Introduction: Oscilloscopes display an image of a voltage vs. time (usually a ve

Question

Introduction:

Oscilloscopes display an image of a voltage vs. time (usually a very small period of time on the order of ms or ms). Compare this to a digital multimeter, which provides a numeric display of dc voltage at an instant of time. It is important to understand how to display an image (a waveform) on an oscilloscope and how the image can be manipulated and measured. This experiment will detail the fundamental steps necessary to view waveforms. Be aware that there are many advanced features available on the TDS 2024 which will be explored in future courses.

Procedure:

The front panel has knobs, buttons and connectors. The knobs are used most often and are similar to the knobs on other oscilloscopes. Option buttons (also called screen buttons, side menu buttons or soft-keys) bring up menus on the display that allow you to access many of the oscilloscope’s features.

Before beginning this lab, make sure there are no probes or cables of any kind attached to the oscilloscope.

Your instructor/TA will identify a BNC connector cable with alligator clips. These are leads which connect to the oscilloscope and the ac source (function generator). There is a silver BNC connector on one end and two clips on the other end. Cables with BNC connectors on each end will also be identified as well as scope probes.

Press the scope power button now. It is located on the top left side of the unit (not on the face). Allow a moment for the grid to display.

Using the BNC-to-clips cable, connect the red clip lead to the upper hook on the scope labeled Probe Comp. It is not necessary to connect the black clip lead as Ground is connected internally.

Attach the BNC (silver) end of the cable to the scope silver connector labeled CH 1.

Press

AUTO

SET

You should see a square wave automatically displayed with suitable vertical (voltage) and horizontal (time base settings). The display contains a great deal of information about the waveform and scope settings. The information near the bottom of the screen is of most importance.

Observe the display information for the calibration wave:

CH1             2.0V             M 500ms

The left-most information (CH1) is the channel number. Next is the number of volts per large division (per centimeter) box, that is, the vertical scale. For this waveform each vertical division is 2.0 V. The time per division (M 500ms) is next. This indicates that the horizontal scale is 500 ms per large division (per centimeter).

Verify now that the numbers above match the display on your screen. If not, go to the * paragraph below.

The scope has the ability to compensate for probes that have a multiplier (X10, etc.). Your BNC connector does not have any multiplier or factor associated with it. Press the button labeled CH 1 MENU. Make sure that the “Probe” menu key says “1X”. If it does not, press the beige colored key to the right of the word “Probe” in the display. Press the button several times until 1X is displayed. Always make sure that the probe multiplier is correct or your measurements will be incorrect. The probe setting is a very common source of error (in this case, the measurements would be ten times too large). At this point it is a good idea to set the coupling to DC.

The

AUTO

SET

button is a very useful way of obtaining automatic and quick waveform display. When you have difficulty obtaining a good display, AUTO SET is a good start. However, in the future, in certain circumstances, AUTO SET might not present the waveforms in the most useful format. Keep this in mind as you use an oscilloscope for many semesters throughout your many courses.

* On occasion, previous users of the scope will change settings and cause the display to be altered. If, at any time, when using the scope, readings or settings seem unusual, press

Default Setup

This will reset scope parameters to the default settings. Press AUTO SET again. Verify now that the numbers above match the display on the scope screen.

A. The Vertical Window

1. VERTICAL POSITION Knob

Center the signal on the display with the POSITION knob located above the CH 1 Menu button. The position knob moves the display vertically and it is calibrated. Turn this knob to observe the effect on the waveform. Typically you will want to adjust the vertical position to place the arrow along the y-axis of the display in line with the x-axis.

2. VOLTS/DIV Knob

Turn the knob above the CH 1 BNC connector which has a yellow band next to it. This knob changes the scale of the vertical display. Notice its impact on the number displayed next to CH1 (on the screen) as well as the change in the waveform display.   This setting varies the vertical scale on which the waveform is presented. This knob vertically expands and shrinks the waveform. It is important to understand that the actual quantity of voltage being displayed is not changing, only the representation. When two waveforms are displayed, for example, the visually smaller one on the display may actually have the larger voltage because of this setting. Note that each channel is color-coded which matches the waveform display color. Some scope probes are also color-coded for ease in matching the displayed waveform to the point in the circuit to which the probe is connected.

Now return to the original waveform by pressing AUTO SET.

B. Time Base

In the HORIZONTAL section of the scope panel there is a knob labeled SEC/DIV. This setting varies the horizontal time scale on which the waveform is presented. Turn the SEC/DIV knob and notice the change it makes in the number shown next to the letter M in the display. This knob can change the horizontal time scale from 2.5 ns/div to 50 s/div in a 1-2-5 step sequence. It horizontally expands/shrinks the waveform.

Change the horizontal setup and notice the horizontal changes to the waveform display and M value.

C. Trigger

Turn the LEVEL knob in the TRIGGER area and notice the changes it makes to the display. The trigger is the signal that causes the oscilloscope to start displaying the waveform.

Press

AUTO

SET

to return to a stable display.

D. To Make Voltage Measurements Automatically

You can measure the following waveform parameters automatically with the oscilloscope:

Frequency

Period

Mean

Pk-Pk

Cyc RMS

Min

Max

Rise Time

Fall Time

Pos Width

Neg Width

Press the MEASURE button. Then press the beige colored button next to the top CH1 None.

Two new items will appear in the display area:

Source

CH1

Type

None

Press the beige colored key next to Type and a parameter will be displayed. First, frequency will be shown (below VALUE). Press the button several times to scroll through the choices until NONE is displayed. You can also return to the MEASURE menu by pressing the beige colored key next to the word BACK in the display. Bes ure the input signal is DC coupled.

For the square waveform displayed, obtain the Freq, Pk-Pk and Cyc RMS values and record below.

Freq

1 kHz

Pk-Pk

5.20 V

Cyc RMS

3.54 V

(Note additional measuring capabilities are available using the cursors, however, these will be discussed at a later time.)

F. Dual Display

Keep channel 1 connected to the PROBE COMP square waveform. Now have your instructor/TA set a waveform on your function generator so that 130 Hz and 2 Vp-p are set for the frequency and amplitude, and connect its OUTPUT to your scope, using a scope probe, to channel 2. Push the AUTO SET button and you should see both waveforms displayed at the same time although it may be difficult to see the waveforms clearly.

To see both waveforms clearly, push the RUN/STOP button. Both waveforms should appear to stop moving. Notice that channel 1 is displayed in yellow and channel 2 is displayed in blue corresponding to the colors beside each VOLTS/DIV knob. The colorized display is useful when you need to observe multiple waveforms.

Press CH 1 MENU twice to turn off channel 1.

Use the scope techniques you have learned to be sure that the probe setting (1X or 10X) is correct. What is your probe?   ______1X ____ If you do not set this correctly, any measurements observed or recorded will be inaccurate. Press RUN/STOP.

Use the measurement techniques you have learned to record information about the signal applied to channel 2. Press the MEASURE button. Then press the key next to the CH2 (displayed on the screen in blue letters).

Freq

130 kHz

Pk-Pk

3.92 V

Cyc RMS

1.36 V

Remove the connection to the function generator and channel 2. Remove the red clip lead attached to the PROBE COMP hook. Press the CH 2 MENU button twice.

G. MEASURING DC VOLTAGES

1. Press the CH1 Menu button on the scope and find the Coupling in the menu. Press the key twice to display Ground. Adjust the position knob (for channel 1) until the zero reference line is horizontally across the middle of the screen, the x-axis.

2. Once the zero level has been established, push the coupling position once to the DC position and set the vertical sensitivity (VOLTS/DIV knob) to 1.000 V. Using a BNC-to-Clip-lead cable connect channel 1 of the scope to the DC 6.0 V power supply (red leads together and the black leads together). Set the power supply to 1.50 V.

3. Record the vertical shift – that is, how many divisions does the waveform move up? ___1.5_________ (Hint: count the divisions)

4. Using the number of divisions and the vertical sensitivity (1 V(/div)) can you verify the 1.5 V? How do you do this? ______________________1.5 V by counting them _______________________

5. Change the vertical sensitivity to 500 mV/ (div). What happens to the trace position? Why? ____________________________________________________________________

6. Return the vertical sensitivity to 1 V/(div). Reverse the leads of the power supply connected to the scope probe. What happens to the display of the waveform? ____________________________________________________________________

For these questions your answers must be very specific, detailed, and quantified.

1. What happened to the square waveform display when you moved the channel 1 VOLTS/DIV knob?

2. What happened to the square waveform display when you moved the SEC/DIV knob?

3. How does an oscilloscope differ from a multimeter (be specific regarding Vp-p, Vrms, Vavg, etc.)?

4. How does the display information (along the bottom of the waveform window) change as the VOLTS/DIV knob is changed?

5. If you have changed settings for volts and time and do not see a clear waveform, what button(s) should you press to “fix” the problem?

6. Are positive and negative voltages indicated on the oscilloscope? If so, how?

AUTO

SET

For these questions your answers must be very specific, detailed, and quantified. What happened to the square waveform display when you moved the channel 1 VOLTS/DIV knob? What happened to the square waveform display when you moved the SEC/DIV knob? How does an oscilloscope differ from a multimeter (be specific regarding V_p-p, V_rms, V_avg, etc.)? How does the display information (along the bottom of the waveform window) change as the VOLTS/DIV knob is changed? If you have changed settings for volts and time and do not see a clear waveform, what button(s) should you press to "fix" the problem? Are positive and negative voltages indicated on the oscilloscope? If so, how?

Explanation / Answer

(1) it will change the level of vertical display.

(2) it will change the level of horizental display

(3)multimeter does not measure the peak voltage, where as oscilloscope does.

(5) using vertical position knob

(6)

Yes

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