Laboratory 2. Oscilloscope Basics

Laboratory 2. Oscilloscope basics

Equipment: Bench equipment, bnc-bnc lead

______

Objective: To investigate the properties and performance of an oscilloscope. To measure signal properties using one channel of the oscilloscope.

Prelab assignment:

1. Go to the Agilent DSO 3000 series oscilloscope home page,

View the video that is posted there.

1. Read the data sheet for this family of oscilloscopes.
   
2. Review this lab handout, and resolve any questions on theory or procedure before coming to lab.

1. Take and pass the quiz posted on Blackboard before coming to lab.

Introduction

An oscilloscope is a device which displays the time domain information of a signal. For example, the signal

could appear on the oscilloscope trace as

From this signal, you could verify the amplitude and frequency of the sine wave. Furthermore, if the nature of the signal were unknown, to you, you could use the oscilloscope to determine the waveshape as well as the amplitude, frequency, and other characteristics.

In this lab, we will investigate the operation and characteristics of the Agilent DSO3062A digital oscilloscope.

Performance

Initial each step as you complete it, in those steps which do not have results to record.

1. The front panel of this oscilloscope is shown below. This oscilloscope is a dual trace scope with a bandwidth of 60MHZ and a maximum sampling rate of 1GSa/second. The oscilloscope has three input ports on the front, which use BNC connectors. These are for Channel 1 input, Channel 2 input, and an external trigger signal.

The oscilloscope output is displayed on an LCD screen on the left of the scope. This output can also be ported to the computer through a USB connector. The oscilloscope is controlled through the set of knobs and pushbuttons on the front panel, which are divided into 5 main sections.

The first button to know is the power button, which is on the bottom left of the device. Locate this pushbutton, and turn the oscilloscope on. As this is a digital device, it will go through a boot-up and self check routine when you turn it on.

2. Next, turn your attention to the HP 3314A function generator on your bench. Connect the function generator output to the scope channel 1 input with a bnc-bnc connector. Turn on the function generator, and set the frequency to 1000 hertz and the amplitude to 1 volt. You can do this by pressing the frequency and amplitude buttons in turn, and adjusting the level with the modify knob and/or the adjacent arrow keys.

Take some time to investigate the function of these arrow keys.

a. What is the function of the up and the down range arrow keys?

b. What is the function of the left and right arrow keys? (Note: use these highlight a digit, and then turn the knob).

VERTICAL CONTROL

3. Once you have set the function generator signal correctly, turn your attention to the oscilloscope.

Find the “Vertical” set of controls. These are the yellow and green buttons of Channels 1 and 2. This section controls the vertical deflection and position on the oscilloscope screen trace. You should see the voltage that you supplying to Channel 1 displayed with a yellow trace on the screen. Find the oval pushbutton labeled “2”. If this key is lit, push the button until the light goes out. You should now see a yellow trace of the sine wave on the screen. If this is not the case, check with the instructor.

The lower buttons control the vertical position of the trace. Adjust the yellow trace so it is centered on the screen. Note that a screen cursor comes as you make this adjustment, and tells you the offset voltage of the current setting. The sine wave should be centered on the screen when this cursor reads 0 volts. Adjust the green trace so that it is at or below the bottom of the screen.

4. This oscilloscope has an autoranging feature. The amplitude setting for thecurrent display is given at the bottom of the screen. Record this setting______. This number refers to the volts per division. (each division is 1 centimeter). Given this scale, record the number of divisions of the sine wave, from it’s positive peak to it’s negative peak______. From the record the peak-peak and peak value of the sinusoidal voltage the you are supplying to Channel 1:

Peak to peak voltage ______. Peak voltage:______

How does this measured value compare with the value that you set on the function generator?

(IMPORTANT NOTE: The autorangiing feature can regularly go to the minimum time scale setting, which results in a reading that appears to be erroneous. Always check the time scale after autoscaling, and verify that this operation was successful. Alternately, set time and amplitude ranges manually.)

1. The larger knob controls the gain of the display. Adjust this knob by one click in the clockwise direction.
   Record the new scale:______
   How many divisions is the sine wave with this setting?______
   
2. Turn this same knob counterclockwise one click back to its original setting, and another click beyond the original setting.
   Record the new scale:______
   How many divisions is the sine wave now? What is its amplitude in volts?______

(make sure to record whether you are giving the amplitude in peak or peak-peak volts).

7. On the function generator, change the sine wave amplitude to 0.83 volts. Measure the amplitude of this sine wave: Number of divisions:______
Amplitude:

1. Note that this knob includes a pushbutton. Click the pushbutton one time, then observe what happens when you rotate the knob:
   
2. Adjust the oscilloscope gain knob until the vertical size of the sine wave is 4 divisions peak to peak (2 divisions peak). Record the Ch1 gain per division:______

What is the measured peak voltage of this wave?______

1. Return the peak amplitude of the sine wave to 1 volt. Note the yellow button between these two knobs. This also is a pushutton. Push it one time. Note the menu on the right side of the display. This menu has 5 options: coupling, BW (bandwidth) limit, Probe, invert, and digital filter. Push the button next to the “invert” option. What happens?
   
2. Push the same button again, so that the invert is off. Now push the “coupling” button. What are the choices for this option?
   
3. GND is short for ground. What does the trace look like when this control is in the “GND” setting?
   

The figure below sbows the analog equivalent of the coupling modes, where a capacitor is inserted into the circuit with a toggle switch. The capacitor has the effect of filtering out the dc component of a wave, so that only the ac component is sent to be displayed on the vertical axis of the scope. When the coupling switch is closed, the capacitor is bypassed and the entire signal is sent to the vertical amplifier and then to the display.

1. For this waveform, what is the difference in the display between the “AC” and “DC” coupling modes?
   
2. Go to the function generator. Next, select offset, and adjust the offset to be 0.25 volts.
   Back at the oscilloscope, select “AC” coupling. How has the oscilloscope trace changed as the offset of the signal is changed?
   
3. Now, change the coupling from “AC” to “DC”. What does the trace show now?
   
4. Note that “DC” coupling means that the entire signal coming into the channel is fed directly to the trace. “AC” coupling means that the DC component of the signal is filtered out before the signal is fed to the trace. Note that the terminology “AC” and “DC” can be somewhat confusing, and remember that “DC” coupling results in the “direct coupling” of the signal to the display.and shows both the dc and ac components of the wave. It is recommended that you use the scope in dc coupling except in the case where the waveform has a small ac component riding on a relatively large dc value.
   
5. Still in dc coupling, observe the displayed trace, and write the equation for the signal.
   

1. Comment on the relationship between the OFFSET value on the function generator and the dc level observed on the oscilloscope:
   

(Remember this relationship as we will need this for future labs.)

1. Keep the offset level at 0.25. Reduce the amplitude setting to 5 millivolts. With the coupling on “DC”, and the vertical deflection at 500mv/div, what does the trace look like:
   Change the deflection to 200mv/div. Can you measure the ripple on the dc?
   What happens to the trace as you change the vertical setting on the scope to 100mv per division?
   
2. Now change the coupling to “AC” Keeping the vertical setting at 100mv/div, what do you see on the scope trace?
   
3. Change the vertical setting to 5mv/div. Note the following:
4. The 1000 hertz signal is present (verify the 1000hertz component by varying the amplitude of the fg).______
5. The displayed signal may also have a significant amount of high frequency noise. Comment on the presence or absence of noise in the signal: ______
6. With high noise, the signal trace is “running” on the screen—is this happening? .______
   If you don’t observe the signal running, notify the instructor to set this up for you.

1. On the menu, select “Digital Filter”. Observe the signal when the filter is both off and on. Comment on the effect.
   Note that a primary purpose of the “AC” setting is to measure the ripple level of signals that are predominantly DC. For the majority of measurements, “DC” coupling should be used.

HORIZONTAL CONTROL AND TRIGGERING

1. Return the function generator to zero offset and a peak amplitude of 1 volt. Set the vertical gain to 500mv per div. Turn your attention to the Horizontal section of the oscilloscope control panel. This section has a large knob on the left, a small knob on the right, and a pushbutton in the center.

PERIOD AND FREQUENCY MEASUREMENT

1. Use the larger horizontal knob to set the horizontal scale to 500 usec/div. You will see the horizontal sensitivity displayed on the bottom of the screen. Use the smaller horizontal knob to set a positive going zero crossing of the sine wave at the far left point on the screen grid.
2. Note one full period of the sine wave—record the number of divisions that a period takes:______
3. Now multiply this by the time per division, to get the period of the sine wave in seconds:______
4. What is the frequency in hertz associated with this period:______
5. How does this compare with the frequency setting of the function generator?

1. Change the horizontal scale to 200 usec/div. Now how many divisions are there per period? ______. What is the period in seconds?______Note that the more accurate measurements can be made when the period is expanded on the screen.
   
2. Change the frequency to 710 hertz, keeping the scale to 200usec/div. How many divisions for a period now?______. What is the measured frequency of this signal?______
   Comment on your ability to measure period accurately when the time scale is at 500 usec/div, 1msec/div, and 2msec/div:
   

TRIGGERING

1. Return the function generator frequency to 1000 hertz. Observe the “T” symbols on the screen—one at the top of the trace, and the other at the left of the trace. T stands for “trigger”, and this cursor shows the trigger point on the wave. Turn the smaller horizontal knob, and place the top trigger point on the center hash mark of the screen.
   
2. Now, find the Trigger section of the oscilloscope, and locate the “Level” knob in this section. Adjust this knob until the trigger point on the left of the screen is at 0 volts.
   What is the instantaneous value of the sinusoidal voltage at the point where the wave passes the vertical trigger point?______
3. Adjust the trigger level to 0.2 volts. What is the effect on the instantaneous value at the vertical trigger point?
   
4. Change the trigger level to 0.7 volts. Print the screen display showing this trace, and attach this to your report.
   (On your computer, start the DSO3000 interface software. Go to the “connect” mode. Note that this gives your computer control over the oscilloscope. You might want to scroll down the DSO controller, and Select the Virtual Panel to explore this.)
   From the display window, you can either export or print this display. Label this printout “Figure 1” and include an appropriate caption on the figure, something like “Step 26 result showing 1volt (peak), 100 hertz sine wave with triggering at 0.7 volts.”
   

1. Close the computer interface and return control to the oscilloscope. Adjust the trigger level up to 1.5 volts. What happens when the trigger point goes above the peak value of the input voltage?
   
2. This oscilloscope goes to a free-running trigger when this signal does not hit the trigger voltage after a set time. The display now triggers randomly, so that it looks like a running waveform. Go to the “Run Control” Section of the oscilloscope, and push the “single” pushbutton. In the “single” mode, the oscilloscope will display a single trace of the wave. Prove this by adjusting the amplitude of the function generator, and note that the trace does not change. Return the fg to 1 volt amplitude.
3. Record the instantaneous voltage of the waveform at the trigger point______
4. Push the “single” button again, and again record the trigger point.______
5. Repeat three more times______
   Is there a pattern to this set of numbers?
   
6. Push the run/stop button. Now you have returned to continuous triggering, and you will see successive traces with random triggering. Now reduce the trigger voltage down to 0.5 volts. Observe the steady trace. It is important to be able to distinguish between this trace, which is continually resampled traces of a steady voltage always triggered at 0.5 volts (and so the trace does not change) from one of the step 28 traces, which was a trace taken at a single point in time and stored. Verify that your current trace is “live” by varying the amplitude of the fg. Does the trace change as you vary the amplitude?
   
7. Now push the center “Main/Delayed” Button. Turn the “delayed” function on. Observe how you can now adjust the two knobs to zoom in on a portion of the wave. Describe how you might use this function:
   

The figure below shows the analog equivalent of the coupling modes, where a capacitor is inserted into the circuit with a toggle switch. Describe the effect of this capacitor, and identify which position the toggle switch is in to put the oscilloscope into the “AC” mode.

EXTRA CREDIT SECTION (2.5%)

Note that the manual for this oscilloscope in on your computer.
1. Use the cursors to measure the sine wave voltage 0.2 and 0.4 msec after the peak of the sine wave. Set this up and record the values below. Show the instructor the cursor setup, and have him/her initial that this has been done______
Value at 0.2 sec. ______
Value at 0.4 sec. ______

Last mod. 12/28/06 tho