The Oscilloscope. Vision is the art of seeing things invisible. J. Swift ( ) OBJECTIVE To learn to operate a digital oscilloscope.

Transcription

1 The Oscilloscope Vision is the art of seeing things invisible. J. Swift ( ) OBJECTIVE To learn to operate a digital oscilloscope. THEORY The oscilloscope, or scope for short, is a device for drawing calibrated graphs of voltage vs time very quickly and conveniently. Such an instrument is obviously useful for the design and repair of circuits in which voltages and currents are changing with time. There are also many devices, called transducers, which convert some non-electrical quantity such as pressure, sound, light intensity, or position to a voltage. By using a transducer the scope can make a plot of the changes in almost any measurable quantity. This capability is widely used in science and technology. Menu buttons Measurement buttons Trigger controls Vertical controls Inputs Horizontal controls Fig. 1. Oscilloscope front panel with major functional blocks marked. The scope you will use is a very flexible instrument, typical of those available in a research laboratory. It has two channels, so that two different voltages may be plotted simultaneously for direct comparison, and many measurement options. The front panel, shown in Fig. 1, is correspondingly formidable at first glance. Fortunately, the myriad of controls can be considered in several independent groups, some of which are marked in the figure. In the remainder of this section we will examine each group in turn, concentrating on the controls we

2 will need in subsequent experiments. The experimental procedure section will then take you through a series of measurements designed to demonstrate the operation of each section. Display and menus The screen, shown in Fig. 2, displays the signals on the gridded portion and provides other information as shown. Each main grid interval (about 1 cm) is called a division, with the vertical scale given in volts/division and the horizontal in seconds/division shown below the grid. The right column is a menu display for the function selected by main-panel buttons. Menu settings are changed by pushing the adjacent button. If there are several choices, a pop-up will appear. Use the Multipurpose knob to highlight a choice and then push the knob to enter. A round button at the bottom of the column allows you to toggle the menu off to get more display space. zero volts level, chan 1 vertical menu chan 1 zero volts level, chan 2 vertical volts/div chan 1 chan 2 horizontal sec/div trigger channel, level and frequency Fig. 2 Typical screen display with some features identified. Vertical Input voltages are applied at the set of connectors across the bottom of the face. The outer shell of the twist-lock connector (BNC) is the ground reference and the center pin conducts the signal. The supplied cable has banana plugs to connect to other circuit elements. The black plug on the cable is ground and the red is for signal. Because this is a two-channel scope there are two identical sets of vertical controls, one for each trace. Pushing the colored button toggles the channel display on and off for channel 1 or 2. The larger knob sets the vertical scale, while the small one positions the trace. The main menu functions are: PHYS 112 The Oscilloscope 2

3 Coupling: Allows the input circuit to accept all signals when set at DC, or only the timevarying part when set for AC. Ground connects the vertical input to ground, so that you can see where the zero-voltage height is on the screen. Zero level is also marked by the colored arrows. (Using the Ground setting does not connect the external input terminal to ground, so your circuit will not be disturbed.) BW Limit (bandwidth limit): This button allows you to cut off signals with frequencies higher than 20 MHz, so that they do not appear on the display. Most of our signals are at lower frequencies, so leaving this button in will sometimes reduce noise without losing any information. Volts/Div: Coarse allows you to set the vertical scale in standard steps. Fine gives continuous variation. The actual scale factor is always displayed on the screen. Probe: Pushing this brings up another menu that lets you set up the input to use voltage or current probes, and to set the scale factor. Here you will always use 1X Voltage connections. Invert: Inverts the input signal so that increasing positive voltages are plotted downward, rather than upward. This should be Off for normal operation. Horizontal The horizontal system controls the time scale of our plots. The main control, labeled Scale, works much like the vertical sensitivity controls, with a series of fixed settings. When set for 1 ms/division, as in the figure, each one centimeter division on the screen corresponds to one millisecond. The position knob works just like its vertical counterpart. Fig. 3. A continuous input waveform and four successive sweeps on the scope screen. The trigger is set for positive slope with the trigger level at the dashed line. PHYS 112 The Oscilloscope 3

4 Trigger The trigger system is used to start successive sweeps at corresponding points on the input waveform on each successive sweep. This operation is indicated schematically in Fig. 3. The Level knob controls the voltage at which the trigger starts the sweep, or you can push the Level knob to trigger at the half-way point. Menu in the Trigger block brings up the main menu for the trigger system. The selections are: Type: Usually set for Edge. Source: Select Ch1 or Ch2 to start the sweep from the main signal. Ext triggers off a signal applied to the Ext Trig connector. This is useful when the inputs are very small or noisy, but the experiment produces a strong signal that occurs in a fixed time relation to the signal of interest. Triggering on AC Line is useful if you want to study something that might be synchronized with the AC power. Slope: Specifies whether the signal should have a positive or negative slope at the trigger voltage. Mode: Auto runs the sweep continuously. Normal starts a sweep only when a trigger occurs, which can be useful with intermittent signals. Coupling: Usually left on DC. If triggering is unstable in particular situation you can try other options. Set Trigger Holdoff: The default is 500ns, which usually works. Other functions The buttons across the top do many tricks of variable utility. Their use will be explained as needed. You can also refer to the Tektronix user manual, available on the lab computers as a PDF. EXPERIMENTAL PROCEDURE Turn on the scope with the switch on top and allow it to go through its start up checks. Connect the function generator to channel 1 on the scope, matching the red and black (ground) leads. Set the function generator for about 1 khz triangle-wave output with the variable amplitude knob at mid-range. PHYS 112 The Oscilloscope 4

5 You should see a signal on channel 1, and nothing on channel 2, so use the channel 2 button to toggle it off if necessary. Adjust the channel 1 and horizontal scale knobs to get a clear display of a few cycles of triangle wave on the screen. Try changing the various controls on the function generator and observing the changes in the output. (The scope is difficult, although probably not impossible, to damage electrically so don't worry about the effects of improper settings.) In this exercise, you are plotting many different signals on the same scales. You can also try plotting the same signal on many different scales by leaving the function generator set, and changing the vertical and horizontal scale controls on the scope. Play with various combinations until you are confident of the effects of the various voltage and time scale controls on the scope and the function generator. The Autoset feature is sometimes useful to get a starting point for a measurement. When you push the Autoset button the scope will try to set sweep time, trigger etc. to display a few cycles of whatever signal is on channel 1, along with some wave form parameters. While not always successful, the feature works well for steady, low-noise signals. Triggering Now examine the triggering controls. Set the function generator for a 1 khz triangle wave output and adjust the scope scales so that two or three cycles roughly fill the screen. Try various settings of the trigger Level control and the horizontal Position control. Note that the trigger voltage level is shown by the colored arrow at the right of the screen, and the trigger time by the orange T-pointer at the top. What happens if the Level is set above or below the voltage range of the signal? Push the trigger Level button to restore a good trigger point. Now display the trigger menu by pushing the button, and look at the effect of switching between Slope Rising and Slope Falling. Summarize your observations in your report. Automated measurements As an example of the measurement functions, we will determine the rise time of the square wave output of the function generator by three different methods. Rise time is defined as the time required for the voltage to change from 10% to 90% of its final value, as shown in Fig. 4. Switch the FUNCTION control to generate square waves and obtain a clear display. By speeding up the sweep enough you should be able to find the 10% and 90% points and read the time between them directly from the calibrated scales. Pushing the Cursor button brings up a menu. Select Type Time, Source Ch1 and push the Cursor 1 button. The Multipurpose knob lets you position the first cursor at the 10% point. Push the Cursor 2 button and set the second cursor at the 90% point. The rise time is displayed in the middle of the menu panel. PHYS 112 The Oscilloscope 5

6 Pushing the Measure button and pushing Ch1 gives a measurement menu. Scroll down and select Rise Time. The scope automatically computes and displays rise time. How do your estimates of the rise time compare? Which method you choose depends on what you are trying to measure and how clear the signal is. You can make multiple measurements on the same signal by selecting them from the menu. To turn off unneeded measurements, just select them again. X-Y mode In the x - y mode, the scope uses one of its input channels, instead of the internal time base generator, to control the horizontal deflection. We will demonstrate this feature by using two function generators to display Lissajous figures, which appear when the inputs are sine waves. The other channel provides vertical deflection, as before, so the scope plots x(t) = A x sinω x t and y(t) = A y sinω y t Connect the outputs of the first generator to channel 1 (x) and the second to channel 2 (y). Set both generators to produce a sine wave at about a hundred Hz and about 2 V amplitude. Push Utility and then select Display on the menu. Select Format XY from the Display menu. (To return to normal select Format YT.) Carefully adjust one function generator frequency until the trace is a steady ellipse. If you only see part of an ellipse, use a longer horizontal time scale so that the scope samples a full cycle. Are f x and f y now equal? Try varying the generator frequency to observe other figures, which occur for certain relationships between f x and f y. Overshoot Settling Time 90% 10% Rise Time Ringing 50% Pulse 50% Width Amplitude Fig. 4. Definition of rise-time and other parameters for a realistic pulse or square wave. PHYS 112 The Oscilloscope 6

7 CH 2 CH 1 red µf R black GND Fig. 5. Circuit for measuring V R in an RC circuit. RC circuit For a more physical example, consider the circuit of Fig. 5. Using the square-wave output of the function generator, the capacitor charges to one polarity, then the source reverses and charges the capacitor to the opposite polarity. The whole cycle repeats at any rate we find useful. The resistor R acts as a transducer, allowing the scope to display a voltage proportional to the current through the capacitor on channel 1. Based on the general properties of RC circuits we expect the current to be large whenever the polarity changes, and to then decay exponentially to zero. Channel 2 will display the driving voltage V S for comparison with this expectation. Wire the circuit for measuring V R as shown in Fig. 6, with R = 1.5 kω. Be careful to connect the grounds as indicated. Set the function generator for a square wave at about 200 Hz. The scope will display V R on channel 1, and V S on channel 2. Set up the scope to trigger on channel 2, and then set the controls so that you can clearly see the current (V R ) and source (V S ) waveforms. You may need to adjust the frequency or amplitude of the function generator output to get a satisfactory display. Ideally, you will be able to see both the decay and the flat area that defines the zero level of current. Sketch the two waveforms, being careful to show the time relation between them. Power supply The scope is also useful for studying the behavior of circuits. The circuit shown in Fig. 6 is a simplified version of a power supply, a device used to convert alternating current from the power company to direct current to supply an electronic device. The crucial component is a diode, shown with an arrow and bar symbol. Diodes allow current to flow in one direction, but block reverse flow. To demonstrate this phenomenon, wire the circuit of Fig. 6, but omit the capacitor for now. Set the function generator for a sine wave at a hundred hertz or so. Use the scope to sketch the voltage across the resistor, being careful to mark the zero level. Are your PHYS 112 The Oscilloscope 7

8 observations consistent with the idea that a diode allows current flow in only one direction? What happens if you reverse the diode in the circuit? CH 2 CH 1 red black 0.47 µf 1 k GND Fig. 6 Model of a power supply circuit. The diode should be one of the devices labeled rectifier diode on the circuit board. Now connect the capacitor as shown, set the function generator to a frequency of Hz, and sketch the new waveform. Has the addition of the capacitor changed the waveform substantially? Raise the function generator frequency to 5-6 khz and sketch the waveform again, carefully noting the zero-volt level. Can you describe the result as a small variation about a positive value? In a real power supply the resistor would be the load we wish to drive, and the capacitor would be chosen to be large enough to make the variations ( ripple ) as small as needed for the particular application. Other signals (optional) Finally, if you have time at the end, you can look at musical signals. We have adapters that will let you connect the analog output of your audio device to the scope. PHYS 112 The Oscilloscope 8