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Guide to Measurement and Instrumentation -- Electrical Indicating and Test Instruments



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1 Introduction

2 Digital Meters

3 Analogue Meters

4 Oscilloscopes

  • Channel
  • Single-ended input
  • Differential input
  • Time base circuit
  • Vertical sensitivity control
  • Display position control

5 Summary

6 Quiz

1. Introduction

The mode of operation of most measuring instruments is to convert the measured quantity into an electrical signal. Usually, this output quantity is in the form of voltage, although other forms of output, such as signal frequency or phase changes, are sometimes found.

We shall learn in this section that the magnitude of voltage signals can be measured by various electrical indicating and test instruments. These can be divided broadly into electrical meters (in both analogue and digital forms) and various types of oscilloscopes. As well as signal level voltages, many of these instruments can also measure higher magnitude voltages, and this is indicated where appropriate. The oscilloscope is particularly useful for interpreting instrument outputs that exist in the form of a varying phase or frequency of an electrical signal.

Electrical meters exist in both digital and analogue forms, although use of an analogue form now tends to be restricted to panel meters, where the analogue form of the output display means that abnormal conditions of monitored systems are identified more readily than is the case with the numeric form of output given by digital meters. The various forms of digital and analogue meters found commonly are presented in Sections 2 and 3.

The oscilloscope is a very versatile measuring instrument widely used for signal measurement, despite the measurement accuracy provided being inferior to that of most meters. Although existing in both analogue and digital forms, most instruments used professionally are now digital, with analogue versions being limited to inexpensive, low-specification instruments intended for use in educational establishments. Although of little use to professional users, the features of analogue instruments are covered in this section because students are quite likely to meet these when doing practical work associated with their course. As far as digital oscilloscopes are concerned, the basic type of instrument used is known as a digital storage oscilloscope. More recently, digital phosphor oscilloscopes have been introduced, which have a capability of detecting and recording rapid transients in voltage signals. A third type is the digital sampling oscilloscope, which is able to measure very high-frequency signals. A fourth and final type is a personal computer (PC)-based oscilloscope, which is effectively an add-on unit to a standard PC. All of these different types of oscilloscopes are discussed in Section 4.

2. Digital Meters

All types of digital meters are basically modified forms of the digital voltmeter (DVM), irrespective of the quantity that they are designed to measure. Digital meters designed to measure quantities other than voltage are, in fact, digital voltmeters that contain appropriate electrical circuits to convert current or resistance measurement signals into voltage signals.

Digital multimeters are also essentially digital voltmeters that contain several conversion circuits, thus allowing the measurement of voltage, current, and resistance within one instrument.

Digital meters have been developed to satisfy a need for higher measurement accuracies and a faster speed of response to voltage changes than can be achieved with analogue instruments.

They are technically superior to analogue meters in almost every respect. The binary nature of the output reading from a digital instrument can be applied readily to a display that is in the form of discrete numerals. Where human operators are required to measure and record signal voltage levels, this form of output makes an important contribution to measurement reliability and accuracy, as the problem of analogue meter parallax error is eliminated and the possibility of gross error through misreading the meter output is reduced greatly. The availability in many instruments of a direct output in digital form is also very useful in the rapidly expanding range of computer control applications. Quoted inaccuracy values are between _0.005% (measuring d.c. voltages) and _2%. Digital meters also have very high input impedance (10M-Ohm compared with 1_20 K-Ohm for analogue meters), which avoids the measurement system loading problem (see Section 3) that occurs frequently when analogue meters are used.

Additional advantages of digital meters are their ability to measure signals of frequency up to 1 MHz and the common inclusion of features such as automatic ranging, which prevents overload and reverse polarity connection, etc.

The major part of a digital voltmeter is the circuitry that converts the analogue voltage being measured into a digital quantity. As the instrument only measures d.c. quantities in its basic mode, another necessary component within it’s one that performs a.c.-d.c. conversion and thereby gives it the capacity to measure a.c. signals. After conversion, the voltage value is displayed by means of indicating tubes or a set of solid-state light-emitting diodes. Four-, five-, or even six- figure output displays are used commonly, and although the instrument itself may not be inherently more accurate than some analogue types, this form of display enables measurements to be recorded with much greater accuracy than that obtainable by reading an analogue meter scale.

Digital voltmeters differ mainly in the technique used to affect the analogue-to-digital conversion between the measured analogue voltage and the output digital reading. As a general rule, the more expensive and complicated conversion methods achieve a faster conversion speed. Some common types of DVM are discussed here.

Voltage-to-Time Conversion Digital Voltmeter

This is the simplest form of DVM and is a ramp type of instrument. When an unknown voltage signal is applied to input terminals of the instrument, a negative slope ramp waveform is generated internally and compared with the input signal. When the two are equal, a pulse is generated that opens a gate, and at a later point in time a second pulse closes the gate when the negative ramp voltage reaches zero. The length of time between the gate opening and closing is monitored by an electronic counter, which produces a digital display according to the level of the input voltage signal. Its main drawbacks are nonlinearities in the shape of the ramp waveform used and lack of noise rejection; these problems lead to a typical inaccuracy of _0.05%. It’s relatively inexpensive, however.

Potentiometric Digital Voltmeter

This uses a servo principle, in which the error between the unknown input voltage level and a reference voltage is applied to a servo-driven potentiometer that adjusts the reference voltage until it balances the unknown voltage. The output reading is produced by a mechanical drum-type digital display driven by the potentiometer. This is also a relatively inexpensive form of DVM that gives excellent performance for its price.

Dual-Slope Integration Digital Voltmeter

This is another relatively simple form of DVM that has better noise-rejection capabilities than many other types and gives correspondingly better measurement accuracy (inaccuracy as low as _0.005%). Unfortunately, it’s quite expensive. The unknown voltage is applied to an integrator for a fixed time, T1, following which a reference voltage of opposite sign is applied to the integrator, which discharges down to a zero output in an interval, T2, measured by a counter.

The output-time relationship for the integrator is shown in Fgr.1, from which the unknown voltage, Vi, can be calculated geometrically from the triangle as

Vi = Vref T1=T2 +

Voltage-to-Frequency Conversion Digital Voltmeter

In this instrument, the unknown voltage signal is fed via a range switch and an amplifier into a converter circuit whose output is in the form of a train of voltage pulses at a frequency proportional to the magnitude of the input signal. The main advantage of this type of DVM is its ability to reject a.c. noise.

Digital Multimeter

This is an extension of the DVM. It can measure both a.c. and d.c. voltages over a number of ranges through inclusion within it of a set of switchable amplifiers and attenuators. It’s used widely in circuit test applications as an alternative to the analogue multimeter and includes protection circuits that prevent damage if high voltages are applied to the wrong range.

Fgr.1 --- Output Time Vi applied Vref applied

T2 T1

3. Analogue Meters

Despite the technical superiority of digital meters, particularly in terms of their greater accuracy and much higher input impedance, analogue meters continue to be used in a significant number of applications. First, they are often preferred as indicators in system control panels. This is because deviations of controlled parameters away from the normal expected range are spotted more easily by a pointer moving against a scale in an analogue meter rather than by variations in the numeric output display of a digital meter. A typical, commercially available analogue panel meter is shown in Fgr.2. Analogue instruments also tend to suffer less from noise and isolation problems, which favor their use in some applications. In addition, because analogue instruments are usually passive instruments that don’t need a power supply, this is often very useful in measurement applications where a suitable main power supply is not readily available.

Many examples of analogue meters also remain in use for historical reasons.

Analogue meters are electromechanical devices that drive a pointer against a scale. They are prone to measurement errors from a number of sources that include inaccurate scale marking during manufacture, bearing friction, bent pointers, and ambient temperature variations.

Further human errors are introduced through parallax error (not reading the scale from directly above) and mistakes in interpolating between scale markings. Quoted inaccuracy values are between _0.1 and _3%. Various types of analogue meters are used as discussed here.

Moving Coil Meter

Fgr.3 --- Pointer N S Moving coil Spring Iron core

Permanent magnet

A moving coil meter is a very commonly used form of analogue voltmeter because of its sensitivity, accuracy, and linear scale, although it only responds to d.c. signals. As shown schematically in Fgr.3, it consists of a rectangular coil wound round a soft iron core that is suspended in the field of a permanent magnet. The signal being measured is applied to the coil, which produces a radial magnetic field. Interaction between this induced field and the field produced by the permanent magnet causes torque, which results in rotation of the coil.

The amount of rotation of the coil is measured by attaching a pointer to it that moves past a graduated scale. The theoretical torque produced is given by:

T = BIhwN

… where B is the flux density of the radial field, I is the current flowing in the coil, h is the height of the coil, w is the width of the coil, and N is the number of turns in the coil. If the iron core is cylindrical and the air gap between the coil and pole faces of the permanent magnet is uniform, then the flux density B is constant and Equation (2) can be rewritten as:

T = KI

… that is, torque is proportional to the coil current and the instrument scale is linear.

As the basic instrument operates at low current levels of one milliamp or so, it’s only suitable for measuring voltages up to around 2 volts. If there is a requirement to measure higher voltages, the measuring range of the instrument can be increased by placing a resistance in series with the coil, such that only a known proportion of the applied voltage is measured by the meter. In this situation the added resistance is known as a shunting resistor.

While Fgr.3 shows the traditional moving coil instrument with a long U-shaped permanent magnet, many newer instruments employ much shorter magnets made from recently developed magnetic materials such as Alnico and Alcomax. These materials produce a substantially greater flux density, which, in addition to allowing the magnet to be smaller, has additional advantages in allowing reductions to be made in the size of the coil and in increasing the usable range of deflection of the coil to about 120_. Some versions of the instrument also have either a specially shaped core or specially shaped magnet pole faces to cater for special situations where a nonlinear scale, such as a logarithmic one, is required.

Moving Iron Meter

As well as measuring d.c. signals, the moving iron meter can also measure a.c. signals at frequencies up to 125 Hz. It’s the least expensive form of meter available and, consequently, this type of meter is also used commonly for measuring voltage signals. The signal to be measured is applied to a stationary coil, and the associated field produced is often amplified by the presence of an iron structure associated with the fixed coil. The moving element in the instrument consists of an iron vane suspended within the field of the fixed coil.

When the fixed coil is excited, the iron vane turns in a direction that increases the flux through it.

The majority of moving-iron instruments are either of the attraction type or of the repulsion type. A few instruments belong to a third combination type. The attraction type, where the iron vane is drawn into the field of the coil as the current is increased, is shown schematically in Fgr.4a. The alternative repulsion type is sketched in Fgr.4b. For an excitation current, I, the torque produced that causes the vane to turn is given by:

T = I 2 dM 2dy , where M is the mutual inductance and y is the angular deflection. Rotation is opposed by a spring that produces a backwards torque given by

Ts = Ky:

At equilibrium, T = Ts, and y is therefore given by

[...]

The instrument thus has a square-law response where the deflection is proportional to the square of the signal being measured, that is, the output reading is a root-mean-squared (r.m.s.) quantity.

The instrument can typically measure voltages in the range of 0 to 30 volts. However, it can be modified to measure higher voltages by placing a resistance in series with it, as in the case of moving coil meters. A series resistance is particularly beneficial in a.c. signal measurements because it compensates for the effect of coil inductance by reducing the total resistance/ inductance ratio, and hence measurement accuracy is improved. A switchable series resistance is often provided within the casing of the instrument to facilitate range extension. However, when the voltage measured exceeds about 300 volts, it becomes impractical to use a series resistance within the case of the instrument because of heat-dissipation problems, and an external resistance is used instead.

Fgr.5 --- Clamp-on jaws Measured current Secondary winding Rectifier ;Meter

Clamp-on Meters

These are used for measuring circuit currents and voltages in a noninvasive manner that avoids having to break the circuit being measured. The meter clamps onto a current-carrying conductor, and the output reading is obtained by transformer action. The principle of operation is illustrated in Fgr.5, where it can be seen that the clamp-on jaws of the instrument act as a transformer core and the current-carrying conductor acts as a primary winding. Current induced in the secondary winding is rectified and applied to a moving coil meter. Although it’s a very convenient instrument to use, the clamp-on meter has low sensitivity and the minimum current measurable is usually about 1 amp.

Analogue Multimeter

The analogue multimeter is now less common than its counterpart, the digital multimeter, but is still widely available. It’s a multifunction instrument that can measure current and resistance, as well as d.c. and a.c. voltage signals. Basically, the instrument consists of a moving coil analogue meter with a switchable bridge rectifier to allow it to measure a.c. signals, as shown in Fgr.6. A set of rotary switches allows the selection of various series and shunt resistors, which make the instrument capable of measuring both voltage and current over a number of ranges. An internal power source is also provided to allow it to measure resistances as well.

While this instrument is very useful for giving an indication of voltage levels, the compromises in its design that enable it to measure so many different quantities necessarily mean that its accuracy is not as good as instruments that are purposely designed to measure just one quantity over a single measuring range.

Fgr.6

Measuring High-Frequency Signals with Analogue Meters

Fgr.7 --- Bridge rectifier; Moving-coil meter

One major limitation in using analogue meters for a.c. voltage measurement is that the maximum frequency measurable directly is low-2 kHz for the dynamometer voltmeter and only 100 Hz in the case of the moving iron instrument. A partial solution to this limitation is to rectify the voltage signal and then apply it to a moving coil meter, as shown in Fgr.7. This extends the upper measurable frequency limit to 20 kHz. However, inclusion of the bridge rectifier makes the measurement system particularly sensitive to environmental temperature changes, and nonlinearities significantly affect measurement accuracy for voltages that are small relative to the full-scale value.

Calculation of Meter Outputs for Nonstandard Waveforms

The two examples given here provide an exercise in calculating the output reading from various types of analogue voltmeters. These examples also serve as a useful reminder of the mode of operation of each type of meter and the form that the output takes.

Fgr.8

Example 1

Calculate the reading that would be observed on a moving coil ammeter when it’s measuring the current in the circuit shown in Fgr.8.

Example 7.2 Calculate the reading that would be observed on a moving iron ammeter when it’s measuring the current in the circuit shown in Fgr.8.

Solution:

A moving iron meter measures r.m.s. current.

4. Oscilloscopes

The oscilloscope is probably the most versatile and useful instrument available for signal measurement. While oscilloscopes still exist in both analogue and digital forms, analogue models tend to be low specification, low-cost instruments produced for educational use in schools, colleges, and universities. Almost all oscilloscopes used for professional work now tend to be digital models. These can be divided into digital storage oscilloscopes, digital phosphor oscilloscopes, and digital sampling oscilloscopes.

The basic function of an oscilloscope is to draw a graph of an electrical signal. In the most common arrangement, the y axis (vertical) of the display represents the voltage of a measured signal and the x axis (horizontal) represents time. Thus, the basic output display is a graph of the variation of the magnitude of the measured voltage with time.

The oscilloscope is able to measure a very wide range of both a.c. and d.c. voltage signals and is used particularly as an item of test equipment for circuit fault finding. In addition to measuring voltage levels, it can also measure other quantities, such as the frequency and phase of a signal.

It can also indicate the nature and magnitude of noise that may be corrupting the measurement signal. The most expensive models can measure signals at frequencies up to 25 GHz, while the least expensive models can only measure signals up to 10MHz. One particularly strong merit of the oscilloscope is its high input impedance, typically 1 MO, which means that the instrument has a negligible loading effect in most measurement situations. As a test instrument, it’s often required to measure voltages whose frequency and magnitude are totally unknown.

The set of rotary switches that alter its time base so easily, and the circuitry that protects it from damage when high voltages are applied to it on the wrong range, make it ideally suited for such applications. However, it’s not a particularly accurate instrument and is best used where only an approximate measurement is required. In the best instruments, inaccuracy can be limited to _1% of the reading, but inaccuracy can approach _5% in the least expensive instruments.

Fgr.9 --- -3 dB Bandwidth G

Voltage gain 0.707G 10,000 1000 100 10 Frequency (Hz)

The most important aspects in the specification of an oscilloscope are its bandwidth, rise time, and accuracy. Bandwidth is defined as the range of frequencies over which the oscilloscope amplifier gain is within 3 dB* of its peak value, as illustrated in Fgr.9. The _3-dB point is where the gain is 0.707 times its maximum value. In most oscilloscopes, the amplifier is direct coupled, which means that it amplifies d.c. voltages by the same factor as low-frequency a.c. ones. For such instruments, the minimum frequency measurable is zero and the bandwidth can be interpreted as the maximum frequency where the sensitivity (deflection/volt) is within 3 dB of the peak value. In all measurement situations, the oscilloscope chosen for use must be such that the maximum frequency to be measured is well within the bandwidth. The _3-dB specification means that an oscilloscope with a specified inaccuracy of _2% and a bandwidth of 100MHz will have an inaccuracy of _5%when measuring 30-MHz signals; this inaccuracy will increase still further at higher frequencies. Thus, when applied to signal-amplitude measurement, the oscilloscope is only usable at frequencies up to about 0.3 times its specified bandwidth.

Rise time is the transit time between 10 and 90% levels of the response when a step input is applied to the oscilloscope. Oscilloscopes are normally designed such that bandwidth _ rise time = 0:35:

Thus, for a bandwidth of 100 MHz, rise time = 0.35/100,000,000 = 3.5 ns.

All oscilloscopes are relatively complicated instruments constructed from a number of subsystems, and it’s necessary to consider each of these in turn in order to understand how the complete instrument functions. To achieve this, it’s useful to start with an explanation of an analogue oscilloscope, as this was the original form in which oscilloscopes were made and many of the terms used to describe the function of oscilloscopes emanate from analogue forms.

Analogue Oscilloscope (Cathode Ray Oscilloscope)

Analogue oscilloscopes were originally called cathode ray oscilloscopes because a fundamental component within them is a cathode ray tube. In recent times, digital oscilloscopes have almost entirely replaced analogue versions in professional use. However, some very inexpensive versions of analogue oscilloscopes still exist that find educational uses in schools, colleges, and universities. The low cost of basic analogue models is their only merit, as their inclusion of a cathode ray tube makes them very fragile, and the technical performance of digital equivalents is greatly superior.

The cathode ray tube within an analogue oscilloscope is shown schematically in Fgr.10.

The cathode consists of a barium and strontium oxide-coated, thin, heated filament from which a stream of electrons is emitted. The stream of electrons is focused onto a well-defined spot on a fluorescent screen by an electrostatic focusing system that consists of a series of metal discs and cylinders charged at various potentials. Adjustment of this focusing mechanism is provided by a focus control on the front panel of an oscilloscope. An intensity control varies the cathode heater current and therefore the rate of emission of electrons, and thus adjusts the intensity of the display on the screen. These and other typical controls are shown in the illustration of the front panel of a simple oscilloscope given in Fgr.11. It should be noted that the layout shown is only one example. Every model of oscilloscope has a different layout of control knobs, but the functions provided remain similar irrespective of the layout of the controls with respect to each other.

Application of potentials to two sets of deflector plates mounted at right angles to one another within the tube provide for deflection of the stream of electrons, such that the spot where the electrons are focused on the screen is moved. The two sets of deflector plates are normally known as horizontal and vertical deflection plates, according to the respective motion caused to the spot on the screen. The magnitude of any signal applied to the deflector plates can be calculated by measuring the deflection of the spot against a cross-wires graticule etched on the screen.

Fgr.10 --- Heater; Cathode Focusing plates; Deflection plates; Fluorescent screen

Fgr.11 --- Trigger level; Trigger; Trigger slope; On Off H V Horizontal input Vertical input Ext. sync.

Channel --- One channel describes the basic subsystem of an electron source, focusing system, and deflector plates. This subsystem is often duplicated one or more times within the cathode ray tube to provide a capability of displaying two or more signals at the same time on the screen.

The common oscilloscope configuration with two channels can therefore display two separate signals simultaneously.

Single-ended input --- This type of input only has one input terminal plus a ground terminal per oscilloscope channel and, consequently, only allows signal voltages to be measured relative to ground. It’s normally only used in simple oscilloscopes.

Differential input --- This type of input is provided on more expensive oscilloscopes. Two input terminals plus a ground terminal are provided for each channel, which allows the potentials at two nongrounded points in a circuit to be compared. This type of input can also be used in single-ended mode to measure a signal relative to ground by using just one of the input terminals plus ground.

Time base circuit --- The purpose of a time base is to apply a voltage to the horizontal deflector plates such that the horizontal position of the spot is proportional to time. This voltage, in the form of a ramp known as a sweep waveform, must be applied repetitively, such that the motion of the spot across the screen appears as a straight line when a d.c. level is applied to the input channel.

Furthermore, this time base voltage must be synchronized with the input signal in the general case of a time-varying signal, such that a steady picture is obtained on the oscilloscope screen.

The length of time taken for the spot to traverse the screen is controlled by a time/div switch, which sets the length of time taken by the spot to travel between two marked divisions on the screen, thereby allowing signals at a wide range of frequencies to be measured.

Each cycle of the sweep wave form is initiated by a pulse from a pulse generator. The input to the pulse generator is a sinusoidal signal known as a triggering signal, with a pulse being generated every time the triggering signal crosses a preselected slope and voltage level condition. This condition is defined by trigger level and trigger slope switches. The former selects the voltage level on the trigger signal, commonly zero, at which a pulse is generated, while the latter selects whether pulsing occurs on a positive or negative going part of the triggering waveform.

Synchronization of the sweep wave form with the measured signal is achieved most easily by deriving the trigger signal from the measured signal, a procedure known as internal triggering. Alternatively, external triggering can be applied if the frequencies of the triggering signal and measured signals are related by an integer constant such that the display is stationary. External triggering is necessary when the amplitude of the measured signal is too small to drive the pulse generator; it’s also used in applications where there is a requirement to measure the phase difference between two sinusoidal signals of the same frequency. It’s very convenient to use 50-Hz line voltage for external triggering when measuring signals at mains frequency; this is often given the name line triggering.

Vertical sensitivity control --- This consists of a series of attenuators and preamplifiers at the input to the oscilloscope.

These condition the measured signal to the optimum magnitude for input to the main amplifier and vertical deflection plates, thus enabling the instrument to measure a very wide range of different signal magnitudes. Selection of the appropriate input amplifier/attenuator is made by setting a volts/div control associated with each oscilloscope channel. This defines the magnitude of the input signal that will cause a deflection of one division on the screen.

Display position control --- This allows the position at which a signal is displayed on the screen to be controlled in two ways. The horizontal position is adjusted by a horizontal position knob on the oscilloscope front panel, and similarly a vertical position knob controls the vertical position. These controls adjust the position of the display by biasing the measured signal with d.c. voltage levels.

Digital Storage Oscilloscopes

Digital storage oscilloscopes are the most basic form of digital oscilloscopes but even these usually have the ability to perform extensive waveform processing and provide permanent storage of measured signals. When first created, a digital storage oscilloscope consisted of a conventional analogue cathode ray oscilloscope with the added facility that the measured analogue signal could be converted to digital format and stored in computer memory within the instrument. These stored data could then be reconverted to analogue form at the frequency necessary to refresh the analogue display on the screen, producing a non-fading display of the signal on the screen.

While examples of such early digital oscilloscopes might still be found in some workplaces, modern digital storage oscilloscopes no longer use cathode ray tubes and are entirely digital in construction and operation. The front panel of any digital oscilloscope has a similar basic layout to that shown for an analogue oscilloscope in Fgr.11, except that the controls for "focusing" and "intensity" are not needed in a digital instrument. The block diagram in Fgr.12 shows typical components used in the digital storage oscilloscope. A typical commercial instrument was also shown earlier in Figure 5.1. The first component (as in an analogue oscilloscope) is an amplifier/attenuator unit that allows adjustment of the magnitude of the input voltage signal to an appropriate level. This is followed by an analogue-to-digital converter that samples the input signal at discrete points in time. The sampled signal values are stored in the acquisition memory component before passing into a microprocessor. This carries out signal processing functions, manages the front panel control settings, and prepares the output display. Following this, the output signal is stored in a display memory module before being output to the display itself. This consists of either a monochrome or a multicolor liquid crystal display (see Section 8). The signal displayed is actually a sequence of individual dots rather than a continuous line as displayed by an analogue oscilloscope. However, as the density of dots increases, the display becomes closer and closer to a continuous line. The density of the dots is entirely dependent on the sampling rate at which the analogue signal is digitized and the rate at which the memory contents are read to reconstruct the original signal. As the speed of sampling and signal processing is a function of instrument cost, more expensive instruments give better performance in terms of dot density and the accuracy with which the analogue signal is recorded and represented. Nevertheless, the cost of computing power is now sufficiently low to mean that all but the least expensive instruments now have a display that looks very much like a continuous trace.

In addition to their ability to display the magnitude of voltage signals and other parameters, such as signal phase and frequency, most digital oscilloscopes can also carry out analysis of the measured waveform and compute signal parameters such as maximum and minimum signal levels, peak-peak values, mean values, r.m.s. values, rise time, and fall time. These additional functions are controlled by extra knobs and push buttons on the front panel. They are also ideally suited to capturing transient signals when set to single-sweep mode. This avoids the problem of the very careful synchronization that is necessary to capture such signals on an analogue oscilloscope. In addition, digital oscilloscopes often have facilities to output analogue signals to devices such as chart recorders and output digital signals in a form compatible with standard interfaces such as IEEE488 and RS232.

The principal limitation of a digital storage oscilloscope is that the only signal information captured is the status of the signal at each sampling instant. Thereafter, no new signal information is captured during the time that the previous sample is being processed. This means that any signal changes occurring between sampling instants, such as fast transients, are not detected. This problem is overcome in the digital phosphor oscilloscope.

Digital Phosphor Oscilloscope

This newer type of oscilloscope, first introduced in 1998, uses a parallel-processing architecture instead of the serial-processing architecture found in digital storage oscilloscopes. The components of the instrument are shown schematically in Fgr.13. The amplifier/attenuator and analogue to-digital converter are the same as in a digital storage oscilloscope. However, the signal processing mechanism is substantially different. Output from the analogue-to-digital converter passes into a digital phosphor memory unit, which is, in fact, entirely electronic and not composed of chemical phosphor as its name might imply. Thereafter, data follow two parallel paths. First, a microprocessor processes data acquired at each sampling instant according to the settings on the control panel and sends the processed signal to the instrument display unit. In addition to this, a snapshot of the input signal is sent directly to the display unit at a rate of 30 images per second.

This enhanced processing capability enables the instrument to have a higher waveform capture rate and to detect very fast signal transients missed by digital storage oscilloscopes.

Digital Sampling Oscilloscope

The digital sampling oscilloscope has a bandwidth of up to 25 GHz, which is about 10 times better than that achieved by other types of oscilloscopes. This increased bandwidth is achieved by reversing the positions of the analogue-to-digital converter and the amplifier, as shown in the block diagram in Fgr.14. This reversal means that the sampled signal applied to the amplifier has a much lower frequency than the original signal, allowing use of a low bandwidth amplifier. However, the fact that the input signal is applied directly to the analogue-to-digital converter without any scaling means that the instrument can only be used to measure signals whose peak magnitude is within a relatively small range of typically 1 volt peak-peak. In contrast, both digital storage and digital phosphor oscilloscopes can typically deal with inputs up to 500 volts.

Personal Computer-Based Oscilloscope

A PC-based oscilloscope consists of a hardware unit that connects to a standard PC via either a USB or a parallel port. The hardware unit provides signal scaling, analogue-to digital conversion, and buffer memory functions found in a conventional oscilloscope. More expensive PC-based oscilloscopes also provide some high-speed digital signal processing functions within the hardware unit. The host PC itself provides the control interface and display facilities.

The primary advantage of a PC-based oscilloscope over other types is one of cost; the cost saving is achieved because use of the PC obviates the need for a display unit and front control panel found in other forms of oscilloscopes. The larger size of a PC display compared with a conventional oscilloscope often makes the output display easier to read. A further advantage is one of portability, as a laptop plus add-on hardware unit is usually smaller and lighter than a conventional oscilloscope. PC-based oscilloscopes also facilitate the transfer of output data into standard PC software such as spreadsheets and word processors.

Although PC-based oscilloscopes have a number of advantages over conventional oscilloscopes, they also have disadvantages. First, electromagnetic noise originating in PC circuits requires the hardware unit to be well shielded in order to avoid corruption of the measured signal.

Second, signal sampling rates can be limited by the mode of connection of the hardware unit into the PC.

5. Summary

This section looked at the various ways of measuring electrical signals that form the output of most types of measuring instruments. We noted that these signals were usually in the form of varying voltages, although a few instruments have an output where either the phase or the frequency of an electrical signal changes. We observed that varying voltages could be measured either by electrical meters or by one of several forms of oscilloscopes. We also learned that the latter are also able to interpret frequency and phase changes in signals.

Our discussion started with electrical meters, which we found now mainly existed in digital form, but we noted that analogue forms also exist, which are mainly used as meters in control panels. We looked first of all at the various forms of digital meters and followed this with a presentation on the types of analogue meters still in use.

Our discussion on oscilloscopes also revealed that both analogue and digital forms exist, but we observed that analogue instruments are now predominantly limited to less expensive versions used in education markets. However, because the students at which this guide is aimed are quite likely to meet analogue oscilloscopes for practical work during their course, we started off by looking at the features of such instruments. We then went on to look at the four alternative forms of digital oscilloscope that form the basis for almost all oscilloscopes used professionally. We learned that the basic form is known as a digital storage oscilloscope and that even this is superior in most respects to an analogue oscilloscope. Where better performance is needed, particularly if the observed signal has fast transients, we saw that a new type known as a digital phosphor oscilloscope is used. A third kind, known as a digital sampling oscilloscope, is designed especially for measuring very high-frequency signals.

However, we noted that this could also measure voltage signals that were up to 1 volt peak to-peak in magnitude. Finally, we looked at the merits of PC-based oscilloscopes. In addition to offering oscilloscope facilities at a lower cost than other forms of oscilloscopes, we learned that these had several other advantages but also some disadvantages.

6. Quiz

-1. Summarize the advantages of digital meters over their analogue counterparts.

-2. Explain the four main alternative mechanisms used for affecting analogue-to-digital conversion in a digital voltmeter.

-3. What sort of applications are analogue meters still commonly found in?

-4. Explain the mode of operation of a moving coil meter.

-5. Explain the mode of operation of a moving iron meter.

-6. How does an oscilloscope work?

-7. What are the main differences between analogue and digital oscilloscopes?

-8. Explain the following terms: (a) bandwidth and (b) rise time. In designing oscilloscopes, what relationship is sort between bandwidth and rise time?

-9. Explain the following terms in relation to an oscilloscope: (a) channel, (b) single ended input, (c) differential input, (d) time base, (e) vertical sensitivity, and (f) display position control.

-10. Sketch a block diagram showing the main components in a digital storage oscilloscope and explain the mode of operation of the instrument.

-11. Draw a block diagram showing the main components in a digital phosphor oscilloscope.

What advantages does a digital phosphor oscilloscope have over a digital storage one?

-12. Illustrate the main components in a digital sampling oscilloscope by sketching a block diagram of them. What performance advantages does a digital sampling oscilloscope have over a digital storage one?

-13. What is a PC-based oscilloscope? Discuss its advantages and disadvantages compared with a digital oscilloscope.

-14. What are the main differences among a digital storage oscilloscope, a digital phosphor oscilloscope, and a digital sampling oscilloscope? How do these differences affect their performance and typical usage?

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Updated: Tuesday, 2014-03-25 2:21 PST