AC Millivoltmeter Block Diagram

Measurement of radio frequency voltage is difficult especially when the magnitude is very small and the frequency is very high. Naturally when small magnitudes are to be measured, we certainly need amplification of the signal, that too at the signal frequency.

Radio frequency signal amplification is complicated and requires special devices, and has several restrictions on the component size, orientation, and shielding is very important. In spite of all the precautions taken the bandwidth restriction again poses a problem. Stability of the gain of the amplifier is yet another problem. For accurate measurement care is to be taken in the design of the instrument considering all the above constrains.

R.F. millivolt meter can be either of the amplifier rectifier type or the rectifier amplifier type. thee amplifier rectifier type again can be the average reading type, peak reading type or true R.M.S. reading type.

In amplifier rectifier type of instrument initially the radio frequency signal will be amplified in a wide band amplifier. The output of the amplifier will be used in a detector to obtain an indication in the indicating instrument, which reads the value of the input voltage by proper calibration of the instrument.

In the rectifier amplifier type of radio frequency voltage measurement, the input signal will initially be rectified. The rectified D.C. will be amplified in a D.C. amplifier. The output of the D.C. amplifier will be connected to an indicating instrument which will be calibrated in terms of the input signal.

In this blog the following topics are covered :

Classification of amplifier rectifier type of A.C. Millivoltmeters
Principle of working of the amplifier rectifier type of A.C. Millivoltmeter
Applications of amplifier rectifier type of A.C. Millivoltmeter
Principle of working of the Rectifier amplifier type of A.C. Millivoltmeter
Types of Probes used with the rectifier amplifier type of instrument
Applications .

Amplifier Rectifier Type A.C. Millivoltmeter :

In this type of instrument the input signal will first be amplified in a wide band amplifier. The output of the wide band amplifier will be used in a rectifier to get a D.C. output which will be indicated in an indicating instrument.

(a) Classification :

Depending on the type of rectifier arrangement used in the circuit we have three types of A.C. millivoltmeters as listed below:
(i) The average reading type
(ii) The peak reading type
(iii) The true R.M.S. reading type

(i) The Average Reading Type :

In this type the dial calibration will be made multiplying the average output of the rectifier by the form factor 1.11, for the sinusoidal waves. For square wave inputs the reading will be more than the true R.M.S. value by 11%. For triangular wave inputs the reading will be less than the true R.M.S. value by 4%.

(ii) The Peak Reading Type :

In this type the detectors response will be quick for increasing amplitudes of the input signals. i The response will be slow for the decreasing amplitudes. As it is the peak detector that is employed, it "holds" the peak indefinitely. Practically a suitable decay will be obtained by properly choosing the time constant selecting the value of capacitor and load resistor in the peak detector.

(iii) The True R.M.S. Reading Type:

The true R.M. S. type indicates the R.M.S. value of the input signal for any type of waveform. It is for this purpose the amplifier and the detector must be confined to their linear region of operation. The true R.M.S. indication will be specified for a given maximum value of "crust factor", which is less than 5. The crest factor will be more than 5 in special cases like the pulse train that have a low duty ratio. Therefore the true R.M.S. meter is generally specified for the crest factor value of 5, for an input signal having an R.M.S. value that equals the full scale deflection.

(b) Description of the Block Diagram:

The block diagram of the a.c. millivoltmeter is shown in Figure. From the block diagram it can be seen that the input signal to be measured will reach the wide band amplifier through a calibrated attenuator.

The output of the wide band amplifier is supplied to the linearized detector. The output of the detector is given to the indicating instrument. Two more detectors which are optional have been shown in the block diagram to indicate the three types of arrangements that can be hand in this type of instrument. The two other blocks of detectors are shown to have been connected in dotted lines. The choice of the detector is optional. Further the output of the wide band amplifier can be terminated over a socket as an optional output terminal, as is shown in the block diagram.
Block Diagram of AC Millivoltmeter
(c) Measurement Method :

The zero setting of the indicating instrument is to be adjusted correctly to zero after the normal warming time of the instrument. The signal input is applied to the input terminals. The range switch is initially is to be set to the largest range. By adjusting the range switch, a reading close to the full scale reading is to be obtained which accurately gives the magnitude of the input signal.

(d) Applications :

Noise analysis
Turbulance studies
R.M.S. power measurement in power systems using thyristor.

(e) Limitations:

The rectifier amplifier type of A.C. millivoltmeters performance is limited by the bandwidth of the amplifier. They can be used up to around 5 MHz, beyond which their accuracy will be poor.

Secondary Storage devices in Computer

The Secondary Storage devices in a Computer are as follows.

Magnetic Tape:

Magnetic tape memories are analogous to the familiar audio tape recorders. Basically a magnetic tape drive consists of a spool on which a magnetic tape is wound. The tape is ½" wide, made of plastic and finely coated with magnetic oxide. The tape is transported across a set of magnetic heads and is taken up on another spool. These heads are mounted between the two spools and are used to write and read information from the tape.

The magnetic medium on the tape surface can be magnetized in one or the other direction by the magnetic head. These two directions of magnetisations represent 0 and 1. For writing on the tape, current is sent in one direction through a coil on the head. A magnetic field is created in the gap in the head. This field in the gap magnetizes a point below it on the tape. The direction of current through the coil of the head determines whether a 1 or 0 is written.

Most tapes have 9 horizontal rows called Tracks on half an inch wide tape. Eight tracks are used to store a coded character called a byte. The 9th track is used for 'error detection' called a parity bit.

The 9 bits for a character including the parity bit is written widthwise (Transverse) in one column. All the 9 bits for a character in one column is recorded simultaneously by the heads.

Tapes in 10.5" dia. have a length of 2500 feet. The data densities may be 800, 1600 or 6250 character/inch. The speeds are 75, 125 or 200 inch/sec.

While recording data on tapes, many records are combined as a block and written on to the tape. Two successive blocks of records are separated by an inter block gap of 0.5" width where nothing is recorded.

The data can be read or written only in a serial fashion, one character after other. Thus if a particular data is at the end of a tape, it can be read only after the earlier parts are read. Thus records in a file can only be organized sequentially in a tape.

Magnetic disk:

These are random access devices where a particular data can be accessed directly (at random). There are two types of magnetic disks. They are floppy disk and the hard disk.

Floppy disks

These are also called diskettes. They are thin flexible disks made of mylar with a coating of a magnetic material (Iron oxide). They are circular disks kept in a square protective jacket of size 3.5". A typical floppy disk can store 1.44 MB of data. (A 51/4 inch floppy disk used earlier could store 360 KB or 1.2 MB of data). A sliding metal shutter protects the disk from finger prints, dust and dirt. Floppy disks contain a write protect mechanism. A plastic button can be slided into either of the two positions. In one position the disk can only be read and no data can be written to it. (write protected).

The surface of the disk is divided into a number of concentric circles called tracks. Each track is divided into pie-shaped wedges called sectors. Two or more sectors combine to form a cluster.

There is a hole at the centre to enable the disk to be held properly by the drive spindle of the floppy drive.

Data are recorded on the tracks. The disc can be rotated by rotating the spindle of the drive. A read/write head is held in physical contact with the floppy. The head can move in a radial direction and can be placed above any track on the disk.

The movement of the head to position it on a particular track is controlled by a servo mechanism, using a stepper motor. To read or write data in a particular point, the read/write head is moved to the desired track and the disk is rotated until the proper sector is brought below the head. By the linear movement of the head and the rotation of the disk, data can be accessed at random.

Hard disks

A hard disk is made of aluminum or other metals or metal alloys. The disk is coated on both sides with magnetic material. To increase the storage capacity, several disks are packed together and mounted on a common spindle of the disk drive to form a disk pack. The term cylinder is used in case of a disk pack to refer to a collection of like numbered tracks on all surfaces of the disk.

A Magnetic Disc Memory

Each surface of the disk contains several tracks and sectors, A set of read/write heads assembled on a common arm can move in a radial direction and can be placed on any cylinder. In a particular position of the read/write heads, the data that can be accessed in all the surfaces is thus a cylinder. The disks are often addressed by surface address, cylinder address and sector address.

While writing, when all tracks at a given head position are filled up, the heads move to the next cylinder's position. This will reduce the head movements necessary. While reading, given the address in terms of surface, cylinder and sector, the read/write head moves to the required cylinder and the disk rotates until the proper sector comes below the R/W head. The data can now be read from the given surface. Thus data can be efficiently read using the cylinder surface concept.

The disk pack is sealed in an airtight dust free chamber along with the read /writes heads. The disks can rotate at a high speed of 3600 rpm or more.

Each disk may have 1000 or more tracks on its surface with a bit density of 10,000 bits per inch of a track.

The time taken to position the read/write heads over a cylinder is called the 'seek'time. The time taken to rotate the disk until the required data are brought below the read/write head is called 'search' or 'latency' time. The sum of search and seek times is called 'access' time.

Optical Disks

Laser beam is used to read or write data in optical disks. These disks have very large capacity. For example a 5.25" optical disk stores 650 MB of data. Only one surface of the disk is used for recording. It is relatively inexpensive and has a long life of at least 20 years.

Some of these disks are re-usable. In these types of disks, the data stored can be erased and new data can be recorded. Ordinary type disks are non re-usable in the sense the data recorded can not be changed by the user.

There are three types of optical disks. They are CD-ROM (Compact disk read only memory), WORM (write once read many) and Erasable Optical disks.


In the case of compact disk read only memory type, the manufacturer writes data at the time of manufacturing. Users can only read the data. New data cannot be written into it. The disk is made of a resin, such as polycarbonate. It is usually coated with aluminium and is highly reflexive. Data are recorded by pointing a laser beam to a spot on the disk. The laser beam produces a tiny pit at the spot representing a bit 1. Thus a spot exposed to the laser beam record is and spots not exposed to laser beam record 0s. While reading, laser of low intensity is used. Depending on the light reflected from a spot, a photo diode reads the data of a spot. If a spot contains a pit, it spreads the light so that the photo diode receives little reflected light, whereas light reflected from spots where there are no pits, the photo diode receives more reflected light. These two conditions will represent either a 1 or 0, as sensed by the photo diode.

CD-ROM drives vary in speed. The original CD-ROM format transfers data at a maximum rate of only 150 kilo bits per sec (150 kbps). A faster CD-ROM drive is 2X (double speed) drive. It can transfer 300 kbps. Some of the latest drives can transfer 3.6 million bits per sec. (3.6 Mbps).

WORM (Write Once Read Many)

In this type of optical disk, the users can write their data on a blank disk, but only once. However the data can be read as many times as desired. Such a disk writes a 1 by melting or burning the temperature sensitive dye coating on the disk. They have very large capacity and al e used to store archival data.

Erasable (Re-usable) Optical disks

It is a Read/Write optical disk memory. Users can read data recorded in it or erase it and write new data onto it. These disks are coated with a magnetic material. The writing or reading is done by a magneto optical system.

The read /write head contains laser beam and a current coil. When a current is sent through the coil, a magnetic field is produced. This field cannot magnetize a spot on the disk at room temperature. But if a laser beam is focused onto the spot, the magnetic properties of the coating varies and now if a current is also sent through the coil, then because of its field, a tiny magnet is produced at the spot representing a 1. All other spots will be i Os. While reading, a polarised laser beam is employed. The light reflected from a spot with the tiny magnet will be rotated by a few degrees which can be sensed by the optical system to represent a bit 1.

To erase a bit, the spot is heated by a laser beam in the absence of the magnetic.field. The tiny magnet disappears. After erasing the tiny magnets, we can again record new information on the disk. These disks are more costly.

Digital Video Disk (DVD)

Digital video disks are designed to work with a video player and television. Unlike compact disk (CD) ROMs, they can store very huge volumes of data. Thus a single disk can store an entire digitized movie. They have storage capacities of the order of 17 GB. With higher data transfer rates such as 12 Mbps, DVDs use a typical format for recording.

DVD - ROM : Digital video disk ROM with a different format is used for computer data. They need a special drive, DVD-ROM drive and a computer. DVD-ROM drives can also play DVD videos and existing CD-ROM even though they use a different format.

A Read/Write version called DVD-RAM enables computer users to create DVD-ROM disks containing upto 2.6 GB of data.

Block Diagram of Personal Computer System

Block Diagram of various components in a Personal Computer System:

To function properly the computer needs both hardware and software. The hardware (H/W) consists of the mechanical and electronic devices, which we can see and touch. The software (S/W) consists of programs, the operating systems and the data that resides in the memory and the storage devices.

The input unit accepts data and instructions from outside through various input devices such as Key-boards, Mouses etc.
The processor (CPU) performs the arithmetical and logical operations and also controls the entire operation of the computer system.
The output (O/P) unit communicates the results of processing to the outside world through various output devices such as monitor, printer etc.
The storage unit stores the information in storage devices like hard-disk, floppy disk etc.

A block diagram of a computer which shows the interaction of various components is as shown in figure.
Block Diagram of Personal Computer

System Unit :

It includes the CPU, storage units such as RAM (Random Access Memory), ROM (Read Only Memory), hard disk drive, CD Drive, floppy disk drive etc.


The C.P.U of a computer performs all the arithmetic operations, takes logical decisions and coordinates activities of all the other units of a computer. The C.P.U is thus rightly called the heart and nerve centre of a computer. To perform these actions C.P.U has the following sub units.

1) Arithmetic and Logic Unit (ALU)
2) Memory unit
3) Control unit


The arithmetic and logic unit (ALU) performs all the arithmetical and logical operations. Thus when two numbers are to be added, these are sent from memory to ALU, where the addition takes place and the result is put back into the memory. The same way other arithmetical operations like subtraction, multiplication, division etc. are performed.

For logical operations also, the numbers to be compared (whether they are equal, whether one is less than the other, whether one is greater than the other etc.) are sent from memory to the ALU, where the comparison takes place and the result is returned to the memory. The result of a logical operation is either True or False. Thus if we compare the numbers 5 and 7 as 5 > 7 ?, the result is false. If we check the relation 5 < 7, the result is True. Such logical operations provide the capability of decision making to the computer.

Control Unit:

The control unit is the most important part of the CPU as it controls and co-ordinates the activities of all other units, such as ALU, memory unit, input and output units. This unit interprets instructions and transfers data from main memory to the ALU for processing.

Under the direction of a program, the control unit performs four basic operations.

1. Fetch : Getting the next program instruction from the computer's memory.
2. Decode : Figuring out what the program is telling the computer to do.
3. Execute : Performing the requested action, such as adding two numbers, deciding which one of them is larger etc.
4. Write back : Writing the results to an internal register (a temporary storage location) or to memory.

The four steps form a 'machine cycle' or a processing cycle and consist of two phases. - The Instruction cycle (fetch and decode) and the Execution cycle (execute and write back). Today's microprocessors can go through these four steps millions of times per second.

Memory Unit:

The memory unit stores all the instructions and data which are needed at the time of processing. It is also known as primary storage or main storage or intermediate access memory.

The main memory or primary memory of a computer is made of semi conductors. It is used to store data and instructions needed immediately by the CPU, to store intermediate results of processing. The memory can be thought of as 'Cells'. Each of these cells is further broken down into smaller parts known as 'bits'. A bit is a binary digit which is either 0 or 1.

Measure of Memory:

A group of 8 bits is called a Byte. One byte can store one character. The smallest addressable unit of memory is a byte. Bigger units are kilo Byte (KB), Mega Byte (MB), Giga Byte (GB) etc.
1KB = 2'° = 1024 bytes
1MB = 210 KB = 1024 x 1024 Bytes
1GB = 210 MB = 1024 x 1024 x 1024 Bytes.
The memory is usually organized into words of fixed lengths. Each word has the same number of bits, called the word length. Small machines may have word size of 1 or 2 bytes. Large machines have word sizes 4 or more bytes.

The memory addresses are consecutive numbers starting with address 0, 1, 2,   Thus at address 0, we find the first word, at address 1, the second word and so on.

A memory with 4096 locations with each location having 16 bits is called a 4096 word 16 - bit memory or 4K 16 bit memory (4K = 4 x 2'° = 4 x 1024 = 4096).

Random Access Memory (RAM):

The read and write memory of a computer is called RAM. Users can either read from it or write information into it. It can be accessed at random. However it is a volatile memory, since its contents will be erased when power goes off.

RAM is further classified into Static and Dynamic RAM.

Static RAMs store the information as long as power is on. Dynamic RAMs lose the information in a very short time (within milli sec.) even if power supply is on. So they need refreshing circuitry to periodically refresh its contents.

Static and dynamic RAMs use MOS (Metal Oxide Semi conductor) technology. Now-a-days CMOS (Complementary MOS) technology is used.

Read Only Memory (ROM):

We can only read from this memory, but we can not write new data into it. The contents of the ROM is decided by the manufacturer at the time of manufacturing. They are generally used to store fixed programs like monitor, assembly, debugging 111. programs etc. They are non volatile. That is they retain the information even if the power supply goes. off.

There are different types of ROMs.

1. Programmable Read Only Memory (PROM): They can be programmed (data recorded into it )by the user, but only once it can be written.

2. Erasable Programmable Read Only Memory (EPROM): In this case, the data recorded can be erased and new data can be written into it. The stored data is erased by exposing it to high intensity ultraviolet light for about 20 minutes.

3. Electrically Erasable PROM (EEPROM): The contents of such a chip can be erased and reprogrammed on the board itself on a byte by byte basis. Intel 2815A is a 16K (2K x 8) EEPROM. It is also used as a back up to RAM whose contents may be lost in a power failure. When power returns EEPROM memory can be used to replace the lost contents of the RAM.

Secondary Storage Devices:

Primary storage is expensive because each bit is represented )y a high speed device, such as a semi conductor which is costly. often it is necessary to store large volumes of data (Millions, billions or trillions of bytes). Therefore slower and less expensive storage units are used for computer systems. These are called secondary storage devices. Magnetic tapes, floppy disks, hard disks, optical disks etc. are used as secondary storage devices. Although both disks and tapes are widely used, the trend is towards disk storage for active files and magnetic tape for back up and archival storage.

Data are stored in them in the same binary codes as in main storage and are made available to main storage when needed, at the time of processing.

Data stored in the primary memory is volatile and its contents will be lost when power goes off. Secondary storage devices are cheaper and also the data stored in it is not volatile.

Characteristics of Secondary storage devices:

Non Volatile: The data recorded in the secondary storage devices like magnetic disk, magnetic tape, compact discs (CD-ROM) etc. are permanent and not volatile. That is, once recorded, it is not lost due to a power failure as in the case with random access memory (RAM) used in the main memory of the computer.

Massive capacity: Very large amounts of data can be stored in these devices. Thus for example a CD ROM can store 650 Mega Bytes (MB) or more of data in a single disc. Magnetic tapes, hard disks etc. can also store very huge volumes of data. Organizations using large data bases need secondary storage. They can also be used to store historical data or archival data.

Cost effective: The cost of secondary storage devices like floppy disk, hard disk, CD ROM etc. is reducing as a result of mass production techniques employed in their manufacture. They are very cheap storage devices.

Modular Expandability: There is no restriction in the amount of data being recorded as secondary storage is easily expandable. Data scan be categorized and stored on different tapes, CD ROMs, floppy disks etc. Whenever necessary the data contained in the secondary storage (disk or tape) can be loaded into the main memory by using appropriate discs or tapes.

Basic Operation of a Computer

What is a Computer?

Computers are growing in popularity day by day. It is the most powerful tool man has ever invented. Computers have made a great impact on our everyday life. Their presence is felt at almost every working place, viz., homes, schools, colleges, offices, industries, hospitals, banks, retail stores, railways, airlines, researches and design organizations and so on.

A computer is typically a programmable computing machine. Older computers were used for tough calculations and used only by scientists and technical engineers. The trend then was to design big and high performance computers to manage huge data and calculate difficult problems. They were very expensive and thus only big organizations could buy them. Technical developments in design and fabrication of semi conductor devices, today, have made possible manufacture of powerful micro computers. The cost of these computers has come down drastically so that even small organizations and individuals can afford them. These computers which are very fast can be utilized not only for computation, but also to locate and retrieve data, to control some processes and machines, to calculate and show certain quantities to send or receive e-mail, to search the web to see a movie and so on. We can also store, retrieve and transmit text, graphics, pictures and sound.

Within limitations imposed by their designers, computers can do whatever they are programmed to do and no more. They cannot "think" for themselves, but their method of dealing with the problem is similar in many respects to the human thought process.

Consider a simple task like deciding whether to take an umbrella when you go to work in the morning. You open the door and look at the sky. You take that information into your brain and store it. You may not be aware of it, but you are. You then compare it to previous experiences and decide; if it is raining or very cloudy, an umbrella is needed. So you pick up one and go out. This can be summarized in more general terms as follows.

1. You retrieve information through senses: feeling, smelling, tasting, hearing or touching.
2. You keep the information in your memory.
3. Other information about similar circumstances are retrieved from another part of your memory.
4. You take a decision logically, for example, comparing two pieces of information.
5. You act on the basis of the decision arrived at, for example, speak, walk, use your hands or look for more information.

Though it may seem silly to break down each action to this extent, this is exactly how a computer works. It gets data or information through some form of input. This information is stored in memory. But, unlike the human brain which usually remembers all the information (through the eyes, ears, nose tongue and touch) the computer retains only the information it has been programmed to store. Any other information is lost. If a computer has been programmed to compare two bits of data, it will do so and take an appropriate decision. The result is transmitted to the output unit of the computer.

A computer, therefore, follows a series of instructions that have been programmed into its memory by the user and operates on data that are presented to it by the user.

The data input may be in the form of:
Typed (linguistic) data or instructions in computer's language. The linguistic raw data can be numeric, alphabetic or alphanumeric.
  It may include audible, visual or audio-visual analogic data.

A computer can process information only in accordance with the program put into it. It can process data only according to the instructions given in the program, but it cannot think by itself and cannot deal with incomplete information, erroneous data etc., as it has no common sense. The programmer should anticipate such conditions and program accordingly.

What makes today's computers so valuable is that they are very fast and reliable. Even the low cost personal computers can perform several million operations per second and can do so for years. Also they can store huge amounts of information and move this information from one place to another at incredible speeds.

In simple words, a computer is an electronic device which processes information, based upon the instructions provided and generates the desired output. Like any other system, a computer also requires an input which is processed to produce the desired output (refer Figure). Computer needs two kinds of inputs. One, the basic or raw data and two, a set of instructions containing the methodology to process this data. This set of instructions is called a program or software.

Basic Operation of a Computer
The five major characteristics of computers which make them so powerful and useful are:

1. Speed: They work at an incredible speed. The speed is measured in terms of instructions per second (IPS). A modern computer can process information at the speed of a couple of million instructions per second (MIPS).

2. Accuracy: In addition to speed, computers are also highly accurate. They either give correct answer or do not answer at all. However a computer is capable of doing only what it is instructed to do. If faulty instructions are provided for processing data, obviously faulty answers will be produced.

3. Consistency: Computers are highly consistent. They never get bored by repetitive work. Hence they are ideal for carrying out repetitive and voluminous work.

4. Storage capacity: Today's computers can store huge amounts of data. Once recorded, a piece of information is never forgotten and any information can be retrieved almost instantaneously. Thus a single CD ROM can contain the entire Encyclopedia Britannica and more.

5. Flexibility: Computer is a versatile machine and its use is limited only by your imagination. Today computers can practically be used in any field to great advantage. Unlike other machines which are designed for a particular task, computers are the most general purpose machines. You can use computers to play music, see movies, type letters, send e-mail, diagnose illness, fix problems in complex manufacturing operations, design buildings and bridges and so on and so forth. 

However computers have one major limitation. They are not originally creative and they will never be.

What are Computer Programs?

A computer can do only what a programmer asks it to do. To perform a particular task the programmer writes a sequence of instructions, called a "Program". An instruction is a command given to the computer to perform a certain specified operation on a given data. A set of programs written for a computer is called 'software'. As the computer cannot think on its own, the programmer has to provide a method to solve a problem.

The first step in tackling a problem is to define the problem. Then the programmer writes the procedure how to solve the problem. The procedure must be written in the form of a series of steps in a logical sequence. A precise statement of the procedure required to solve a problem is called an 'algorithm'. Having obtained the algorithm for solving a problem, the algorithm is expressed in a pictorial form called a 'flow chart'. It provides a visual indication of the steps involved in the procedure and the logical sequence in which the steps are performed. The flow chart is very much helpful in writing the actual program which contains the precise computer instructions to be performed in a given order.

Bolometer Method of Power Measurement

In this method of R.F. power measurement, the unknown power will be absorbed in a specially constructed bolometer element of resistance material. The resultant temperature raise is then detected by measuring the change in bolometer resistance using a bridge circuit.

(a) Bolometer:

A bolometer consists of a coaxial cable for the input through which the R.F. power will be fed . The cable has a short tapered section to transform the line impedance to that of the bolometer. A stub provides a ground return path as provided by a capacitor for the R.F. currents.

The bolometer consists of an element, which may be a thermistor or a barretter. A barretter is basically a wire mounted in a catridge similar to a fuse. Its resistance increases, with temperature. A thermistor is a small semiconductor bead with connecting wires. It has a negative temperature coefficient. The bead material is a mixture of manganese and nickel oxides combined with finely divide copper particles to control the resistivity. The bead diameter is about 0.5 cm. There are several shapes of thermistors. They are available as rods disks, washers or flakes.

The bolometer element is connected to the bridge. The bridge will be excited simultaneously by direct current from the voltage source and by the alternating voltage at audio frequency. Thus the bolometer element is simultaneously heated by the D.C. power, audio frequency power and radio frequency power being measured. The resistance of the bolometer is dependent on the total power and is dependent on where this power comes from.

(b) Arrangement for Measurement:

The arrangement for Bolometer method of R.F. Power Measurement is given in Figure.

Arrangement for Bolometer Method

The output from the bolometer element is connected to the arm CD. Exitation of the bridge is by two sources namely the A.F. or low R.F. source through a capacitor C and through the battery supplying the voltage E. The galvanometer G is used to act as the null detector.

(c) Measurement Method: 

The radio frequency power reaches the bolometer element through the coaxial cable. A nominal voltage from the audio frequency source is applied to superimpose upon the radio frequency power being measured. The direct current from the voltage source E will be next adjusted by the use of the rheostat R till the heating of the bolometer causes its resistance to have the value required to balance the bridge. The galvanometer acts as null detector and indicates the balancing of the bridge.

When once the balance is obtained the radio frequency power is switched off. This leads to unbalancing of the bridge as the bolometer element loosing the heat in the absence of R.F. power.

The audio frequency power will now be increased to re-balance the bridge. The resistance of bolometer depends on the total power dissipated irrespective of the frequency. Hence the radio frequency power measured will be directly proportional to the audio frequency power that causes the balance condition.

Taking the voltage from auxiliary supply as V1, before the radio frequency power is switched off, and further taking the voltage from the auxiliary supply after the second balance as V2, the R.F. power is

V22-V12/ 4R1

To obtain the maximum accuracy the audio power initially supplied to the bolometer element should not be large compared to the radio frequency power being measured.

(d) Precautions:

The bolometer and the associated bridge circuit must be properly shielded.

The initial audio power supplied to the bolometer element should not be large compared to the R.F. power being measured.

(e) Applications of RF Power Meter:

They are useful in the measurement of RF power at

The output of transmitters
Cable terminations
Antenna feeders

RF Power Measurement using Thermocouple Instrument

An elementary method of measuring radio frequency power using the thermocouple instrument is shown below. In this method a thermocouple ammeter along with a non-inductive resistor are employed. The tank circuit of the device whose power output is to be measured will be linked through a suitable transformer, to yet another tuned circuit. This tuned circuit consists of the secondary of the transformer a capacitor, a known value of non-inductive resistance and a thermocouple ammeter. The value of the resistor must be known and the resistor must have a power rating which is atleast twice the power that is expected to be measured. It must also have the temperature coefficient that shall not bring any change in its value due to the dissipation of the power being measured.

(a) Description of the Circuit: 

An arrangement for measurement of R.F. power is shown in Figure. In this circuit the L1 is the tuning coil and C1 is the tuning capacitor of the tank circuit of the source whose power is to be measured.
Measurement of RF Power using Thermocouple Ammeter
The tank circuit is linked using the transformer T whose primary is connected to the source tank circuit. The secondary of the transformer is connected to the meter through a capacitor CS and the resistor R. The transformer T is termed the linking transformer and it is tuned in its secondary by the capacitor CS, The resistance R used in the circuit is the non-inductive resistance of the required power rating

(b) Measurement Method:  

The primary of the linking transformer will be connected to the source whose power is to be measured. The capacitor in the secondary of the linking transformer CS, will be adjusted in its value such that it tunes the secondary circuit to the same frequency of source. As the secondary is tuned to the frequency of source and is at resonance, the only resistance in the circuit is the resistance ‘R'. Hence the thermocouple instrument can be calibrated in terms of power. It indicates the power output of the source under that condition.

Audio Frequency Power Meter

Measurement of power at audio and radio frequencies is largely different from power measurement at power frequencies. The frequency and matching of the load impedance play important role at frequencies other than power frequencies.

For audio- frequency power measurement a rectifier type voltmeter will be used choosing proper diodes for the rectifier that offer minimum capacitance. A standard resistance of 600 ohm will be taken and the rectifier type voltmeter will be connected across it.

When 1 W of power is dissipated across this resistance the voltmeter reads 0.775 V. This can be taken as 1 mW over the scale of the voltmeter. This principle is extended to design a Power Output Meter. Such an arrangement is useful only in the frequency range from 20 kHz to 20 kHz. If accuracy is not important its range can be extended up to 100 kHz.

The rectifier type voltmeter cannot be used for Radio Frequency Power Measurement. The reason is the effect of capacitance of the rectifier, the inductance offered by the moving coil and the power consumed by resistor at Radio Frequencies.

Replacing the rectifier instrument with the thermocouple instrument can improve the situation. Even the thermocouple instruments cannot offer error free performance beyond 100 kHz. Special measurement techniques are used for high frequency power measurements.
In this blog the following topics are covered :

1. Measurement of audio frequency power
2. The A.F. Power Output meter
3. R.F. Power measurement using thermocouple instrument
4. Bolometer method of R.F. power measurement

The Audio Frequency Power Output Meter:

The Audio Frequency Power Meter is one that measures directly the absolute power in mW or in dB, with reference to a chosen signal level, derived by an audio frequency system, in to a variable external load. The load will be incorporated in the power output meter itself.

(a) Description of the Block Diagram of the Power Output Meter :

The block diagram of the audio frequency power output meter is shown in Figure. It consists of a variable ratio transformer, a loss adjusting network, meter multiplier and the indicating meter. The indicating meter is nothing but a rectifier type of voltmeter calibrated in terms of dB, with reference to the power allowed in to a load of known impedance contained in the instrument itself and is substituted in place of the original load impedance where the power is developed.

Block Diagram of Power Output Meter

The rectifier type of voltmeter is calibrated from 1-50 mW with an auxiliary scale, reading from 0-17 dB, above the reference level of 1 mW. The input impedance of the meter ranges from 2.5 ohm to 20 k ohm, consisting of forty steps, each step providing 25% of increase.

The internal arrangement of the instrument is such that the variable ratio transformer and the loss adjusting network put together will match the known value of standard resistance across which the voltmeter is connected. Therefore, for the different input impedance settings the meter is matched and will give the power output directly in watts.

The range selector switch changes the voltmeter's multipliers to get the higher range of power.

(b) Measurement Method :

To measure the output power of an audio frequency source the regular load to the source will be disconnected. Knowing the load value the power output meter's impedance switch will be adjusted to the load value. The range selector switch will be adjusted such that the required multiplier is connected to the meter. Now the indication given by the meter is the power output given by the source.

(c) The dB Scale :

The power output meter's scale will also be calibrated in decibels. One milli watt (1mW) is taken as the reference power and this is taken as '0' for the dB scale as the power output meter is frequently used for measurement of the power output of radio receivers during the standard tests etc., The 50 mW power is taken at the lowest useful power output from a receiver or the like to be of practical value of the listener. 50 mW is calibrated as 17 dB and powers less than 1 mW are given the negative reading in dB.

The decibel scale can be used for direct measurement of the gain in dB provided that the impedance of the power output meter is properly adjusted for the load impedance of the source giving the signal. The dB scale is graduated to read from -10 dB to +17 dB with a reference of '0' dB at 0.775 V which is the voltage that is developed across the standard resistance of 600 ohm when 1 mW of power is dissipated across it. To read dB over the scale the range selector switch will have to be kept in '0' dB position. For all other range switch settings algebraic sum of the scale reading and the range switch indication must be taken.

(d) Design Variations :

Transistorized power output meters have been designed with better accuracy over a better frequency range. These use a high power wire wound resistor of the same value of the load of the source in place of the load. Different values of such resistances are made available which could be selected by a rotary switch and multiplier switch.

The voltage across the resistor is given as input to a 'transistor amplifier which drives the meter. An attenuator will also be ganged with the resistance selector switches to provide a direct reading of output power on the meter. The frequency range of such output meters can be extended up to 100 kHz with a little reduction in the accuracy.

The current trend is to use a digital power output meter that displays numerically the power output over a display.

(e) Applications of A.F. Power Output Meter :

It can be used for the measurement of audio power in the output of radio receivers, audio stages of T.V. receivers and Public address amplifiers.
It can be used as the output indicator in conducting standard tests on communication receivers and radio receivers.
It can be used as a null indicator in A.C. bridges working at audio frequencies.
It can be used as an output indicator in transmission measuring set in telephone lines.
It can be used to determine the source impedance of the equipment under test by finding the load in to which maximum power is delivered.

(f) Specifications of Audio Power Output Meter :

Power range: 0.1 mW to 10 W in four ranges
Impedance range: 2.5 ohm to 20 k ohm in 40 steps
Impedance Multiplier: xl, x10, x100 and x1000
Frequency range: 20 Hz to 20 kHz useful with reduced accuracy up to 100 kHz
Power supply: 230 V 50 Hz A.C.

Measurement of Phase using CRO

Measurement of Phase using CRO:

Cathode ray oscilloscope is used to determine the phase difference between two signals of the same frequency. The procedure is to apply one of the signals to the horizontal deflecting plates and the other to the vertical deflecting plates. The result is a pattern that reflects the character of the phase difference. Typical pattern for a phase angle is shown below in Figure.

The phase difference q between the two signals is given by

Sin θ = ± B/A

Where A is the maximum vertical deflection produced and B is Y intercept.

Irrespective of the relative amplitudes of the applied signal voltages the ellipse provides a simple means of finding the phase difference between the signals. The gains of vertical and horizontal amplifiers can be adjusted to get the ellipse in a square marked by the coordinate lines on the graticule. If the major axis is in the first and third quadrants the phase angle is between 0" and 90° or 270° and 360°. If the major axis passes through the second and fourth quadrants the phase angle is between 90° and 100° or between 180° and 270°.

Measurement of Time Interval using CRO:

Time intervals of short duration can be measured using CRO. The sweep is triggered by the pulse that serves as a time reference. The length or duration of this driven sweep is adjusted so that both the pulses are visible on the screen. If the approximate sweep speed is known, a rough measurement of the time interval separating the two pulses can be determined from the linear distance separating the pulses on the screen.

Greater accuracy can be had by markers (or indexes or cursor). Makers can be superimposed upon the cathode ray trace. These markers have their origin in a sine wave oscillator of accurately known frequency. The waveform from this oscillator is then modified by amplification, clamping and differentiating until a series of sharp pulses are obtained.

The operator or user has to adjust the position of the marker until it coincides with the observed pulse, the time interval is given by the setting of the position control.

Measurement of Frequency using CRO

A CRO can be used in number of ways to measure frequency when two sources of identical frequencies are available. The frequencies may also have harmonic relationship with each other. The CRO establishes the harmonic relationship without the necessity of generating the harmonic. The three methods commonly used for measurement of frequency using CRO are listed below.

1. Lissajous figures.
2. Spot wheel pattern.
3. Gear wheel pattern.

(a) Lissajous Figures Method :

This is the simplest method to determine the frequency relation of two sine wave voltages. One voltage is used to produce vertical deflection, while the second is used to give the horizontal deflection. lf the ratio of the two frequencies involved can be expressed by an integer, or by a ratio of integers the pattern obtained on the CRO is dependent on the relative phase of the two signals. (Phase measurement can also be made in this way).

The pattern will be stationary when the ratio of the two frequencies is correctly a ratio of integers. The ratio of horizontal to vertical frequency is given by the number of times the figure (pattern) is tangent to the horizontal line divided by the number of times its end is tangential to the vertical line. This rule is valied when the forward and return traces do not coinside. When the phase difference is 0° and 180° they do coinside, and the rule is not applicable in those conditions.

When the frequency ratio of the two signals is not exactly equal then the pattern will rotate or barrel or wave about as i f the relative phase of the two deflecting signals is continuously changing. If the frequencies differ very much we see only a luminous rectangle on the screen.

Lissajous figures for different frequency ratios and phase angle are shown below in Figure.

Lissajous figures method

It can be observed from the figures that when the phase difference is `W, the patterns observed are interesting. When the frequency ratio is 1 we get a circle, for a ratio of 1:2 we get a figure of eight and so on. The patterns observed for ratios 2:3 and 3:4 are slightly complicated. To determine the frequency ratio of such complicated patterns the following procedure is used. If a tangent is drawn against the top edge of the pattern for 2:3 pattern it will make contact with the pattern at two places. Similarly drawing a vertical tangent along the vertical side will contact at three places. So it is evident that the horizontal tangencies correspond the vertical frequency in their number and the vertical tangencies correspond the horizontal frequency, hence the ratio 2:3. The same rule can be applied to pattern of 3:4 frequency ratio, or in general any pattern of this type. It is to be noted here that the pattern not only depends on the frequency of the voltage applied to the deflecting plates but also on their phase. So the circle can be obtained only if the ratio is 1 and phase difference is 90°. If the ratio is I and the phase is not specified a straight line inclined to right or left, an ellipse inclined to right or left, or a circle can appear.

Lissajous figures become difficult to interpret if the frequency ratio is large as complicated patterns will result. Alternate methods are available as described here under.

(b) Spot Wheel Method :

In this method a circular sweep is produced using the low frequency source and a R.C. combination in series. When the mid-point of the series combination is taken as reference the voltages out of the other two terminals will have a phase difference of 90°. This is called the phase splitting network. The resistance is so chosen that its value is equal to the capacitive reactance at the working frequency. Then the magnitudes of the voltages produced across the two will be equal. This R.C. series phase splitting network satisfactorily works only at one frequency.

The arrangement for split circle or spot wheel method of measurement is shown below along with typical pattern in Figure.

Spot Wheel Method of Measurement of Frequency

The low frequency is used with the phase splitter to get the circular sweep. The high frequency signal is used to modulate the intensity of the electron beam. This can easily be done by applying the high frequency voltage to the Z-modulation terminal of the oscilloscope. This terminal is made available usually on the rear side of the instrument and is accessible through an opening. The resultant pattern consists of a circle of spots in which the ratio of the high frequency to the low frequency is equal to the number of spots divided by a suitable integer 'n' that is less than the number of spots. The pattern will be the same for different values of 'n' so that the proper value of this integer must be determined from other information such as prior knowledge for approximate frequency.

(c) Gear Wheel Method :

An R.C. series phase splitting unit is used in this method to produce a circular sweep supplied by the low frequency signal. The high frequency signal is given to the radial deflecting electrode. This causes the beam to move regularly in and out in accordance with the amplitude and frequency of the high frequency wave. The number of teeth in the wheel pattern that results, divided by the number of times a radial line from center outwards that intersect the pattern, gives the frequency ratio. The Gear wheel pattern is shown below Figure.

Gear Wheel Method

Measurement of Voltage and Current using CRO

(a) Voltage Measurement using CRO :

Both d.c and a.c can be measured on CRO. The CRO will be switched on and a horizontal line is obtained using the internal sweep. The unknown d.c voltage is applied to the vertical input terminals placing the input switch to d.c mode. On application of the voltage the horizontal line shifts. This displacement is measured using the scale marking on the graticule. The magnitude of the unknown voltage can be calculated, by multiplying the displacement obtained in meters with deflection factor.

Unknown voltage = deflection factor x displacement in m. To measure the magnitude of alternating voltage, the CRO is switched on and the horizontal line is obtained. The unknown voltage is applied to the vertical deflecting plates. Now the sweep is withdrawn. A vertical line is displayed by the CRO. The length of this line is measured using the scale marking on the graticule. This length I multiplied by the deflection factor gives the peak to peak voltage of the applied voltage. 1/2 deflection factor gives the peak amplitude of the unknown voltage. 

(b) Measurement of Current using CRO : 

Measurement of current is an indirect method. The unknown current is allowed to flow through a known value of resistance R. The voltage developed across this resistance is applied to the vertical deflection plates of CRO. The displacement obtained is measured and the voltage drop across the known value of resistance is estimated. Now the ratio of the voltage drop across the resistor and its resistance gives the magnitude of current.

Deflection Sensitivity of CRT

The deflection sensitivity of a cathode ray tube is the deflection on the screen given by the beam in meters per volt of the applied voltage.

This is different from the deflection sensitivity of CRO. The CRO's deflection sensitivity includes the gain of the amplifiers between the input terminals and the deflection plates.

The deflection sensitivity of the cathode ray tube of any CRO can be obtained by applying known voltage to the deflecting plates directly. Connection to the deflection plates can be made from terminals provided for this purpose on the frame of the CRO and are accessible from the protecting covers, through a window like opening.

The following block diagram shows the arrangement for the determination of the deflection sensitivity of the CR Tube.

(a) Vertical deflection sensitivity :

To obtain vertical deflection sensitivity, the internal sweep can be used to produce the horizontal line on the CRO. The CRO is to be set up for operation and the horizontal line is to be adjusted in line with one of the horizontal lines on the graticule. A d.c power supply is taken and its voltage is applied between the vertical deflecting plates. A multimeter with good sensitivity or a digital voltmeter is connected across the power supply to indicate the voltage applied to the vertical deflecting plates. On application of the voltage, the horizontal line shifts either up or down depending on the polarity of the applied voltage on the deflecting plates. The voltage is adjusted to get a convenient displacement of the horizontal line. Now the displacement in meters can be noted from the marking on the graticule. From this the deflection sensitivity can be calculated and expressed as meters/volt.

(b) Horizontal deflection sensitivity :

In this case the internal sweep is not necessary. The sweep mode switch can be put to external mode. The vertical deflecting plates will be supplied with a low voltage alternating voltage. The screen will display a vertical line. This line is aligned with one vertical line on the graticule. A d.c voltage from a power supply is applied to the horizontal deflecting plates. The voltage applied is measured with a multimeter or the like. Application of the voltage to the horizontal deflecting plates will shift the vertical line either to left or right. This depends on the polarity on the deflecting plates of the applied voltage. The voltage will be adjusted to get convenient deflection over the scale. The voltage required for that displacement will be noted. From that the deflection sensitivity can be calculated and expressed in m/V.

Procedure for Operation of CRO

The following is the general procedure in switching on a CRO.

1. Keep the vertical shift and horizontal shift controls in their middle positions.

2. Keep the intensity control in its minimum.

3. Keep the sweep mode switch in line position.

4. Now switch on the CRO. Wait for sometime till the CR Tube warms up. Though the rest of the circuit is consisting of semi-conducting devices the CR Tube is a vacuum tube and requires some warming time.

5. Connect the output of the source through the probe recommended by the manufacturer to the vertical input of the CRO.

6. Adjust the vertical input attenuators to get the required amplitude of the signal.

7. The sweep can now be internal and also in the auto mode that does not require fine adjustment. Adjust the sweep frequency selector to get a single cycle or two cycles as per the requirement.

Note : CROs in which triggered sweep circuit is arranged the scope will not give any display without vertical input signal, if the internal trigger mode is selected. The reason is that the sweep generator takes the trigger pulse from the input signal after processing. When the input itself is not present the trigger pulse will not also be present with the result that the sweep is not triggered and we do not get the pattern.

8. Adjusting the sweep frequency, vertical gain, are to be done executing care, and observing the result of adjustment. Careless adjustments will not only result in waveforms that are irrelevant, but also consume lot of time to obtain the expected pattern.

9. In order to utilise the instrument over long periods without spoiling the CR tube the intensity must be kept at the barest minimum possible to get a good visual pattern of the waveform. Spot should never be allowed to stay on at a particular part of the screen with high intensity as it would burn the screen.

10. To obtain best results free from interferences the CRO must be connected to a proper earth terminal using a good earth wire from the instrument.

In the beginning there will be some little difficulty but when one is used for the panel and system it will be very easy and the steps explained above may be altered without causing damage to the equipment. However the procedure is only to switch on and to get the horizontal line. Various measurements require different adjustments and modes of operation that can be learned on regularly using the instrument.

CRO Front Panel Controls

The Front Panel and User Controls in a CRO : 

The front panel of a single trace CRO is shown below. This CRO uses a CR Tube of 8 X 10 cm size. The front panel controls can be seen in Figure. To the right top we find three sockets marked with a square wave, a triangular wave and a sine wave near them. From these sockets we can take voltage of the corresponding waveform for calibration work. Just below the three sockets is the intensity control. Underneath intensity control we have the focus control. The astigmatism, trace rotation controls are below the focus control. The main on off switch is located below the trace rotation control. A LED indicator is provided to indicate the "switched on" condition of the instrument.

The rest of the control panel is divided into two sections. To the left of the panel we have the vertical section and the horizontal section is on the right hand side. In the vertical section to the left side the vertical input socket is provided. The mode switch / input selector is below the vertical input socket marked A.C. ground and D.C. Depending on the nature of the input signal we select the position of the switch and each time when the switch is operated ground connection is established to discharge the input attenuator as explained in vertical section of chapter this. The coarse attenuator for the vertical amplifier is marked VOLTS/CM and has 12 steps; 5 mV/cm to 20 V/cm. The fine attenuator is located above the coarse attenuator slightly towards the right side. The vertical shift control is to the left of the vertical control just below the CRT frame. A magnification switch with markings X 1 and X 5 is below the coarse attenuator that magnifies the input signal. A switch marked CT (component tester) OSC (oscilloscope) is just below the vertical mode switch. This switch converts the oscilloscope to a component tester when depressed.

In the horizontal section of the control panel, below the CRT frame we have horizontal shift control. The Fine frequency control is slightly to the left and below this control to right is the coarse frequency control marked in TIME/CM. These two controls vary the sweep frequency. A magnifier (multiplier) switch is provided below the coarse frequency switch with markings X I and X 5. The various modes as marked at the switch positions, in which the sweep could be operated, are seen near the push type switch column. Above this switch column is the trigger level control. The external trigger pulses or X input signals can be given in, through the socket below the switch column. Below the X input socket is an attenuator control that adjusts the X input signal under external mode. A LED is provided to the left of the X attenuator to indicate if the CRO is in Triggered mode.

The C B E terminals and an additional terminal are meant for utilizing the CRO for the component tester, transistor tester functions. These terminals along with a GND (ground) terminal are located just below the vertical section of the control panel.