Infrared transducers

Chapter Infrared transducers

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • range from about 9 kHz to several hundred gigahertz, into ac signals that are amplifiedby the receiving apparatus. A transmitting antenna converts powerful alternating cur-rents into EM fields, which propagate through space.There are a few significant differences between receiving antennas and trans-mitting antennas designed for a specific radio frequency. The efficiency of an an-tenna is important in transmitting applications, but not so important in reception.Efficiency is the percentage of the power going into a transducer that is convertedinto the desired form. If the input power to a transducer is Pinwatts and the outputpower is Poutwatts, the efficiency in percent, Eff%, can be found using the followingequation:Eff %100 Pout /PinProblem 32-2In a transmitting antenna, 75 W of RF power are delivered to the transducer, and 62W are radiated as an EM field. What is the efficiency of the transducer?To solve this problem, plug the numbers into the formula. In this particular case, Pin75 and Pout62. Therefore,Eff%10062/751000.8383 percentAnother difference between transmitting and receiving antennas is the fact that,for any given frequency, transmitting antennas are often larger than receiving antennas.Transmitting antennas are also more critical as to their location. Whereas a small loopor whip antenna might work well indoors in a portable radio receiver for the frequency-modulation (FM) broadcast band, the same antenna would not function well at thebroadcasting station for use with the transmitter.Still another difference between transmitting and receiving antennas involvespower-handling capability. Obviously, very little power strikes the antenna in a wirelessreceiver; it can be measured in fractions of a microwatt. However, a transmitter mightproduce kilowatts or even megawatts of output power. A small loop antenna, for exam-ple, would get hot if it were supplied with 1 kW of RF power; if it were forced to dealwith 100 kW, it would probably melt.Infrared transducersMany wireless devices transmit and receive energy at infrared (IR) wavelengths,rather than at radio wavelengths. Infrared energy has a frequency higher than thatof radio waves, but lower than that of visible light. Infrared is sometimes called heatradiation, but this is a misnomer. Some wireless devices transmit and receive theirsignals in the visible-light range, although these are encountered much less oftenthan IR devices.The most common IR transmitting transducer is the infrared-emitting diode(IRED). You learned about this type of diode in chapter 20. A fluctuating direct currentis applied to the IRED. The current causes the device to emit IR rays; the fluctuationsin the current constitute the modulation, and produce rapid variations in the intensity614 Wireless and personal communications systems