# Thông tin thiết kế mạch P3

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## Thông tin thiết kế mạch P3

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THE AMPLITUDE MODULATED RADIO RECEIVER The electromagnetic disturbance created by the transmitter is propagated by the transmitter antenna and travels at the speed of light as described in Chapter 2. It is evident that, if the electromagnetic wave encounters a conductor, a current will be induced in the conductor. How much current is induced will depend on the strength of the electromagnetic ﬁeld, the size and shape of the conductor and its orientation to the direction of propagation of the wave....

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2. 80 THE AMPLITUDE MODULATED RADIO RECEIVER Figure 3.1. (a) The envelope detector circuit. The diode ‘‘half-wave’’ rectiﬁes the AM wave and the RC time-constant ‘‘follows’’ the envelope with a slight ripple. (b) The input signal to the envelope detector. (c) The output signal of the envelope detector. Note that (1) when the voltage is rising the ripple is larger than when the voltage is falling. A longer time constant will help reduce the ripple; however, it will also increase the likelihood that the output voltage will not follow the envelope when the voltage is falling causing ‘diagonal clipping’. (2) In practice, the carrier frequency is much higher than the modulating frequency, hence the ripple is much smaller than shown.
3. 3.2 THE BASIC RECEIVER: SYSTEM DESIGN 81 Figure 3.1. (continued ) To recover the ‘‘message’’ we require a circuit which will follow the envelope of the amplitude of the carrier. Such a circuit is called an envelope detector and it consists of a diode and a parallel RC circuit as shown in Figure 3.1(a). The input signal to the circuit is most appropriately represented by an ideal current source connected to the primary of the transformer. This ideal current source represents all the currents induced in the antenna by all the radio stations broad- casting signals in free space. The signal is coupled to the parallel-tuned LC circuit which selectively enhances the amplitude of the signal whose carrier frequency is the same as the resonant frequency of the LC circuit. In Figure 3.1(b), only the enhanced modulated signal is shown at the input of the envelope detector. Because the diode conducts only when the anode has a positive potential compared to the cathode, only the positive half of the signal appears across the output resistor. Because the capacitor is connected in parallel with the resistor, when the diode conducts the capacitor must charge up to the peak value of the voltage. When the input voltage is less than the voltage across the capacitor, the conduction is cut off and the capacitor starts to discharge through the resistor with the voltage falling off exponentially. With the proper choice of time-constant RC, the output voltage waveform will have the form shown in Figure 3.1(c). This waveform is essentially the envelope of the carrier signal with a ripple at a frequency equal to the carrier frequency. A low-pass ﬁlter can be used to remove the ripple. The circuit shown in Figure 3.1(a) has been used with success as a practical receiver with the resistor R replaced by a high impedance headphone. Needless to say, such a simple circuit has its limitations. The power in the circuit is supplied entirely by the transmitter and naturally it is at a very low level, especially as the distance between the transmitter and the receiver increases. Secondly, the ability of the LC tuned circuit to suppress the signals propagated by all the other transmitters is limited and therefore such a receiver will be subject to interference from other stations. These limitations can be overcome by using the superheterodyne conﬁg- uration described below.
5. 3.3 THE SUPERHETERODYNE RECEIVER: SYSTEM DESIGN 83 design with sharp cut-off characteristics. The output of the intermediate-frequency ampliﬁer which then goes to the envelope detector consists of the intermediate frequency and its two sidebands. The envelope detector removes the intermediate frequency, leaving the audio-frequency signal which is then ampliﬁed by the audio- frequency ampliﬁer to a level capable of driving the loudspeaker. It is clear that there will be a very large difference between the signal from a powerful local radio station and a weak distant station. To help reduce the difference an automatic gain control (AGC) is used to adjust the signal reaching the envelope detector to stay within predetermined values. The most interesting signal processing step in the system takes place in the frequency changer or frequency mixer or simply the mixer [1]. There are two basic types of mixers: the analog multiplier and the switching types. The analog multiplier frequency changer simply multiplies the radio-frequency signal and the local oscillator so that when the modulated carrier current is im ðtÞ ¼ Að1 þ k sin oS tÞ sin oC t ð3:3:1Þ and the local oscillator signal is io ðtÞ ¼ B sin oL t ð3:3:2Þ the output of the mixer is iðtÞ ¼ Að1 þ k sin oS tÞ sin oC t Â B sin oL t ð3:3:3Þ 1 iðtÞ ¼ 2 ABð1 þ k sin oS tÞ½cosðoL À oC Þt À cosðoL þ oC Þt ð3:3:4Þ iðtÞ ¼ 1 AB½cosðoL À oC Þt À cosðoL þ oC Þt þ k sin oS t cosðoL À oC Þt 2 À k sin oS t cosðoL þ oC Þt ð3:3:5Þ iðtÞ ¼ 1 ABfcosðoL À oC Þt À cosðoL þ oC Þt 2 þ 1 k½sinðoL À oC À oS Þt þ sinðoL À oC þ oS Þt 2 À 1 k½sinðoL þ oC À oS Þt þ sinðoL þ oC þ oS Þtg: 2 ð3:3:6Þ The spectrum of Equation (3.3.6) is shown in Figure 3.3. It should be noted that this has been simpliﬁed for clarity. The product formation in Equation (3.3.3) is not a precise process and tends to create a large number of frequencies due to sub- and higher harmonics present in both the radio-frequency and local oscillator signals. The radio-frequency and local oscillator signals are usually present in the output as well. It is important to keep all the unwanted signals outside the frequency band of the intermediate frequency and, failing that, to reduce their amplitude to a very low value. It can be seen that the mixing operation gives two additional carriers and their sidebands at frequencies corresponding to the sum (oL þ oC ) and difference (oL À oC ) of the local oscillator and carrier frequencies. The required signal at
6. 84 THE AMPLITUDE MODULATED RADIO RECEIVER Figure 3.3. A simpliﬁed spectrum of the output from a frequency changer which uses a nonlinear device. the difference frequency (intermediate frequency) can now be ﬁltered out by the intermediate-frequency stage of the receiver. It should be noted that the mixing operation does not affect the sidebands. To clarify the changes in frequency that take place as the signal proceeds through the system, the AM broadcast band (600 kHz– 1600 kHz) is used as an example in Table 3.1. The frequency changer or mixer presents two immediate problems: the choice of the local oscillator frequency and the design strategy of the mixer itself. (1) It can be seen from Table 3.1 that the local oscillator frequency has been chosen to be higher than the incoming radio-frequency signal. There is a very good reason for this. The ratio of the maximum to the minimum capacitance TABLE 3.1 Radio frequency (kHz) Low-frequency end High-frequency end Incoming signal, fc Æ fs 600 Æ 5 1600 Æ 5 Local oscillator, fL 600 þ 455 ¼ 1055 1600 þ 455 ¼ 2055 Intermediate frequency, fk 455 455 Image frequencya, fim 1055 þ 455 ¼ 1510 2055 þ 455 ¼ 2510 Output, intermediate-frequency ampliﬁer, fk Æ fs 455 Æ 5 455 Æ 5 Envelope detector, fs 0–5 0–5 a The image frequency is the frequency of the unwanted signal which, when combined with the local oscillator frequency, will give the intermediate frequency. Normally the radio-frequency ampliﬁer should suppress the image frequency but this may be difﬁcult if the signal from the desired station is very weak and the image signal is very strong.
9. 3.4 COMPONENTS OF THE SUPERHETERODYNE RECEIVER 87 Figure 3.4. A typical radio-frequency ampliﬁer. The load RL represents the input resistance (impedance) of the circuits which are driven by the ampliﬁer. the load can be represented by a resistance RL in parallel with the tuned circuit. To simplify the analysis of the circuit, the winding resistance r in series with the inductance is transformed into an equivalent shunt resistance (refer to Figure 2.50) Rp where o2 L2 p Rp ¼ ð3:4:1Þ r and Lp ¼ L ð3:4:2Þ when Q ) 1. The ampliﬁer load RL combined in parallel with Rp is now the resistive part of the collector load. The new equivalent circuit is shown in Figure 3.5, where Req ¼ Rp kn2 RL : ð3:4:3Þ It can be seen from Figure 3.5 that: (1) The emitter resistor Re has not been bypassed to ground with a capacitor. (2) At the frequency of resonance, the parallel LC circuit in the collector circuit will behave like an open circuit. The equivalent collector load is Req . (3) Because the inductor is connected directly between þVcc and the collector, the dc voltage on the collector is þVcc .