Interactivity: Powers of Ten This activity calculates the powers of ten. Web Links A Review of Logarithms http://www.sosmath.com/algebra/logs/ log1/ log1.html
Content 4.1 Background for Studying Frequency-Based Cable Testing 4.1.4 Decibels The decibel (dB) is a measurement unit important in describing networking signals. The decibel is related to the exponents and logarithms described in prior sections. There are two formulas for calculating decibels: The variables represent the following values: The first formula describes decibels in terms of power (P), and the second in terms of voltage (V). Typically, light waves on optical fiber and radio waves in the air are measured using the power formula. Electromagnetic waves on copper cables are measured using the voltage formula. These formulas have several things in common. Enter values for dB and Pref to discover the correct power. This formula could be used to see how much power is left in a radio wave after it has traveled over a distance through different materials, and through various stages of electronic systems such as a radio. To explore decibels further, try the following examples using the flash activities: Interactive Media Activity Interactivity: Calculating Gain This activity allows the user to enter the final voltage and the reference voltage to get gain in decibels. Interactive Media Activity Interactivity: Using Decibels This activity allows the user to enter a value for the decibels and a value for the reference power resulting in the final power. Web Links Decibels http://arts.ucsc.edu/ems/music/ tech_background/TE-06/teces_06.html
Content 4.1 Background for Studying Frequency-Based Cable Testing 4.1.5 Viewing signals in time and frequency One of the most important facts of the information age is that data symbolizing characters, words, pictures, video, or music can be represented electrically by voltage patterns on wires and in electronic devices. The data represented by these voltage patterns can be converted to light waves or radio waves, and then back to voltage waves. Consider the example of an analog telephone. The sound waves of the caller’s voice enter a microphone in the telephone. The microphone converts the patterns of sound energy into voltage patterns of electrical energy that represent the voice. If the voltage patterns were graphed over time, the distinct patterns representing the voice would be displayed. An oscilloscope is an important electronic device used to view electrical signals such as voltage waves and pulses. The x-axis on the display represents time, and the y-axis represents voltage or current. There are usually two y-axis inputs, so two waves can be observed and measured at the same time. Analyzing signals using an oscilloscope is called time-domain analysis, because the x-axis or domain of the mathematical function represents time. Engineers also use frequency-domain analysis to study signals. In frequency-domain analysis, the x-axis represents frequency. An electronic device called a spectrum analyzer creates graphs for frequency-domain analysis. Experiment with this graphic by adding several signals, and try to predict what the output will look like on both the oscilloscope and the spectrum analyzer. Electromagnetic signals use different frequencies for transmission so that different signals do not interfere with each other. Frequency modulation (FM) radio signals use frequencies that are different from television or satellite signals. When listeners change the station on a radio, they are changing the frequency that the radio is receiving. Web Links Time and frequency relationship http://www.see.ed.ac.uk/~dil/ed-only/ demos/doc/demonstrations/ node3.html
Content 4.1 Background for Studying Frequency-Based Cable Testing 4.1.6 Analog and digital signals in time and frequency To understand the complexities of networking signals and cable testing, examine how analog signals vary with time and with frequency. First, consider a single-frequency electrical sine wave, whose frequency can be detected by the human ear. If this signal is transmitted to a speaker, a tone can be heard. How would a spectrum analyzer display this pure tone? Next, imagine the combination of several sine waves. The resulting wave is more complex than a pure sine wave. Several tones would be heard. How would a spectrum analyzer display this? The graph of several tones shows several individual lines corresponding to the frequency of each tone. Finally, imagine a complex signal, like a voice or a musical instrument. What would its spectrum analyzer graph look like? If many different tones are present, a continuous spectrum of individual tones would be represented. Interactive Media Activity Interactivity: Fourier Synthesis In this activity, the user will draw sine waves by selecting the amplitude, frequency, and phase of each.
Web Links Analog and Digital Signals http://www.smartcomputing.com/editorial/ article.asp?article= articles%2F1992%2Ffeb92%2F0215% 2F92n0215%2Eas
Content 4.1 Background for Studying Frequency-Based Cable Testing 4.1.7 Noise in time and frequency Noise is an important concept in communications systems, including LANS. While noise usually refers to undesirable sounds, noise related to communications refers to undesirable signals. Noise can originate from natural and technological sources, and is added to the data signals in communications systems. All communications systems have some amount of noise. Even though noise cannot be eliminated, its effects can be minimized if the sources of the noise are understood. There are many possible sources of noise: Noise that affects all transmission frequencies equally is called white noise. Noise that only affects small ranges of frequencies is called narrowband interference. When detected on a radio receiver, white noise would interfere with all radio stations. Narrowband interference would affect only a few stations whose frequencies are close together. When detected on a LAN, white noise would affect all data transmissions, but narrowband interference might disrupt only certain signals. If the band of frequencies affected by the narrowband interference included all frequencies transmitted on the LAN, then the performance of the entire LAN would be compromised. Interactive Media