Solid State Shortwave Receivers For Beginners - The Listeners Guide

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A.M. Signal It will be helpful to first consider what exactly an AM. signal consists of. For the sake of this example we will consider a 14MHZ carrier wave which is modulated by a 1kHZ audio tone. Looking at this in terms of the frequencies that are produced at the output of the transmitter we have the arrangement shown in Fig.27. Apart from the carrier wave, two other signals are produced, one 1kHZ above the carrier wave, and one 1kHZ below it. If a 2kHZ audio signal were to be used as the modulating signal, these signals would be 2kHZ above and below the carrier wave. Their spacing from the carrier wave is always equal to the modulating audio frequency. The one above the carrier is called the upper sideband, and the one below it is called the lower sideband. In a practical signal consisting of transmitted speech, there would be many frequencies in each sideband, but the two sidebands would still be symetrically grouped around the carrier. Such a signal might look something like Fig.28. 72
At the detector of the receiver the two sidebands react with the carrier signal to produce the original audio frequencies at the output. The phasing of the two sidebands is such that they do not react with one another. One drawback of A.M. is that if two transmitters are operating close to one another in terms of frequency, the two carriers will react with one another to produce an audio output. For instance, if the carrier waves are 3kHZ apart, a 3kHZ audio signal will be produced. Furthermore, as the carriers are stronger than the sidebands, this 3kHZ signal will be extremely strong, and it is quite likely that neither signal will be completely comprehendable. This effect is easily demonstrated by tuning over the M.W. band after dark. One or two examples can usually be found without too much effort. The amateur bands are extremely crowded in recent times, and the widespread use of A.M. is no longer really feasable on these bands. Instead S.S.B. is used, and what this consists of is suppressing the carrier and one sideband at the transmitter, and only transmitting one of the sidebands. Hence the name, single sideband. It is possible to use an ordinary diode detector to demodulate an S.S.B. signal, but only if an oscillator at the receiver is used to replace the missing carrier signal. There is no need to replace the missing sideband, even if this were a feasable proposition, as both sidebands contain the same information, and only one of them is needed to react with the car- - rier to produce the proper audio output. An ordinary diode detector plus an oscillator makes a far from ideal S.S.B. demodulator, and some reaction between the sideband components results with a consequent loss of audio quality. Fot proper S.S.B. demodulation a completely different type of detector is required. This form of detector is known as a product detector, and it uses the heterodyne principle. A product detector is a form of mixer, and this has the S.S.B. signal injected at one of its two inputs, and an oscillator signal at the other input. Four signals will be generated at the output. These are the original two inputs, the sum of the two, and the difference between the two. In a practical situation the oscillator is adjusted to the frequency of the suppressed carrier wave, the difference signal is then the required audio signal. A simple example would be if a 3.7MHZ suppressed carrier is modulated by a 1kHZ tone to produce a lower sideband signal at 3.699MHZ. If the oscillator is adjusted to 3.7MHZ, and these signals are fed into the product detector, outputs at 1kHZ (3.7 - 3.699 = 0.001MHZ, or 1kHZ), 7.399MHZ (3.7 + 3.699 = 7.399MHZ), 3.699MHZ, and 3.7MHZ are produced. The last three signals are at R.F. and are easily filtered out to leave the required audio signal. 73
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Solid State Short Wave Receivers Fo
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SOLID STATE SHORT WAVE RECEIVERS FO
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CONTENTS Page CHAPTER 1 Frequency S
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CHAPTER 1 There is a strange fascin
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Propagation The reasons for the ama
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41 Metres 7.1 to 7.3 MHZ 31 Metres
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e. A length of wire is connected to
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Crystal Set CHAPTER 2 Ultra Simple
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Components List for Fig.6 T1 Denco
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To those unfamiliar with this metho
Page 23 and 24: The completed chassis and panel are
Page 25 and 26: crystal set is not very high. This
Page 27: ted it would look something like th
Page 30 and 31: eceivers it does have too low a val
Page 32: to have on. connection earthed. In
Page 35 and 36: It is very worthwhile experimenting
Page 37 and 38: layer, acts as the dielectric of th
Page 39 and 40: level that reaches the detector of
Page 41: CHAPTER 3 General Purpose Receivers
Page 44 and 45: The output of Tr1 is developed acro
Page 47 and 48: Components List for Fig. 18 Resisto
Page 50 and 51: Components List for Fig.19 Resistor
Page 55 and 56: A.F. noise from Tr2 from reaching s
Page 57: The gain of the I.C. is fixed at ap
Page 60: Like MOSFETS, CMOS I.C.s can be dam
Page 63: CHAPTER 4 Portable Receivers One mi
Page 66: Normally the telescopic aerial shou
Page 74 and 75: As should be apparent by now, if th
Page 78 and 79: the mains earth (regardless of whet
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Page 82 and 83: Components List for Fig.30 R1 3.3k
Page 84 and 85: Components List for Fig.32 R1 3.3k
Page 86 and 87: Apart from increasing the sensitivi
Page 89 and 90: Components List for Fig.36 R1 390 o
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Solid State Shortwave Receivers For Beginners - The Listeners Guide

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