Thông tin thiết kế mạch P2

AMPLITUDE MODULATED RADIO TRANSMITTERA radio signal can be generated by causing an electromagnetic disturbance and making suitable arrangements for this disturbance to be propagated in free space. The equipment normally used for creating the disturbance is the transmitter, and the transmitter antenna ensures the efficient propagation of the disturbance in free space. To detect the disturbance, one needs to capture some finite portion of the electromagnetic energy and convert it into a form which is meaningful to one of the human senses...
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Thông tin thiết kế mạch P2 Telecommunication Circuit Design, Second Edition. Patrick D. van der Puije Copyright # 2002 John Wiley & Sons, Inc. ISBNs: 0-471-41542-1 (Hardback); 0-471-22153-8 (Electronic) 2 AMPLITUDE MODULATED RADIO TRANSMITTER2.1 INTRODUCTIONA radio signal can be generated by causing an electromagnetic disturbance andmaking suitable arrangements for this disturbance to be propagated in free space.The equipment normally used for creating the disturbance is the transmitter, and thetransmitter antenna ensures the efficient propagation of the disturbance in free space.To detect the disturbance, one needs to capture some finite portion of the electro-magnetic energy and convert it into a form which is meaningful to one of the humansenses. The equipment used for this purpose is, of course, a receiver. The energy ofthe disturbance is captured using an antenna and an electrical circuit then convertsthe disturbance into an audible signal. Assume for a moment that our transmitter propagated a completely arbitrarysignal (that is, the signal contained all frequencies and all amplitudes). Then no othertransmitter can operate in free space without severe interference because free space isa common medium for the propagation of all electromagnetic waves. However, if werestrict each transmitter to one specific frequency (that is, continuous sinusoidalwaveforms) then interference can be avoided by incorporating a narrow-band filter atthe receiver to eliminate all other frequencies except the desired one. Such acommunication channel would work quite well except that its signal cannotconvey information since a sinusoid is completely predictable and information, bydefinition, must be unpredictable. Human beings communicate primarily through speech and hearing. Normalspeech contains frequencies from approximately 100 Hz to approximately 5 kHzand a range of amplitudes starting from a whisper to very loud shouting. An attemptto propagate speech in free space comes up against two very severe obstacles. Thefirst is similar to that of the transmitters discussed earlier, in which they interferewith each other because they share the same medium of propagation. The secondobstacle is due to the fact that low frequencies, such as speech, cannot be propagated 1718 AMPLITUDE MODULATED RADIO TRANSMITTERefficiently in free space whereas high frequencies can. Unfortunately, human beingscannot hear frequencies above 20 kHz which is, in fact, not high enough for freespace transmission. However, if we can arrange to change some property of acontinuous sinusoidal high-frequency source in accordance with speech, then theprospects for effective communication through free space become a distinctpossibility. Changing some property of a (high-frequency) sinusoid in accordancewith another signal, for example speech, is called modulation. It is possible tochange the amplitude of the high-frequency signal, called the carrier, in accordancewith speech and=or music. The modulation is then called amplitude modulation orAM for short. It is also possible to change the phase angle of the carrier, in whichcase we have phase modulation (PM), or the frequency, in which case we havefrequency modulation (FM).2.2 AMPLITUDE MODULATION THEORYIn order to simplify the derivation of the equation for an amplitude modulated wave,we make the simplification that the modulating signal is a sinusoid of angularfrequency os and that the carrier signal to be modulated (also sinusoidal) has anangular frequency oc. Let the instantaneous carrier current be i ¼ A sin oc t ð2:2:1Þwhere A is the amplitude. The amplitude modulated carrier must have the form i ¼ ½A þ gðtފ sin oc t ð2:2:2Þwhere gðtÞ ¼ B sin os t ð2:2:3Þis the modulating signal. Then i ¼ ðA þ B sin os tÞ sin oc t ð2:2:4ÞThe waveform is shown in Figure 2.1. The current may then be expressed as i ¼ ðA þ kA sin os tÞ sin oc t ð2:2:5Þwhere B k¼ : ð2:2:6Þ A 2.2 AMPLITUDE MODULATION THEORY 19Figure 2.1. Amplitude modulated wave: the carrier frequency remains sinusoidal at oc w ...