This lab will not be assessed against any of the assessment criteria

Introduction:

In this lab you will be getting to know the Cathode Ray Oscilloscope. This is an instrument that is used to measure oscillating voltages in real time. Its main component is a cathode ray tube that accelerates a beam of electrons towards a scintillating screen. The electron beam is deflected by the potential difference that is applied across parallel plates inside the tube. The amount of deflection is a direct measure of the voltage. In this way graphs of voltage vs time can be obtained.

Getting to know the Oscilloscope

          Motion of the beam

Turn the oscilloscope on and adjust the SEC/DIV knob to about 100 ms. You should see a dot sweep across the screen. How long does it take to cross the screen and how long for one division? Change the SEC/DIV knob to a lower setting. How low do you need to adjust the SEC/DIV knob so that the dot sweeping across the screen appears as a horizontal line across the screen? This horizontal movement of the beam is controlled by a potential difference across horizontally aligned plates. This potential difference increases linearly in time which increases the deflection of the beam sweeps the dot across the screen.

Measuring DC Voltage (Direct Current)

Attach the probe to Channel 1 and set the VOLTS/DIV control to 0.5 V. Move the AC-DC-GND switch to GND. Now adjust vertical position to so that the beam traces across the central horizontal axis. You have now set the ground reference voltage to zero volts -- you are ready to measure voltage.

Flick the AC-DC-GND switch to DC. Connect the leads to one of the batteries and record the voltage. Reverse the direction of the connections and repeat the measurement.

Now set the switch to AC and repeat. Do you obtain the same results?

Measuring AC Voltage (Alternating Current)

In this part you will use the oscilloscope to measure the period and amplitude of an AC Voltage. Connect the leads of the oscilloscope to the signal generator. Set the signal generator to produce a sine wave with the frequency at 1KHz and the amplitude at about half the maximum.

Measure and record the amplitude of the wave that is produced (you may need to adjust the VOLTS/DIV and SEC/DIV knobs to obtain a precise reading. It may also be helpful to adjust the horizontal position (Sec D) to move the wave until a peak is on the vertical axis. If the wave is not steady you can adjust the Trigger level (Sec E).

Measure and record the period of the wave. It may be helpful to adjust the horizontal position so that at least one peak is on a vertical line. Try changing the SEC/DIV knob to a setting where there is a second peak on a vertical line, then count the number of periods between these two peaks. This may give you a more accurate measurement for the period. From the period find the frequency of the wave. How does it compare with the setting on the signal generator?

Repeat the above measurements with the signal generator set to a significantly different frequency and amplitude.

Repeat the above measurements with a square wave.

Connect leads from CH2 to a second signal generator. Set the CH1-Both-CH2 switch (Sec B) to Both. Adjust the amplitude of the second signal generator until the amplitude of the wave is the same as the amplitude of the wave from the first signal generator. (the VOLTS/DIV knob should be set the same on CH1 and CH2 and both waves should be sinusoidal. Also make sure that that GND on CH2 is adjusted to the central horizontal line). Adjust the frequencies so that one is double the other.

Change the Add-Alt-Chop switch (Sec C) to each of the three settings. Describe what they do by drawing the display in each of the three cases. Press the CH2 Invert Button (Sec C). Describe what it does. Try tuning both signals so that they have the same frequencies, describe the patterns you see with Add-Alt-Chop set to the Add setting.

Lissajous Figures

In the above setting you were able to view two waves at once with the voltage shown in the vertical direction and time in the horizontal direction. It is possible to view the voltage of one wave on the vertical direction and simultaneously view the voltage of the second wave in the horizontal direction. The resulting curves are called Lissajous figures. Set the SEC/DIV to its first setting (XY). Now the timed sweep is bypassed and you should see the Lissajous figures. Draw the figures when the frequencies of the waves in the two channels are equal and when they are in a ratios of 2:1 and 3:1 and 4:1. Can you explain what you observe?

Charging and Discharging a Capacitor

Change the signal generator to produce square waves. Set the SEC/DIV control so that there are several periods on the screen. Connect a capacitor and a resistor in series across the signal generator and measure the voltage across the capacitor using the probe from CH1 (black lead connected to the capacitor side). Sketch the pattern you observe and describe which part of the curve corresponds to charging the capacitor and which corresponds to discharging the capacitor. For the portion of the curve which is discharging measure how long it takes the pulse to decay to e^-1 of its original value (e^-1 ?5/8). This is called the time constant and should be equal to RC where R is the resistance of the resistor and C is the capacitance of the capacitor. How close is your result for the time constant to the expected value?

Generating Potential

Connect the leads from Ch1 to each end of a coil of wire. Move a bar magnet in and out of the coil. Draw a sketch of what you see (You may need to lower the VOLTS/DIV to see anything interesting). What is the largest voltage you can generate?
 

Connect the leads from Ch1 to a microphone. Observe and sketch the signal from a whistle, a tuning fork, a human voice etc. Can you reproduce a sinusoidal wave?