LAB 4

Oscilloscopes

Prelab

Read the lab exercise and familiarize yourself with the procedure.
Watch the YouTube video explaining the basics of using the oscilloscope.
- Focus on methods used to adjust what the oscilloscope displays
- The content beyond 8:30 will not be covered in the lab
Bring a USB key to the lab session.
- USB keys 8GB or less are more likely to work well than larger ones.

Purpose

To learn how oscilloscopes are used view time varying signals.

To learn how to use function generators.

Theory

Function Generator

The function generators, so named because of their ability to generate various functions (sine, square, triangle, and arbitrary are typical) at various frequencies and amplitudes. The sine function, which generates a sine wave, is very useful for simulating AC signals, or mimicking the response of components to AC signals. Sine waves are also useful in the testing of radio, audio, and amplifier circuits. The square function is typically used to mimic a fast on-off switching. It is useful to view the forced response of a circuit where power is suddenly applied, and to then view the natural response after the power is cut. The other functions may have specific applications, but will not be used in the lab.

The function generator used in the lab has the ability to generate the above functions at an amplitude (voltage) of 0-20V and a frequency of up to 10Mhz or so depending on the model. The function generator is typically used as the source in transient circuits and generates a voltage that oscillates with the chosen amplitude (voltage), frequency, and pattern.

Function Generator similar to those used in the lab

Oscilloscope

The oscilloscope is a measurement device, not unlike a multimeter. Rather than measuring a parameter at a certain instant in time, an oscilloscope shows a time varying depiction of the circuit behavior. This is especially useful when studying non-linear circuit elements such as capacitors and inductors. In addition to non-linear circuit elements, voltage sources may also be time-varying. It is important to remember that the oscilloscope does nothing to alter the circuit behavior in any way, it is only a way to view what is happening.

The oscilloscope typically displays time on the x axis, and voltage on the y- axis. An AC source will produce a sine wave on the display, as its voltage swings positive and negative over time. The frequency and voltage of the AC source will determine the appearance of the sine wave on the display. The oscilloscope will display a value for the Volts per division, and Time per division. This is the scale of the graph represented on the display grid. By multiplying the number of divisions by the value of the divisions, you can calculate the voltage of the waveform, or the period/frequency of the waveform.

The oscilloscope allows you to modify the appearance of the AC waveform by stretching it horizontally to see a more detailed picture at a certain time range, or compressing it horizontally to see a longer period of time all at once. You may additionally stretch the signal vertically, or compress it vertically to achieve different views of the change in voltage.

Digital oscilloscopes include measurement tools to reduce the need for the calculation described above, instead you can highlight a certain range, either horizontally or vertically and have the oscilloscope output the value.

Lastly, and most importantly, oscilloscopes allow you to view multiple time varying signals simultaneously. This is particularly useful to observe the input waveform (typically an AC source) and the response of a circuit element such as a capacitor. Being able to view an input and the output at once is very useful in calculation of many circuit parameters.

Oscilloscopes used in the lab

Explanation of Oscilloscope display

BNC Cables

Both of the above pieces of test equipment typically connect via BNC cable, this is a special cable designed to work at high frequencies and be less susceptible to EMF from the environment. This ensures that you get the truest representation of the behavior of your circuit without losses and without picking up any interference.

To attach BNC Cables, align the collar with the jack on the device and press firmly to slide the collar over the jack. Turn the collar to lock in place.

Equipment & Components

Resistor.
Capacitor.
Protoboard.
BNC T-Connector.
BNC to BNC cable.
Alligator to BNC cable.
Oscilloscope Probe.
Function Generator.
Oscilloscope.

Figures

Fig 4.1 Oscilloscope displaying a 1.5kHz sine wave @ 12Vpp

Fig 4.2 Oscilloscope displaying the same signal with V/div scale increased

Fig 4.3 Oscilloscope displaying the same signal with t/div scale increased

Fig 4.4 Measurement function turned on (displayed values can be changed)

Fig 4.6 time cursors turned on.

Fig 4.7 Oscilloscope CH2 enabled shoeing original signal and response across an RC circuit.

Procedure

Setting the Function Generator

The function generators let us create an on demand signal of our choosing. In this lab they will be used to create a signal to look at with the oscilloscope, but in future labs we will use them more in depth for the purposes described in the theory section above.

Power on the function generator.
Set the function to Sine using the knob or buttons.
Set the frequency to 1.5kHz using the base and multiplier dials/buttons.
- Set the dial to 1.5, and the multiplier to x1000 to get 1500Hz.
Set the amplitude to about the middle of the range, it'll be adjusted later.
Set any other features (DC Offset/Sweep) to off or 0 if not already off.

Connecting the Oscilloscope to the Function Generator

The BNC cables allow a shielded connection for the oscilloscope to view the signal from the generator while introducing minimal loss and noise.

Attach the T-Connector to the function generator.
- Use the port labelled "main" or "HI".
Attach the BNC cable from one side of the T to CH1 on the oscilloscope.
Power on the oscilloscope.
Press 'Default Setup' on the oscilloscope.
Press the AUTO button on the oscilloscope.
Press the lit up CH1 button, and then the button next to DC until AC is selected.

Using the Horizontal, Vertical, and Trigger Adjustment Knobs

The scale and position knobs allow you to zoom in or out, stretch or contract, and look forward and backward in time on the signal under inspection. Different sections or views may help you discover different behaviours. Altering the scale allows us to see very fast waves along with very slow ones on the same display. These adjustments knobs do not change the signal under inspection, jut your view of them.

The oscilloscope should show a sine wave.
- If it doesn't double check the function generator setup.
Press the 'Measure' button in upper section of the control buttons on the oscilloscope.
- This should open a display of red values indicating a few important test parameters.
- You do not have edit these, except that CH1 Vpp should be shown.
Adjust the Amplitude knob on the function generator to get 12Vpp
- You can use the vertical scale, or use the measure button.
- The measure button is bottom left in the "MENU" section on the front of the oscilloscope.
- If the wave stretches beyond the display window, the oscilloscope will not be able to measure the amplitude, you can alter the vertical scale to shrink the wave into the oscilloscope display.
You may have to adjust the trigger level if the wave becomes unstable
- Remember that the trigger level must be between Vmin & Vmax
- The trigger adjust knob is near the bottom right corner of the scope.
If your wave is already steady, try moving the trigger as a test.
- make a note of when the wave is no longer stable on the screen.
- You can press in on the trigger knob to return the trigger level to the 0V position.
You should now have a stable 1.5kHz 12Vpp sine wave on the display.
Try turning the Horizontal knob (time/div) and observe the effect.
Try turning the Horizontal position knob and observe the effect.
- You can press the position knob to return the wave to "0 ms".
Try turning the Vertical knob (Volts/div) and observe the effect.
Try turning the Vertical position knob and observe the effect.
- You can press the position knob to return the wave to "0 V".
Set the Horizontal scale to 200 μs/div with the horizontal knob.
Set the Vertical scale to 2V/div with the vertical knob.
- Note: these are suggested scales for a good view of the wave, changing these values in no way changes the signal under observation.
- The horizontal and vertical scale only allow you to zoom in or out on the wave as desired. You can choose others if you see fit.
Your display should look like Fig 4.4.
Before pressing Print, it is generally a good idea to press the menu button to turn off any extra displays the clutter the view. Keep this in mind for future prints as well.
Insert your USB key and Press "PRINT".
You may want to remove the USB key and check that you image saved using your laptop.

Using the Second Channel

The dual channel mode is the most useful oscilloscope function. It will allow you to see an input wave alongside the response to that signal in a different part of the circuit. This will be used in the coming labs, but we will just experiment with viewing 2 waves simultaneously here. Two waves can have different vertical scales (voltage) but not two different horizontal scales (time). This is because we are most concerned with time dependent behaviour when using the oscilloscopes and knowing that the waveforms are "in-sync" is crucial to any assumptions we make based on the observations.

Attach your BNC to alligator leads to the other end of the T-connector.
Attach the other BNC to Scope Probe to CH2 port on the oscilloscope.
- Make sure the slider on the probe is set to 1x.
Clip the leads from the function gen. across the resistor and capacitor.
Clip the leads from the oscilloscope across just the capacitor.
- The two black alligator clips should connect to the same point.
Press the button above CH2 to turn it on (a blue line should appear).
You can also press the AUTO button to have the oscilloscope guess at a good view of the waves.
Press the lit up CH2 button, and then the button next to DC until AC is selected.
Using the skills built above, adjust the display for channel 2 so that the scale in both cases matches in all respects.
- You want to be sure that your two waves have a common scale so that you can compare them directly.
- A common error in the use of oscilloscopes is to not consider the scale indicated by the Horizontal or Vertical knobs.
- Notice the scales DO NOT match in Fig 4.7. This would lead a naive observer to make incorrect assumptions about the behaviour of the circuit. It if very important to keep the scales in mind if they are not the same on both channels.
- You should just make a qualitative observation about the nature of the yellow and blue waves here, you don't have to print this display.
Adjust the vertical scale so that the displays take up a major portion of the window, the scales do not need to match.
Press print again.

You should have 2 images, and any qualitative notes that you made along the way.

No datasheet/signature is required for this lab.

Results

Include the digital prints from the oscilloscope in your report along with a description in a few sentences of what they are showing.
From the scale and wave shown, give the inspected Vpp. From the scale and wave shown, give the inspected Frequency.
- Be sure to show the multiplication used to arrive at the inspected values.
Explain how the CH1 and CH2 signals differ in the final print in: size, shape, Vpp, frequency, and any other parameters you feel are relevant.

Questions

Why is it important to be aware of the Volts/Div and Time/Div when viewing a waveform on the oscilloscope ?
- Which is responsible for the scale on which axis?
What is the purpose of the trigger knob, and how is it used ?

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