Voltage (measured on the vertical axis)

Transcription

1 Operate a Digital Storage Oscilloscope Name(s) It is important to understand these basic features of the oscilloscope. VOLTAGE measured on the vertical axis. TIME measured on the horizontal axis. TRIGGER tells the voltage and time trace where to start on the screen. CURSORS measure the differences in voltage or time Voltage (measured on the vertical axis) Oscilloscopes measure voltage. When you connect scope leads to a voltage source, it causes the voltage trace to go up. The higher the voltage, the farther up on the screen the voltage trace goes. How far the trace goes up or down depends on how much voltage is available, and where you set the Volts per Division. If you set volts-per-division too low, the trace may go up off the scale. This won't hurt anything; you will just see a blank screen. If you set the volts-per-division too high you will not see small changes in the voltage trace. Your challenge is to adjust the scale so the changes in the voltage signal are easy to watch. Most scopes have 8 horizontal lines, or divisions for the vertical scale. Vertical and horizontal divisions are called graticules. Obtain a lab scope, the operating manual for that scope, and an electro-trainer test board. Hook the two scope leads together. With no difference in voltage between the two leads the scope should display 0 (zero) volts. 1) How can you tell where the scope will display the zero line? (draw the symbol used by your scope) Adjust the zero line towards the bottom of your screen. Hook the scope leads to the Red (+) and Black (-) of the Electro-Trainer switched to 12 Volts Regulated (Be sure to plug the lab-scope test leads to channel 1 and the common (ground) jacks of the lab-scope)

2 As the Electro-Trainer produces a constant, DC voltage the vertical trace should stay in the same place across the entire screen. If you cannot see a line, the trace is either off the screen to the top, or to the bottom. 2) Most Lab scopes will measure both AC and DC volts. What do AC and DC stand for? Set channel 1 to measure AC volts. 3) How many AC volts does the scope display? Set channel 1 to measure DC volts. Set channel 1 to measure 5 Volts/Div (Volts per Division) Reverse the positive and negative scope leads (hook positive lead to negative terminal and negative lead to positive terminal) 4) What happened to the voltage trace? Is it off the top of the screen, or off the bottom of the screen. Reconnect the positive lead to the Electro-Trainer positive, and negative to ground. Adjust the volts per division so the voltage trace is near the top of the screen. (keep the zero line one or two graticules from the bottom of the screen) 5) What is the voltage scale that you used? (VOTLS/DIV) Select a smaller voltage scale such as 1 Volts/Div. 6) Where did the voltage trace go? Tech Tip: If you are not sure which voltage scale to use, set the scope to a high voltage scale (like 5 or 10 Volts/Div). This will locate the pattern on screen. You can then easily adjust the volts-per-division so the pattern fills most of the screen. For the test be ready to show how to change the voltage scale, the zero position, and how to set DC volts for channel 1.

3 Time (measured on the horizontal axis) The oscilloscope voltage trace is drawn to show changes in voltage over a length of time. Most lab-scopes will measure seconds (S), milliseconds (ms), and microseconds (µs). 1.0 S = 1 S (one second).001 Second = 1 ms (one millisecond or one thousandth of a second) S = 1 µs (one microsecond or one millionth of a second) When hooked to 12 volts Direct Current (DC) we see a straight line across the horizontal axis because the voltage is not changing. When the voltage is changing, we need to adjust the time-per-division scale to see the the changes. Obtain a Ford MAP sensor for the Electro-Trainer board. (This MAP sensor rapidly turns a voltage signal on and off.) Hook up +12v and -12v of the MAP sensor to the Electro-Trainer. (switch on 12V regulated). Hook the positive scope lead to the MAP sensor DATA terminal and the negative scope lead to ground. Adjust the Volts/Div and the Zero position so the voltage trace comes close to the top and bottom of the screen. 7) How many Volts/Div should you use to view this voltage trace? This sensor is turning On and Off a voltage signal at a regular frequency. 8) Adjust the Time/Div to 2 ms and draw the pattern here Adjust the Time/Div to faster (towards microseconds) and slower (towards seconds) to see how the pattern changes.

4 Trigger tells the voltage and time trace where to start on the screen. Triggering is necessary in order to be able to see a stable pattern. An un-triggered signal will roll across the screen making it difficult to observe. Lab-scopes have trigger point that tells the display where to start. Locate and explore the trigger menu 9) What are the options for setting the trigger on your lab-scope. The TRIGGER LEVEL control changes how high up on the waveform the trigger point is. Select Auto trigger for your lab-scope Move the trigger level below Zero volts. 10) What happens to the pattern when the trigger level does not cross the waveform? The TRIGGER SLOPE control changes whether the trigger point is on the rising edge, or the falling edge of a wave. Positive means the signal will only trigger on a rising edge of a waveform, and Negative means the signal will only be trigger on the falling edge. Set the time scale to show one or two on-off cycles. Adjust the trigger slope from positive to negative Observe how the pattern changes. 11) Draw one of the patterns below. (Show the symbols that indicate trigger level and trigger slope in your drawing.)

5 Cursors measure the differences in voltage or time. Once a voltage trace is captured, you can turn on and adjust cursors to precisely measure changes in voltage and time. Adjust your TIME/DIV to show one complete cycle (from ON to OFF and back ON) Trigger on the down slope (triggers as voltage drops towards zero) Adjust zero volts to be one or two graticules off the bottom of the scope Adjust VOLTS/DIV to show the top of the voltage trace near the top of the screen Experiment with the voltage and time cursors. Notice numbers that indicate the voltage change between the voltage cursors lines, and indicate length of time between the time cursor lines. 12) What is the difference in voltage from ON to OFF for this signal? 13) How long does the voltage trace stay turned OFF? 14) Draw the pattern below. Show symbols that indicate trigger level, trigger slope, VOLTS/DIV, TIME/DIV difference in Voltage cursors and length of time the signal is turned OFF in your drawing Voltage, Time, Trigger slope, Trigger level and Cursors are critical for capturing and measuring the many types of patterns used by computer control sensors and actuators. This lab is only an introduction; I encourage you to practice with the lab-scope at every opportunity. Learning to use this tool takes practice.