Definition of detection

Chapter Definition of detection

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

  • The noise figure (NF) of a receiver is specified in various different ways. Thelower the noise figure, the better the sensitivity. You might see a specification such as“NF: 6 dB or better.”Noise figure depends on the type of active amplifier device used in the front end ofa receiver. Gallium-arsenide field-effect transistors (GaAsFETs) are well known for thelow levels of noise they generate, even at quite high frequencies. Other types of FETscan be used at lower frequencies. Bipolar transistors tend to be rather noisy.Definition of detectionDetection, also called demodulation, is the recovery of information such as audio, avisible image (either still or moving), or printed data from a signal. The way this is donedepends on the modulation mode.Detection of AM signalsAmplitude modulation is done by varying the instantaneous strength of the carrier wave.The fluctuations follow the waveform of the voice or video data. The carrier itself is ac,of a frequency many times that of the modulating audio or video. The modulation peaksoccur on the positive and negative swings of the carrier. This is shown in Fig. 27-4.504 Data reception27-4An AM signal. The heavy lines show the output after passingthrough the envelope-detector diode or transistor.If you feed the AM carrier current directly into a headset, fax machine, or whatever,you can’t recover the data because the positive and negative peaks come by in suchrapid succession that they cancel each other out.A simple way to get the modulating waveform out of an AM signal is to cut off half ofthe carrier wave cycle. The result (Fig. 27-4) is fluctuating, pulsating dc. The pulsationsoccur at the carrier frequency, the slower fluctuation is a duplication of the modulating au-dio or video. This process can be done by passing the signal through a diode with a small