CDA 3201L Lab #3
Welcome to CDA3201L Lab #3! The purpose of this lab is to introduce you to the use of an
oscilloscope. You’ve likely used some kind of debugger while writing a program in C or Python –
an oscilloscope can help you debug your circuits in much the same way. These devices can be
especially helpful when working with large, complex circuits to check voltage levels or timing of
different signals. For example, you can take a “snapshot” of a signal based on a specific event.
You can zoom in to see different parts of the signal in more detail, take measurements, and
even transfer the captured waveforms to a PC for further processing or graphing.
To prepare for Lab #3, carefully read and work with your lab partner to complete the prelab
BEFORE you come to lab.
An inverter is a logic gate that takes a single input and has one output. The inverter will flip the
input signal; that is, if the input is a logic low, the output will be high, and if the input is logic
high, the output will be low. The diagram and truth table for the inverter are shown here:
For 5V logic, the “high” level is about 5V, and the “low” level is about 0V. So, if the input is 0V,
the output will be about 5V. If the input is 5V, the output will be about 0V. The change is not
instantaneous – there is a brief switching delay. This is listed in the datasheet as tPLH or tPHL,
which stands for “propagation time from low to high” or “propagation time from high to low”.
The same holds true for logic gates besides inverters – NAND, NOR, etc.; they all take time to
transition when a change on the input causes a change on the output. The delay also depends
on what is connected to the output and environmental conditions. Datasheets will list the “test
conditions” such as CL (capacitive load), temperature, and supply voltage.
PQ-1: Check your datasheet for the 74LS00 Quad NAND – what is the typical (Typ)
propagation delay for these gates? What is the maximum (Max) delay? Be sure to
Understanding these delays is critical to designing functional logic circuits. We will go into more
detail later this semester. For now, we will just be measuring these properties using an
oscilloscope and a special circuit that makes these delays more obvious, called a ring oscillator.
A ring oscillator is a kind of circuit whose output constantly changes between two states: LOW and
HIGH. A diagram of a simple ring oscillator comprised of 3 inverters is shown below:
Let’s assume the output of U2 is initially LOW (Z = 0), and walk through the events that happen
next. The output of U2 is fed back into the input of U0. The output of U0 will be HIGH, and is
fed into U1. The output of U1 will be LOW. Finally, this is fed into U2, and the circuit output will
be HIGH (Z = 1). If we assume tPLH = tPHL = 10 ns, signal propagation will take 3 x 10 = 30 ns.
PQ-2: Continue the exercise – what is the sequence of events when the output of U2 is
HIGH? How much time (in total) will have passed since Z = 0?
Note that this only works with an odd number of inverters (3, 5, 7, …), otherwise it will not
oscillate. We would like to measure its period of oscillation or, alternatively, its frequency (recall
that period and frequency are inverses). That is where the oscilloscope comes in. As you know,
NAND gates can be used to make inverters. To build the ring oscillator, you will need to use
three NAND gates, connected in the following manner:
This circuit also has an EN or enable signal which can be used to turn the oscillation on or off.
PQ-3: Show that when EN = 1, it enables the ring oscillator, and when EN = 0, the output
does not oscillate.
PQ-4: In general, what will the period of oscillation be, as a function of the number of
gates (G) and gate delay (tp)? (Hint: think about your answer to PQ-2!)
To prepare for your lab, use the provided datasheet to plan out which pins on the 74LS00
package need to be connected to realize the NAND-gate ring oscillator on your breadboard.
Have a TA check this before you begin constructing your circuit.
TAs will begin by walking you through the basic use of the MSO 2014 Oscilloscopes in the lab,
including the setup instructions (copied below for your reference).
1. Turn on the oscilloscope (this takes a while). Ensure your probe is connected to Channel 1.
Connect the test lead to PROBE COMP and the reference lead above that:
Press the Autoset button. You should see a square wave oscillating between 0V and 5V:
2. Press the yellow menu button to bring up the channel 1 probe settings. You should see this
on the screen. Check the number displayed in the Probe Setup box. Here, it says 10x.
If it says 10X, continue with Step 3; otherwise, skip to Step 4.
3. Press the button below Probe Setup so the menu item is selected. Twist the Multipurpose A
knob counterclockwise so the Probe Setup reads 1x. Press the button below Probe Setup
again to save the setting, and press the yellow Channel 1 button to exit the menu. Press the
Autoset button again so the square wave is clearly visible on the screen.
4. Notice in the bottom left of the screen, it shows a 1 with yellow background and a “2.00V”.
This represents the number of Volts per vertical division. We also call this “Volts per div”.
Since it is a 5V square wave, the “1” should be 2.5 divs higher than the “0”. The frequency of
the signal is also displayed on the bottom right of the screen.
You can adjust the Volts per div manually by turning the “Scale” knob below the yellow
Channel 1 button. You can also adjust where the wave is displayed on the y-axis by turning
the “Position” button for Channel 1.
5. You can also measure the time between any two parts of the signal using the cursors. To
use the cursors, press the Cursor button. Two vertical lines should appear on the screen,
labeled A and B. You can adjust the position of each cursor independently using the
Multipurpose A and Multipurpose B knobs.
The position (in time) of each cursor is shown in the box on the top right of the screen. The
difference (delta) is also displayed, showing the actual time between cursors. In this case, it
reads 976 uS. The cursors can be aligned to the edges of the square wave (one falling edge,
one rising edge) to find the period of oscillation (in this case, it is a 1kHz square wave).
Now, you are ready to construct your NAND-gate based ring oscillator circuit from the prelab.
Implement this circuit on your breadboard using a single 74LS00 Quad NAND chip and use an
oscilloscope to measure the period of oscillation. In your report, include the following: 1) the
period (measured with cursors), 2) computed frequency, 3) measured frequency (as shown in
the bottom right of the screen), 4) a picture of the oscilloscope screen showing the cursors and
frequency, 5) answers to the PQs from the prelab. Is the measured period what you expected
based on the datasheet? Discuss your findings in the report.
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