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Unformatted text preview: Page 1 Version 1.09 UNIT 5 Time Varying Signals: The basics of frequency measurements and signal encoding. Your Name Date of Submission CHEMISTRY 6158C Department of Chemistry University of Florida Gainesville, FL 32611 (Note: Much of the material in this handout was rewritten/updated in 2001 by graduate student Andrew K. Ottens.) Page 2 Version 1.09 Introduction We have already covered two of three electrical domains, analog and digital. In this unit we will focus on the last of the three - time-based signals. Time-based measurements are subdivided into three basic areas: event counting, frequency measurements and period measurements. Event counting is simple to understand; you need only count the number of times a specific event occurs. An example is the Geiger counter, a simple instrument designed to count beta particles from radioactive decay. The clicks you hear using a Geiger counter represent individual high-energy electron collisions. You simply count discrete events. Some might even say counting truly falls in the digital domain, but counting per unit time, discussed below, is one of the two main methods for encoding time-based signals. When we record radioactivity, we are measuring counts within a certain time frame - counts per minute lets say. This allows us to compare results between sample and standard. Suppose we wanted to determine the approximate age of a bone fragment. Living organisms maintain a relatively constant level of 14 C. Once the organism dies, however, this amount decreases with time. 14 C decays to 14 N by converting a neutron to a proton with release of an electron (a beta particle). If we count beta particles released from the bone fragment within a certain time frame, lets say ten minutes, we can compare that value to a standard. Our standard 14 C sample may produce 3500 counts in 10 minutes and our bone fragment 800 counts in ten minutes. Using the known rate of decay for 14 C we can estimate the bone fragments age. The number of counts per ten- minute interval is considered the frequency of beta emission, events per unit time. Whenever we concern ourselves with the number of events per unit time we talk about the frequency of events. With communications technology, often frequency encoding is used to transmit information across long distances, such as radio communication with distant space probes. As discussed in class, radio broadcasts use AM or FM modulation to carry voice information over a carrier wave all having to do with frequencies. Back during the Cold War the United States flew U2 missions over the Soviet Union. If you installed a camera on a U2 spy plane, you would want to be able to control many of its functions. Shutter speed, how long the shutter remains open, determines how exposed the film is. Night images require the shutter to remain open longer to allow for more light to hit the film. Therefore, we would need to know how long the shutter remains open. If the shutter motor is powered by a three-volt source, we could measure how long...
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