EE1427 Engineering Science Laboratory Guide
EE1427 ENGINEERING SCIENCE LABORATORY TASKS
1.0PREREQUISITES
In order to prepare for the experiments to be conducted in this laboratory exercise, please familiarise yourself with the following:
This link will enable you to familiarise yourself with oscilloscope functions. While the scope featured is the old style cathode ray scope (CRT), the functions are applicable to all types of scope.
This link contains the manual for the present TFT scopes in use in CG04. Pay particular attention to “Understanding Oscilloscope Functions”, “Operating Examples” and Application Examples”.
Pay particular attention to the left half of the Test and Measurement Unit, i.e. to the Frequency Generator and the Frequency Counter.
2.0LAB REQUIREMENTS
For this lab, you will need a breadboard which should have been purchased from the lab technician, along with the necessary tools required for manipulating components and wires.
You will also need your lab book, in which you will record all your work, which should include calculations, graphs and diagrams. After each section, your lab book must be signed by the module leader.
It is also recommended that you bring with you a USB memory stick, as you will need to save captured images from the oscilloscope, and use them in your reports.
Finally, you will also need a degree of care and vigilance. The lab is a dangerous place, so please adhere to the safety protocols that have been laid out previously.
3.0LAB TASKS
Week 1 – Task 1 Oscilloscope Familiarisation
Week 2 – Task 2 RCL Filter Circuits: Frequency and Time Domain Study
Week 3 – Task 3 RCL Filter Circuits: Resonance Study
4.0REPORT AND MARKS
This lab will contribute towards your final mark for the Engineering Science module, so it is important that you try and get as higher mark as possible. This can be achieved by the standard of work you will produce. I expect a detailed typed up formal report in addition to the notes made in your lab books as you carry out each procedure, with clearly labelled circuit diagrams, graphs, calculations and I also want to see your work backed up by relevant theory. My assistants and I will be monitoring you closely throughout the duration of the lab, and I will take into consideration the way you conduct yourselves in the lab when it comes to marking your reports.
Finally, please feel free to ask questions if you become stumped, as I and my assistants will only be too willing to help. However, we will not give you any answers, nor will we construct circuits. All the best!
5.0TASK 1 – OSCILLOSCOPE FAMILIARISATION
5.1 INTRODUCTION
The aim of the first task is to introduce the basic operational procedures of the oscilloscope so you will learn how the “scope” functions and how to set it up with the optimal operating settings in the most common measurement and diagnostic conditions.
The “cathode ray oscilloscope” is one of the most versatile tools available to an electrical engineer for investigating circuits, but have been replaced in the lab with the modern “digital storage oscilloscope”. Its great versatility demands a basic level of understanding on the part of the user. Without sufficient understanding, a scope can be virtually useless or even dangerous.
In order to help you develop a degree of familiarity with the scope and its functions, the following procedures have been designed. They should be followed so that familiarisation with the scope can be effectively gained.
For each of the aspects involved in this experiment, note down all your observations in your lab books, as well as any parameters that you are asked to measure.
5.2 EQUIPMENT REQUIRED
1 Tektronix TDS2002 Digital Storage Scope (Figure 5.1)
1 USB memory stick
BNC-BNC leads (Figure 5.2)
BNC-croc clipped leads (Figure 5.3)
Frequency Generator and Counter
Breadboard
Figure 5.1
Figure 5.2
Figure 5.3
5.3 INITIALISATION
The type of scope installed in the lab is the Tektronix TDS2002 digital storage oscilloscope. These scopes are small, lightweight, benchtop packages that can be used to take ground-referenced measurements. To switch on the scope, push the button located to the left on the top surface of the device. After a few seconds, the main scope screen will appear which has a black background with dotted horizontal and vertical lines.
The scope has two channels, (so that two signals can be displayed simultaneously) Channel 1 (CH1) and Channel 2 (CH2). Channel 1 and its related controls have a yellow designation. This also means that when a signal is connected to the CH1 input, a yellow trace will appear. Channel 2 is designated with the colour blue.
First, it is necessary for the yellow (CH1) trace to be visible in the centre of the screen in order to initialise Channel 1. This is achieved by first pressing the yellow CH1 MENU until in the bottom left of the main screen you see yellow text saying “CH1” followed by a voltage value. If in the same part of the screen you see blue text, (CH2, voltage value), press the blue CH2 MENU button until this disappears. To initialise Channel 1, press the AUTO SET button, which is located in the top right hand corner of the scope controls. You should now see the yellow Channel 1 horizontal trace in the centre of the screen.
Can you initialise Channel 2 in the same way?
Next, you are to display a signal from the Function Generator on Channel 1 of the scope screen. Connect a BNC-BNC (BNC stands for Bayonet Neill Concelman) cable from Ch1 (Channel 1) of the scope to the OUTPUT socket on the Function Generator, which is the lower of the three BNC sockets on the far lower left of the workstation. On the frequency generator, locate the 50Ω/600Ω switch, and make sure it is out (50Ω selected). On the DISPLAY button, make sure F/G is selected, which allows the frequency to be displayed on the digital readout of the frequency counter (upper left quadrant). Select the sinusoidal function on the generator and select the X100K frequency multiplier button. Use the dial to obtain a frequency of 20kHz.
To the left of the scope control panel, there are five buttons that go from top to bottom. On the right of the scope screen there are five parameters that correspond to the five function buttons. If these five parameters are not displayed, press the yellow CH1 MENU button in order to bring them up. From top to bottom, the functions are:
i) Coupling – set this to AC by pressing the button to the right
ii) BW limit – set to off
iii) Volts/div – set to Coarse
iv) Probe – set to 1x
v) Invert – set to off
Note that the coupling button, when set to Ground, displays a horizontal line (yellow for CH1, blue for CH2) which can be moved vertically by turning the VERTICAL POSITION dials. This function can be used to select an alternative zero reference if desired. Experiment with this and make a note in your lab books regarding your observations.
5.4 BASIC CONTROLS
Figure 5.4
Figure 5.4 shows the screen of the scope, and what the axes represent. Use the SEC/DIV (seconds per horizontal division) dial and VOLTS/DIV (volts per vertical division) dial for Channel 1 (yellow) of the scope controls to obtain a trace on the scope of at least 2 periods and by adjusting the AMP button on the frequency generator, obtain a waveform that fills at least two-thirds of the screen. Make a note in your lab book of the VOLTS/DIV and SEC/DIV settings which are located at the bottom of the screen, and use them to visually measure the waveform’s frequency and peak-to-peak voltage. Take the following measurements:
(i)Positive peak to ground voltage
(ii)Negative peak to ground voltage
Adjust the VERTICAL POSITION, HORIZONTAL POSITION, VOLTS/DIV and SEC/DIV buttons, and by observing the changes made to the waveform on the screen, explain what these controls do in your lab book.
Next press the MEASURE button, which is at the top of the scope controls. Use the function buttons on the left of the scope controls to set the “Source” to CH1 (top button). Press the second function button several times, and make a note of all the parameters that this button cycles through.
5.5 DC OFFSET
Locate the OFFSET button on the Function Generator. Pull this button to the “out” position, and turn the button clockwise and anticlockwise. Make a note of what is happening on the screen of the scope as you vary the DC offset. Next, press the yellow CH1 MENU button and select DC coupling. Observe what happens when you adjust the “offset” button of the frequency generator. Can you explain what is being added to the waveform on the scope screen? Push the OFFSET button back in once you have finished with this part of the experiment, and reselect AC coupling on the scope.
5.6 USING TWO CHANNELS
Construct the circuit below on your breadboard. Vinis to be supplied by the Function Generator. Use Channel 1 of the scope to measure an input sinusoidal waveform of 25kHz with a peak to peak voltage of 7.5V.
Figure 5.5
On the scope controls, press the blue CH2 MENU button in order to allow two waveforms to be displayed at the same time using the scope’s second channel, Channel 2. Waveforms displayed on Channel 2 are blue in colour, and any related parameters have blue text. Attach a BNC cable to the Channel 2 input of the scope, and connect the two crocodile clipped ends to measure the output of your circuit across the resistor. Adjust the VOLTS/DIV for Channel 2 to obtain a suitable trace on the screen. In your lab books, draw accurately what you see, not forgetting to note down the SEC/DIV and VOLTS/DIV for both channels. What is the overall peak-to-peak gain of the circuit, which is a ratio of the output and input voltages? Using the following formula, calculate this gain in decibels.
Gain in dB = 20 log10 (Vout/Vin)
Next use the CH1 and CH2 MENU buttons to find the peak to peak values of both waveforms, and use this to recalculate the gain. Then press the red MATH MENU button. Press the Operation button in order to select +. Describe what you see, and explain what is being displayed on the scope. Do the same for when – is selected. Then press the MATH MENU to switch off the red trace.
To save you from drawing what you see on the scope screen, the Tektronix TDS2002 Digital Storage Scope comes with PC based software via a serial link. Switch on your bench PC, and log in. Open the “OpenChoice Desktop” program. Once the new window has opened, hit the “Select Instrument” button, which should open a new dialog box. Select “ASRL1::INSTR”, and then back in the main window select “Get Screen”. After a few seconds, the image from the scope will be downloaded onto the PC interface program. Once this has been done, you can save the image onto a USB stick, or on the desktop should you with to email it to yourself. This image should go in your main report.
5.7 TRIGGER FUNCTIONS
The TRIGGER function controls allow the oscilloscope display to be synchronised with the signal you want to investigate. To provide a more stable trace on the scope screen, modern oscilloscopes have a function called the trigger. When using triggering, the scope will pause each time the sweep (which is the steady motion of the trace across the screen) reaches the extreme right side of the screen. The scope then waits for a specified event before drawing the next trace. The trigger event is usually the input waveform reaching some user-specified threshold voltage in the specified direction (going positive or going negative).
The effect is to resynchronise the timebase to the input signal, preventing horizontal drift of the trace. In this way, triggering allows the display of periodic signals such as sine waves and square waves. Trigger circuits also allow the display of nonperiodic signals such as single pulses or pulses that don't recur at a fixed rate.
Press the TRIG MENU button located on the right of the scope controls. This activates the trigger functions which should appear on the right side of the screen. Leave the “Type” on “Edge”. Switch the source between CH1 and CH2 (ignoring Ext, Ext/5 and AC Line). Can you describe what is happening?
Vary the TRIGGER LEVEL dial for both CH1 and CH2 sources. Describe what happens and draw what you see.
Next, while still in the TRIGGER menu, switch the “Source” function to “Ext” (for external). What can you see on the scope? Draw in your lab books what you see. Fetch another BNC-croc clips cable. Connect the BNC end of this cable to the EXT TRIG (trigger input) socket on the scope. Attach the crocodile clips to the input of your circuit. Draw what is displayed on the scope screen. Does what you see on the screen look familiar? Why?
Now attach the crocodile clips from the trigger input BNC cable to the output of your circuit (across the resistor). Draw and explain what you see.
Can you explain the triggering function based on what you have observed?
LAB BOOKS MUST BE SIGNED AT THIS POINT
6.0TASK 2 – RCL CIRCUITS: FREQUENCY AND TIME STUDY
6.1 INTRODUCTION
In this task, you will investigate the performance of some basic filter circuits. Filters to select different frequency ranges are widely used in electronics, communications, control systems etc. A common example is the tone control provided in audio systems. This is a filter circuit which can be adjusted to emphasise or reduce different frequencies and so give different qualities to the music.
Figure 6.1
The engineering properties of a filter can be represented in the frequency domain which shows how the attenuation (or AC voltage ratio) varies with frequency. The AC voltage ratio is a complex quantity and is usually represented in its polar form i.e. magnitude and phase angle.
Time domain representation is also possible. This shows the relation between the input and output time waveforms when a transient signal (e.g. step or ramp waveform) is applied at the input.
6.2 CR FILTER CIRCUIT – FREQUENCY DOMAIN RESPONSE
Theory
Figure 6.2 shows the circuit configuration for a CR filter.
Figure 6.2
Kirchhoff’s Voltage Law for the circuit of Figure 6.2 states that
The bar above the variables indicates that the voltages are complex quantities having angles as well as magnitudes. Also lower case letters (v, i) are used for quantities which vary with time, whereas upper case letters (V, I) are used for quantities which DO NOT vary with time (e.g. average, RMS, peak values).
Capacitors are affected by the type of current that goes through them. It has a resistance (impedance) called ‘reactance’ and is dependant on the frequency of the voltage across it. The reactance of the capacitor is denoted by Zc, where
Ohm’s Law is also applicable
So the AC voltage ratio equation is given by
Putting CR = τ, where τ is the time constant, and expressing numerator and denominator in their separate polar forms gives
This can be evaluated by ‘polar’ division i.e. divide magnitudes and subtract angles. Note the special condition that arises when ω= 1/τ. This important frequency is called the critical frequency, fc, where
ωc = 2πfc = 1/τ
Circuit
Connect a C = 1.0μF capacitor (non-electrolytic, non-polarised) and 170Ω<R<230Ω (record these values in your log book) onto your breadboard to resemble the circuit shown in Figure 6.2. Use an appropriate cable to connect the Function Generator, from which you should use the 50Ω BNC output. Use another cable to connect the circuit output to CH2 of the scope, and another to display your input on CH1.
Draw a table in your log books with six columns headed by:
Frequency (Hz)
Log10f
vin (V)
vout (V)
Gv (= vout/vin)
dB = 20log10Gv
Figure 6.3
Set the function generator to input a sinusoidal waveform into your circuit, which you can verify on the oscilloscope, with a peak to peak voltage between 2.5V and 3.0V. Vary the frequency from about 30Hz up to 30kHz, using a suitable scale. Record your results in tabular form and immediately plot a control graph of log10f against 20log10Gv. Your graph should be similar to that of Figure 6.3.
If you examine your control graph you will see that for some intervals there is a need to take additional measurements to fill the gaps between some sets of points, particularly when there are conditions of special interest, such as the value for fc, i.e. the -3dB frequency. Do this while the equipment is set up, and you may need to plot another graph to find out if the new results are good enough.
It is particularly important to locate fc, which is the frequency at which the voltage ratio is 1/√2 (= -3.0dB) as its value will be needed later in the experiment. Take additional measurements to focus in on that part of the graph, at around -3.0dB. Plot a graph for this range and read off the value for fc.
Deduce your values of fc and τ from the graph. These are related directly to the product RC, and as a consequence there is no need to know the separate values of R and C. Later on in the experiment the product RC will also be evaluated from the time domain tests, so you must keep the same two components for consistency.
Calculations and Comments
Each part of the tests described above should be fully accounted for, both in your lab books and in your final formal reports.