Sunday, 3 December 2017

VHF Tuner, RF Tuner Block Diagram

The block schematic of a TV tuner is shown below. At the antenna input terminals, the tuner must have input impedance equal to the characteristic impedance of the aerial feeder so that the matching provides a maximum power transfer and a voids reflections on the line the standard impedance is 300 Ω, corresponding to the twin wire ribbon feeder commonly used. The balun matches this impedance to the 75 Ω input impedance of the RF amplifier. It consists of a ferrite core upon which are four tightly coupled and evenly spaced bifilal windings of a couple of tums each, in the form of two quarter wave lines each of 150 Ω, that provide by series connection a 300 Ω balanced impedance on one side and a parallel connected 75 Ω impedance unbalanced, on the other side, by grounding of one terminal.

Block diagram of RF Tuner
A pair two small capacitors 470 pF each are included in each lead to prevent or block the dc path from chassis to antenna and prevent damage due to lightning the 2 Meg shunting resistors discharge any static accumulated charge on these capacitors.

IF Trap & HP Filter

Unwanted spurious signals in the IF range of 33 to 40 MHz are blocked by the IF trap and high pass (HP) filter as the rejection of these by the low QRF circuits, may not be adequate due to broad bandwidth particularly on the lowest channel number 2 viz, 47-54 MHz. It is very difficult to reject them once they reach the mixer and IF stages. The trap usually consists of an HP filter with pass beyond 40 MHz in the form.
Tuner input Balun & Filters
RF Amplifier:

This provides a gain to the input signal ensuring better signal-to-noise ratio, isolation to the local oscillator radiation and better image selectivity. Its primary function is to provide adequate gain (20 dB) to weak signals maintaining a good signal-to-noise (S/N) ratio at the mixer. The mixer generates more noise because of its heterodyne function.

The equivalent noise voltage at the input of the RF amplifier sets a limit to the minimum signal that can be received. The noise voltage is visible on the screen of the picture as shown background of black and white randomly moving spots. The noise voltage is typically around 10 µV. The RF stage is best suited for AGC because the signal level is small and the gain control is most effective here producing minimum distortion. Application of AGC to RF stage is usually delayed suitably in order to maintain good S/N ratio at weak signals.

The stage provides isolation by acting as a buffer between the local oscillator and antenna terminals to minimize radiation from the local oscillator. These irradiations can be a source of considerable interference to neighbouring receivers showing as diagonal line patterns on the screen. The field strength should be less than 100 µV/m at 100 ft in the VHF band or as per regulations prevailing. A judicious placement of the coupling coils windows, use of RF chokes, feed-through capacitors in the tuner supply lines, viz, Vcc, B+, filament supply wires and proper tuner case shielding, are all important to bring down the radiation below specified limits. Separate chassis ground return is generally often employed for tuner circuits. Feed through capacitor construction shown in figure, provides very efficient by passing and decoupling because of its coaxial construction. Reception of image frequency corresponding to (fs  + 2IF) which also produces the IF at the mixer, should also be adequately suppressed by the RF tuned circuits.

RF Response:

The RF amplifier has a pass band broad enough to pass the channel selected and also allow for the variation in the local oscillator fine tuning and variation due to the AGC voltage that should not affect the RF gain. Double tuned filters with a suitable dip between peaks can provide broad bandwidth of over 11 MHz and also good transient response. Below figure shows a typical response with a dip of 1dB between the peaks.

Mixer:

The mixer produces the IF signal by heterodyning the RF signal with the local oscillator frequency. As there are two carriers in the RF signal, viz, picture carrier and sound carrier, two Ifs are produced, the picture IF equal to 38.9 MHz and the sound IF equals to 33.4 MHz. The local oscillator frequency is higher than the RF carriers so that for channel 4 for example, the Low frequency is (62.25 + 38.9)101.15 MHz; and for channels , it is (176.25 + 38.9) 215.15 MHz.

Local Oscillator:

This provides the local oscillator frequency which should be essentially stable free from drifts due to temperature aging of components or small changes in the supply voltages, etc. It should have minimum harmonic content. The oscillator frequency control is basically the fine tuning control of the receiver. This may be manual varactor tuned or automatic fine tuning using varactor diode bias control from the frequency discriminator at the IF.

Feed Through Capacitor

Function of TV Tuner

A TV Tuner is a completely sealed and isolated unit which receives UHF/VHF signals from the antenna. It consists of RF amplifier, local oscillator and mixer stages. It is also called the front end of the TV receiver.

The following are the functions of  TV Tuner to be performed:

1. It selects the channel to be received by switching pre-tuned circuits in the RF stage and the oscillator.
2. It matches the impedance of the line at its input and amplifies it to maintain a good signal-to-noise ratio. The gain is controlled by AGC voltage, to suit the input signal strength.
3. It converts the RF into IF by mixing it with the local oscillator frequency, to feed into the video IF amplifier.
4. It blocks the interfering antenna pick-up signals in the IF range and prevents them from entering the receiver and mix up with the local oscillator and hence to the IF amplifier.
5. It isolates the local oscillator signals from the antena, due to the RF amplifier acting as a buffer, preventing radiation and interference to other receivers.
6. It rejects the image frequencies by means of the RF selective circuits.

Depending upon the tuning range required, the tuner is designed as a single channel, multichannel  VHF or UHF tuner. The VHF tuner may be a step-switch type tuner, using preset tuned circuits on a turret or a rotary switch. The UHF tuner uses continuous tuning usually employing transmission line or strip-line tuned circuits.

Factors affecting Tuner Design:

The following are the factors that affect the design of a TV Tuner:

1. Choice of IF and Local Oscillator Frequencies: The following are the merits of choosing intermediate frequencies (IF) close to 40 MHz are
a. High image rejection ratio.
b. Reduced local oscillator radiation.
c. Ease of detection.
d. Good selectivity at the IF stages.
The local oscillator frequency is kept higher than the channel carrier frequency since this results in a relatively narrow oscillator frequency range. In the 625 – B monochrome system, the picture IF = 38.9 MHz and sound IF = 33.4 MHz.

2. Need for an RF Amplifier Stage: At very high frequencies, the problems of image signals, radiation from the local oscillator through the antenna circuit and conversion loss at the mixer are such that a stage of amplification prior to the mixer is desirable, That is, an RF amplifier stage is desirable. Also, this stage feeds enough RF signal into the mixer for a clean picture without snow.

3. Coupling Networks: Parallel tuned networks are used to accomplish the desired selectivity in RF and IF sections of the receiver. For minimum power loss, the coupling network should match the output impedance of one stage to the input impedance of the following stage.

Wednesday, 22 November 2017

Electrocardiogram of the Heart

An Electrocardiogram of the Heart is the recording of small electric waves being generated during heart activity. The term electrocardiograph is derived from three Greek words 'electro' meaning related to electrical activity, 'cardio' meaning heart, and 'graph' meaning to write. It is used worldwide as a simple way of diagnosing heart conditions.

Electrical activity of heart: In the heart there are specialized cells called pacemaker cells. When the heart muscle is at rest, the pacemaker cells are negatively charged and when the heart contracts they are positively charged. These cells produce small electrical signals b changing their electrical from positive to negative and back. The heart cells have the ability to spread its electrical charge to its adjacent cells. This initial trigger is enough to produce a chain reaction. Thus the electrical activity of heart starts at the top of the heart and spreads down. So this tiny electric shock spreads down to heart causing it to contract. The contraction of heart results in the pumping of blood.

For the cardiovascular system to work properly, the atria and ventricles must operate in proper time relationship. During each cycle of ECG, the action potential in the heart originates near the top of the right atrium at a point called the pace maker or sino-atrial node (SA node). Pacemakers are specialized cells that generate action potentials at a regular rate. It is necessary that to initiate the heart beat, the action potentials generated by the pacemakers propagate in all directions along the surface of atria. These propagating waves terminate at a point near the center of the heart called AV node (Atrio-ventricular node). As told earlier, the action of atria and ventricles must have a proper timing relationship. This is provided by some special fibers at the AV node. These fibers act as a delay line to provide the timing between the atria and ventricles. So after the electrical excitation passes through the delay line, it is spread rapidly to all parts of both ventricles by the bundle of His. Purkinje fibers are special fibers present in the bundle of His.


Usually heart problems can produce a wide range of symptoms. Without the help of ECG it may not be possible to tell whether the symptoms are caused by a heart problem. Symptoms are chest pain, shortness of breath, palpitations, abdominal pain, weakness etc. In some cases ECG reveals a problem that is not cardiac in nature. Ex: Overdose of certain drugs, electrolyte abnormalities etc.

Saturday, 18 November 2017

E-commerce - Challenges, Security Issues, Fundamentals

TRADITIONAL COMMERCE vs E-COMMERCE

Traditional commerce is the term used for the local buying and selling of products. here there is no intermediary service such as internet or others. Some of the products involved in sale or purchase are fashion clothing, food products, jewelry and other products which are available in the local market. While in a E-commerce, sale or purchase of books, online CD'S, delivery of software, advertising and promotion of travel and other services takes place. Also online tracking of shipments is done through E-commerce.

But we can also combine both traditional commerce and E-commerce. The combination of both includes online banking, sale or purchase of automobiles and insurance products. Investment can be also done through the traditional and electronic commerce.

CHALLENGES OF E-COMMERCE

E-commerce can help to increase the profit that is increasing sales and decreasing cost. Some of the challenges faced by e-commerce is explained below.

E-commerce can help to increase the profit, that is increasing sales and decreasing cost. The advertisement provided on the web can help small industries for the purpose of promotion. It will reach all the customers in the world. This help in the increased sale and thus the cost of product can be reduced. Also, the business can use e-commerce in their purchasing process to identify the new suppliers and business parties. This is also a challenge faced by an E-commerce. When we create a site for the purpose of E-commerce, the following factors should be considered.
1. Why do we need to create a site?
2. Who is the visitor for the site?
3. Why do we want our own website for E-Commerce?

SECURITY ISSUES IN E-COMMERCE

Security issues in E-commerce can be classified according to secrecy, integrity and necessity.

1. Necessity Threats: This type of threats will happen due to delay or denial threat. It will disrupt the normal computer processing. A computer that has affected necessity threats slows the processing speed.
2. Secrecy Threats: It is one of the most high security threat. It prevents the unauthorized information disclosure. But the privacy is to the protection of individual rights to non disclosure.
Example: Sniffer programs.
3. Integrity Threats: Unprotected banking transactions are a type of integrity threat. Here the deposited amount transmitted over the internet may sometimes subject to integrity violations may leads to integrity threat.
Example: Cyber Vandalism (replacing data with porn or others).

E-COMMERCE SUCCESS

E-Commerce not only helps to increase the profit by increasing sales and decreasing cost. But also increases the purchasing opportunities for the buyer. This process helps to identify new parties and business suppliers. Thus negotiation of price and delivery terms can be made easier. This is one of the success factors for E-Commerce. Another factor is the buyer can select from a wide range of choices and information available about the product in the web. Also they can customize the level of details about a particular product.

The involvement of E-commerce in the social welfare is also a success factor for the E-commerce. The welfare of the society includes tax refunds, public retirement and so on. This type of electronic payment helps the customer to audit and monitor. This type of transaction can be done at home. Thus we can avoid the transportation cost and save time.

E-COMMERCE FUNDAMENTALS AND APPLICATIONS

1. Electronic Cash: Electronic Cash must be able to pass transparently across international borders and can be automatically converted to recipient country's currency. It must have a monetary value. Companies offering E-Cash are
a. Check Free: Check Free provides software that permits users to pay all their bills with online electronic checks.
b. Internet Cash: Merchants provide cash rather than credit cards to pay for products for online customers.
c. Pay Pal: Pay Pal provide payment processing services to business and for individuals.
d. Cyber Cash: Cyber Cash includes credit card, micro payment and check payment services.

2. Electronic Payment: Electronic Payment is the first introduced money transfer method. In Early days, it was labeled at Electronic Fund Transfer. The transfer of fund is done through electronic terminals, telephone or computers.

3. Electronic Wallets: It includes credit cards, electronic cash, owner identification and owner address information. Examples are Agile Wallet, Microsoft Wallet and E-Wallets.

4. Stored Value Cards: It consists of a microchip or a plastic card with a magnetic strip. Commonly used stored value cards include Prepaid Phone and Bus Card.
   example: Magnets strip cards, Smart Cards, Mondex Smart Card etc.

Website Development

Web Developers are the people involved in developing the website. They consists of a team of workers. Their main aim is to satisfy the client's need and to meet the expectation level of client. The web development team consists of  Web Optimizers,Web Designers,Web Programmers and Content Writers. The main duty of designers is to design the web page. They produce prototypes. Dynamic programming section belongs to Web programmers. Content writers write the contents for the website. Web Optimizers optimize the web pages.

Each Worker in the Web Development Company should have their own duty.The web designers produce the prototypes according to the client's need.They design the pages beautifully to attract the visitors.Content Writers write the contents for the website..They construct the sentences simply in order to understand the visitors quickly.Programming is an important part in website development and is done by the web programmers.Optimization is done by the Web Optimizers.Optimizers are the key people in website development.Their duty is to increase the page rank.

Web developers work in co-operation in order to fulfill clients demand.The prototypes built by the developers are reviewed by the team workers and send to client.After demonstration,client will send the feedback of website.According to the feedback,they redesign the web page.After designing ,programming and content writing,optimization for website is done.

Sunday, 16 July 2017

Isolation Amplifiers

Isolation amplifiers are used to provide electrical isolation and an electric safety barrier to the patients during measurements. Some patients are highly susceptible to electrical shock hazards. Under certain conditions, these shock hazards can cause very severe effects to the patient. So it is required to protect the patient. The isolation amplifiers can provide very high insulation in the range of about a lack mega ohms! There exist a common mode voltage which are the potential difference between instrument ground and signal ground during measurement. The data acquisition components need to be protected from this common mode voltages because as the magnitude of it increases, the chance of instrument destruction increases. Even if the magnitude of the common mode voltage is low, there is a possibility of noisy representation of the signal under investigation.

Isolation amplifiers are very useful when we need to amplify low level signals in multichannel applications. Also they eliminate measurement errors caused by ground loops.

There are two broad classifications of isolation amplifiers. Some isolation amplifiers provide input-to-output isolation without channel-to-channel isolation. It offers only one isolation barrier for a multichannel instrument. Some other type of isolation amplifiers provide both input-to-output isolation and channel-to-channel isolation.

There are two isolation amplifier specifications.
1. The amplifier isolation breakdown voltage: It is defined as the absolute maximum common mode voltage that the isolation amplifier can handle without damage.
2. The Amplifier CMRR: It is defined as the degree to which the common mode voltage will disrupt the normal mode component measurement.

In addition to these, the frequency of the common mode voltage is also important. Higher frequency common mode voltages can create difficulty for many isolation amplifiers due to the effect of parasitic capacitance.

The basic block in figure which illustrate the working of an isolation amplifier is shown below.


Input amplifier amplifies the input  and the amplified output is applied to a modulatior which modulates the signal by using AM, PWM or any other technique. The isolation barrier is generally an energy converter where the electrical energy of the modulator is converted to some other form of energy (a non-electrical energy). Then the signal is modulated and finally amplified by the output amplifier.

The standard symbol of an isolation amplifier is shown in figure.


Isolation Amplifiers using Optical Isolation

Normally we use optical isolation barrier in order to reduce the effect of EMI (Electromagnetic Interference) as the optical fibers are not susceptible to EMI.  In Isolation amplifier using Optical isolation, the transducer converts the physiological information into electrical form which is amplified by the isolation amplifier. The amplified output is applied to a voltage to frequency converter. It is done because after voltage to frequency conversion the signals will be in digital form which is ideal for optical transmission. The FOT (Fiber Optic Transmitter) transmits the signal through the optical fiber cable which can provide a high degree of isolation. At the receiving end, the signal is received by a FOR (Fiber Optic Receiver) which feeds the signal to FVC (Frequency to Voltage Converter) to get the original signal.

Bioelectric Amplifiers

As the name implies, the bioelectric amplifiers are used to amplify the bioelectric signals. The bioelectric signals measured from various body parts are having an amplitude ranging from mVs to µVs. So it is necessary to amplify the extremely low amplitude signals for the analysis of the biological data. It is for this purpose that the bioelectric amplifiers are used. Usually we use operational amplifiers as bioelectric amplifiers due to its high gain and other versatile features.
The bioelectric amplifiers have some properties.

1. The gain of the bioelectric amplifier may be low, medium or high depending on the type of amplifier and the signal to be amplified. For example the low gain amplifiers are used for the measurement of action potential, medium gain amplifiers are used for the amplification of ECG waveform and the high gain amplifiers are used for EEG signal amplification.

2. The bioelectric amplifiers may be ac coupled or dc coupled.

3. The frequency response of a bioelectric amplifier range from very low frequency to high frequency range.

4. Bioelectric amplifiers have differential input and single ended output.

5. High CMRR and extremely high input impedance.

Two important parameters of bioelectric amplifiers are noise and drift. We have to avoid the effect of both these parameters on bioelectric amplifiers. Drift is the change in output due to the change in temperature. Noise is the thermal noise generated in electronic devices. Both these problems can be avoided by proper design to make the bioelectric amplifier more effective.

LVDT - Advantages and Disadvantages


As shown in figure, LVDT consists of a transformer with one primary winding and two secondary windings. Here the secondary windings are connected in such a way that their induced voltages oppose each other. An alternating current is driven through the primary, causing a voltage to be induced in each secondary proportional to its mutual inductance with the primary. An iron core is located in between the primary and secondary windings.If the core is at the central position, the voltages in the two secondary windings are equal and hence the output is cancelled. When the core is displaced in one direction, the voltage in one coin increase as the other decreases causing the output voltage to increase from zero to maximum. This voltage is in phase with the primary voltage. When the core moves in the other direction, the output voltage also increases from zero to maximum, but its phase is opposite to that of primary. Thus the magnitude of the output voltage will be proportional to the distance moved by the core and it is why the device is called linear. The phase of the voltage indicates the direction of the displacement.

Advantages of LVDTs

1. The sliding core does not touch inside the tube and hence it can  move without friction.
2. As friction is avoided, the output will be accurate in almost all cases.
3. There is no sliding or rotating contacts which improves the efficiency.
4. Very high resolution
5. Simple in construction
6. Easy to align and maintain
7. Light in weight

Disadvantages of LVDTs

1. Large displacements are required for differential output
2. Sensitive to stray magnetic field
3. It is sensitive to temperature variations.

Tuesday, 20 June 2017

Capacitive and Inductive Transducers

Capacitive Transducers:

Capacitive transducer is a measurement device in which variations in pressure upon a capacitive element proportionally change the element’s capacitive rating and thus the strength of the measured electric signal from the device. As we know here the capacitance of the transducer varies with respect to the external stimulas. Normally a capacitive transducer uses a stationary plate and a movable plate. The stationary and the movable plates are separated by aair or vacuum dielectric. The movable plate changes its position under the influence of an external stimulus. As we know the capacitance depends on the area of plates, the movement of plate causes a change in capacitance. This is the basic working principle of a capacitive transducer.

Advantages of Capacitive transducers:

Compared with optical, piezo-resistive and inductive transducers, capacitive transducers have many advantages.
1. Low cost,
2. Less power usage,
3. Good stability,
4. High resolution,
5. Better speed,
6. Good frequency response
7. They require very little force to operate..

Disadvantages of Capacitive transducers:

1. The performance is severely affected by dirt and contaminants as they change the dielectric constant.
2. They are sensitive to temperature variations
3. Metallic parts must be insulated from each other

Biomedical Application:

1. It is mainly used for blood pressure measurements.
2. Also used for displacement or position measurement.
3. They can detect motion, acceleration, flow and many other variables.

Inductive Transducers:

The inductance of a coil can be changed either by changing its physical dimension or by changing the effective permeability of its magnetic core. In transducers using inductive elements we normally use the second principle. The effective permeability can be changed by moving a core, which is having a permeability higher than, the air through the coil. But the disadvantage with basic type of inductive transducer is that the inductance of coil is not related linearly to the displacement of coil if large displacements occur accidently. It is to avoid this disadvantage that we use LVDTs. The existing inductive transducers have a relatively small output power and often require amplification of their outputs.

Transducers and Sensors

A transducer is a device that performs the conversion of one form of variable into another. Normally in biomedical applications, the transducer input is in non-electrical form and the output will be in electrical form. Actually transducers and electrodes are a part of a group of devices called sensors. Major difference between transducers and electrodes is that transducers make use of some transducible element for the measurement. But the electrodes directly measure the signals. A sensor means a device used to measure a particular parameter either by changing the desired signal to electrical signal or by changing ionic flow into electron flow.

Two types of principles are involved in the process of converting non-electrical variable into electrical signal. Depending on these principles the transducers are mainly classified into two types. - Active transducers and Passive transducers

ACTIVE TRANSDUCERS:

An active transducer is one which gives its output without the use of excitation voltage or modulation of a carrier signal. One property of active transducer is that it converts non-electrical energy into electrical energy and vice-versa.
The various types of active transducers are
a.Magnetic induction type transducers
b. Piezoelectric type transducers
c. Photovoltaic type transducers
d. Thermoelectric type transducers

A. Magnetic induction type Transducers:

We know that when an electrical conductor is moved in a magnetic field in such a way that flux through the conductor is changed, a voltage is induced. The induced voltage will be proportional to the rate of change of magnetic flux which in turn is proportional to the conductor movement.. The induced emf is given by, e = Blv

Biomedical Applications:
1. These type of transducers are used in electromagnetic flow meters to measure blood flow.
2. Also used in heart sound microphones.

B. Piezoelectric type transducers:

Piezoelectricity is the ability of some materials to generate an electric field or electric potential in response to applied mechanical stress. The effect is related to a change of polarization density within the materials volume and is also reversible-means the production of stress or strain when an electric field is applied. A piezoelectric crystal (such as Quartz) can produce a voltage under deformation by compression or tension. This is called piezoelectric effect. This effect is the basic working principle of piezoelectric type transducers. So they convert displacement or pressure into an electrical variable.

Biomedical Application:
Mainly used in pulse sensing measurements.

C. Photo voltaic type of transducers:

Photoelectric effect is the ejection of electron from a metal or semi conductor surface when it is illuminated by light or any other radiation of suitable wavelength. So a photoelectric transducer generates electrical voltage in proportion to the radiant energy incident on it.

Biomedical Application:
Used in pulse sensors.

D. Thermoelectric type transducers:

These types of transducers are based on the seeback effect. It states that when two junctions of a thermocouple are maintained at different temperatures, an emf is generated which wil be proportional to the temperature difference between the junctions. Thermocouples are widely used type of temperature sensor for measurement and control. They are inexpensive, interchangeable and are supplied with standard connectors and can measure a wide range of temperatures. Also we can use thermistors as active transducers. Here its resistance value changes with change in temperature. The material used in a thermistor is usually a ceramic or polymer.

Advantages: Small size, Low cost, Fast response, Wide temperature range, High accuracy

Disadvantages: Thermistors are unsuitable for wide temperature ranges, Less stable at high temperatures, Non-linear temperature-resistance curve.

Biomedical Application:
1. Used in the measurement of physiological temperature
2. In biotelemetry systems to measure temperature.

PASSIVE TRANSDUCERS:

Passive transducers convert the physiological parameter (such as blood pressure, temperature etc) into an electrical output using a DC or AC excitation voltage. One important property of the passive transducers is that they are not reversible. Passive components such as resistors, capacitors, inductors are used to make the passive transducers. They require an external power to operate and the output is a measure of some variation in the passive components.

Wheatstone Bridge
Many transducers make use of the principle of Wheatstone bridge. In many biomedical transducers using Wheatstone bridge, all four resistances are equal under balanced condition. The balanced condition is varied when any of the resistances varies. Normally during the measurement, it is designed in such a way that the resistance value of a particular resistance varies and hence balance is lost. By analyzing this change in resistance, the parameter can be measured indirectly.

Friday, 9 June 2017

Electrodes in Biomedical Instrumentation

Electrodes are devices that convert  ionic potentials into electronic potentials.  The type of electrode used for the measurements depends on the anatomical location of the bioelectric event to be measured. In order to process the signal in electronic circuits, it will be better to convert ionic conduction into electronic conduction. So simply bio-electrodes are a class of sensors that transdues ionic conduction into electronic conduction. The purpose of bio-electrodes is to acquire bioelectrical signals such as ECG, EMG, EEG etc.
Electrodes are mainly classified into two. They are perfectly polarized electrodes and perfectly non-polarized electrodes. There are a wide variety of electrodes which can be used to measure bioelectric events. The three main classes of electrodes are Microelectrodes, Body Surface electrodes and Needle electrodes.

A. Microelectrodes

Microelectrodes are electrodes with tips having tips sufficiently small enough to penetrate a single cell in order to obtain readings from within the cell. The tips must be small enough to permit penetration without damaging the minute cell. The main functions of microelectrodes are potential recording and current injection. Microelectrodes are having high impedances in mega ohn range because of their smaller size. Microelectrodes are generally of two types. With the use of a microelectrode or an array of microelectrodes, researchers can gather all sort of information regarding living organism.
a. Metal type b. Micropipette type

a. Metal microelectrode: Metal microelectrodes are formed by electrolytically etching the tip of fine tungsten to the desired size and dimension. Then the wire is coated almost to the tip with any type of insulating material. The metal-ion interface takes place where the metal tip contacts the electrolyte. The main features of metal microelectrodes are
1. Very good S/N ratio
2. Strong enough to penetrate
3. High biocompatibility

b. Micropipette: The micropipette type of microelectrode is a glass micropipette with its tip drawn out to the desired size. The micropipette is filled with an electrolyte which should be compatible with the cellular fluids. A micropipette is a small and extremely fine pointed pipette used in making microinjections. A commercial type of micropipette is shown in figure below.


B. Body Surface Electrodes:

Surface electrodes are those which are placed in contact with the skin of the subject in order to obtain bioelectric potentials from the surface. Body surface electrodes are of many sizes and types. In spite of the type, any surface electrode can be used to sense ECG, EEG, EMG etc. The various types of body surface electrodes are discussed below. Major body surface electrodes are

1. Immersion electrodes: They are one of the first type of bioelectric measuring electrodes. Immersion electrodes were simply buckets of saline solution in which the subject placed his hands and feet. So it was not a comfortable type of measurement and hence it was replaced with plate electrodes.

2. Plate electrodes: These electrodes were separated from subject’s skin by cotton pads socked in a strong saline solution. The plate electrodes have generally smaller contact area and they do not totally seal on the patient. The electrode slippage and displacement of plates were the major difficulties faced by these type of electrodes because they have a tendency to lose their adhesive ability as a result of contact with fluids on or near the patient. Since these types of electrodes were very sensitive, it led to measurement errors.

3. Floating electrodes: These types of electrodes can eliminate the movement errors (called artifacts) which is a main problem with plate electrodes. This is done by avoiding any direct contact of the metal with the skin. So the main advantage of floating electrodes is mechanical reliability. Here the conductive path between the metal and the skin is the electrolyte paste or jelly.

4. Disposable electrodes: Normally plate electrodes, floating electrodes etc can be used more than one time. This requires the cleaning and cares after each use. We can use disposable electrodes which can be used only once and be dsposed after the use. These types of electrodes are now widely used.

5. Suction electrodes: These type of electrodes are well suited for the attachment to flat surfaces of body and to regions where the underlying tissue is soft, due to the presence of contact surface. An advantage of these type of electrodes is that it has a small surface area. These types of electrodes are mainly used for the measurement of ECG. Suction electrodes used a plastic syringe barrel to house suction tubing and input cables to an AC amplifier.


6. Ear clip & Scalp electrodes: These type of electrodes are widely used in the measurement of EEG exclusively. Scalp electrodes can provide EEG easily by placing it over bare head. A typical ear clip electrode is shown in figure below. The most common method for EEG measurement is 10 – 20 electrode placement system and here we use scalp electrode usually. They can avoid measurement errors and movement errors. During labour internal monitoring may be needed and is usually in the form of an electrode placed under the baby’s scalp. It is called fetal scalp electrode which is used to monitor baby’s heartbeat while still in uterus.

C. Needle Electrodes:

To reduce the interface and noise (artifact) caused due to electrode movement, during the measurement of EEG, EMG etc we can use small sub-dermal needle electrodes which penetrate the scalp. Actually the needle electrodes are not inserted into the brain. They nearly penetrate the skin. Generally they are simply inserted through a small section of the skin just beneath the skin parallel to it.
The needle electrodes for EMG measurement consist of fine insulated wires placed in such a way that their tips are in contact with the muscle, nerve or other tissues from which the measurement is made. The needle creates the hole necessary for insertion and the wires forming the electrodes are carried inside it. A typical EEG needle electrode is shown in figure.

One of the main advantage of needle electrodes is that they are less susceptible to movement errors than surface electrodes. Also the needle electrodes have lower impedances when compared to surface electrodes as it makes direct contact with the sub-dermal tissues or intracellular fluid.

Friday, 2 June 2017

CRO and its Applications

An oscilloscope is previously called as an oscillograph. It can be informally known as a scope, CRO (for cathode-ray oscilloscope), or DSO (for the more modern digital storage oscilloscope). It is a type of electronic test instrument that allows observation of constantly varying signal voltages. The output of a CRO will be usually as a two-dimensional graph of one or more electrical potential differences using the vertical or y-axis, plotted as a function of time (horizontal or x-axis). Many signals (e.g. sine, cosine etc) can be converted to voltages and can be displayed this way. By changing the mode of CRO into transfer characteristics, we can see the transfer characteristics of signals. The transfer characteristics of a signal give the variation of the output wave with respect to the variation to the input wave. The Signals may be either periodic or repeat constantly, so that the multiple samples of a signal which is actually varying with respect to time can be displayed as a steady picture in the CRO. Many oscilloscopes (storage oscilloscopes) are able to capture non-repeating waveforms for a specified period of time, and are able to produce a steady display of the captured segment.

Oscilloscopes are mainly used to observe the correct wave shape of an electrical signal. Oscilloscopes are usually calibrated so that the two axes voltage and time can be easily read as well as possible by the eye. This will yields to the measurement of the peak-to-peak voltage of a waveform. It also allows checking the frequency of periodic signals, the time between pulses, the time taken for a signal to rise to full amplitude, which is usually called as the rise time, and relative timing of several related signals.

The applications of Oscilloscopes are in the fields of sciences, engineering, medicine, and telecommunications industry. For the maintenance of electronic equipment and laboratory work, general-purpose instruments are used. Special-purpose oscilloscopes are used for analyzing an automotive ignition system or to display the waveform of the heartbeat as an electrocardiogram. If we consider some practical example, some computer sound software allows the sound being listened to be displayed on the screen as by an oscilloscope. Thus CRO can be used in many applications in the emerging world technologies. Most of all, the CRO can be considered as the eye of an electronic engineer. In other words an electronic engineer cannot see his outputs without the help of a CRO.

Some special storage CRTs are used to maintain a steady display of a single brief signal in case of advanced storage oscilloscopes. By digital storage oscilloscopes (DSOs) with thin panel displays, fast analog-to-digital converters and digital signal processors, CROs were later largely outdated. DSOs without integrated displays are known as digitizers. Digitizers are available at lower cost, and it uses a general-purpose digital computer to process and display the required waveforms.