PHY 2624 - Engineering Physics II

Introduction to the Oscilloscope

 

Leader: _____________________________  Recorder: ___________________________

Skeptic: _____________________________ Encourager: _________________________

 

Materials

Digital Oscilloscope (w/ power cord)

Function Generator (w/ power cord)

BNC cable

 

Introduction

      In this lab exercise we will familiarize ourselves with one of the most important of all measurement tools, the oscilloscope.  The oscilloscope is essentially a voltmeter which can measure potential differences that change with time.  Potentials that change with time are often referred to as AC potentials or AC wave forms.  Most oscilloscopes now use digital technology, but the principles of using either digital or older analog oscilloscopes are essentially the same. 

      The voltage, V(t),  which is measured by the oscilloscope is used to steer the trace on the display up and down.  To account for measuring different amplitude signals, the potential is amplified and the amount of amplification is one of the things the user controls.  If this was all that an oscilloscope did, then you would see a line going up and down on the display.  In order to see the wave forms, the oscilloscope spreads out the display horizontally by applying a ramp voltage as shown in figure 1.  The rate at which the voltage is ramped is a second, important control adjusted by the user.

      In this lab, we will use a function generator to generate various wave forms, and we will make measurements on these wave forms and familiarize ourselves with the important controls on an oscilloscope.

 

Figure 1  Ramp voltage

Procedure

      Begin by turning on the oscilloscope and the function generator.  Connect the Output of the function generator to Channel 1 of the oscilloscope with the length of BNC cable. The BNC connector on the cable is known as a male and the connector on the oscilloscope is known as a female.  To connect the BNC cable, align the slot on the male connector with one of the knobs on the female connector on the oscilloscope.  Press the male connector onto the female, and turn clockwise until it catches. 

      There are three sets of controls you need to use to manage normal data acquisition with the oscilloscope.  These include the vertical sensitivity, the time base, and the trigger.  Verify that the sine wave output of the function generator is selected, and then press the 1 kHz button.  Adjust the gross and fine buttons so that the display on the function generator reads about 1500 Hz.

      These oscilloscopes have a nice feature that they will set themselves up automatically to find data.  If the oscilloscope isn’t already displaying a signal, press the Auto Set button on the upper right of the oscilloscope.  The oscilloscope should now show a display known as a trace of the output of the function generator.

 

Q1)  Sketch a picture of the trace.

 

 

 

Q2)  Below the trace is information used to help you make measurements.  Record the information listed below the trace.

 

 

We will now make sense of that information at the bottom of the trace.

 

Amplitude

      We are displaying a sine wave.  To describe the sine wave, we would like to measure the amplitude and the frequency.  The number on the lower left can be used to determine the amplitude.  The oscilloscope display looks like a piece of graph paper.  Each major grid line usually referred to as a division on the oscilloscope corresponds to a certain number of volts.  The Volts/Div is the number given on the lower left of the display.  This is known as the vertical sensitivity of the oscilloscope.  Each major division is subdivided into five subdivisions.  Count the number of divisions from the t axis to the peak of the sine wave and multiply that by the scale sensitivity.  This will give you the amplitude of the sine wave.  It is easier to count the divisions if the peak lines up on one of the grid lines.  Above the S/Div knob in the group of controls labeled Horizontal Sensitivity is a knob labeled position.  Turn that knob back and forth.

 

Q3)  What does turning the position knob do?

 

 

You can use that knob to get a peak to line up with a grid line.

 

Q4)  Record the amplitude of the sine wave that you determined.

 

      It can be difficult to determine if the sine wave is centered evenly on the axis, so we often measure the peak to peak amplitude.  Count the number of divisions from the bottom of the sine wave to the top and multiply by the V/Div.

Q5)  Record the peak to peak amplitude

 

 

Q6)  How is the peak to peak amplitude related to the amplitude?

 

      On the function generator is a knob labeled Amplitude.  Turn the amplitude knob to the left.

 

Q7)  What happens to the signal from the function generator?

 

      You now probably have a very small trace on the display of the scope.  Turn the V/Div knob under CH1 so that the trace fills the screen.

 

Q8)  Determine and record the amplitude of the sine wave now.

 

 

Period and Frequency

      Now we will measure the period of the sine wave.  It is probably easiest to measure the period by determining the distance between successive peaks.  The middle number on the display below the trace is the horizontal sensitivity.  Use the horizontal and vertical position knobs to align the peaks so that their location is easy to determine.  Count the number of divisions between the peaks and multiply by the horizontal sensitivity to obtain the period. You should be able to determine the number of divisions to at least a tenth of a division. 

 

Q9)  Record the period of the sine wave.

 

 

 

Q10)  How is the period related to the frequency?

 

 

Q11)  Use the period to determine the frequency.

 

 

Q12)  The frequency is also shown directly on your display.  Record that value.

 

 

Q13)  Does the value you determined in Q11) agree with the value you recorded in Q12)?

 

 

Q14)  How do these values compare to the frequency given by the digital readout on the signal generator?

 

Q15)  Calculate the percent difference between the frequency you determined form the oscilloscope and the frequency displayed by the function generator.

 

 

Press the 10k button on the function generator.  This will increase the frequency by a factor of 10.

 

Q16)  Sketch the appearance of the trace.

 

 

 

Adjust the horizontal sensitivity so that the trace shows two or three cycles of the sine wave.

 

Q17)  Record the new horizontal sensitivity.

 

Press the 100 button on the function generator.

 

Q18)  Sketch the appearance of the trace.

 

 

 

Adjust the horizontal sensitivity so that the trace shows two or three cycles of the sine wave.

 

Q19)  Record the new horizontal sensitivity.

 

 

Measurement Cursors

      Now, we will use another feature of the oscilloscope, namely its measurement cursors. Press the button near the top labeled Cursor.  The cursors can measure either potential or time.  A menu appears on the right side of the display and the choices are made by the buttons immediately to the right of the display.  Press the Type button until Voltage is chosen.  The source should be CH1.  If not press that button until it reads CH1.  You should now have two horizontal lines displayed on the screen.  The vertical position knob under CH1 controls cursor 1 and the vertical position knob under CH2 controls cursor 2.

      Adjust cursor 1 so that it sits at the peak of the sine wave and adjust cursor 2 so that it sits at the valley.  The display records the potential difference between the time axis and each cursor as well as the potential difference between the points marked by the cursor labeled as Delta.

 

Q20)  Record the peak to peak amplitude given by the cursors.

 

 

Press the Type Button so that Time is chosen.  You should now have two vertical lines.  Use the vertical position buttons to adjust the cursors to two successive peaks of the sine wave.

 

Q21)  Record the time between the two successive peaks.

 

 

Q22)  Determine the frequency from your measured time.

 

 

Q23)  Does its value agree with the measured value shown on the display.