EECS 202 – INTRODUCTION TO ELECTRICAL ENGINEERING
LAB 2: INTRODUCTION TO INSTRUMENTATION
By Alan V. Sahakian
Revised by Arthur R. Butz
© Northwestern University 2000,2001,2007,2008
To learn how to make simple electronic measurements
To learn about four basic instruments: the oscilloscope, function generator, multimeter (data acquisition/switch unit) and power supply
To begin to become familiar with the instrumentation which will be used in the upper-division EE courses
Please note: Questions which should be answered in your lab report are indicated by an asterisk (*).
Electronic instrumentation is used to make measurements of voltage, current, resistance, and other parameters in electronic circuits and systems. With appropriate transducers, quantities such as force, pressure, temperature, position, etc. can also be measured. Two basic instruments are used to make such measurements: the Oscilloscope and the Digital Multimeter.
In its most basic form, the oscilloscope (scope) displays voltage as a function of time. Oscilloscopes may be analog or digital. In the case of an analog scope, a cathode ray tube (crt) is used as the display element. A crt operates by creating a glowing spot on the screen where an electron beam is striking it. The electron beam can be deflected both vertically (y) and horizontally (x) by voltages applied to internal deflection plates (or currents applied to external deflection coils). In a common analog scope, the vertical deflection plates are driven by an adjustable amplifier, the input of which is the signal to be examined. The horizontal deflection plates are driven by a sawtooth waveform, which makes the beam sweep horizontally. Both the vertical sensitivity and the horizontal sweep rate can be adjusted to various calibrated values. The horizontal sweep is "triggered" from the vertical signal by setting a threshold and slope. The triggering makes successive recurrences of an event appear on the screen at the same location.
Analog scopes are limited in that the trigger event must occur at the beginning of the event to be viewed. Sometimes we would like to see what happened before the trigger event. A digital scope makes this possible. Digital scopes also provide waveform memory, and the ability to make calculations on stored waveforms. They also can be interfaced with a host computer to download data and upload settings.
The scope which we will be using is an Agilent Technologies DSO6012A digital oscilloscope. The DSO6012A uses the General Purpose Interface Bus standard (GP-IB, which is also referred to as IEEE-488) to connect to a computer. If you look at the left rear of the DSO6012A you can see the GP-IB cables which interconnect it with the computer and the rest of the instruments.
This is a powerful unit which samples its inputs at up to 2 GHz (2 billion samples per second). Using the Nyquist criterion we would expect that this scope could therefore sample signals having components up to 1 GHz, but to make the design of the anti-aliasing low-pass filter less complex and to provide more accurate voltage measurement, the bandwidth of the scope is limited to 100 MHz instead. You can view a description of the scope here and detailed specifications here (PDF file). *What is the resolution (number of bits) used in the A/D converter of the DSO6012A?
The Digital Multimeter:
The Multimeter is a general-purpose device which can measure voltage, current, resistance, and with the addition of a thermocouple, temperature. Like the scope, the multimeter can be either analog or digital, but at this point in time virtually all new multimeters are digital multimeters (DMMs).
The DMM uses a very high resolution A/D converter to yield a large number of digits of meaningful display. The HP (Agilent) 34970A DMM which we have as part of the lab setup has a basic resolution of up to 6 ½ (six and a half) digits. This means that a voltage of 2 volts can be measured with a resolution of 500 nanovolts (500 billionths of a volt). It is this high resolution which makes DMMs preferable to analog multimeters. You can view a description of the DMM here and detailed specifications here (PDF file).
The 34970A is also designed to be interfaced to a computer using GP-IB. This instrument is very versatile, but unfortunately it is somewhat confusing due to this versatility. It is capable of measuring up to 20 separate inputs under computer control. We have brought out six of these 20 inputs to BNC connectors on a small box for your use.
The Function Generator:
A function generator produces voltage waveforms of several types: sine wave, square wave, ramp wave, and sometimes other waveforms. These waveforms are useful for testing circuits, for example measuring frequency response, or generating the clock input to a digital circuit.
The Agilent 33220A function generator in the lab is a digital instrument, which is very much like the CD player in that it has a digitally stored representation of waveforms and it uses a D/A converter to make them. It is very accurate, for example it can generate a 1 kHz sine wave with a frequency resolution of 1 micro Hertz (one part in 1 billion). It can also generate sine waves whose frequencies are being swept, and arbitrary waveforms which are downloaded into it through the GP-IB interface from the host computer, and it can be programmed to generate noise, which is a randomly-changing voltage. You can view a description of this instrument here and detailed specifications here. *What is the maximum sample rate of the 33220A?
The Power Supply:
A power supply generates stable voltages to power electronic circuits. High-quality power supplies can also be set to limit the current which they will deliver, which is very useful when powering up a new circuit (in case there is a short circuit or other problem current limiting can prevent the circuit from being destroyed). The power supply which we have in lab is an HP (Agilent) E3631A triple output unit, which is GP-IB controlled. The three outputs are 0 to 6V, 0 to +25V, and 0 to -25V. A description of this device can be found here and detailed specifications here. For maximum current outputs for the power supply, just refer to the device board. The data for this feature are written on it. * What is the maximum current which the E3631A can deliver on each of its three outputs?
In this experiment you will simply generate signals (waveforms) and dc voltages and make some measurements on them.
Step 1: Generating and viewing a sine wave
In this step you will use the function generator to generate a sine wave, and you will use the scope to view the sine wave.
First, be sure that all the instruments and the computer are turned on.
Connect the scope probe to the channel 1 input of the scope (the scope has two input channels). Connect a BNC-to-cliplead cable from the "Output" terminal of the function generator to the scope probe (black-to-black). Set the function generator to generate a 10 kHz sine wave with an amplitude of 1 V peak-to-peak by using the following keystrokes:
Freq, enter number 10 on the keypad, then units kHz
Ampl, enter number 1 on the keypad, then units Vpp
Press the "Output" button on the function generator. Set the oscilloscope to give a display by pressing autoscale.
Next you will use the cursor function of the oscilloscope to make voltage and time measurements on the displayed waveform. Press the Cursors button on the scope and rotate the knob next to the Cursors button counterclockwise. You should see a vertical cursor (line) representing "X1". Move this cursor to the first crest of the displayed sine wave. Next, press the button below the scope screen which has the label "X2". This selects the second cursor for measurements relative to the first cursor. Now rotate the cursor knob clockwise and you should see the second cursor moving from the center of the screen. Move the second cursor to the second crest of the sine wave. Note that the screen reads out the time positions of the cursors (relative to the center of the screen), and also their time difference, and the inverse of the time difference. *Record the time difference and its inverse. Do these values make sense?
Next you will make voltage measurements using the cursors. Press the "X…Y" button below the screen, to toggle to control of vertical or voltage cursors. Using the cursor knob, move the first ("Y1") voltage cursor (a horizontal line) down to the bottom (trough) of the waveform. Press the "Y2" button, and move the second voltage cursor to the top (crest) of the waveform. Note that the screen reads out the voltages of the two cursors (relative to the center line) and also their voltage difference. *Record the voltage difference. Does this value make sense? It will be a factor of two too high because the function generator is expecting a load impedance of 50 Ohms but the scope probe is presenting in effect an infinite impedance. You will learn more about source and load impedances in the later courses.
Step 2: Viewing the Spectrum of a Signal on the Scope:
The scope can perform Fourier Transforms on signals which it has recorded using an algorithm called the Fast Fourier Transform (FFT). This lets us view the spectrum of a signal (a plot of the power in a signal as a function of frequency).
Press the Math button on the scope. Press the Function button below the screen until you get FFT. The screen should now show a Power Spectrum of the signal. Note that there is a single peak, which corresponds to the 10 kHz signal. Note that the spectrum is displayed with a linear frequency (horizontal) scale and a logarithmic amplitude (vertical) scale (in decibels, dB). The "Span" and "Center" frequencies are indicated. By pressing "More FFT" at the bottom of the screen you can view the vertical scale in units of dB per half-inch division. *Print the signal and spectrum display, and indicate the scale. You may also want to save the graphics file for your report. Your TA can help you interpret the scale. Note that by pressing the "1" button you can toggle between display of both signal and FFT, on the one hand, and display of FFT alone on the other.
Next, adjust the frequency of the sine wave being generated by the function generator as follows. On the function generator press Freq and toggle between Freq and Period to get Freq. You should see the 10 kHz which you entered. To make it convenient to adjust the frequency you will use the control knob. The display should be highlighting one digit of the frequency. This is the digit which will be controlled by the knob. Press the left or right arrow key to move this digit to the second position (the 1kHz position). Now you can increase or decrease the frequency in increments of 1 kHz by simply rotating the knob.
Slowly increase the frequency to 300 kHz and *describe what you see on the scope, relating to both the signal and FFT displays, and print the display.
Decease the frequency to 50 kHz. Change the function generator waveform to a square wave. Note that a square wave has a spectrum which has its power concentrated at the fundamental frequency and the odd harmonics. Does the square wave show a very narrow overshoot spike at the discontinuities? If not then turn up the "Intensity" (knob below screen) until it does. Now increase the horizontal sweep (knob near top of device) until the shape and length of the overshoot spike is clear. *Print that transient and be sure to note the time scale on the printout.
Now return to the original horizontal sweep and decrease the intensity until the overshoot spike goes away but the horizontal lines are still visible. *Print the display, including the scale.
Change the function generator waveform to a ramp waveform, leave the intensity as is, and *print.
Change the function generator waveform to "Noise". Note that noise is often described by its "color" which is an audio term. White noise has equal power per Hertz of bandwidth, i.e. a "flat" spectrum, and if you were to listen to it then it would sound very harsh (this is like the noise you hear between FM stations). Pink noise is filtered white noise which has equal power per octave (doubling) of bandwidth (i.e. less power per Hertz at higher frequencies). The function generator purports to generate white noise. On the FFT menu set the "span" to 10 MHz, the "center" to 5 MHz, and use the Horizontal control if necessary to cause the FFT or spectrum to display from extreme left to extreme right. What you see here will not strike you as "flat". The reason is that here we are dealing with a random process and the flatness holds only in an average sense. *Describe and print the waveform and the spectrum (you may have to turn the positioning knob below the "1" button to show the spectrum apart from the signal).
To see such flatness as you can at this point, take into account that the specs for the waveform generator say that the noise has roughly a 10 MHz bandwidth. Set the span to 50 MHz and the center to 25 MHz and use the Horizontal control if necessary. Now you should see the noise tending to drop off above 10 MHz and a suggestion of flatness below 10 MHz. *Describe and print the waveform and the spectrum with these new settings.
Step 3: Using the Power Supply and DMM
In this step you will simply program the power supply to generate a stable 3.3 V and use the DMM to measure this voltage.
Connect the DMM Channel 1 input to the 6 V Power Supply output. Note that the convention throughout most of the world is that Red represents Positive, and Black Represents Negative, thus you should connect the red clip lead to the Positive (red) power supply binding post, and the black clip lead to the Negative (black) power supply binding post.
On the DMM, use the knob to select Channel "101" which is channel 1. Press the DMM Measure button and use the knob to select DC Volts. Press measure again and use the knob to select Auto Range. Press Measure again and use the knob to select "6 ½ digits" of resolution. Finally press measure again to begin measurement. (Your TA will be happy to help you if you get stuck. The DMM can be a bit confusing.)
Next, adjust the power supply to generate 3.3V on the 0-6V output as follows. On the power supply, press the +6V button. Now press the Display Limit button. As with the function generator, the Power supply lets you select a digit of the value to modify using the left and right arrow keys. Select the second digit (the digit to the right of the decimal point) and use the knob to adjust the voltage to 3.3 V. Finally, press Output On/Off to turn on the power supply output. *What voltage is the power supply actually delivering? How stable is this voltage?