Instructional Activities:

In this set of activities, students will attempt to communicate using hand signals or assemble an operational amplifier to play their iPods through a small speaker. The focus of activities will be the concepts of signal and noise and the trade-offs involved in amplifying the signal. Students are not expected to have a background in electronics. Student groups will need access to computers or another data collection device with voltage probes for the final investigation.

1. INTRODUCTION (Class Discussion and Semaphore activity)

Open with a discussion of students’ list of instruments that utilize other types of proxy variables from the final question on the AFM student worksheet. This should serve as the introduction to the idea of an electrical signal being used as a proxy variable. Instrumentation that has been used in the course to date relying on electrical signals can be discussed (for example, the majority of Vernier probes). A second connection to the AFM lesson is that very small changes in data may not be noticeable. Examples of more common transducer devices that use an electric signal—such as an electric guitar—could lead students to suggest that the signal is amplified in some way.

2. Use 'Semaphore' to Teach Signal to Noise

The students first meet for about 3 minutes to decide their technique for accurate delivery/receipt of the signal.  During the activity, the students must be absolutely silent.  Feel free to add any "noise" to the activity as you walk between groups, etc.  Set students up in teams of 3 students each.  One student should be the "receiver" on one end of the classroom and another the "transmitter" on the other end of your classroom. Have the 3rd student record what the receiver interprets the signal to be.   The transmitters should use semaphore to send the signal,which is a randomly generated haiku (keeps the students from guessing what the message is). Have students meet again to discuss the accuracy of the message.  Students generally want to trade jobs and try again. Alternatively, students could try Morse code to send and receive messages.  See "Extensions" below.

Resources needed:  Semaphore signs, Haiku for Semaphore, Student Guide for Evaluating Signal to Noise using Semaphore, Random Haiku Generator site

3. Signal and Noise (PowerPoint, Student Activity)

• What is signal? What is noise?

  After students have completed the semaphore activity, ask them how successful the sending and receiving of the message was.  If they tested the OpAmp with various inputs, ask them to describe the sound quality emanating from the speaker. How does it compare to their car stereos? IPod headphones? Why is it different? The conversation should transition to an introduction of signal and noise using the PowerPoint Intro to Signal & Noise. To clarify the potential confusion over these common terms, students are asked to build and then refine an operational definition from examples.

   Slide 2 Classroom Signal: teacher’s voice Noise: sound from all of the other students talking

   Slide 3 Commodities traders Signal: buy or sell order Noise: Audible & Visual background confusion from other traders

   Slide 4 Urban Astronomer Signal: Light from star Noise: light pollution

  For slide 5, students should work in groups to formally write a definition of signal and noise that is shared & discussed. From the provided examples, 'signal' should be loosely understood as:  the information of interest one is trying to observe. At this point, students’ definition of noise will probably revolve around something interfering or coming between the signal and the receiver to make observation difficult.

   Slide 6 introduces a different type of noise with a low-res webcam and a poor analog tv signal. For both devices, the noise similarly makes the signal (the visual image) difficult to observe. What is different, however, is that both of these devices are interpreting a signal and presenting it to us for observation. Rather than a third entity coming between the signal and the observer, the noise is inherent to the device itself through the grain of the low- res webcam or the poor reception of the analog tv.

   Slide 7 pushes the examples to include “noise" where no signal is being transmitted such as a static filled tv or a radio tuned between stations. Students should discuss how to alter their definition of “noise" to include devices displaying random, meaningless data or corrupting the signal with this data.

   Slide 8 asks students to identify the signal and noise for motion data collected by a sonic ranger (motion detector). The signal is the position and time data with the associated calculations for velocity and acceleration, while the “noise" is the spike on the graph. This spike represents random, meaningless data interpreted by the motion detector as a position value. If students are familiar with motion detectors, they should be able to brainstorm possible causes for the noise in the data—such as another object (a hand or book) being read by the motion detector, classroom sound interfering with the detector, etc.

   Slide 9 shows a zoom in of same data collection during the initial motionless period. Students should be asked to identify the signal and noise for this section as well. The signal again is the position and time data interpreted by the motion detector. Questions to ask the students: How does this noise compare to the spike in the last example? (happens continuously, very small fluctuations in signal) How can we account for this noise? (most likely part of the device itself, not an outside interference) How does this noise impact our measurements? (limits the precision of our data, can only be confident of position data with a certain range of uncertainty) How do calculations affect this noise? (Students should be able to notice that small fluctuations in the data are exaggerated and amplified as calculations are performed for velocity and acceleration graphs).

  The final slide (10) introduces the main source of noise within electronics—random electron movements related to thermal energy.

• How can we measure signal & noise?

  The student worksheet Quantifying Signal and Noise for a Radio Example provides a simplified approach to estimate signal to noise ratio. Students numeric answers for the worksheets may vary slightly depending on how closely they adhere to the time tick marks to pull values. The student worksheet provides ranges of expected answers. It is important to clarify confusion with the process at this point, as the next activity is more open ended in terms of how students will estimate signal and noise. Teacher Background: Examples of the differing means used to calculate signal to noise ratios for different disciplines beyond the scope of this lesson’s goals can be found here.