Thông tin thiết kế mạch P5

THE FREQUENCY MODULATED RADIO RECEIVERIn amplitude modulation, the frequency of the carrier is kept constant while its amplitude is changed in accordance with the amplitude of the modulating signal. In frequency modulation, the amplitude of the carrier is kept constant and its frequency is changed in accordance with the amplitude of the modulating signal. It is evident that, if a circuit could be found which will convert changes in frequency to changes in amplitude, the techniques used for detecting AM can be used for FM as well....
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Thông tin thiết kế mạch P5 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) 5 THE FREQUENCY MODULATED RADIO RECEIVER5.1 INTRODUCTIONIn amplitude modulation, the frequency of the carrier is kept constant while itsamplitude is changed in accordance with the amplitude of the modulating signal. Infrequency modulation, the amplitude of the carrier is kept constant and its frequencyis changed in accordance with the amplitude of the modulating signal. It is evidentthat, if a circuit could be found which will convert changes in frequency to changesin amplitude, the techniques used for detecting AM can be used for FM as well. In Section 4.3.3.4, three frequency-to-amplitude conversion circuits werediscussed and their performance in terms of linearity and dynamic range wereexamined. It therefore follows that the FM receiver must have the same basic form asthe AM receiver. The structure of the FM receiver is as shown in Figure 5.1. The superheterodyne technique is used in FM for the same reasons it is used inAM; it translates all incoming frequencies to a fixed intermediate frequency at whichthe filtering process can be carried out effectively. The antenna is responsible for capturing part of the electromagnetic energypropagated by the transmitter. The basic rules of antenna design apply but, becausein commercial FM radio the frequency of the electromagnetic energy is between 88and 108 MHz, it is practical to have antennas whose physical dimensions are withintolerable limits. The radio-frequency amplifier raises the power level to a point where it can beused in a mixer or frequency changer to change the center frequency to a lowerfrequency – the intermediate frequency (IF). The mixer in conjunction with the localoscillator translate the incoming radio frequency to an intermediate frequency of10.7 MHz. There is nothing special about an intermediate frequency of 10.7 MHzexcept that it is a relatively low frequency at which the required values of Ls and Csare large enough to reduce the effects of circuit strays. It is at this fixed frequencythat filtering to remove the unwanted products of the mixing process and other 143144 Figure 5.1. The block diagram of the domestic FM receiver showing frequency ranges and bandwidths. 5.1 INTRODUCTION 145interfering signals and noise takes place. The filtered signal then proceeds to theamplitude limiter. The need for the limiter becomes evident when one recalls that theFM signal is usually converted into an AM signal in the discriminator before it isdetected. This means that any variation in the amplitude of the FM signal will besuperimposed on the proper signal from the discriminator and hence will causedistortion. The amplitude limiter very severely clips the signal to a constantamplitude and also filters out the unwanted harmonics that are produced by thelimiter. The signal then proceeds to the frequency discriminator (frequency-to-amplitude convertor) and onto the envelope detector. The audio-frequency amplifierraises the output of the envelope detector to a level suitable for driving aloudspeaker. Although the structures of AM and FM receivers are similar, there are veryimportant differences which require different design and construction approaches.These are the following: 1. The higher carrier frequencies (88–108 MHz) used in FM requires small values of both L and C in the tuned circuits used. This means that stray inductances and capacitances will constitute a larger percentage of the designed value and hence have a much greater effect on all tuned circuits. Although measures can be taken to incorporate the effects of fixed circuit strays in the design, there are other changes in element values, due to such factors as temperature and vibration, which can cause sufficient drift to necessitate retuning of the receiver during a program. The local oscillator is most vulnerable to stray elements, especially because it has to operate at a frequency 10.7 MHz above the carrier frequency. To ensure the stability of the oscillator, high circuit Q factors, negative temperature-coefficient capacitors and automatic frequency control (AFC) are used. Some radio-frequency amplifiers for FM front-ends use distributed parameter circuit components such as coaxial and transmission lines. 2. With the intermediate frequency set at 10.7 MHz, the band from which image interference can originate is from 109.4 to 129.4 MHz. This frequency band is reserved for aeronautical radionavigation systems. It follows that one FM station cannot cause image interference for another but the aeronautical radionavigation systems can. As in AM, image interference can be reduced by differentially amplifying the desired signal relative to the image signal. A high Q factor tuned radio-frequency amplifier is used for this purpose. 3. The use of a high Q factor tuned amplifier in the radio-frequency stage requires a very stable local oscillator frequency which will accurately track the incoming radio frequency and produce a minimum variation from the selected intermediate frequency. The local oscillator by itself is not capable of this but, used in conjunction with the AFC and other stabilizing measures, it can perform satisfactorily. 4. The ideal i ...