Unformatted text preview: Lecture 2
Radio Technology RFID & GPS systems Sound Creation Sounds are created by the motion of air particles in space. A mechanical force must be provided to the surrounding air to create sound. The mechanical force compresses air and the difference in pressure between the compressed air and the uncompressed air causes the compression to propagate away from the source of the sound. Sound Creation As a sound propagates, stationary observers, sense movement In humans, as areas of compressed and expanded air propagate toward our ears, we perceive the changes in air pressure as the "peaks and trough" reach us. Another name for peaks and troughs is sound waves. Wave Types Mechanical Waves are governed by Newton's law and exist only within a material medium like water, air and rock Electromagnetic waves are visible light, ultraviolet light, radio and television waves, microwaves, x-rays and radar waves. Electromagnetic waves require no material medium to exist. Sound to Signal Sound is measurable and quantifiable. A microphone can measure sound by recording the changes in air pressure at a particular location. Like a human ear, a microphone has sensitive moving parts that can be used to sense the changes in air pressure peaks and troughs at a particular point in space. Microphone create electrical signals or audio signals. Audio Signals describe the local changes in air pressure at the microphone at a particular time. Sounds to Signals The figure on the next slide shows a measurement of pressure created by a transient sound (a book dropping to the ground) as it is converted into an electrical voltage. As time changes, so does voltage. The vertical axis is the voltage. The horizontal axis represents time. Loud sounds create larger vertical displacement. The louder the sound, the more energy or particles are displaced. As the sound diminishes, the signal "dies" out slowly. This is called the decay of a signal. 0.25 0.2 0.15 0.1 0.05 0 -0.05 -0.1 -0.15 -0.2 -0.25 10 20 30 40 50 60 70 80 90 100 Oscillations Oscillating systems move particles back and forth about the origin in a repeated motion.
x Displacement 0 Frequency is a property of oscillatory motion. Frequency is measured by the number of oscillations within one second or the number of times particles move back and forth about the origin in one second. Examples of Oscillation Systems Earthquakes oscillation or movement of buildings after an earthquake String instruments a guitar and a violin use oscillating strings to create sound. Baseball bats after the batter hits the ball, the bat oscillates causing the batter to feel a vibration in his/her hands. Radios use an electronic device to oscillation the radio waves into space. Properties of Oscillatory motion Frequency is defined as the number of oscillations per second. Frequency is measured in Hertz (Hz). 1 hertz = 1 oscillation per second Period is defined as relating to frequency and is measured by the time required to complete one oscillation or cycle. Amplitude is dependent on how the motion is generated. Amplitude is the magnitude of the maximum displacement of the particle in either direction. Properties of Oscillatory motion
Amplitude Frequency = number of cycles per second 90 360 0 180 0 270 Amplitude 1 Cycle Properties of Oscillatory motion Pure Sine Wave a single frequency in which the signal passes from input to output and back to input via a feedback loop with no change in amplitude or phase. Complex Waves multiple frequencies in which the signal passes from input to output and back to input via a feedback loop with no change in amplitude or phase. Frequency Content of Waveforms All audio waveforms are made up of a sum of pure tones at different frequencies. When designing audio systems, we need to know the range of possible frequencies of the signals being produced. When determining a signal's bandwidth, or the possible range of frequencies, we need to know how quickly the signal may change. Audio Information The amount of information required to represent an audio signal is determined by the limitations of the human auditory perception and the audio signal we are representing. The standard used by audio manufactures of audio systems is between 20 hz and 20,000 hz. 20 hz is a low bass note 20,000 hz is a high pitch sound and is beyond the range of most human hearing. Signal Processing Signal processing is the interpretation and management of signals. Sounds and images can be manipulated and interpreted. For analog signals, signal processing involves the modulation and demodulation of signals for telecommunication systems like radio. For digital signals, signal processing involves compression, error checking and error detection. Radio Waves Radio technology is the basis for most of the "wireless" devices we use today cell phones, CB radios, first responders communication systems, television, GPS, RFID tags and wireless LANs. Radio waves fall into the electromagnetic wave classification. Radio waves are electrical and magnetic energy that travel through space. At certain frequencies along the Electromagnetic radiation spectrum, radio waves appear. See chart on following slide.. Frequencies that make up part of the electromagnetic radiation spectrum Ultra-low frequency (ULF) -- 0-3 Hz Extremely low frequency (ELF) -- 3 Hz - 3 kHz Very low frequency (VLF) -- 3kHz - 30 kHz Low frequency (LF) -- 30 kHz - 300 kHz Medium frequency (MF) -- 300 kHz - 3 MHz AM Radio High frequency (HF) -- 3MHz - 30 MHz Very high frequency (VHF) -- 30 MHz - 300 MHz - FM radio and TV Ultra-high frequency (UHF)-- 300MHz - 3 GHz TV, mobile phones, wireless LANs and two-way radios Super high frequency (SHF) -- 3GHz - 30 GHz microwave devices, wireless LANs, and radar. Extremely high frequency (EHF) -- 30GHz - 300 GHz Radio Waves Radio signals in the Medium frequency (AM radio) range can travel long distances because the signals reflect off the ionosphere, however, this occurrence is not stable or reliable. Also, high bandwidth messages are unable to travel at these frequencies. Geosynchronous satellites and cheap highperformance, high-frequency electronics have given way to new radio applications. Radio Technology Radio Waves Radio waves are used to move information from one place to another. Voice, music and pictures (TV). Radar sends out radio signals to locate objects within the path of the signal. Space explorers uses radio to communicate with earth Mars Rovers GPS systems use radio signals to locate the position of an object on earth. Cellular and Cordless phones frequently use radio signals instead of wires. Radio Communication System Design cell phone Transmitter power at the base station or cell tower and the mobile unit or cell phone. Transmitter frequency how far the signal needs to travel, types of interference it will incur and the direction of the signal. Receiver Sensitivity is measured by the amount of power at the receiver needed to produce a usable signal. Antenna size and location there is a direct correlation between antenna size and the amount of power received. Broadcast Radio Transmitter takes the audio signal and converts it to a sine wave and sends it as a radio signal. Receiver receives the radio signal and decodes the sine wave to an audio sound. Broadcast Radio Tuner A tuner in a car radio separates one radio station from another. A capacitor/inductor oscillator (capacitors hold electrical charges and inductors resist any changes in the flow of energy) act as the tuner. Radio stations send thousands of sine waves at different frequencies. A radio station located at the top of the dial sends out move waves per second than those located at the bottom of the dial. Tuners use a process called resonance to locate the radio station. The capacitor and inductor will resonate at one frequency and the sine wave matching this frequency will be amplified so you can hear the music. All other sine waves generated by other radio stations will be overlooked until you decide to change the station. Antenna radio signals, or sine waves are captured by antennas Radio Waves Radio waves carry information on the carrier signal. Information can be coded on the wave. An example of this is the dot and dash of the telegraphy. Information can be impressed on the wave through the process of modulation. The modulated signal is called sideband. Frequencies can be added to the wave. Two of the most common frequencies used in sending radio signals are Amplitude Modulation (AM) and Frequency Modulation (FM). FM has less noise. Source: The Columbia Encyclopedia, Sixth Edition. 2001-05 How radio signals work . . . The modulated carrier is also amplified, then applied to an antenna that converts the electrical signals to electromagnetic waves for radiation into space. Such waves radiate at the speed of light and are transmitted not only by line of sight but also by deflection from the ionosphere. "There are certain differences between AM and FM receivers. In an AM transmission the carrier wave is constant in frequency and varies in amplitude (strength) according to the sounds present at the microphone; in FM the carrier is constant in amplitude and varies in frequency. Because the noise that affects radio signals is partly, but not completely, manifested in amplitude variations, wideband FM receivers are inherently less sensitive to noise. In an FM receiver, the limiter and discriminator stages are circuits that respond solely to changes in frequency. The other stages of the FM receiver are similar to those of the AM receiver but require more care in design and assembly to make full use of FM's advantages. FM is also used in television sound systems. In both radio and television receivers, once the basic signals have been separated from the carrier wave they are fed to a loudspeaker or a display device (usually a cathode-ray tube), where they are converted into sound and visual images, respectively." The Columbia Encyclopedia, Sixth Edition. 2001-05 RFID Radio Frequency Identification systems use high, medium and low radio frequencies to identify and track objects. RFID systems are identified by the type of radio frequency they use to send information. See chart of the next slide. RFID systems consists of three components: reader, tag and a host computer. The reader sends a message, via a radio frequency, to the tag requesting identification information. The tag responds, via a radio frequency, with ID information. The reader then sends the information it received from the tag to the computer. The host computer keeps track of the tags location and systematically files any information about the tag (billing, arrival time, product information ect.). RFID Frequency Bands and Applications
Frequency Band System Characteristics Example Applications
Low 100-500 kHz Intermediate 10-15 MHz High 850-950 MHz 2.4-5.0 GHz Short read range Inexpensive Access control Animal identification Inventory control Access control Smart cards Medium read range Long read range High reading speed Line of sight required Expensive Railroad car monitoring Toll collection systems Source: http://www.cwt.vt.edu/faq/rfid.htm#Intro RFID Below is a simple diagram of a RFID system Radio Frequency Reader Radio Frequency RFID Tag Host Computer RFID Systems There are two types of RFID systems Active RFID systems use batteries as a source of power. The advantages of this type of system is that the RFID tag can transmit information over large distances. The drawbacks to this system are: 1) they are expensive, 2) the life span of the energy source is limited 3) very expensive. Passive RFID systems the energy source comes from the reader. These tags remain passive until they connect with the reader. They are cheaper, smaller and have an indefinite life span. The disadvantages are: 1) they must be close the reader to work and 2) they need a high powered reader to work. RFID RFID tags have memory chips. RFID tags can be classified by the type of memory cell used. Read only devices the tag can only transmit information coded in the memory Read and write devices the tag can transmit information coded in the memory and can write information to the memory. GPS GPS is a satellite navigational system that uses radio signals to locate the position of people and objects anywhere on earth. GPS systems use ground receivers that send radio signals to satellites orbiting the earth. GPS systems use a trilateration process to calculate longitude, latitude and altitude. Trilateration GPS Civilian applications include the tracking of people, cars and animals. First responders use the system to locate people in trouble. People have navigational systems in their cars to help them with directions. Military applications include precision guided munitions and locating group troupes (situational awareness). Drawbacks to the system include: slow response time, GPS receivers do not work inside or between tall buildings or large objects. Components of GPS Systems Satellites 24 satellites orbit the earth in such a pattern that they are evenly distributed above the earth so that 4 satellites are always in view. Receivers the receiver determines the time in which it receives a radio signal from the satellite. The delay between the transmission and reception times, along with the speed of a radio wave transmission is used to determine the distance between the satellite and the receiver. System Control Center maintained by the US military, paid for by the US taxpayer and in times of emergency can be shut down at a moments notice. Discussion Questions posted in Blackboard Lecture 2 - week 1 question In a broadcast radio system, how is information sent from the radio station to your car radio? What is the difference between AM and FM radio? Lecture 2 week 2 question You're an audio system manufacturer, from an engineering perspective, what do you need to know to help you build a quality audio system? References Cyganski, David, Orr, John A; Information Technology Inside and Outside, Prentice Hall, 2001 The Columbia Encyclopedia, Sixth Edition. 2001-05. - Oscillator http://www.cwt.vt.edu/faq/rfid.htm#Intro Halliday, Resnick, Walker; Fundamentals of Physics/ 7th edition, John Wiley & Sons, Inc. 2005 ...
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- Spring '08
- FM radio, radio waves, RFID Systems