All experiments in the laboratory will be performed at a test bench which has several basic electronic instruments permanently installed. They include: a dc power supply, a waveform generator, a digital voltmeter (DVM), and a digital oscilloscope. A DVM is really a universal meter which can also measure current (both dc and ac) and resistance, with high precision. Other instruments, such as an analog oscilloscope or an analog universal meter, and circuit components (e.g. resistance or capacitance substitution boxes) can be obtained from the stock room, as needed.
An oscilloscope is a basic electronic test instrument which displays images of voltage varying with time. There are two basic types of oscilloscope: analog and digital. An analog oscilloscope displays signals in real time, using amplified input voltage to deflect vertically an electron beam in the Cathode Ray Tube (CRT) while a ramp "time base" signal sweeps it at a predetermined speed horizontally. A digital or digitizing oscilloscope samples the input waveform amplitude and stores the digital information for display on a CRT screen. The analog oscilloscope has been largely replaced today by a digital oscilloscope but professionals still prefer the analog instruments for some applications because it shows an image of a waveform in real time so that you see what is actually coming to the input at a given moment. A great advantage of the digital scope is that digitized waveforms can be stored and viewed independently of the changing input signal, can be easily processed (e.g. added to or subtracted from another signal) or sent to a computer or a printer. It is also easier to measure the period or the amplitude of a signal which is often displayed numerically on the screen. Historically, analog oscilloscope were developed first, and digital oscilloscopes were initially rare and expensive but thanks to a remarkable progress in digital technology their prices declined dramatically. Today, major instrumentation companies, such as Tektronics and HP, make today only digital scopes.
An oscilloscope uses an electron beam to write a pattern on a phosphorescent screen. The deflection in the vertical direction is proportional to the magnitude of the signal being measured while usually the horizontal deflection increases linearly with time which is produced by an internal time-base generator. The beam scans from left to right at a rate determined by the setting of the time-base control on the front panel. Once the beam completes its scan to the right hand side of the screen it is turned off and quickly retraces its position to begin the process anew. The graticule on the screen is calibrated in centimeters and the time-base control provides selectable trace speeds in centimeters per second, centimeters per millisecond or centimeters per microsecond. Thus the oscilloscope displays the instantaneous value of the applied signal versus time.
An important circuit component in an oscilloscope electronics is trigger. Trigger starts the electron beam sweep across the screen at predetermined input voltage level. Is purpose is to show only signals of interest (that have a certain amplitude) and to reject noise and prevent it form blurring the image. The trigger level, which cab be positive or negative, is adjusted manually by turning a knob on the scope front panel. This is internal trigger operation. There is a terminal, near the trigger adjustment knob, for connecting to an external trigger source, and a switch that disconnects the trigger circuit form the scope input signal to an external signal. External trigger function is very useful for measuring time relations between different signals, for example between a signal form a microprocessor and its clock pulse.
Another method of comparing timing of two signals (also their amplitude) is to use two scope inputs. Typically the oscilloscope provides two independent vertical inputs so that two signals can be observed simultaneously to permit comparison. Two of the choices require, alternate and chopped, some explanation. The persistence of the trace on the screen depends on the particular phosphor that coats the screen and is typically about 1/20 of a second which is comparable to the persistence of the human eye. When displaying two rapidly changing signals on the screen 1*** the most straightforward method is to view each signal on alternating complete sweeps across the screen. The persistence of the eye and the phosphor coating of the screen make it appear as if both signals are displayed simultaneously. This method does not work for very slowly-varying phenomena where the sweep speed of the beam is several milliseconds or more per centimeter because only a small band is illuminated at each instant rather than the complete trace. Furthermore, this band contains only one signal. In this case use the alternate mode which samples the two channels at a rapid rate and provides a simultaneous view of both signals.Another point about the vertical channel controls needs some explanation, namely the coupling mode which offers a choice of AC or DC. The amplifiers in the vertical channel of the oscilloscope can amplify signals from 0 Hz to many MHz. The deflection of the electron beam is proportional to the instantaneous value of the input signal. The ability to respond to signals down to 0Hz is necessary to be able to observe and measure slow phenomena but is a disadvantage when observing small AC signals superimposed on a larger DC voltage. The large DC level requires setting of the vertical amplifier to a low gain in order to keep the deflection of the beam within the limited display space of the screen. But that also will decrease the AC signal, which may be barely visible. In order to view only the AC component of the composite AC+DC signal, the oscilloscope has the AC input through a blocking capacitor. This permits setting the gain of the vertical channel amplifiers and the horizontal sweep rate to properly display the AC signal.
A limitation of the oscilloscope, and many ac-powered instruments, is that one input terminal used for measurement is connected to the chassis ground so that voltage measurements cannot be made across an arbitrary terminal pair.
Analog oscilloscope accuracy In this section we present the abridged specifications for the vertical and horizontal channels of a typical analog oscilloscope that you will encounter in the labs and in industry, to get idea of the capability of the instrument and its accuracy. The student should refer to the manual for the instrument to get a more complete idea of its capabilities.
|Table 1. Abridged specifications for an analog oscilloscope.|
|Vertical Deflection System|
|Deflection Factor Range||2 mV/division to 5 V/division|
|Deflection Factor Accuracy:
+15° C to +35° C
|Step Response Rise Time: 0°C to +35°C
5 mV/Division to 5 V/Division
|3.5 ns or less|
|Bandwidth (-3 dB): 0°C to +35°C
5 mV/Division to 5 V/Division
|Dc to at least 100 MHz|
|AC Coupled Lower Limit of Bandwidth||10 Hz or less at -3 dB|
|Maximum safe input voltage||400 Volts|
|Horizontal Deflection System|
|Calibrated Sweep Range||0.5 sec/division to 0.05 µsec/division|
|Sweep Accuracy: +15°C to +35°C||±2%|
We observe from this table that the oscilloscope is a versatile instrument that can be used to voltage signals from the millivolt to tens of volts range. These signals can be slowly varying such that it takes a sweep time of several seconds to display them, or they can be varying rapidly such that one horizontal sweep corresponds to 0.4 msec. The accuracy of these measurements is ±2% to ±5% which is adequate for general work in the design and development of electronic equipment. It is not of the accuracy required for reference standards. 2***
The purpose of the digital oscilloscope is the same as that of the analog oscilloscope, namely, to display the instantaneous value of input signals versus time on a screen. Whereas the analog scope does this in a straightforward manner by amplifying the input signals and using them to deflect the beam in the vertical direction and using a linear time-base to deflect the beam in the horizontal direction, the digital oscilloscope is based on digital signal processing techniques. The input signal (or signals) is sampled at a rate high enough to be faithfully reconstructed 3*** and the samples are converted into numbers that can be operated on (processed) by a digital computer. This seeming increase in complexity of implementation has the advantage for the user of making it possible to display the result of arithmetic operations, such as sum and difference, on the signals, as well as measuring time difference between points on the signal or the values corresponding to selected points on the signals. This is possible because the data to be displayed on the screen is collected internal to the instrument as an array of numbers indexed by a variable that is related to the x-axis position. Furthermore, data can be transferred to an attached computer via an instrument bus interconnection for plotting, inclusion in a document, or subsequent processing. From an implementation standpoint, it allows the instrument manufacturer to leverage the tremendous advances made in computers and integrated circuits to provide many new and useful features. The influence if the computer on the digital oscilloscope is evident by the extensive use of pull-down menus to access the many functions.
In this section we present and discuss the abridged specifications of a typical, modern, digital oscilloscope such as those found in our laboratories.
|Table 2. Abridged specifications for a digital oscilloscope.|
|Bandwidth (-3dB)||Dc to at least 100 MHz|
|Ac-coupled Bandwidth||10 Hz to at least 100 MHz|
|Range||2 mV/division to 5 V/division|
|Verniers||Accuracy about ±3.5% ***5|
|Single cursor vertical accuracy||±1.2% of full scale ±0.5% of position value|
|Dual cursor vertical accuracy||±0.4% of full scale|
|Rise time||3.5 ns ***6|
|Dynamic range||±32 V or ±8 division, whichever is less|
|Sweep speeds||5 s/division to 2 ns/division|
|Accuracy||±0.01%±0.2% of full scale ±200 ps|
|Horizontal resolution||100 ps|
|Cursor accuracy||±0.01%±0.2% of full scale ±200 ps|
|Display System (8 cm vertical by 10 cm horizontal)|
|Resolution||255 vertical by 500 horizontal points|
|Sampling||8 bit samples, 20 MSamples/sec|
From the display system specifications we see that there are 32 display points per centimeter in the vertical direction by 50 display points per centimeter in the horizontal direction which is somewhat finer than the size of the spot painted by an analog oscilloscope. The measurement ranges for the digital oscilloscope are comparable to those of a comparable analog oscilloscope. It should be noted that the accuracy of the horizontal system is better than that of the analog oscilloscope because it is based on the accuracy of the clock that controls when the samples are taken rather than on the linearity and accuracy of the sawtooth waveform that sweeps the CRT beam.
The digital multimeter is a multi-function instrument that can measure ac and dc voltage or current, and resistance. It appears in various incarnations as a laboratory instrument and as a general purpose test instrument out in the field. The instrument used in the laboratory ***7 is based on pre-processing circuitry for each measuring function that converts the input to a dc voltage between ±12 volts. An analog to digital converter produces a digital representation of this dc voltage which is processed by an internal computer to produce an indication for the digital output display. A DC amplifier with a switch selectable gain of 10 or 100 or an attenuator with a gain of 0.1 or 0.01 is used to scale input voltages so that any input range corresponds to 10 volts full scale. This normalized input signal is then sampled and digitized by the analog to digital converter and processed by the computer. Thus, the underlying instrument is a dc voltmeter. Current measurements are achieved by passing the unknown current through a known value of resistance and measuring the resultant voltage developed across this resistance. Resistance measurement is based on passing a known current through the unknown resistor and measuring the resultant voltage drop. High resistance values, where the internal lead resistance is negligible in comparison with the resistance-under-test, are measured using a two-terminal method in which the same two terminals are used to deliver the current to the resistor and to sense the developed voltage. Low resistance values are measured using a four-terminal circuit in which the current to the resistor is supplied via one pair of terminals and the voltage across the resistance is sensed via another pair of terminals that are closer to the resistor. Furthermore, there is negligible current through these voltage sensing leads so that there is no additional voltage drop across the unknown resistor due to the voltage sensing leads.
AC voltage and current is measured by using a true RMS ac-to-dc converter to convert the ac signal to a dc voltage which is measured as before.
Unlike the oscilloscope and many other ac powered instruments which use chassis ground as one of the input terminals, this digital multimeter employs floating measurement(i.e. isolated) circuits and optical coupling to the display unit to permit both inputs of the instrument to be floating.
The following table presents abridged specifications for the digital multimeter.
|Agilent 34401A digital multimeter|
|DC Voltage||<±(0.0030% of reading +0.0030% of range)|
|DC Current ***8||±(0.005% of reading +0.01% of range)|
|Resistance ***9||±(0.002% of reading +0.001% of range)|
|True RMS AC Voltage ***10||±(0.04% of reading +0.02% of range)|
|True RMS AC Current ***11||±(0.1% of reading +0.04% of range)|
|A/D Linearity for Voltage||0.0002% of reading + 0.0001% of range|
The underlying instrument in the analog multimeter is a dc microammeter and therefore its fundamental measurement is dc current. DC currents larger than that necessary to achieve full-scale deflection of the basic meter are measured by shunting the meter with a resistor whose resistance is a known fraction of the internal resistance of the meter.
DC voltages are measured by applying them across a known large-valued resistor in series with the microammeter and measuring the resultant current. The current corresponds to voltage (Ohm’s law) which is shown on the appropriate meter scale. The more sensitive the underlying microammeter the larger the series resistance across which the unknown voltage is applied and the smaller the loading effect of the measuring instrument.
Resistance can be measured by using the microammeter to sense the current flow through the unknown resistor due to a known voltage source. Alternatively, the series ohmmeter shown in Fig.1 consists of a known resistance in series with an unknown voltage source. In use, this ohmmeter is “zero-adjusted” by short-circuiting the input and adjusting the shunt across the microammeter to indicate zero ohms at full scale deflection of the meter. When an unknown resistor is placed in series with the known resistor the current in the loop is reduced resulting in a smaller deflection of the meter needle. The scale can be calibrated directly in ohms with zero being at the right hand side (full-scale deflection) and infinite resistance at the left hand side. The ohmmeter is a low accuracy method for measuring resistance but it is quick and simple, and sufficient for many purposes.
AC voltages and currents are measured by using diodes to rectify the ac signals thereby producing pulsating, unidirectional signals whose average value is proportional to the peak value of the ac signal. The meter is calibrated in terms of the rms value of a sine wave.
Constant voltage dc power supplies are common and useful laboratory instruments. Recall that an ideal voltage source is defined as a two terminal element that maintains a prescribed voltage across its terminals regardless of the applied load. In the case of a power supply it is desired that the output voltage be a specified constant over an output current range from zero to several amperes. This is accomplished using electronic voltage regulators to reduce the source impedance to a very small fraction of an ohm ***12.
The dual or triple power supplies at each laboratory bench consist of multiple, independent supplies which can be connected in different configurations. These instruments provide two identical, floating ***13 voltage sources with a shared common terminal that can be used to provide positive and negative voltages with respect to the common terminal. The instrument panel has usually switches that allow to operate the two units independently, in parallel (to provide more current) or in series. In the latter case the common terminal of the two units can be used as the common terminal (or ground) of the supplied circuit. In this case three wires have to be run between the power supply and the circuit: positive, negative and common. Such double polarity supply is often required for electronic circuits.
A triple power supply provides an additional independent, floating source.
(Due on second lab meeting)
Equipment needed from the stockroom: an analog universal meter (AVM), an analog oscilloscope with the manual, the manual for digital oscilloscope at your bench, leads.
1.1 Familiarize yourself with the instruments at your bench.
Leave the digital scope for later. The dual dc power supply output consists of two independent units which can be connected in different configurations (such as series or dual polarity) or used independently. Check one of these units by connecting a DVM and AVM across its terminals. Turn the voltage adjustment knob to get several different voltage values (for example 1.5V, 14V, 30V. Compare readings on the power supply display with readings on the DVM and the AVM. Note readings of the voltmeters on different ranges of the instruments.
NOTE: A DVM range is set by push buttons and an AVM range by its rotary switch. Make sure also that both instruments are set for DC measurement.
Which voltmeter range should be selected to read best a given voltage? Comment on the precision of voltage measurements on different ranges of the DVM and the AVM and note if its readings agree with the power supply display.
1.2 Configure the power supply.Next, configure the power supply for dual polarity voltage, such as +15V and -15V with respect to ground, which is required in many electronic circuits. Using the DVM, measure voltages between the ground (common) and "+", ground and "−" terminal, and also between "+" and "−" terminals.
We start with a simpler traditional analog oscilloscope, which is easier to understand and to operate. This will introduce us to the general oscilloscope measurement fundamentals which apply also to the digital scope.
Perform the following tests:
2.1. Time measurements
Observe a sine wave, a triangle and a square wave from the waveform generator at your bench. Measure a sinewave period and compare it with the generator frequency for a low frequency (between 50 Hz and 500 Hz), and a high frequency(>100 kHz). Expand the image of the sinewave by selecting appropriate vertical (volts/division) and horizontal (time/division) gains. Experiment with internal trigger by adjusting its level and observing the starting (leftmost) point of the waveform. Note that setting trigger too high makes the waveform disappear (when the trigger level higher that the input amplitude).
Estimate the precision with which you can measure the period by considering a fraction of the horizontal scale divisions that you can read reliably.
Try internal and external triggering (for the latter use a “sync” or trigger output of the signal generator). Using internal triggering adjust the trigger level and observe its effect on the starting point of the displayed waveform. Make notes and sketch observed waveforms for your report.
2.2. Voltage measurements
Set the waveform generator to obtain a sinewave with a frequency of a few hundred hertz. Connect the oscilloscope to the generator (watch those ground leads! -polarity is important here!). Connect also the DVM to the same terminals. From the oscilloscope image measure the signal amplitude. Note the number of vertical divisions and volts/division scope setting. The latter should be selected so that the image is expanded on the screen for easier and more precise reading. The color inner knob on the range switch must be turned all the way to the “calibrated” position”.
Note a reading on the DVM for the same signal that you observe on the scope. The meter should be set for an appropriate range of AC voltage measurement.
Next, without changing the scope settings or the generator frequency, repeat the measurements described above for a triangular wave and for a square wave.
Prepare a table showing the amplitudes (from the scope) and the DVM readings for the three waveforms. Do the scope and the voltmeter readings agree for the three waveforms? What does the voltmeter show? Compare appropriate values from the scope and DVM measurements by showing their difference in percent in the table.
HINT: Refresh your memory about rms for different waveforms.
2.3 Scope AC and DC inputs
A switch at the scope input has three positions: GROUND, DC, and AC. In GROUND position the scope internal circuit is disconnected from the input terminal and connected it to ground. This helps to find zero voltage level on the display. DC position connects the input terminal directly to the scope circuit while the AC setting makes this connection through a capacitor. Thus in the AC input mode any dc voltage, which may be present in the measured signal, does not show on the display (a capacitor represents an open circuit for dc).
To see the effect of different input modes perform these tests:
a) a) Set the input switch to GROUND and by adjusting the VERTICAL POSITION knob move the image (a straight horizontal line) to the middle position on the display grid.
b) Switch the scope input to DC and observe a sine wave or a triangular wave. Check if the wave is centered on the zero (ground) level adjusted previously. Now add some dc bias to your signal, adjust the dc level, and observe the waveform position on the scope display. (There is a knob on the waveform generator for setting dc or “offset’ voltage, which is a dc voltage added to the generated waveform).
c) Switch to AC input and observe if changing dc bias affects the image.
d) Switch the waveform generator to produce a square wave (without dc bias) at high frequency (>5 kHz) and observe it in both DC and AC input modes. Repeat the same at low frequency (about 50 Hz).
Make notes of your observations and sketch waveforms seen in tests b) and d). Explain what you see. Why the square wave looks "strange" only in AC mode at low frequency?
2.4 Frequency range of instruments
Measuring instruments are designed to operate within certain voltage and frequency range and should not be used outside their design specifications. The oscilloscopes in our laboratory are capable of operating at fairly high frequency (tens of megahertz). What about the DVM and the AVM? To check their useful frequency range, do the following:
- Connect an oscilloscope, DVM, and AVM to the waveform generator at the same time.
- Set the waveform generator to a sine wave with the amplitude of a few volts. Start with a frequency of a few tens of hertz and measure the voltage with all three instruments.
- Increase frequency by a decade (a factor of ten) and measure the voltage with the three instruments again. Repeat this procedure until you reach the maximum frequency of the waveform generator.
You should notice that readings of voltmeters drop with increasing frequency while the scope indicates almost the same voltage (the generator output voltage may somewhat vary with frequency too). Adjust frequency to find its values where the readings of DVM are only about 5% below the voltage indicated by the scope. Repeat the same for AVM. This way you can find the useful frequency ranges for each voltmeter, defined here as the frequency range where a voltmeter reading is different by no more than 5% from the “true value” assumed to be shown by the oscilloscope.
A digital oscilloscope can be a great tool, provided that you know how to use it. It has many functions and menus which have to be programmed before the desired mode of operation is obtained. This may be daunting for a beginner who has to spend time studying the manual before making any measurements. Like with a computer, it is best to start with simple operations and move to more complex tasks as you gain experience. Familiarity with an analog scope should help, because front panels and basic functions of digital oscilloscopes are made to resemble those of analog scopes.
Here are some of the most common problems that beginners have with digital scopes:
- It is often not obvious how to access various functions of the instrument, many of which are programmable from the front panel buttons. Consult the manual.
- The waveform you see on the screen does not necessarily represent the input signal. It may be a waveform stored from a prior measurement.
All this can be sorted out and clarified with experience which you are about to acquire.
- You may be easily "fooled" by a digital scope if you let it "think" for you by selecting the "auto" mode in which the voltage and time scales are set by the instrument program. On occasions a noise may look like a nice signal or a momentary transient voltage like a continuous wave.
Repeat measurements 2.1 and 2.2 from the previous section using the digital scope at your bench. Note that you do not need to count the scale divisions to get the signal amplitude or the period; the instrument displays this information for you. You can also activate the vertical and horizontal cursors and check the values of amplitude and period using the cursor position displays.
Describe briefly the measurement procedure and the results, including sketches of waveforms. Include the table of voltage vs. frequency from part 2.4. Address the topics and answer the questions printed in bold letters in the manual.
In particular discuss these problems:
Add any observations or conclusions you wish to make; they enhance your report.
Do not forget to number figures and tables and to give them captions (titles). Number all pages of the report.