CSC270    Lab #6

Experiment #1: the Oscilloscope

Studying the calibration waveform

• Connect Probe 1 to the calibration test-point, as shown in the picture below:
  

• Press AutoScale
• Observe the waveform. It should look like this picture:

Moving the waveform on the screen.

To move it horizontally, find the Horizontal section on the front panel, and the Delay button. Turn it and observe how the waveform can be translated horizontally.

To move it vertically, find the Vertical section, and turn the Position knob in the Probe 1 column. See how you an move the waveform up and down.

Changing the scale of the waveform.

To change its horizontal scale, locate the Time/Div knob in the the Horizontal section. Play with it and see how you can "zoom" in and out, closer or farther away from the waveform. Observe what happens to the edge of the waveform as you get to 1 microsecond, 500 nanoseconds... per division. What does this mean?

To change the vertical scale, locate the Volts/Div button in the Vertical section, and play with it. See how you can squeeze the waveform, or make it grow larger than the screen. Why would we want to make the waveform very large? Observe what happens to the top and bottom of the waveform at 200mV, 100mV and 50mV? What does this mean?

Measuring waveforms

Let's start with the timing first.
• Using the previous features, make one period of the waveform fit the whole screen.
• Align the left edge of the waveform to one vertical grid line.
• Measure the number of grid squares separating this edge from the similar edge, one period away.
• Locate the units associated with one horizontal grid square: 100 µs. Multiply this number by the number of squares to get the period.

                 1 grid square = 100 µs
                 1 period = T = _______ squares = _________ µs
  

• Compute the frequency, defined as 1 over the period:

                 frequency = 1/T = 1 / _______ =  ________ Hz
  

  Let's now measure the voltages.

• Move the waveform so that its bottom is aligned with one of the horizontal grid lines.
• Measure the number of squares separating the bottom from the top of the waveform.

                 bottom to top of waveform = _________ squares
  

  Record the unit for vertical squares: ____ V/square, and calculate the peak-to-peak voltage (V p-p):

                 waveform height = _____ squares @ ____ V/square
  
                                 = _____ Volts
  
  

Measurements: The Easy Way!

Could the scope have measured all this for us? Of course!
• Press Auto Scale again to make the scope synchronize itself with the waveform.
• At the top left of the front panel, locate the Measure section.
• Press Time
• Look at the bottom of the screen:

             Source   +---- Time measurements ----+  clear   Next
             1 2 3 4  |   Freq   Period  Duty Cy  |  Meas    Menu
  

• Press the grey button at the bottom of the screen that is right next to Period.
• and... Magic!
• Read the period measured by the scope:

             Period =  _________ µs.  
  

  How close is it to the one you measured?
• Now press the button under the Freq display...
• More magic!
• What is the frequency reported by the scope?

             Freq = ________ KHz
  

• Press the Duty Cycle button. Record the duty cycle:

             Duty cycle = _______%
  

• Press Next Menu
• Press Rise Time
• How fast does the signal go from low to high?

             Rise time <= ________ µs
  

More easy measurements

Let's see what the scope can do with voltage measurements.
• Locate the Measure section at the top left of the front panel again, and press Voltage this time.
• At the bottom of the screen, locate the Clear Meas knob and press it. This will clear any previous measurements.
• Press V P-P to instruct the scope that you want to measure peak-to-peak voltages.

           V P-P = _____________ Volts
  

• Press V avg to measure the average voltage.

           V avg = _____________ Volts
  

• Press Next Menu
• Make sure the Show Meas option is On.
• Press V Max and V Min to measure the min and max voltages.

           V Max = ________ Volts
  
           V Min = ________ Volts
  

• Press V top and V base to measure two different quantities:

           V Top = ________ Volts
   
           V Base= ________ Volts
  

• Explain the difference between V Top and V Max.

Exercise

Find everything there is to know (frequency, period, rise-time, voltage peak-to-peak, average voltage, top voltage, max voltage) about the 1 KHz clock signal on the digital kit.

Capturing Irregular Waveforms

Being certain to ground the lead (with the alligator clip) to the ground test point, connect Probe 1 to Test Point 3 on the HP waveform generator. The picture below shows Probe 1 connected to Test Point 1. Use it as example of how to connect the probe to TP 3.

• Press Auto-scale.
• What a mess!
• Make sure you take a short pause to learn more about what is going on...

  ...

• Welcome back. Play with the holdoff button and increase the delay to about 15 ~ 17 µs until you get a stable waveform, similar to that shown below:
  
• What would you say the period of this waveform is? Find the period of the signal using the automatic feature you played with earlier. Is the value indicated by the scope similar to the one you measure on the screen? Why?
• The best way in such a case is to use cursors to measure the exact timing. Press Cursors in the Measure section at the top left of the panel.
• Press the button next to t1 at the bottom of the screen.
• Turn the knob next to the Cursors button and move the vertical cursor to the first transition of the waveform.
• Press t2 and move the second vertical cursor to the same transition, a period away.

  Note: it is very likely that the waveform will "jump" around as you move the cursor. That's okay. Move it until you can pin it down to the same transition the t1 cursor rests on.

• Look up t1, t2, Delta-t and 1/Delta-t at the bottom of the screen.

           t1 = _______    t2 = _______   
  
           Delta-t = _______ 1/Delta-t = ________
  

• What is the frequency of the signal?

           freq = _________  Hz
  

• Use the cursors in the same fashion to measure the high and low voltage levels of the waveform. What is the height of the waveform?

           V p-p = __________ Volts
  

Exercise

Find everything there is to know (frequency, period, voltage peak-to-peak) about the signal present on Test Point #4 of the HP kit.



Experiment #2

Wire up a D-flip-flop ( SN74LS74) such that its Q-bar output is connected to its D input, and its clock input is connected to the Clock signal of the kit. Set this signal to 1 Hz (1 cycle per second). Also connect the clear and preset inputs to Vcc. Clear is an active low signal that sets the output Q to 0 when activated. Preset is an active low signal that sets the output Q to 1 when activated. Both clear and preset are typically signals used to initialize the flip-flop when the circuit is first powered up, or when it needs to be reset.

Connect both the Clock and Q signal to logic indicators.

Does this all seem familiar? What is the frequency at which the Q output oscillates? Find all there is to know about the Q output signal (as with the signals on the test board and the 1 Hz clock from above) and compare the answers to what you found about the 1 Hz clock signal earlier.

High frequency oscillations

Set the frequency of the Kit's clock signal to 100 Khz. Connect the probes of the oscilloscope to clock and Q (which should still be connected to the logic indicators). Observe the two signals with the scope. Is the relationship between clock and Q still the same? Looking at the scope and at the logic indicators, do you see there the answer to a question from a previous lab where you were asked to comment on the intensity of the logic indicator when the clock signal was beating at a rate of 100 KHz? Refine your explanations

What is the duty cycle of the clock signal (amount of time it is high divided by a period)? Of the Q output? (Hints: use the scope to tell you the answer!)