< reprint > Modulation and Demodulation Circuit detailed

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Amplitude modulation and detection circuits

Radio and radio communication is the use of modulation technology to add low-frequency sound signal to the high-frequency signal emitted. The process of restoring in the receiver is called demodulation. wherein the low frequency signal is called the modulation signal, the high frequency signal is called the carrier. The common method of continuous modulation is amplitude modulation and FM, and the corresponding demodulation method is called detection and frequency identification.

Here we first introduce the amplitude modulation and the detection circuit.

(1) Amplitude modulation circuit

Amplitude modulation is to make the amplitude of the carrier signal varies with the amplitude of the modulated signal, and the frequency and phase of the carrier is constant. The circuit that can complete the amplitude modulation function is called amplitude modulation circuit or amplitude modulation device.

Amplitude modulation is a nonlinear frequency change process, so its key is to use diodes, three-stage pipe and other non-linear devices. According to the modulation process in which circuit can be three-stage tube amplitude modulation circuit into the collector amplitude modulation, the base amplitude modulation and transmitter amplitude modulation 3 species. The following is an example of the collector amplitude modulation circuit.

Figure 6 is the collector amplitude modulation circuit, the amplitude carrier generated by the high-frequency carrier oscillator by T1 added to the transistor base. The low frequency modulated signal is coupled to the collector by T3. C1, C2, C3 are high-frequency bypass capacitors, R1, and R2 are bias resistors. The LC parallel circuit of the collector is resonant on the carrier frequency. If the static working point of the transistor is selected in the curved part of the characteristic curve, the transistor is a nonlinear device. Because the collector current of the transistor varies with the modulation voltage, the 2 signals in the collector achieve amplitude modulation due to the nonlinear effect. Because the LC resonant circuit is tuned to the carrier's base frequency, the amplitude modulation wave output can be obtained in the secondary of the T2.

(2) Detector circuit

The function of the detector circuit or detector is to remove the low frequency signal from the amplitude modulation wave. Its working process is just the opposite of amplitude modulation. The detection process is also a frequency conversion process, but also the use of non-linear components. Diodes and transistors are commonly used. In addition, in order to remove the low-frequency useful signal, it must also filter out high-frequency components, so the detector circuit usually contains a nonlinear component and filter two parts. The following is an example of a diode detector to illustrate its work.

Figure 7 is a diode detector circuit. VD is a detector element, and C and R are low-pass filters. When the input of the modulated signal is large, the diode VD is intermittent operation. At the positive half-week, the diode is on, the C is charged, the negative half-week and the input voltage is small, the diode is cut off and C is discharged to R. The voltage obtained at both ends of R contains a lot of frequency components, through the capacitor C filter In addition to the high frequency portion, and then through the DC capacitor C 0 DC effect, at the output can be restored low-frequency signal.

< reprint > Modulation and Demodulation Circuit detailed