5th I & E Electronic Measurement (Module1 & 3) Performance attributes of measurements The essential purpose of instruments is to sense or source things in the physical world. The performance of an instrument can thus be understood and characterized by the following concepts: ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ Connection to the variable of interest. The inability to make a suitable connection could stem from physical requirements, difficulty of probing a silicon wafer, or from safety considerations (the object of interest or its environment might be hazardous). Sensitivity refers to the smallest value of the physical property that is detectable. For example, humans can smell sulfur if its concentration in air is a few parts per million. However, even a few parts per billion are sufficient to corrode electronic circuits. Gas chromatographs are sensitive enough to detect such weak concentrations. Resolution specifies the smallest change in a physical property that causes a change in the measurement or sourced quantity. For example, humans can detect loudness variations of about 1 dB, but a sound level meter may detect changes as small as 0.001 dB. Dynamic Range refers to the span from the smallest to the largest value of detectable stimuli. For instance, a voltmeter can be capable of registering input from 10 microvolts to 1 kilovolt. Linearity specifies how the output changes with respect to the input. The output of perfectly linear device will always increase in direct proportion to an increase in its input. For instance, a perfectly linear source would increase its output by exactly 1 millivolt if it were adjusted from 2 to 3 millivolts. Also, its output would increase by exactly 1 millivolt if it were adjusted from 10.000 to 10.001 volts. Accuracy refers to the degree to which a measurement corresponds to the true value of the physical input. Lag and Settling Time refer to the amount of time that lapses between requesting measurement or output and the result being achieved. Sample Rate is the time between successive measurements. The sample rate can be limited by either the acquisition time (the time it takes to determine the magnitude of the physical variable of interest) or the output rate (the amount of time required to report the result).
Ideal instruments As shown in Fig. 1.3, the role of an instrument is as a transducer, relating properties of the physical world to information. The transducer has two primary interfaces; the input is connected to the physical world (DUT) and the output is information communicated to the operator. (For stimulus instruments, the roles of input and output are reversed—that is, the input is the information and the output is the physical stimulus of the DUT.) The behavior of the instrument as a transducer can be characterized in terms of its transfer function—the ratio of the output to the input. Ideally, the transfer function of the instrument would be simply a unit conversion. For example, a voltmeter’s transfer function could be “X degrees of movement in the display meter per electrical volt at the DUT.” Ideal instruments. A simple instrument example. A common example of an instrument is the mercury-bulb thermometer Since materials expand with increasing temperature, a thermometer can be constructed by bringing a reservoir of mercury into thermal contact with the device under test. The resultant volume of mercury is thus related to the temperature of the DUT. When a small capillary is connected to the mercury reservoir, the volume of mercury can be detected by the height that the mercury rises in the capillary column. Ideally, the length of the mercury in the capillary is directly proportional to the temperature of the reservoir. (The transfer function would be X inches of mercury in the column per degree.) Markings along the length of the column can be calibrated to indicate the temperature of the DUT. . A mercury-bulb thermometer.
Some alternate information displays for an electrical signal. Types of instruments Although all instruments share the same basic role, there is a large variety of instruments. As mentioned previously, some instruments are used for measurements, while others are designed to provide stimulus. Figure 1.3 illustrates three primary elements of instruments that can be used to describe variations among instruments. 1. The interface to the DUT depends on the nature of the physical property to be measured (e.g., temperature, pressure, voltage, mass, time) and the type of connection to the instrument. Different instruments are used to measure different things. 2. The operator interface is determined by the kind of information desired about the physical property, and the means by which the information is communicated. For example, the user of an instrument that detects electrical voltage may desire different information about the electrical signal (e.g., rms voltage, peak voltage, waveform shape, frequency, etc.), depending upon the application. The interface to the instrument may be a colorful graphic display for a human, or it may be an interface to a computer. Figure 1.5illustrates several possible information displays for the same electrical signal. 3. The fidelity of the transformation that takes place within the instrument itself—the extent to which the actual instrument behaves like an ideal instrument—is the third element that differentiates instruments. The same limitations of human perception described in the introduction apply to the behavior of instruments. The degree to which the instrument overcomes these limitations (for example, the accuracy, sensitivity, and sample rate) is the primary differentiator between instruments of similar function. Electronic instruments Electronic instruments have several advantages over purely mechanical ones, including: Electronic instruments are a natural choice when measuring electrical devices. The sophistication of electronics allows for improved signal and information processing within the instrument. Electronic instruments can make sophisticated measurements, incorporate calibration routines within the instrument, and present the information to the user in a variety of formats. Electronic instruments enable the use of computers as controllers of the instruments for fully automated measurement systems.
Oscilloscope: The oscilloscope is, a voltmeter. Instead of the mechanical deflection of a metallic pointer as used in the normal voltmeters, the oscilloscope uses the movement of an electron beam against a fluorescent screen, which produces the movement of a visible spot. The movement of such spot on the screen is proportional to the varying magnitude of the signal, which is under measurement. Basic Principle: • The electron beam can be deflected in two directions: the horizontal or x-direction and the vertical or y-direction. • Thus an electron beam producing a spot can be used to produce two dimensional displays. Thus CRO can be regarded as a fast x-y plotter. • The x-axis and y-axis can be used to study the variation of one voltage as a function of another. Typically the x-axis of the oscilloscope represents the time while the y-axis represents variation of the input voltage signal. • Thus if the input voltage signal applied to the y-axis of CRO is sinusoidally varying and if x-axis represents the time axis, then the spot moves sinusoidally, and the familiar sinusoidal waveform can be seen on the screen of the oscilloscope. • The oscilloscope is so fast device that it can display the periodic signals whose time period is as small as microseconds and even nanoseconds. • The CRO. Basically operates on voltages, but it is possible to convert current, pressure, strain, acceleration and other physical quantities into the voltage using transducers and obtain their visual representations on the CRO. Cathode Ray Tube (CRT): • • The cathode ray tube (CRT) is the heart of the CRO. CRT generates the electron beam, accelerates the beam, deflects the beam, and also has a screen where beam becomes visible, as a spot. The main parts of the CRT are: i) Electron gun ii) Deflection system iii) Fluorescent screen iv) Glass tube or envelope v) Base A schematic diagram of CRT, showing its structure and main components is shown in the Fig.1.