UNIVERSITY OF PENNSYLVANIA

DEPARTMENT OF ELECTRICAL ENGINEERING

 

ESE UNDERGRADUATE LABORATORY

ESE 205: Electrical Circuits and Systems I Laboratory

 

 

AC Waveform – Generation and Measurements

 

Objective

To introduce the function generator/arbitrary waveform generator and the oscilloscope. You will familiarize yourself with different waveforms and learn how to select a waveform and adjust its frequency and amplitude. You will use the oscilloscope to display the waveforms and to measure their characteristics (amplitude, frequency, phase, rise and fall times, period and offset voltage). 

Background

1. Function Generator:

Up to now we have worked with DC (direct current) voltage and current sources (i.e. power supplies), whose values are constant. In this lab you will become familiar with sources that vary as a function of time (called AC or alternating current sources). There are many different AC waveforms. The one you are most familiar with is the sinusoid as shown in Figure 1.

Figure 1: A sinusoidal waveform (AC signal)

Others are the pulse train, triangular and ramp waveforms. The questions we would like to answer in this lab are how to generate these AC voltages and how to measure or display them. We will be using two different instruments: (1) function or waveform generator and (2) oscilloscope. Both are among the most important instruments in electronics. It is essential that you know how to use both instruments well.

In previous labs we have used the digital multimeter (DMM) to measure DC currents and voltages. The DMM in the AC Mode can be used to measure the RMS value of a waveform (root mean square). However, there are many other attributes of an AC signal besides the RMS value that are important such as the exact shape, frequency (or period), offset voltage, phase, etc. as is shown for a sinusoid in Figure 1.

2. Oscilloscope:

One of the most used instruments in the lab is the oscilloscope which allows you to display ("see") the waveform as a function of time in a similar fashion as is done in Figure 1. Carefully read the following "Tutorial on the Oscilloscope."

Pre-Lab Assignment

1.
Read the tutorial on "Function Generator/Arbitrary Waveform Generator" on the ESE Undergraduate Homepage.

2.
Explain briefly the principle of the HP33210 function generator (max. 10 lines).  

3.
Write down the different steps involved for the following procedure:

  • To change the output impedance from 50 Ohm to a High Z (explain the different steps involved for the HP33120A using the menu function).
  •  To generate a pulse train with a duty cycle of 25%, between 0 and 5V and frequency of 10kHz (explain step by step).

4. Explain how a waveform is drawn on the screen of an oscilloscope (max. 10 lines).

5. Explain what "Triggering" is. (max. 10 lines) 

In-lab Assignment

A. Equipment:

  • 1. HP function generator/waveform generator (HP 33120A)
  • 2. HP digital oscilloscope HP54600
  • 3. HP Scope Probe
  • 4. Blue box with cables and connectors
  • 5. PC with sound card and speakers

B. Procedure

1. Identify the three main blocks of the HP 54600 oscilloscope:

    • Vertical deflection system: two input channel coax connectors, vertical scale (V/div) and position knobs
    • Horizontal deflection system: time base with knob in Time/div
    • The trigger system: trigger source, mode, slope/display buttons and trigger level knob

Also notice the measure and storage panels.

The following exercises are intended to guide you through the basic functions of the oscilloscope. Try out other functions and experiments with the different settings. This is a learning experience.

2. Select, display, measure and listen to a sinusoidal waveform:

a. Use the HP 33120A function generator to create a sinusoidal waveform with a peak-to-peak amplitude of 1Vpp and frequency of 1 kHz. If necessary, review the tutorial on the "Function Generator/Arbitrary Waveform Generator".

b. Connect the OUTPUT of the function generator to the INPUT of the oscilloscope (Channel 1) using a coaxial cable. Push the AUTOSCALE button on the Measure panel. You can switch the input channel 2 off by pressing the button marked 2 on the vertical panel; then push the Off/On key underneath the display window.

c. Change the scale (V/div) of channel 1 (V/div KEY) (vertical panel) and note the display changes. Try out a few other settings. You can now change the time base as well (Time/div on the horizontal panel). Read the peak to peak value of the sinusoid using the scales of the scope display (shown at the top left corner in V/div). Notice the difference with the setting on the function generator. Explain the difference (hint: output impedance of the function generator is 50 Ohms - see Function/Waveform Generator Tutorial).
NOTE: In case the value of the displayed waveform is off by a factor of 10, check the probe setting. Push the button labeled "1" (channel 1) just above the "Position" knob. This will bring up a menu at the bottom of the screen. At the right hand side you will see Probe 1 10 and 100. Make sure that this is set to 1 (unless you use a probe).

d. Connect the output of the function generator to the input of the speaker. Use a coax splitter with alligator clips to connect the function generator output to the speaker. Use a T-connector so that you can also display the waveform on the oscilloscope while listening to the sinusoidal waveform. Do not turn the volume up too high (to prevent a cacophony of sounds in the lab). Change the frequency of the signal and record how low and high a frequency you can hear.
fLO= ... Hz;  fHI= ... kHz.

You can now disconnect the speaker if you like quietness.

 

3. Trigger Modes:

 

These exercises will help you understand the trigger function.

 

a. Display on channel 1 a sinusoid of a few kilohertz and a few Vpp in magnitude.

b. Select the trigger SOURCE key (on the trigger panel); you will notice a series of choices displayed at the bottom of the screen. Push the key underneath the word Channel 2. This will select channel 2 as the trigger source. Notice and record what happens. Why? Next, select channel 1 as the trigger source.

c. Now change the trigger mode by pressing the MODE key and selecting Auto (with the keys at the bottom of the display). Turn the trigger LEVEL knob to change the trigger level (on the trigger panel) and notice what happens. Can you explain it? What happens when the trigger level exceeds the peak voltage of the sinusoid? Next, select Norm trigger mode (at bottom of the display).

d. Select trigger Slope/Coupling key on the trigger panel. Switch between the positive and negative going slopes. Note the effect on the display.

4. Measure functions:

 

You will learn how to use the scope to give you the amplitude and time characteristics of the waveform.

 

a. Select a square wave on the function generator with an amplitude of 1Vpp, offset voltage of 0.5V (so that the waveform lies between 0 and 1V), frequency of 1.25 Mhz and a 25% duty cycle.

b. Push the VOLTAGE key on the measure panel. Select one of the keys at the bottom of the display to measure the peak-to-peak voltage (Vpp), average (Vavg) and RMS (Vrms) voltage. Compare and record the Vpp to the RMS values. What is the relationship between both? Now, push the Next Menu button at the bottom of the display. You can now measure the Vmax, Vtop, Vmin and Vbase values. Record these values in your notebook. What is the difference between Vtop and Vmax? Also measure the overshoot of the signal.

c. Push the TIME key on the measure panel in order to measure the frequency, period, rise and fall times. Choose the timebase such that you see one or two periods on the screen.  Select the appropriate keys to measure the frequency (compare with the setting on the function generator), the period and duty cycle. Record these values. Then go to the Next Menu to measure the rise and fall times.

d. Use the cursors to measure time or voltage differences. Push the CURSOR key on the measure panel. Two vertical position-controllable cursors appear and can be used to make time measurements anywhere along the displayed waveform. Use the cursors to measure the pulse width and pulse period. Calculate the duty cycle and compare with the measurement result in part c. Experiment on your own. Similarly, two horizontal cursors are available for precision voltage measurements.

e. Delay function:

In order to zoom in on a specific part of the waveform you can use the delay function. Experiment with this feature. Push the MAIN/DELAYED button on the horizontal panel. Next, select Delay and notice the display. Change the timebase (Time/div) to further zoom in on the rise time of the waveform. This feature is convenient to look at the detailed structure of a waveform. You can go back to the regular display by pushing the Main display.
 

5. Modifying a waveform (modulating):

 

Frequency sweep

 

Create a sinusoidal waveform of amplitude 1.25Vpp whose frequency sweeps between 50 Hz and 5000 Hz. Consult the user's manual at your station to find out how to do so. Observe the waveform on the oscilloscope. Display the waveform on the oscilloscope. Connect the output of the function generator to the speaker and listen to the effect of the frequency sweep.

 

Change the frequency

 

Change the waveform to a square wave, just play with it.... Do you see and hear the effects? (Note: don't turn the volume up too high).

Make a print out using the BenchLink software. Start BenchLink by clicking on the "Connect to Scope" icon on the desktop. If necessary review the tutorial on "Using BenchLink Scope Software to Download and Display Waveforms" available on the EE Undergraduate Homepage.

6. Listen to a heartbeat:

Select the Cardiac waveform on the function generator (see user's manual or the tutorial of the pre-lab on "Function /Waveform generator"). Set the magnitude to 6 Vpp and frequency of 1.5 Hz. Display on the oscilloscope and listen to the heartbeat at the same time. Change the frequency and observe the difference.

7. Arbitrary waveforms:

You will use BenchLink to create your own waveform, store it in the function generator's memory, display it and listen to it.
Start BenchLink from the Start > Programs > ESE Software >BenchLink Consult the tutorial on "BenchLink to Generate Arbitrary Waveforms" if necessary. Use the freehand, a drawing pallets or any other tool to create your own waveform. Be creative and remember you will have to listen to it ... After creating the waveform load it into the function generator's memory (using the I/O menu in BenchLink). Display it on the scope and listen to it at the same time. Change the frequency and amplitude and observe the difference. Make some rough observations in your notebook about the sound quality. Print out the waveform.

8. Scope Probe:


A scope probe is used to display high frequency signals and to reduce noise and ringing on the signal. In the following experiments you will study the effect of using a probe.

a. Scope Probe Adjustment

Probe pins can be easily damaged or broken. Handle the probe with care.

Connect the probe to one of the input channels of the oscilloscope. You need to inform the scope that you are using a 10:1 probe. This is done by pushing the key labeled 1 or 2 on the vertical panel of the scope and then pressing the key at the bottom right side of the display until the 10:1 indicator is highlighted

Attach the tip of the scope to the square wave reference signal at the terminal on the front panel (underneath the display indicated by the square wave icon). View the square wave signal on the scope. If the probe is not properly adjusted the square wave won't have square corners. Use your screw driver to make adjustments on the probe so that the square wave has a flat top. Do this carefully and do not turn the screw too much as this can damage the probe. The probe is now ready to be used.

b. Measuring of a square wave

Set the waveform generator to a square wave with a frequency of 2 MHz and 2 Vpp. Display the square wave on the oscilloscope using a coax cable (black cable). Notice that the square wave is not very clean and that it has a considerable amount of ringing. Make a print out of the screen (use Benchlink software by clicking on the icon "Connect to Scope).

Next use the probe scope to display the signal. Connect the probe input to the output of the function generator. Be extra careful not to bend the probe pin (it is easily damaged). Also, connect the ground of the function generator to the ground connector of the probe. Adjust the vertical scale of the scope and notice the waveform. It should be much cleaner with less ringing. Make a print out for your report.

c. Effect of poorly adjusted probe

 Let’s study the effect of a poorly adjusted probe. Connect the probe input to the output of the function generator (make sure that the ground of the function generator is connected to the probe ground). Select a 2 MHz square wave of 10 Vpp and display it on the scope. Use the cursors on the scope to measure the waveform characteristic: peak-to-peak value, Vtop, Vmax. Record the values in your lab notebook. Now mis-adjust the scope probe by turning the screw in the scope compensation box by about a quarter turn. Notice what happens to the square wave output. Do the same measurements as before, record them in your lab notebook. How does it compare with the measurement of a compensated probe? Now readjust the probe carefully, using the reference square wave signal at the scope terminal.
 

 

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Created by Jan Van der Spiegel: March 12, 1997.
Updated by Sid Deliwala on 9/29/10 :