities, nonverbal
memory.
Occipital lobesVision, color, shape and movement.
Temporal lobes
Speech and auditory processing,language comprehension, long-termmemory.
Diff t i EEG
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Different waves in EEGSlowest but highest
amplitude waves,deepest stages of sleep
it tends to appear during
drowsy, meditative, or
sleeping states.
Predominantly originates
From occipital lobe during
wakeful relaxation with
closed eyes.
associated with active, busy,
or anxious thinking andactive concentration.
relate to neural consciousnes
via the mechanism for
conscious attention
P bl ith EEG
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Problems with EEG
Electrical activity generated by complex systemof billions of neurons.
Difficult to register electrode location.
Artifacts from motion, eye blinks, swallows, heartbeat, sweating
Food, age, time of day, fatigue, motivation of
subject.
Advantages of EEG Many EEG studies have reported reproducible
changes in brain dynamics that are task dependent!
People are able to control their brainwaves via
biofeedback!
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Fig.Basic structure of the heart. RA is the right atrium, RV is the right
ventricle; LA is the left atrium, and LV is the left ventricle. Basic pacing rates