Introduction to Lab Instruments
Contents
1. different from what you have seen in the textbook even though they are in fact the same Follow these steps Display the frequency domain components of the 10 duty cycle pulse and observe the envelope Explain to yourself how this amplitude envelope represents a sync function Change the duty cycle of the pulse to 50 You now have a square wave 50 of the time on and 50 of the time off Find the appropriate display for the the frequency domain components Explain to yourself what they represent and whether they do follow the same sync function envelope Measure the amplitude and frequency of the four first components of the square wave spec trum Calculate these values from the theory and verify that they match e Which of the two pulses requires a wider bandwidth or spectrum to be transmitted the fast one or the slow one 4 Conclusion The experiment today lead you to explore the instruments that you will use extensively in the lab It also required you to explore very basic notions of time domain and frequency domain for two types of signals which are widely found in the theory References 1 P Kumar Digital Signal Processing Laboratory CRC Press 2005 2 Tektronix TDS1000 and TDS2000 Series Digital Storage Oscilloscope User Manual ie Agilent Technologies Agilent 39939220A 20MHz Function Arbitrary Waveform Generator User s Guide Agilent Technologies Inc 2003 4 Tektronix AFG32. t From the front panel press Sine You will see on the bottom right corner of your screen a menu labeled Output Menu Press on the corresponding button Another menu will appear in which you will find Load Impedance Press on it and you will have three options Select High Z From this point on your instrument is set to see a high impedance load on its output To return to the main screen just press on Sine again e Make sure that the Output ON button is lit if it is not press it It is located right above the BNC output connector Otherwise you will not have a signal Note that the Output ON button is lit on Figures 1 a and 1 b When the output is ON and the output impedance is set to high the message Load High Z will appear on the top right corner of the screen 3 2 The Tektronix TDS 2012 Oscilloscope This section will describe some features that if not understood now will potentially cause you to waste precious time during the experiments These are the trigger the MATH FFT function and the use of cursors for measuring voltage time amplitude and frequency Just as a reminder whenever you are in the lab try very hard to avoid pressing the Auto Set button Yes it is convenient and very popular but you will not learn as much about the instrument if you always use it Students tend to assume that whatever is displayed after this button is pressed must be the truth Always read the numbers and see if they make sense The trick is to k
3. 000 Series Arbitrary Function Generators Quick Start User Manual
4. ECE 431 2013 Bruno Korst Fagundes CommLab Introduction to Lab Instruments ECE431 1 Introduction This first experiment is a review of very basic concepts in Electrical Engineering done with the help of common instruments found in a lab In other words you will probe in practice some concepts seen in theory In the process you will review some features of the instruments which you will use in all courses in Communications and Digital Signal Processing In most experiments you will simulate a system implement it on hardware and test it with actual signals and instruments Though this experiment appears to be rudimentary what you practice here will be used in all future experiments You will review the time domain and frequency domain representation of two types of signals sinusoids and pulses You will use three types of instruments e An Arbitrary Signal Generator e An Oscilloscope 2 Suggested Practice and Reading As a learner you will benefit most from the practice in this lab by avoiding the automatic settings of the instruments You as an Engineer must understand what you need from the instrument prior to letting the instrument choose it for you This is to say that you will benefit from learning how to set the instrument first this should take about three experiments and only then go for the automatic features If you can explain what the automatic features do go ahead and use them There is no standard background r
5. e a square wave something is wrong 3 3 1 Generating Displaying and Measuring a Sine Wave Set your signal generator to output a 1V KHz sine wave and connect its output to the inputs of the oscilloscope using a T connector and BNC to BNC coaxial cables On your oscilloscope do the following tasks e Make the sine wave display on the screen then make it stop using the trigger e Measure its amplitude and frequency in the old fashioned way i e eye balling by using the per division numbers shown on the display Write your reading down e Measure amplitude and frequency using the Cursors horizontal and vertical e Measure amplitude and frequency using the Measure function of the scope Compare the numbers Now set your oscilloscope to display the frequency domain by pressing the MATH button From now on the amplitude is displayed in dB with 1V m as the reference Now do the following e Measure the amplitude and frequency of the sinusoid by inspection eye balling e Measure the amplitude and frequency using cursors e Calculate which amplitude value of the sinusoid would give you 0dB Change the amplitude on the signal generator to the value you have calculated and verify that the amplitude on the oscilloscope is 0dB Just for your own entertainment turn the trigger off or set it to a wrong value and make the signal run across the display Now press the Run Stop button Great the signal has stopped Now d
6. eading to prepare you to use the instruments Your best option is to explore the instruments either during the experiments or at the laboratory in your free time A good reference is 1 in particular the second half of the book which is dedicated to explaining some of the instruments found in a teaching lab The websites for manufacturers such as Agilent and Tektronix do offer literature about their instruments which may be helpful Manuals for the instruments 2 3 4 are available both at the websites and at the lab 3 The Arbitrary Signal Generator In this lab you will use the Tektronix AFG3021 It is called arbitrary because it allows for waveforms to be designed and edited either on the instrument or on a computer and then loaded onto the instrument Some of your experiments will utilize arbitrary i e made up waveforms Below are two settings which you will need throughout the course 3 1 Setting the Output Impedance Since you will implement your systems on a DSP platform all your input signals will be presented to an analog to digital converter prior to being processed by the DSP The input of the converter is a high impedance input By default your signal generator assumes a 502 impedance at their output This is to say that you must adjust the output of the signal generators to see a large impedance This is done by choosing a High Z output option as described below 3 1 1 Tektronix AFG 3021 e Turn the instrument on
7. isconnect the input to the scope or turn the signal generator off altogether You will see that your signal is supposedly still there when it no longer exists This is why you should avoid this button unless you are trying the catch a feature that happens every once in a while on your signal such as a glitch a sudden variation or a spurious noise burst 3 3 2 Generating Displaying and Measuring a Pulse On your signal generator select Pulse and set your output to be a 1Vpp 1KHz pulse with 10 duty cycle This represents a short fast pulse When you are sure you have the right signal displayed on the oscilloscope in time domain press on the MATH button and switch your display to the frequency domain You will explore the frequency domain representation of a pulse in this section From your knowledge of Signals and Systems you should know what to expect from the frequency domain display About 90 of students say it s a sinc when asked what a square wave looks like in the frequency domain That is what the math says and it refers to the amplitude envelope formed by the frequency components of the signal In general when you use the oscilloscope to display the frequency domain it will perform a Fast Fourier Transform FFT on the signal and will display the amplitude in terms of power i e no negative amplitude and will display only the positive frequency components This is to say that your display will appear a little
8. make the signal stop on the display Note that the signal continues to be collected and displayed but it is now steady on your display If the signal changes in amplitude frequency or a feature appears the display will show it The improper way to make a signal stop is to press the Run Stop button In the exercise section of this document you will do an exercise to understand why using Run Stop will not be appropriate 3 2 2 The Math FFT Function This scope allows you to see your signal in the frequency domain by means of an FFT operation You can select the Math mode by pressing a big fat red button right in the middle of the instrument Figure 2 Big Fat Red Button Math Mode Figure 3 below shows you the time domain for two input channels and the corresponding frequency domain for one of those channels Both inputs are a 1KHz 1Vpp sine wave M Pos 53 50mMs MEASURE Hi Freq 1000hHe a Time Domain Two channels b Frequency Domain One Channel Figure 3 Tektronix TDS 2012 Oscilloscope After pressing the Math Menu button you must select what type of operation you want the instrument to perform For the case of an FFT you also must select the channel Source that you want to display Figure 3 b is showing the FFT mode for Channel 1 On FFT mode the horizontal control works opposite to its time domain operation This is to say that if you want to increase the Hertz per Division reading you should turn the h
9. now what to expect from the instrument The general rule is if you understand what the Auto Set button does to get the readings go ahead and use it 3 2 1 The Trigger In general when you connect a signal source to your oscilloscope the signal runs across your screen In order for you to take a reading you should make it stop The proper way to make it stop is to set up a voltage level at which the oscilloscope starts to collect and display the signal In other words you should set up a trigger point This is done by pressing the Trig Menu button 1 it happens requency Period Phase Menu Amplitude Level Menu Run Mode Menu Output a Pressing Sine Output ON b Finding Output Menu Output ON a Load High Z High Limit 10 00 Y NolseAdd Off Low Limit 10 00 V Noise Level 10 9 Load High Z Invert Off c Selecting High Z Figure 1 Tektronix AFG 3021 found at the right most portion of your front panel right under a rotating knob As you press it a soft menu appears on the right side of the display Select the trigger Source i e channel 1 or 2 we will not use any other and a small arrow will appear further to the right of the display The colour of the arrow is the same as the channel chosen as the source This arrow indicates your trigger level Rotate the knob that is over the Trig Menu button and the arrow will move Position the arrow within the voltage span of your signal This will
10. orizontal scale knob clockwise In this case what the instrument is actually doing is selecting a different sampling rate for the input at every click of the horizontal scale knob You should be careful when using this knob to zoom in as you may end up with aliasing depending on the type of signal you have this will likely happen in the exercise section below The proper way to zoom into the signal in the frequency domain is to use the FFT Zoom soft key Note also that as you turn the horizontal scale knob at the bottom of the screen the number displayed changes as it represents the horizontal scale units of Hz per Division Figure 3 b shows the frequency component at 1KHz 3 2 3 Using Cursors In order for you to perform specific or relative measurements between points of a signal the instrument allows you to place cursors on the desired points and presents you with the values at each cursor and the calculation for the difference between the cursors Cursors can measure differences in time vertical cursors and amplitude horizontal cursors or in Math mode they can measure differences in magnitude and frequency After pressing the cursors button on the top part of the front panel you will be presented with a menu to select the signal source Channel 1 2 or math and the type of measurement you want 3 2 4 Using the Measure Feature The Measure button is found on the top part of the front panel This feature is very convenient but y
11. ou should try to develop an intuition whether the numbers displayed make sense This usually comes with practice For instance if your measured output from the DSP board is a 20 Vpp sine wave it should be obvious that this makes absolutely no sense The power supply feeds the board with only 12 Voc and the output of the codec cannot handle signals greater than 2 7 Vpp In that case consider that maybe your 10x probe option is on If you have a rough idea of what to expect you will question the displayed value right away Again knowing what to expect is very important To use the feature press on the Measure button A soft menu will appear on the right area of the display You must select the Source of the measurement i e channel 1 2 or Math and the type of measurement Vpp Vrms frequency etc When you select it the measurement will also appear on the right side of the display Keep in mind that if the 10x 100x or 1000x probe factor is set then the measurement will be scaled by it To disable it you must go to the channel menu CH1 Menu for instance and disable the probe factor 3 3 Exercise Now that you are an expert in the use of the signal generator and the oscilloscope you must do a few exercises to consolidate what you have learned so far Start by connecting the signal generator directly to the oscilloscope Make sure you connect the proper output of the signal generator to the oscilloscope if you want a sine wave and you se
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