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Unit-4

MISCELLANEOUS INSTRUMENTS

1

Amplitude Distortion • Distortion is 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), then

the 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 experience

linear distortion

• The linear distortion will not change the shape of a single sinuosoid, but

will usually change the shape of a multi-tone signal

2

• Amplitude distortion is 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 = βIb

• 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 f1 and f2 are present at the input, resulting in the creation

of several other frequency components, whose frequencies include

(f1 + f2 ), (f1 - f2 ), (2f1 - f2 ), (2f2 – f1), and in general (mf1 ± nf2) for

integer m and n

Amplitude Distortion (-contd.)

3

• Generally the strength of the unwanted output falls rapidly as m and n

increase

• Amplitude distortion is measured with the system operating under

steady-state conditions with a sinusoidal input signal

• When other frequencies are present, the term "amplitude" 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, or selective level

voltmeters

• Some wave analyzers have the facility of automatic frequency

control, in which the tuning automatically locks to the signal


4

• 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.


...DDDD 2

4

2

3

2

2

5

Fig. (4.1) Graph of a Waveform and the distorted versions of the same waveform


6

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 7

Basic Wave Analyzer (-contd.)

Fig. (9.1a) Basic Wave Analyzer

8

Basic Wave Analyzer (-contd.)

9

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

10

• 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.)

11


12


13

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 can‟t 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 oscillator

in 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 meter

circuit

• 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 14

• 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 oscillator

frequency, 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 be

measured

Heterodyne Wave Analyzer (-contd.)

15

• 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


16

• 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 of

zero 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 control

of the local oscillator

• The bandwidth, which is controlled by the active filter, can be

selected at 200 Hz, 1 kHz, and 3 kHz


17


18


19

Harmonic Distortion Analyzer Fundamental 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 frequency 20

• The bridge is unbalanced for the harmonics, 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 be

balanced 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 (Vacuum

Tube 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.)

21

• The harmonic distortion output can then be measured with a meter

• For balance at the fundamental frequency:

C1 = C2 = C, R1 = R2 = R, R3 = 2R4

(iii) Bridged T -Network Method:

• As shown in fig. (9.7), L & C's are tuned to the fundamental

frequency, and R is adjusted to bypass fundamental frequency

• The tank circuit being tuned to the fundamental frequency, the

fundamental 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


22

• 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 are

simpler 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


23


Fig. (9.5) Resonance Bridge

24


Fig. (9.6) Wien’s Bridge Method 25


26


27

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; the

instrument 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 28

(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 tuned

oscillator

• As the oscillator sweeps from fmin to fmax of 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.)

29

• 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 a

super-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)


30

• 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


31


32

Basic Spectrum Analyzer Using Swept Receiver Design

33


34


Fig. (9.12) Test Waveform as seen on X-axis (time) & Z-axis (frequency)

Fig. (9.13) RF Spectrum Analyzer 35

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 the

capacitor can be calibrated to read Q directly

• At resonance XL = XC and 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

36

• 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 Ω

• 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 EC and 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.)

37

• 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

38

Q-METER (-contd.)

Fig. (10.7) Circuit Diagram of a Q-meter

39

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 Rsh is to introduce an additional resistance in the tank

circuit, as shown in fig. (10.8)

• To make the Qobs value as close as possible to Qact , Rsh should be

made as small as possible (Rsh value of 0.02-0.04 Ω 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

R

R1

R

RR

Q

Q

RR

LQand

R

LQ

shobsact

shsh

obs

act

sh

obsact

40

Q-METER (-contd.)

41

• 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; this

characteristic 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/f2 against 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

1If

)CC(L4f

1or,

)CC(L2

1fand

L4Slope,therefore,4

SlopeL

42

• 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 in

fig.(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 is

found 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.)

43

• 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-METER (-contd.)

LC2

1f

)CC(L2

1f

s1

1

)CC(L2

1f

s2

2

)CC(L2

12

)CC(L2

1

s1s2

3

C4CC 21

s

44

Q-METER (-contd.)

45

Examples Ex. 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 µH)

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 & f1 was

half the second resonance frequency. Determine the value of f2 at the

second resonance (given L=40 µH) (Ans. 2.8 MHz) 46

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 hooked

by 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 surgery 47

• 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 a

chair 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.)

48

• 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 are

open 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. These

waves 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

EEG (-contd.)

49

Fig. (1) The cerebrum contains the frontal, parietal, temporal and occipital lobes 50

Fig. (2) The 10–20 electrode system for measuring the EEG

51

Fig. (3) A man undergoing an EEG, wearing a cap equipped with electrodes

52

Fig. 4(a) Four types of EEG waves

Fig. 4(b) When the eyes are

opened, alpha waves disappear

53


Normal In adults who are awake, the EEG shows mostly alpha waves and beta

waves.

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 each

flash of light, but the brain waves are normal.

Abnormal The two sides of the brain show different patterns of electrical

activity. 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.

54


Abnormal The EEG records changes in the brain waves that may not be in

just 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 adults

who 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. 55

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.

EEG (-contd.)

56

Electrocardiography

• An electrocardiogram (ECG or EKG) is an electrical recording of the

heart activity over time and is used in the investigation of heart

disease

• British physiologist Augustus D. Waller was the pioneer of

electrocardiography and in 1887 published the first human

electrocardiogram

• In 1903 Dutch physiologist, Willem Einthoven, transformed this

curious 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 ECG 57

Fig. (1) 58

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, III

respectively

• 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

Electrocardiography (-contd.)

Fig. (2)

59

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 switchAmp ADC

Signal

processorMonitor

PrinterStorage

LA

LL

RA

RL


60

Fig. (4) Schematic representation of normal ECG


61

Types of ECG Recordings

• Bipolar Leads record voltage between electrodes placed on wrists & legs (right leg is grounded)

• Lead I records between right arm & left arm

• Lead II: right arm & left leg

• Lead III: left arm & left leg

Fig. (5) 62

Fig. (6) Einthoven‟s triangle. Lead I is from RA to LA, lead II is from RA to

LL, and lead III is from LA to LL.

0IIIIII

63

Causes of Cardiac

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) 64

P wave: (Depolarization of both atria)

• Relationship between P and QRS helps distinguish various cardiac arrhythmias

• Shape and duration of P may indicate atrial enlargement

PR interval: (from onset of P wave to onset of QRS)

• Normal duration = 0.12 – 0.2 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

Elements of the ECG

65

• 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 sec

T 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 rate

•https://www.youtube.com/watch?v=FThXJUFWUrw

Elements of the ECG (-contd.)

66

https://www.youtube.com/watch?v=RYZ4daFwMa8

67

Ultrasound System

• 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 human

ear 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 68

Ultrasound System: Basic Functionality

69


• 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 70

• 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 sound

waves, 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 technician


71

Ultrasound System: System Components

72

• 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 delayed

so that the resulting sound waves can be focused along the desired

scan line to produce a narrowly focused beam at the desired focal

point


73

• 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 focal

point

• 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)


74

• 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 the

transducer 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 out

band noise

• In B (Brightness) mode, demodulation followed by envelope detection

and log compression is the most common practice


75

• Several 2D noise reduction and image enhancement functions are

also performed in this mode

• In spectral mode, a windowed Fast Fourier Transform (FFT) is

performed 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 the

transducer

• 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


76

• In these systems, a separate set of transducers are used for

transmission and reception

• Due to large immediate reflection from the surface of the

transducer, 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


77


78

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 sound

wave 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

Ultrasound System: Imaging Modes

79

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 sound

wave

• 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 the

Doppler 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 determined


80

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 magnitude

Power 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:

• It shows the spectrum of the measured velocity in a time varying manner

• Both PW & CW Doppler systems are capable of showing spectral

Doppler


81

M-mode:

• This display refers to scanning a single line in the object and then

displaying the resulting amplitudes successively, which shows the

movement 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 higher

loss, the depth of imaging is limited

• Some modern ultrasound systems switch between harmonic and

conventional imaging based on depth of scanning


82

• 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 Young‟s

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


83

Basic Ultrasound Machine

Basic Ultrasound Machine Components:

• Central Processing Unit (CPU)

• Transducer probe

• Transducer Pulse Controls

• Display

• Keyboard/Cursor

• Disk Storage

• Printers

84

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 cerebral hemispheres of rabbits and monkeys.

– 1924 - German Psychiatrist Hans Berger discovered alpha waves in humans and coined the term “electroencephalogram”

– 1950s - Walter Grey Walter developed “EEG topography” - mapping electrical activity of the brain.

Human Brain

Frontal Lobes Personality, emotions, problem solving.

Parietal lobes Cognition, spatial relationships and

mathematical abilities, nonverbal memory.

Occipital lobes Vision, color, shape and movement.

Temporal lobes Speech and auditory processing,

language comprehension, long-term memory.

Different waves in EEG Slowest 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 and

active concentration.

relate to neural consciousness

via the mechanism for

conscious attention

Problems with EEG

• Electrical activity generated by complex system

of billions of neurons.

• Difficult to “register” electrode location.

• Artifacts from motion, eye blinks, swallows, heart

beat, 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!

89

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

are also shown.

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