SInternational Standards

The international standards are defined by international agreement. They represent certain units of measurement to the closest possible accuracy that production and measurement technology allow. These standards are periodically checked by absolute measurements in terms of the fundamental units. These standards are maintained at international bureau of weights and measure and are not available to the ordinary users for measurements. International ohm: It is defined as the resistance offered by a column of mercury having a mass of 14.4521 grams, uniform cross-section areas length of 106.300 cm, to the flow of constant current at the melting point of ice. International ampere: It is an unvarying current, which when passed through a solution of silver nitrate in water deposits silver at the rate 0.00111800 grams/sec (g/s).

The primary (basic) standards are maintained by national standards laboratories in different parts of the world. A primary standard is a standard that is accurate enough that it is not calibrated by or subordinate to other standards. The main function of the primary standards is the calibration and verification of secondary standards. These standards are not available for use outside the national laboratories.

Secondary standards are the basic reference standards used in industrial measurement laboratories. These standards are maintained by the particular involved industry and are checked locally against other reference standards in area. Secondary standards are generally sent to the international standards laboratories on a periodic basis for calibration and comparison against the primary standards. They are then returned to the industrial user with certification of their measured value in terms of the primary standard.

Working Standards

Working standards are the principle tools of a measurement laboratory. They are used to check laboratory instruments for accuracy and performance. These standards are used to perform comparison measurements in industrial application. For example, manufacturers of components such as capacitors, resistors etc. use a standard called a working standard for checking the component values being manufactured, e.g. a standard capacitor for checking of capacitance value manufactured.

Cathode-Ray Oscilloscope

A cathode ray oscilloscope or simply CRO or oscilloscope is one of the extremely useful and the most versatile tool used in the sciences, medicine, engineering and telecommunication industry. These are commonly used to observe the exact wave shape of an electrical signal. In addition to the amplitude of the signal, an oscilloscope can show distortion, the time between two events (such as pulse width, period, or rise time) and relative timing of two related signals. General purpose oscilloscopes are used for maintenance of electronic equipment and laboratory work. Special purpose oscilloscopes may be used for such purposes as analyzing aircraft cockpit instruments, automotive ignition system or to display the waveform of the heartbeat. In electronics engineering/telecommunication industry, the cathode ray oscilloscopes are used extensively for design, build and test of electronic circuits. The engineers and technicians studythe wave shapes of alternating currents and voltages as well as for measurement of voltage, current, power and frequency. The oscilloscope allows the user to observe the amplitude of electrical signals as a function of time on the screen. Originally all oscilloscopes used cathode ray tubes(CRTs) as their display element and linear amplifiers for signal processing. However, modern oscilloscopes have LCD or LED screens, fast analog-to-digital converters and digital signal processors. Some oscilloscopes use storage CRTs to display single events for a limited time. These days oscilloscope peripheral modules are available for general purpose laptop or desktop personal computers which allows the laptop or desktop computers to be used as test instruments. Two to three decades ago, the oscilloscopes were quite bulky and were generally bench top devices. But most modern oscilloscopes are lightweight, portable instruments that are compact enough to be easily carried by an engineer or a technician. Special purpose oscilloscopes may be rack mounted or permanently mounted into a custom instrument housing. Oscilloscope allows us to observe the constantly varying signal, 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). Although an oscilloscope displays voltage on its vertical axis, any other quantity that can be converted to a voltage can be displayed as well. In most instances, oscilloscopes show events that repeat with either no change or change slowly. Whatever is the type of an oscilloscope, whether CRT or LCD or LED screen, its front panel normally has control sections divided into Vertical, Horizontal, and Trigger sections. There are also display controls and input connectors. In this chapter, we shall study the general purpose oscilloscopes. Special purpose oscilloscopes are covered in Chapter 8.

Types of Time Base Circuits

A time-base circuit is a circuit which generates a saw-tooth waveform. It causes the spot to move in the horizontal and vertical direction linearly with time. When the vertical motion of the spot produced by the Y-plates due to alternating voltage, is superimposed over the horizontal sweep produced by X-plates, the actual waveform is trace on the screen. Following two types of time base circuits are important from the subject point of view: 1. Free running time base generator. A circuit, in which the periodic sawtooth waveform is generated, without the application of any signal, is called a free running time base generator. Such a circuit is required to display a periodic waveform. It may be noted that in a free running time base generator, the sweep time (Ts ) must be larger than the period of the waveform to be displayed. 2. Triggered time base generator. A circuit, in which the linear waveform with a prescribed duration of time is generated by the application of a trigger signal, is called a triggered time base generator. Such a circuit is required to display widely separated narrow-width pulser or a waveform which may not be periodic but may occur at irregular intervals. The operation of the circuit may be understood from the condition, that the switch (S) is initially assumed to be closed. At this instant, the capacitor voltage and hence the output voltage (vO) is zero, when the switch (S) is opened, the capacitor voltage starts towards the supply voltage (V) in accordance with the equation, VS = V (1 – e –t/RC) ...(i) However, the capacitor is never allowed to charge to a voltage equal to the supply voltage. It is because of the fact that after a time interval (TS), when the sweep voltage has attained the value (VS), the switch (S), again closes. The resulting sweep voltage waveform is as shown in Fig. 7.17 (b). The sweep-speed error for the exponential sweep circuit is given by the relation, eS = V V S It is evident from the above relation that smaller the value of sweep voltage (VS), lower will be the value of sweep-speed error and hence higher is the value of linearity. Due to this reason, the exponential charging sweep circuit is used only in those applications, which require lower sweep voltages (usually of the order of volts or tens of volts). If t/RC << 1, then we can expand equation (i) as follows:

Current Probe

This probe provides a method of inductively coupling the signal to the CRO input. The direct electrical connection between the test circuit and CRO is not necessary. This probe can be clamped around a wire carrying an electrical current without any physical contact to the probe. Thus the magnitude of current with a frequency range from d.c to 50 MHz can be measured using this probe. The current sensor consists of two parts: A conventional transformer for transforming alternating current to voltage, and a Hall effect device for converting direct current to a voltage. The current probe is shown in Fig. 7.52. A magnetic core with a removable piece is used as the coupling element for the current probe. The wire carrying the current to be measured is inserted in the center of the magnetic core and acts as a primary of a transformer. The core is the ferrite U shaped and work as secondary of the transformer. Because of the electromagnetic induction principle, whenever current flows through primary, the e.m.f gets induced in the secondary. This is fed to the CRO input via termination circuitry. When d.c current flows through the wire, it will not appear at the secondary. In addition to this flux in core may increase causing the saturation of the core which is undesirable. This provides inaccurate measurements. To avoid this problem Hall Effect sensor and a feedback amplifier is added to the probe.

Oscilloscope Specifications and Performance

The oscilloscope specifications are necessary when choosing a particular oscilloscope for a particular application. It is necessary to look in detail at the specifications list to see whether the instrument meets its requirements. The oscilloscope specifications and performance are given below: 1. Bandwidth. The bandwidth specification tells you the frequency range the oscilloscope accurately measures. As signal frequency increases, the capability of the oscilloscope to accurately respond decreases. By convention, the bandwidth tells you the frequency at which the displayed signal reduces to 70.7% of the applied sine wave signal. (This 70.7% point is referred to as the “–3 dB point,” a term based on a logarithmic scale.) 2. Rise Time. Rise time is another way of describing the useful frequency range of an oscilloscope. Rise time may be a more appropriate performance consideration when you expect to measure pulses and steps. An oscilloscope cannot accurately display pulses with rise times faster than the specified rise time of the oscilloscope. 3. Vertical Sensitivity. The vertical sensitivity indicates how much the vertical amplifier can amplify a weak signal. Vertical sensitivity is usually given in millivolts (mV) per division. The smallest voltage a general purpose oscilloscope can detect is typically about 2 mV per vertical screen division. 4. Sweep Speed. For analog oscilloscopes, this specification indicates how fast the trace can sweep across the screen, allowing you to see fine details. The fastest sweep speed of an oscilloscope is usually given in nanoseconds/div. 5. Gain Accuracy. The gain accuracy indicates how accurately the vertical system attenuates or amplifies a signal. This is usually listed as a percentage error. 6. Time Base or Horizontal Accuracy. The time base or horizontal accuracy indicates how accurately the horizontal system displays the timing of a signal. This is usually listed as a percentage error. 7. Sample Rate. On digital oscilloscopes, the sampling rate indicates how many samples per second the ADC (and therefore the oscilloscope) can acquire. Maximum sample rates are usually given in megasamples per second (MS/s). The faster the oscilloscope can sample, the more accurately it can represent fine details in a fast signal. The minimum sample rate may also be important if you need to look at slowly changing signals over long periods of time. Typically, the sample rate changes with changes made to the sec/div control to maintain a constant number of waveform points in the waveform record. 214 Electronic Measurements & Instrumentation 8. ADC Resolution (Or Vertical Resolution). The resolution, in bits, of the ADC (and therefore the digital oscilloscope) indicates how precisely it can turn input voltages into digital values. Calculation techniques can improve the effective resolution. Record Length. The record length of a digital oscilloscope indicates how many waveform points the oscilloscope is able to acquire for one waveform record. Some digital oscilloscopes let you adjust the record length. The maximum record length depends on the amount of memory in your oscilloscope. Since the oscilloscope can only store a finite number of waveform points, there is a trade-off between record detail and record length. You can acquire either a detailed picture of a signal for a short period of time (the oscilloscope “fills up” on waveform points quickly) or a less detailed picture for a longer period of time. Some

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Dr. R.S. SEDHA

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