2 Oscilloscope Familiarization

Transcription

1 Lab 2 Oscilloscope Familiarization What You Need To Know: Voltages and currents in an electronic circuit as in a CD player, mobile phone or TV set vary in time. Throughout the course you will investigate two types of signals, periodic and transient. The vibrations of the string of a guitar for example produces sound, pressure waves in the air. This changing pressure induces vibrations and produces time dependent voltage in a microphone. This signal is intensified in an amplifier, and a loudspeaker converts it to sound at the end, much louder than the original sound. Periodic Signal The sound of a vibrating string produces a periodic voltage in the microphone. If the time period of the signal is T then, V ( t + T ) = V ( t), see diagram below. The waveform repeats every period T. The periodic signal is characterized by its time period T or frequency f=1/t and by its peak-to peak value (p-p value). Instead of frequency, the angular frequency ω=2πf is also often used. The household supply voltage varies with time according to a simple harmonic function. V = V0 cos( ωt + α) V o is the amplitude of the voltage and α is a phase constant. 2-1

2 Transient Signal This is an example of a transient or one shot signal. There may be long periods of time when nothing happens and then suddenly you get activity for a moment and then nothing again. The example below is a heart-beat pulse. Another example of a transient pulse would be an earthquake monitor. With certain electronic events, the time interval when something happens can be VERY short, on the order of milliseconds. The shape of periodic signals and transient signals can be investigated with an oscilloscope. What does an oscilloscope do? An oscilloscope is easily the most useful instrument available for testing circuits because it allows you to see the signals at different points in the circuit as a function of time. The best way of investigating an electronic system is to monitor signals at the input and output of each system block, checking that each block is operating as expected and is correctly linked to the next. What follows is a brief outline of the main functions of the oscilloscope (scope)

3 The Oscilloscope Basic Design The function of an oscilloscope is extremely simple: it draws a V/t graph, a graph of voltage against time, voltage on the vertical or Y-axis, and time on the horizontal or X-axis. Many of the controls of the oscilloscope allow you to change the vertical or horizontal scales of the V/t graph, so that you can display a clear picture of the signal you want to investigate. 'Dual trace' oscilloscopes display two V/t graphs at the same time, so that simultaneous signals from different parts of an electronic system can be compared. You can check out how the oscilloscope works online at

4 As an example we can see the image of sounds on the oscilloscope. This is the image of a tongue click. Here is a detailed layout of the oscilloscope (or scope) you will be using; G W INSTEK Digital Storage Oscilloscope GDS-1000 series. 2-4

5 Oscilloscope Functions A scope is a very sensitive voltage measuring instrument. The scope gives numerical readouts as points on a graph. There are only 3 basic operations which it performs, so don t let the number of knobs put you off! These 3 basic operations are important to remember. 1) It will display graphically, along the vertical y-axis, the voltages put into its vertical input channel. 2) It will display graphically, along the horizontal x-axis, the voltages put into its horizontal input channel either; (a) external; (your input signals) (b) internal; (scope supplies the voltage signals). 3) Triggering, which tells the scope when to start displaying the signal. Once you know the basic functions it will be easy to interpret these knobs on ANY oscilloscope. Learn the function of the instrument, not the particular knobs which will change from instrument to instrument you encounter in your career. Turning on the channel (AC signal... most of your signals will be AC.) Turning on the XY mode for a DC input signal In this mode of operation, the scope is acting like a voltmeter and is used to display a time independent voltage. This is a static voltage or DC voltage. 2-5

6 In the diagram above, the oscilloscope is operating in XY mode with 1.5V applied to X-input and -1.0V applied to the Y-input. The X and Y scales are both set to 0.5V per division. Mostly you will have an increasing internal voltage applied to the horizontal x-axis (which acts like a time scale) and an external signal applied to the vertical axis. With no input to the vertical channel all you get is a horizontal line across the screen with the internal input applied to the x-channel. (See next page for picture of input signal to X-channel for time scale.) See Lissajous figures in the appendix 2 for better use of XY mode. When the X-channel is used as a time axis, this is refered to as sweep or time base operation of the scope. Now you are measuring a time dependent voltage. (AC voltage.) Vertical and Horizontal input channels You have a horizontal x-axis = CH 1 position and a horizontal scale knob = CH1 Volts/Div knob. You also have a vertical y-axis = CH 2 position and vertical scale knob = CH2 Volts/Div knob. 2-6

7 Vertical scale and position: CH 2 Selecting the vertical scale: turn the VOLTS/DIV knob; left (down) or right (up). The range is 2mV/Div ~ 5V/Div, in increments. To move the waveform up or down turn the vertical position knob for each channel. There is also a method to invert the wave form but you rarely use it. (Invert key). Also you can use XY mode for 2 input signals. Selecting the horizontal scale and offset function: CH 1 Selecting the horizontal scale: turn the VOLTS/DIV knob; left (down) or right (up). The range is 1ns/Div ~ 10sec/Div in increments. The next page has a diagram of the horizontal settings. 2-7

8 Selecting the horizontal offset position This is useful for positioning a waveform in a convenient position to be able to read the wave period along the x-axis. The period is the time for one complete cycle to complete. Selecting the vertical offset position. A similar position knob for the vertical moves the vertical setting position from up or down. It is mostly used to centre the waveform in the up/down position. Find this to adjust the vertical position away from centre to the back. 2-8

9 Signal Generator One of the most important piece of equipment in the lab, aside from the scope, is the signal generator. These are used for testing equipment. They are designed to produce several different forms of input signal, sine wave, square wave, ramp or sawtooth waves and spikes. With certain limits, the parameters associated with the waveform such as voltage amplitude, period, duration of positive and negative portions of a square wave, slope of the ramp are all adjustable. Below are examples of the most common wave forms. 2-9

10 Power Supply The new Instek power supplies are now programmable. This makes their operation more complex than our previous supplies. Simply turning on the supply will not apply power to the output terminals. Follow these steps to use these new supplies. Apply power to the unit via the power button in the lower left hand corner of the supply. Wait for it to boot. After booting, select a channel (one of 3 available) by pressing the [Shift] key (second function key, second row from top, far right) then a numeral [1], [2] or [3], usually ch. 1 is OK. Now set the output voltage for this channel. Press [V set] (3 rd button down from top next to display). Press numeral keys, (e.g. [1] [3] [.] [8]) value for the desired voltage. Don t forget the decimal point. After pressing [Vset] the dial may be used to set desired voltage as well. Then press [Enter] (lower right hand corner of numeric key pad). The selected voltage value will appear in the appropriate channel box at the bottom of the display. Current should be pre set to 200 ma (0.2 amp). Press [I set] then either the numeric value from the key pad or dialing in the current followed by the [Enter] key. Generally, current settings should be preceded by a decimal point. Finally, pressing [Output] will deliver the set voltage to the output terminals of that channel. Pressing [V set] or [I set] while voltage is present at the output terminals will increment the output voltage by 1 volt or 0.1 amp per detent. 2-10

11 What You Need To Do: There are 2 main parts to this lab: 1) XY mode of operation, measuring DC static voltage. 2) Time sweep mode, measuring AC (time dependent) voltage, period and frequency 3) OPTIONAL.. Trigger Control see Appendix 1 for details. WARNING: Do NOT attempt to measure the 110 V, 60 Hz wall outlet power available in the laboratory with the oscilloscope. You will damage the equipment and probably yourself! Part 1 Disconnect any leads (wires) from the scope and signal generator. This section will guide you step by step through the lab. Each channel (horizontal or x) CH 1 and (vertical or y) CH 2 serves two functions, they both have XY mode and sweep mode. Part 1 involves only the XY mode and measurement of DC static voltage for example from a battery. This acts very much like a regular voltmeter. The instantaneous values of the signals (voltages) applied to the X and Y channel determine where the spot will appear on the screen. To operate in XY mode it is necessary to make the following adjustments: A) Turn on the scope lower left button and wait for it to boot up. B) With no lead (no wires) attached to the scope press the MENU key in the horizontal column. Press XY-mode key which is close to the screen at the bottom. The XY should go grey when the key is pressed. C) On the lower left of the display you will see CH 1 (yellow) and CH 2 (blue) numbers for VOLTS/DIV. Set both of these to 1VOLT/DIV, adjust both VOLT/DIV knobs to achieve this. D) Please note that in XY mode the TIME/DIV knob does not function. E) In the menu screen on the display you should set the COUPLING to the DC setting, which looks like a solid line over a dotted line. F) Press MENU and CH 1, you should see a yellow dot on the screen. Use the Vertical Adjustment knobs to place both channels at 0.0V (dot is in the center of the display.) 2-11

12 Here is the display with MENU set to CH 1. G) You are now going to attach a battery to the CH 1 input. I used a 1.5 Volt battery in this demo. Connect the red wire to the positive end of the battery and the other end into the positive (red) plug on CH 1. Attach black wire to the negative end of the battery and the other end to the black plug of CH 1. What do you see? Well for a 1.5 Volt battery I saw this... The yellow dot jumps 1.5 Volt to the right along x-axis. H) Disconnect the red and black from CH 1 and plug them instead into CH 2, what happens? The yellow dot jumps up, 1.5V up along the y-axis. 2-12

13 I) Now connect another red wire from the positive end of the battery and connect the other end to CH 1, what happens. (By the way, both negative plugs for CH 1 and CH 2 are connected internally to each other, they share a common ground.) Now the yellow dot jumps to the right and up both by 1.5 Volts. Ok we are done with the XY mode and DC voltages we will move on to time dependent voltages and the signal generator. Disconnect the battery and all wires from the scope. Part 2 For this experiment we will be dealing with the signal generator and using it as an input for CH 2 of our oscilloscope. A) On the scope, push the MENU key and set CH 1 to DC Coupling. B) Push the MENU key again and set CH 2 to AC coupling. C) Push MENU again and set MAIN to ROLL. D) Plug the wires into CH 2 and then into the output of the signal generator. E) Set the signal generator to SINE waveform, push the button in. F) Press the RANGE button 10 that is 1-10 Hz range. Set the frequency using the COARSE and FINE adjustment knobs, lower left, so that the display reads 001 or 1 Hz. G) You should now see a sine wave on the scope. If you press the RUN/STOP key, the wave will stop moving and you can use the horizontal adjustment knob to move the wave into a position where you can easily measure the time duration of one cycle. You should find it to be 1 Hz, or 1 cycle per second. Try setting the TIME/DIV to 250 ms then 1 Hz would mean 1 cycle completes in 4 boxes on the screen, since 4 x 250 ms = 1000 ms = 1 sec. Notice also you have a frequency displayed on the screen (small green letter f) but all it will say is f < 20 Hz. The frequency needs to be much higher for this to function properly. 2-13

14 Showing 1 Hz H) Try setting the frequency on the signal generator to 4 Hz or 004. Press the RUN/STOP key again and find the period of the wave. For 1 cycle you should see a complete wave in 1 box on the screen. Ok, now set the TIME/DIV on the scope to 500μs. A) Set the signal generator to 1000 HZ, that is push the 1K button in and set the display to 1000, using the COARSE and FINE adjustment knobs. B) Push the RUN/STOP key on the scope and move the wave horizontally to a position where you can measure 1 cycle easily. The wave should look 2-boxes-long, that is 2 x 500μs = 1 ms. C) Look at your frequency read out at the bottom of the screen, small green letter f, it will be far more accurate than the reading on the signal generator and give you a few decimal places, e.g. F = KHz. Now we will move on to triangular and square waveforms. A) Stay with frequency 1000 Hz. Set the signal generator to output a triangle waveform. B) On the scope use TIME/DIV= 500μs and 500mV on the VOLTS/DIV scale. Here is what you see if you ve set it up correctly. 2-14

15 Now set the signal generator to give you a square wave. Still keep freq= 1000Hz. Set the TIME/DIV to about 250μs and use about 500 mv on the VOLTS/DIV scale on your scope. Here is what you see if you ve done it correctly. CONGRATULATIONS!! At this point we have completed our survey of lab equipment and you are now familiar with the most important settings of the oscilloscope and signal generator. This will be very useful in later labs. If you which to continue there are a couple more experiments in the Appendices which your instructor might help you with. 2-15

16 Appendix I: Trigger Function of Oscilloscope What You Need To Know: TRIGGER Option: This will begin a display at a predetermined voltage or slope. Selecting the Trigger Run and Stop mode: Pressing the run/stop trigger key toggles between run and stop mode. The Trigger function configures the conditions by which the scope captures a signal. There are different types of Trigger: 1) Edge: Triggers when the signal crosses an amplitude threshold in either the positive or negative slope. 2) Video: Exacts a sync pulse from a video format signal and triggers on a specific line. 3) Pulse: Triggers when the pulse width of the signal matches the trigger settings. We will be using only the EDGE setting of the trigger. The trigger source can be; 1) CH1 or CH2 input signals 2) Line - AC mains signal or 3) Ext - External trigger input signal. 2-16

17 The Auto Trigger status appears in the upper right corner of the display. You will be using CH 2 input. 1) Auto: Trigger at 1kHz automate or 100kHz 2) Norm: Trigger only at the detection of pulse on Y channel Edge mode for the Trigger using CH 2 input: In the following diagram we show two options for the trigger level, one set at 1mV the other set at 3mV and the resulting display images for the given pulse. Edge mode for Trigger using External (Ext) setting. The externally applied signal is often a sharp pulse or spike, a signal of short duration but fairly large amplitude. The main thing is the timing of the spikes. What follows are examples of the different results you would get by sending in trigger signals at various different times relative to the same input signal. 2-17

18 What You Need Do: A) The settings start out like the Part 2 of this lab. B) On the oscilloscope: Push the MENU key again and set CH 2 to AC coupling. C) Push MENU again and set MAIN to ROLL. D) Plug the wires into CH 2 and then into the output of the signal generator. E) Set the TIME/DIV to 250 μs, set VOLTS/DIV to 500 mv. F) Set the signal generator to SINE waveform, push the button in. G) Set the signal generator RANGE to 1 khz and adjust the knobs so that the display reads to 1000 Hz. H) We have been using the AC mode for CH 2. If we had used DC mode instead there may have been a change in the display. You might have seen the sine-wave move up or down along the y-axis depending on the DC offset of the signal generator. This is a static DC voltage superimposed upon the sine wave signal. I) By using the AC mode for CH 2 (use MENU and CH 2 then push the AC key) we ignore any DC offset the signal generator may have and thereby get a nicely centered sine-wave on the screen. J) Press the MENU button under the TRIGGER then select TYPE > EDGE, K) Select SOURCE = CH 2 The SOURCE button will cycle through CH 1, CH 2, External (EXT) and then LINE. External refers to an external signal which would come through the EXT TRIG BNC connector at the lower right of the scope. LINE refers to 60 Hz mains voltage. L) Select SLOPE/COUPLING choose positive slope (you have to keep pushing the key to cycle through the items in that menu for that key.) Now as soon as you ask for a positive slope notice that the waveform starts over and starts with a positive slope, it starts moving upward in the +y direction. If instead you ask for a negative slope, again the wave-from restarts and this time starts with a negative slope, it starts moving down in the y-direction. In order to use an external trigger you must ask your Lab instructor to assist you. 2-18

19 Appendix II: Extra Exercises for Advanced Students Addition of sine wave and Beats Tools: oscilloscope, signal generator, BNC plugs, microphone, loudspeaker. 1. Determine the time period, frequency and peak-to peak voltage of a signal. 2. Determine the time delay between two AC signals of the same frequency. Calculate the phase difference from the delay time! 2π 2π A = A0 sin( t), B is delayed by t time. B = sin( ( t t)) T T The phase difference is α = 2 π t T. 3. Use the addition function of the oscilloscope to see the sum of these AC signals of the same frequency. Do the same with signals of slightly different frequencies. What do you observe? Determine the frequency of the beat. Beat Frequency: A beat wave is a sound of fluctuating volume caused when you add two sound waves of slightly different frequencies. The sound of a single pitch or tone consists of only one specific wavelength or frequency, represented in the form of a sine wave. The volume you hear depends on the amplitude of the wave. When you add another sound wave of the same pitch, the new volume will be the sum of the two amplitudes, provided the waves are in unison or have the same phase. But if you add two sound waves of slightly different frequencies, the sound you hear will fluctuate in volume according to the difference in their frequencies. 2-19

20 These are called beat frequencies or beat waves. Use the loudspeaker to hear the beat. Determine the highest frequency you can hear. Lissajous Curves Lissajous curves are the family of curves described by the parametric equations x ( t) = a sin( ω t + δ ) y = bsin t Plot a Lissajous Curves with following parameters ω = 1, δ = 0 a b, ω = 1, δ = π/2 a = b, ω = 1, δ = π/2 ω = 2, δ = π/2 Solutions to Lissajous Curves: 2-20

21 Special cases are summarized in the following table, and include the line, circle, ellipse, and section of a parabola. parameters curve, line,, circle,, ellipse, section of a parabola It follows that, gives a parabola from the fact that this gives the parametric equations, which is simply a horizontally offset form of the parametric equation of the parabola. 2-21