chapter10 - C HAPTER 10 C ARRIER SYSTEMS I ntroduction The...

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CHAPTER 10 CARRIER SYSTEMS Introduction The output from a transducer is normally in the form of an analog voltage related to the value of the measured variable. This voltage signal is usually transmitted directly by wire to the remaining elements of the instrumentation system for amplification, filtering, display, etc. In some cases, however, it is desirable to transmit the signal by means of a carrier system. This involves the use of a high frequency voltage source (the carrier) which is modulated by the lower frequency transducer signal. The modulation may be in the form of amplitude (AM), frequency (FM) or phase (PM) modification. Only AM systems will be considered here. A radio transmitter together with a remote receiver is a familiar example of a carrier system. Here the signal (microphone output in the case of a radio station) is used to modulate the amplitude of a high frequency carrier signal. This electrical signal is converted to an acoustical signal, and transmitted. These signals are outside the frequency range of human hearing (40-16,000 Hertz) and are typically in the frequency range: 0.5 and 1.6 MegaHertz for an AM radio station. At the receiver the signal is band-pass filtered, demodulated and fed to a speaker to recover the original acoustic (speech or music) input. An obvious advantage of using a carrier system here is that information can be transmitted over relatively large distances without using wires. Long cables can be a problem because they basically behave like low-pass filters and can attenuate the signal significantly; they also may enhance coupling with local power circuits causing harmonics of 60 Hertz to appear in the measured signal (see chapter on noise). So when we need to make a remote measurement, and the amplifiers and signal conditioning components of the measurement circuit have to be physically well separated from the transducer, we can use these transmitter-receiver systems to pass the signal from the transducer to the rest of the measurement/recording circuit. It should be noted that low frequency acoustic waves attenuate a lot more slowly than high frequency waves and hence can be transmitted over longer distances. An example of this is whales communicating over long distances in the oceans. Before the advent of mechanized shipping, which creates low frequency noise, whales could communicate between oceans using low frequency acoustic signals. There are many low frequency noise sources in the environment. If low frequency signals are transmitted, both the signal and the extraneous noise will be picked up at the receiver. Since both occupy the same frequency range, they cannot be separated by filtering. The role of the modulation here is to move the signal to a frequency region where the background noise is very low. The distances over which the transmission is still possible is inversely related to frequency, and a function of the strength of the transmitted signal.
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chapter10 - C HAPTER 10 C ARRIER SYSTEMS I ntroduction The...

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