Modulation circuit and demodulation circuit

Modulation circuit and demodulation circuit 1. Amplitude modulation circuit The amplitude modulation circuit adds the modulation signal and the carrier signal to a nonlinear component (such as a crystal diode or transistor) at the same time, transforms them nonlinearly into a new frequency component, and then uses the resonant circuit to select the desired frequency content. Amplitude modulation circuits are divided into diode amplitude modulation circuits and transistor base amplitude modulation, emitter amplitude modulation and collector amplitude modulation circuits. Usually, triode amplitude modulation circuits are mostly used. If the modulated amplifier uses a low-power small-signal tuning amplifier, it is called low-level amplitude modulation; conversely, if a high-power, large-signal tuning amplifier is used, it is called high-level amplitude modulation. In practice, high-level amplitude modulation is mostly used, and its requirements are: (1) The modulation characteristics (the relationship between modulation voltage and output amplitude) are required to be linear; (2) The collector efficiency is high; (3) The requirements are low The amplifier circuit is simple. 1. Base amplitude modulation circuit Figure 1 is a transistor base amplitude modulation circuit. The carrier signal is added to the base of BG through the high-frequency transformer T1. The low-frequency modulation signal is connected in series with the high-frequency carrier through an inductor L. C2 is a high-frequency bypass capacitor. , C1 is a low-frequency bypass capacitor, R1 and R2 are biased voltage dividers. Due to the nonlinear effect of the ic=f(ube) relationship curve of the transistor, the collector current ic contains various harmonic components, which are tuned by the collector. The loop selects the amplitude modulation wave. The advantage of the base amplitude modulation circuit is that it requires low power of the low-frequency modulation signal, so the low-frequency amplifier is relatively simple. Its disadvantage is that it works in an undervoltage state, has low collector efficiency, and cannot fully utilize the energy of the DC power supply. 2. Emitter amplitude modulation circuit Figure 2 is an emitter amplitude modulation circuit. Its principle is similar to base amplitude modulation, because the voltage added between the base and the emitter is about 1 volt, while the collector power supply voltage ranges from tens to tens of volts. Volts, the influence of the modulation voltage on the collector circuit is negligible, so the working principles and characteristics of emitter amplitude modulation and base amplitude modulation are similar. 3. Collector amplitude modulation circuit Figure 3 is a collector amplitude modulation circuit. The low-frequency modulation signal is introduced from the collector. Since it works in an overvoltage state, it has high efficiency but the nonlinear distortion of the modulation characteristics is serious. In order to improve the modulation characteristics, you can Introduce nonlinear compensation measures into the circuit so that the input excitation voltage changes with the collector power supply voltage. For example, when the collector power supply voltage decreases, the amplitude of the excitation voltage decreases and will not enter a strong voltage state; conversely, when the collector power supply voltage decreases, When the electrode power supply voltage increases, it increases and does not enter the undervoltage area. Therefore, the amplitude modulator always works in a weak overvoltage or critical state, which can not only improve the modulation characteristics, but also achieve higher efficiency. To achieve this The circuit of the measure is called a double collector amplitude modulation circuit. The modulation characteristics can also be improved by using the dual amplitude modulation circuit of collector and emitter in Figure 4. Pay attention to the same terminal of the transformer. When the modulation signal is in the positive half-wave, although the collector power supply voltage increases, the base bias voltage also becomes positive at the same time, which prevents it from entering an undervoltage operating state; when the modulation signal is in the negative half-wave, , although the collector voltage decreases, the base bias voltage also becomes negative, preventing it from entering the strong overvoltage area, thus maintaining the operation in a critical and weak overvoltage state. Figure 1. Base amplitude modulation circuit Figure 2. Emitter amplitude modulation circuit 2. Amplitude detection circuit The process of extracting the modulated signal from the amplitude modulation wave is called amplitude detection. There are three commonly used detection circuits: small signal square law detection and large signal envelope Full-wave and product detection have the following three requirements for the detector: (1) Detection efficiency (voltage transmission coefficient) If the detector inputs a constant-amplitude high-frequency voltage peak value of Uc and the output voltage after detection is Uo, then the detection efficiency K is defined as: K=Uo/Uc. If the input of the detector is an envelope amplitude modulated wave, the detection efficiency is defined as the ratio of the output low-frequency voltage amplitude UΩ to the input high-frequency voltage envelope amplitude mUc: K=UΩ/mUc where: m is the amplitude modulation coefficient. The larger K means that a larger low-frequency output signal can be obtained under the same input situation, that is, the detection efficiency is high. (2) Detection distortion reflects the degree of compliance between the output low-frequency voltage waveform and the input modulated wave shape. (3) Input resistance Ri The equivalent resistance seen from the input end of the detector is called input resistance Rio. Usually the detector is connected to the output end of the intermediate frequency amplifier, and Ri is regarded as its load. Therefore, the larger Ri is, the smaller the impact on the intermediate frequency amplifier will be. 1. Small signal square law detector Figure 5(a) is a small signal detector circuit. Its characteristics are: (1) The amplitude of the input high-frequency signal ui(t) is on the order of tens of millivolts; (2) Select an appropriate bias voltage so that the operating point Q is on the curved section of the volt-ampere characteristic [see Figure 5 (b)], current flows through the diode throughout the high-frequency signal period. Theoretical analysis shows that the output voltage u2 of the detector is proportional to the input voltage U c, which is why square law detection is named. Its parameters are as follows: (1) Detection efficiency K=UΩ/mUc=Ra2Uc/(1+a1R [Examination question output voltage reaction] In the formula: R is the load resistance of the detector, Uc is the carrier amplitude of the high-frequency amplitude modulated wave, a1 and a2 are coefficients related to the operating point current, and their values ​​at room temperature are approximately: a1=38Io And a2=0.74×10 Io (the unit of Io is ampere) If the operating point current of the detector is selected as Io=20 microamps, R=4.7 kiloohms, Uc=50 millivolts, the detection efficiency is: K=Ra2Uc/ (1+a1R)=(4.7×10