ELEC 241 Lab

Experiment 1.2

The Oscilloscope and Function Generator

Equipment

So far we've measured only constant (or nearly constant) voltages and currents. Although these are important, as signals they are fairly boring. A much more interesting (and information rich) class of signals are time varying voltages and currents. For a slowly time varying signal, we could just write down the values as they change (as we did in plotting the light bulb I-V curve), but for most time varying signals we need something a bit faster. That faster something is the oscilloscope (from the Latin oscillare to swing + Greek skopion to look at).

In order to measure time varying signals, we need a source of time varying signals. The DC power supply is our source of constant voltages, the function generator (from the English function a mathematical relation + generator a machine that produces electricity) is our source for that class of time varying signals known as periodic signals.

Part 1: Viewing Signals with the Oscilloscope

Making a measurement with the Oscilloscope consists of two phases: (1) getting it to display anything at all, (2) getting it to display what you want. If any one of several important controls is not properly set, no display at all will appear, so the first order of business is to get all of these controls into a reasonable state.

The oscilloscopes are Iwatsu model SS-5702 and the function generators are Instek model GFG8020H.


Step 1:

 Set the oscilloscope controls as follows:

  • Power: ON
  • Intensity: Mid scale
  • V Mode: CH 1
  • Vertical Position (both knobs): Mid scale
  • Volts/Div (both knobs): 2V
  • Variable Volts/Div (both knobs): Fully clockwise
  • AC-GND-DC (both switches): DC
  • Polarity: Normal (CH 2 Position IN)
  • Horizontal Position: Mid scale
  • Time/Div: 1ms
  • Variable Time/Div (Time Variable): Fully clockwise
  • Sweep Mode: Auto (Holdoff button pressed in)
  • Trigger Source: CH 1
  • Trigger Level: zero
  • Trigger Coupling: AC

If everything is in order, you should see a blue-green horizontal line through the middle of the screen.

Step 2:

 Set up the function generator to produce a 1kHz sine wave:

  • Power: ON (can't be turned off)
  • Duty: Pressed in and fully counterclockwise
  • Offset: Pressed in
  • Ampl: Pressed in and mid scale
  • Range: 1K
  • Function: Sine
  • Frequency: 1.0


Step 3:

 Connect the function generator's MAIN OUTPUT ($50\Omega$ ) to the oscilloscope's CH 1 input. The easiest way to do this is to connect one end of a BNC patch cord to the function generator and the other to the oscilloscope. This connects the generator's ground and signal terminals to the scope's ground and terminals. If all has gone well, you should see about 10 cycles of a sine wave.

Step 4:

 Now examine the effect of each control. Move the display with the positioning controls. Change the timebase to see what effect is produced. Pull the red X1-X5 switch in the middle of the timebase's position knob (pull out the H POSITION knob). What does this do? What is the effect of changing the slope control from "+" to "-"?

Change the vertical amplifier settings to see the effect that they have. If you turn the red knobs in the center of the vertical amplifier adjustment, you place the oscilloscope in an uncalibrated mode. Do this and see what happens.

Step 5:

 Examine the various waveforms produced by the function generator. Examine the effects of the DUTY and OFFSET controls (Offset must be pulled out to function). Before going on, be certain that you are comfortable with the oscilloscope and function generator. If you are having problems, ask your labbie for help.

Part 2: Quantitative Measurements with the Oscilloscope

In addition to allowing us to view the "shape" of a signal, the oscilloscope can also measure voltage, amplitude, time intervals, and frequency.


Step 1:

 Connect the oscilloscope CH 2 input to the 0-6V output of the DC power supply. For this you can use a BNC patch cord and your BNC to banana plug adapter.

Step 2:

 Set the V MODE switch to CH 2 and the CH 2 AC/GND/DC switch to GND. This disconnects the CH 2 input from the front panel connector and connects it to ground so we can set the zero level. With the CH 2 position control, adjust the trace to the center line of the scale and set the switch back to DC.

Step 3:

 Increase the voltage control until the meter reads 2V. With the VOLTS/DIV switch set to 2 the trace should be even with the first horizontal line above mid screen. Continue to increase the voltage and see how well the scope and meter agree.

Question 5:

 Why would we want to use the oscilloscope to measure a "DC" voltage?

Step 4:

 Set the V MODE switch to CH 1 and "zero" Channel 1 as above. Set the function generator to produce a 2kHz sine wave. Set the TIME/DIV switch to .1ms. Use the Horizontal Position control to align one of the zero crossings of the waveform with one of the vertical scale line. Measure the distance between two successive zero crossings of the same slope and multiply by the Time/Div factor to get the period of the waveform. Using the formula f=1/T, determine the measured frequency of the signal. How does this compare with the nominal frequency?

Diversion:

 There are several ways we can express the amplitude of a signal. For the sine wave $y(t) = A \sin(\omega t)$ the amplitude $A$ is equal to the distance from the positive (or negative) peaks of the waveform to the t-axis. This peak amplitude measurement is equally useful for any waveform which has equal positive and negative peaks.

Arbitrary waveforms may not have this property, so a more general measurement is the peak-to-peak amplitude, the distance between the positive and negative peaks of the signal.

Other measures of a signal's magnitude include average and rms, which we'll talk about later. Since in general these different measures have different values, it is a good idea always to specify which amplitude measurement you are using.

Step 5:

 We can also use the oscilloscope to measure the amplitude of a signal. Disconnect your oscilloscope from the function generator and use a BNC clip lead to connect CH 1 to the CAL output of the scope.

Question 6:

 The calibration output has no place for attaching the black ground (common) lead: why not?

Step 6:

 Sketch this signal's waveform. What is its period? What is its frequency? Adjust the CH 1 Volts/Div switch so that the waveform nearly fills the screen vertically. Use the CH 1 Position control to position the lower peaks of the waveform even with one of the horizontal scale lines. Measure the peak-to-peak amplitude by counting the number of divisions between the upper and lower peaks and multiplying by the Volts/Div factor. Does your measurement of the waveform's amplitude correspond to the stated value? If not, make sure that the vertical amplifier is in the CAL mode.