Experiment 1 - Getting to know your lab bench equipment

EXPERIMENT #1

POWER SUPPLY SETUP AND MEASUREMENT

For this part of the experiment you will be measuring the output of a power supply (Hp3631A) using two other pieces of equipment, a Digital Multimeter (DMM) (Hp34401A) and a Digital Oscilloscope (Hp5460A) that will measure its voltage. The power supply can be considered to be an ideal voltage source, and both of the measuring devices are considered to be perfect measuring devices.

Perfect Voltage Source - a perfect source means that no matter what circuit is connected between its terminals the voltage remains the same. This is much harder to achieve than it seems and much of the circuitry of the supply is dedicated to making sure that this is true over a very wide range of voltages and a very wide range of load resistances. As you learn more about loads and how they affect sources you will see why making a perfect voltage source is actually quite difficult.

Perfect Measuring device - a perfect measuring device is one that does not change the circuit characteristics in anyway when connected to the circuit under test - it simply gives you a measure of the desired parameter, in this case voltage. This is also very hard to accomplish for a wide range of voltages and loads and much of the circuitry inside the DMM is devoted to this.

For this lab we will assume that the power supply is an ideal voltage source and that our multimeters are perfect measuring devices. In later labs, we will investigate what happens when this is not true, for example the batteries that we will be using to run the cars later in the semester are anything but perfect voltage sources.

Figure 1: Configuration of the power supply to be used to supply a voltage to the measurement equipment. In this case, the circuit is simply the digital multimeter and the oscilloscope.

a) power supply



Set up the power supply to output 5V from the output ports supplying the +/- 6V. (see figure 1 for steps) but don't connect it up yet. The display on the power supply will give you 2 numbers indicating the voltage (left) and current (right).




1. The number indicating voltage reads 5.00 because you set the power supply to output 5V but the current reads 0.00 - WHY? What current is being displayed by the power supply?

   
   

   
   
   
   Figure 2: Configuration of the DMM used to probe the voltage from the power supply. In this case the circuit under test is the output of the power supply. The red port of the DMM is connected to the red port of the power supply. Same for black.
   
   

   b) multimeter
   
   

   Probe the voltage from the power supply with the multimeter setup to measure DC VOLTAGE using the red and black cables with the banana plugs at both ends. The red cable is used to connect the red port of the power supply (see figure 1) to the correct red port of the DMM (see figure 2). Same for black. The display on the power supply will give you 2 numbers indicating the voltage (left) and current (right).

   
   
2. What voltage do you read from the multimeter?
   
3. What voltage do you read from the power supply?
   
4. Spin the dial and change the voltage. Does the current number on the power supply change (value displayed on right of power supply display)? Why is the value so small (or zero)?
   
   
   c) oscilloscope
   
   

   Without disconnecting the multimeter, probe the voltage with channel 1 of the oscilloscope using the cable with a BNC connector at one end and two banana plugs at the other end. You should see a straight line appear on the screen of the oscilloscope. One of the most fundamental measurements made using the oscilloscope is obtaining the value of the voltage (or the voltage range if the signal is time varying) of the input signal. To be able to read the voltage from the grid on the screen of the oscilloscope you need to specify the zero reference – it is designated by a small one and an arrow on the side of the display. Reading voltages is simplified if you move the zero reference to the middle of the screen. Once you have located the zero reference you can count the number of large and small divisions and estimate the voltage. But what voltage increment does each large and small division on the screen represent? It is written somewhere on the screen and can be changed by turning one of the knobs near the input port - can you find the indicator and the knob? (This is teacher speak for: you have to find it yourself).

   
   

   Now we will play a simple game - one member of your group will set the power supply to an arbitrary but odd voltage (NOT an even number like 5V say but 3.42V for example). The other member will try to read the voltage off of the oscilloscope.

   
   
5. What voltage do you read by eye from the oscilloscope?
   
   

   Now see what the oscilloscope thinks the voltage level is. Using the VOLTAGE button at the top make a more accurate measurement of the voltage using the scope.

   
   
6. What value is displayed on the screen of the oscilloscope?
   
   d) Comparison
   
   

   Draw in your lab notebook your circuit including all three pieces of equipment. You must get into the habit of always, ALWAYS drawing your circuit - and the circuit includes the test equipment and sources.

   
   
7. Adjust the voltage from power supply to at least four different settings from 0 to 6 V (set to values that are hard to read on the oscilloscope, like 2.45V for example) and fill in the table below with reading taken in the three different ways. Compute the percent error of each measurement by computing: %error = 100*(supplied voltage - measured voltage)/(supplied voltage).
   
8. Is the accuracy of the voltage readings from the three methods uniformly accurate (or inaccurate) for all voltages values across the range of the power supply (0-6V).
   

   
   
   

   Disconnect the DC power supply and multimeters; you're finished using those devices for now.

   
   

   
   FUNCTION GENERATOR SETUP AND MEASUREMENT
   
   
   
   
   

   a) Amplitude and Period measurements
   
   

   Now setup the function generator to output a SQUARE WAVE with 1Khz frequency, 4V peak-to-peak amplitude, and no DC offset. Connect the function generator to channel 1 of the oscilloscope. Play with the knob that sets volts/div on channel 1 and the knob that sets time/div for all channels so that you can see how these affect the scope display. After playing with x-axis (Time/div) and y-axis (Volts/div) resolution knobs a little set the y-axis setting at 1V/div and set the x-axis time setting so that at least one cycle of the square wave is displayed. Position the waveform so that it is easy to measure the amplitude and period.

   
   
   
   

9. Draw the waveform on the gridded paper in your notebook. Your graph should be large and easy to read; don't put more than three or four graphs on a single page. Be sure to label the graph with your setting (volts/div and time/div) so that someone else will know how to read the graph.
   
10. Measure the peak-to-peak amplitude and the period by eye. What are these values? Are they close to the supplied values?
    
11. Measure the peak-to-peak amplitude with the VOLTAGE key and measure the period using the TIME key. What are these values? Are they different from the values specified by the function generator?
    
    b) Offset measurements (AC and DC Coupling)
    
    

    Without changing the amplitude or frequency of the square wave, or changing the settings of the oscilloscope, adjust the function generator's DC offset from 0 to 2V.

    
    
12. Draw the resulting waveform on top of your previous graph. (Be sure to label which waveform (or trace) is which.) What has changed?
    
    

    Now switch the channel 1 coupling from DC coupling to AC coupling. You do this by hitting the button labeled '1' that is just above the BNC connector associated with channel 1. After hitting this button look at the display - there is a menu at the bottom. One of the menu items allows you to set the coupling for channel 1. Push the button once and the coupling should move to AC coupling. If you push the button again you will put channel 1 to ground 'gnd'. You will do this in the next step.

    
    
13. How did the display change? What is the effect of the AC coupling setting?
    
    

    Now switch the coupling of channel 1 to ground.

14. What happens to the display?
    
15. How could you use this feature to find the zero reference of each signal? This is sometimes used to center the signal in the display without having to disconnect the signal.
    
    

    Put the oscilloscope back to DC coupling.

    
    
    c) Duty Cycle Measurements
    
    

    Again, without changing the amplitude, offset, or frequency of the square wave, or changing the settings of the oscilloscope, adjust the duty cycle of the square wave from 50% to another setting.

    
    
16. Draw this waveform on top of the other two.
    
17. What does this adjustment do?
    
18. Measure the duty cycle by eye by counting the intervals in the horizontal direction. What is it?
    
19. Use the TIME button on the oscilloscope choosing the percent duty cycle measurement to get the oscilloscope's estimate of the duty cycle. What is it?
    
20. How do the two values compare to each other and to the value specified by the function generator?
    
    

    Set the duty cycle back to 50%.

    
    

    TRACE TRIGGERING
    

    
    Triggering allows the user to specify how the oscilloscope draws and redraws a signal on the screen. If the signal that you are trying to measure is not a periodic signal it is difficult to capture a picture of the signal. At best you can see a very quick trace and you can store this trace for a while using the store commands which we won't cover today. Can you see why an aperiodic signal would be difficult? Below are two figures that show how a periodical signal is mapped onto the oscilloscopes display in an untriggered and triggered mode.
    
    

    
    

    
    

    Set the function generator to output a sawtooth wave (not a triangle wave) with amplitude 2V peak-to-peak and frequency 1kHz, keeping it connected to channel 1 of the oscilloscope. The sawtooth is a waveform whose rising slope is very different from its falling slope; this will help demonstrate the effects of the different triggering modes.

    
    

    
    

    
    

    Now push the SOURCE button found in the Triggering section on the oscilloscope. Set the trigger source to be channel 1. This means the signal going into channel 1 will determine how the oscilloscope redraws the waveform each time. Adjust the trigger level knob. You will see a horizontal line appear on the scope - this line moves up and down as you turn the LEVEL knob. The position of this line tells the oscilloscope to start redrawing the signal when the value of the function reaches the value specified by this horizontal line ON THE RISING EDGE of the function. Since this is a digital oscilloscope the position where the trigger level intersects the waveform is actually in the center of the screen. Can you see how the function shifts back and forth as you change the level but the intersection remains in the center of the screen.

    
    
21. Draw the sawtooth wave displayed on the oscilloscope along with the line indicating the trigger level set to 2V. Circle all the intersections of this line with the sawtooth wave on the RISING EDGE of the waveform.
    

    
    
    
    

    Now switch the triggering to occur on the FALLING EDGE of the waveform and adjust the trigger level again. This can be changed by pushing the SLOPE/COUPLING button in the Triggering section on the oscilloscope. On the far left of the menu at the bottom of the display are two arrows - one pointing up indicating the RISING part of the periodic function and one point down indicating the FALLING part of the periodic function. Push the down arrow for this part.

    
    

22. Why do you not see a change in the appearance of the display when the level is changed now?
    
    Hint: expand the time scale so that you can expand the sawtooth waveform - look at the FALLING part of the waveform where the signal abruptly transitions from the maximum to minimum amplitudes.
    
    

    Lets see what happens when the scope is not well triggered. Keep the sawtooth wave connected to channel 1, but now set the triggering source to be channel 2 which has nothing connected to it.

    
    
23. What has happened to the display? Why?
    
    
    

    OSCILLOSCOPE”S XY MODE
    

    
    

    Now we will use the oscilloscope in a different mode. We will be using this mode periodically during the semester. You will need to cooperate with one of your neighbors for this part as you will need to share function generators. First connect (or rather don’t disconnect) the function generator to channel 1. Set the function generator to output a 2V peak-to-peak sine wave with frequency 1kHz and no DC offset.

    
    

    Using a couple of cables with BNC connectors at both ends, connect them together using a barrel connector. Connect your neighbor’s function generator to your channel 2. You may both share the function generators by using a T connector on the output of the function generator. Set this function generator with the same settings as the other (2V peak-to-peak and 1kHz frequency). At this point you should see two sine waves on the display of the oscilloscope.

    
    

    Push the MAIN/DELAYED button on the oscilloscope and put the display in XY mode.

    
    
24. What does the display look like? Draw the figure in your lab notebook.
    
25. Why do you get this figure?

Play with the frequencies of the two function generators and see the interesting figures that you get. These are called LISSAJOUS FIGURES. Plot others if you have time.

BEFORE YOU LEAVE TAKE the OSCILLOSCOPE OUT of XY MODE