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Selectivity is the ability of a receiver to respond to a desired signal, but not to undesired ones. This generally means that a receiver must have a frequency window within which it is sensitive, but outside of which it is not at all sensitive. This window is established by a preselector in the early RF amplification stages of the receiver and is honed to precision by a narrowband filter in a later amplifier stage. Suppose you want to receive an LSB signal whose suppressed-carrier frequency is 3.885 MHz. The signal information is contained in a band from 3.882 to 3.885 MHz, a span of 3 kHz. The preselector makes the receiver sensitive in a range of about plus or minus 10 percent of the signal frequency; other frequencies are attenuated. This reduces the chance for a strong, out-of-band signal to impair the performance of the receiver. The narrowband filter responds only to the frequencies 3.882 to 3.885 MHz, so that signals in adjacent channels are rejected. The better this filter works, the better the adjacent channel rejection.
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The signals at a receiver input vary over several orders of magnitude. If you are listening to a weak signal and a strong one comes on the same frequency, you don t want to get your eardrums damaged! But also, you want the receiver to work well if a strong signal appears at a frequency near (but not exactly on top of) the weak one. Dynamic range is the ability of a receiver to maintain a fairly constant output and to keep its sensitivity in the presence of signals ranging from the very weak to the extremely strong. Dynamic range is specified in decibels and is typically over 100 dB.
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All RF amplifier circuits generate some wideband noise. The less internal noise a receiver produces, the better the S/N ratio. This becomes more and more important as the frequency increases. It is paramount at very high frequencies and above (over 30 MHz).
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504 Data reception The noise figure (NF) of a receiver is specified in various different ways. The lower 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 of a receiver. Gallium-arsenide field-effect transistors (GaAsFETs) are well known for the low levels of noise they generate, even at quite high frequencies. Other types of FETs can be used at lower frequencies. Bipolar transistors tend to be rather noisy.
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Definition of detection
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Detection, also called demodulation, is the recovery of information such as audio, a visible image (either still or moving), or printed data from a signal. The way this is done depends on the modulation mode.
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Amplitude 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 peaks occur on the positive and negative swings of the carrier. This is shown in Fig. 27-4.
27-4 An AM signal. The heavy lines show the output after passing through 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 such rapid 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 of the carrier wave cycle. The result (Fig. 27-4) is fluctuating, pulsating dc. The pulsations occur at the carrier frequency, the slower fluctuation is a duplication of the modulating audio or video. This process can be done by passing the signal through a diode with a small
Detection of CW signals 505 enough capacitance so that it follows the carrier frequency. A transistor, biased for class-B operation, can also be used. The rapid pulsations are smoothed out by passing the output of the diode or transistor through a capacitor of just the right value. The capacitance should be large enough so that it holds the charge for one carrier current cycle, but not so large that it smooths out the cycles of the modulating signal. This scheme is known as envelope detection. It is used extensively for reception of AM audio and video.
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