Demodulator circuit principle

1. Working principle of demodulator The demodulator is an important part of the modulated DC amplifier circuit. It restores the amplified AC voltage to DC voltage, and its magnitude and polarity should correspond to the amplitude and phase of the AC voltage. The figure below is the principle circuit of the demodulator. RL is the load and C is the filter capacitor. Its function is to smooth the output DC voltage. The demodulation switch K has the same frequency as the input AC signal Ui. When Ui is positive, the switch is turned on and the output is equal to the input voltage UO. After the smoothing effect of the capacitor C, a smooth DC voltage UO should be obtained. On the contrary, if Ui is negative, the switch is turned on, and when Ui is positive, the switch is turned off, then the output terminal gets a negative pulsating DC voltage UO. 2. Phase-sensitive rectifier-demodulator 1. Half-wave phase-sensitive rectifier The figure below shows a half-wave phase-sensitive rectifier. The transistor BG with Ce reversely connected (inverted) is used as the demodulation switch. The working process is divided into two situations: when Ui When Ua is in the same phase as Ua, if Ua is positive on the left and negative on the right, BG is saturated (when the transistor works inverted, the PN junction of the C and B poles can also reach saturation by applying a forward voltage), and the output is equal to the positive half of the input. wave voltage, so the output is a positive polarity voltage, but when Ui and Ua are inverted, that is to say, when Ua is positive on the left and negative on the right to saturate BG, Ui happens to be negative on the upper side and positive on the lower side, so the output is a negative polarity DC Voltage. Since the rectified output is related to the phase of Ui, it is called a phase-sensitive rectifier-demodulator. Its output can reflect changes in the magnitude and polarity of the input. 2. Full-wave phase-sensitive rectifier-demodulator. The half-wave rectifier can only use the input half-wave voltage, so the demodulation efficiency is low. The figure below shows the full-wave phase-sensitive rectifier circuit. Both BG1 and BG2 use ce reverse-connected transistors, which are respectively controlled by two voltages Ua1 and Ua2 of equal size and opposite phase plates. Assume that Ui and Ua are in the same phase. When the positive half wave occurs, Ua1 is positive on the left and negative on the right, which makes BG1 saturated and turned on. However, when Ua2 is negative on the left and right, BG2 is cut off. At this time, since Ui is also a positive half wave (up and down are negative), the output Umo It is a positive half wave. On the contrary, during the negative half wave, Ua1 and Ua2 are both negative on the left and positive on the right. After GB1 is cut off and BG2 is smoothed by C, a smooth and correct output voltage can be obtained. In the same way, when Ui and Ua are inverse phase, a negative output voltage is obtained. 3. Phase-sensitive amplification and demodulator 1. Half-wave phase-sensitive amplification circuit The above-mentioned phase-sensitive rectifier-demodulator only demodulates the input signal and has no amplification function. The figure below shows the half-wave phase-sensitive amplification circuit. The input signal es is a sinusoidal voltage and has the same phase and frequency as the control voltage Ua. The diode D causes the collector current iC to flow in one direction and Re acts as a current series negative feedback. When es and Ua are both positive half-waves, BG and D is in the on state, then the collector current iC flows through the load RL, and the output voltage UO is a negative half-wave, as shown in the dotted line of the UO waveform diagram. After the smoothing effect of C, the solid line DC voltage is obtained. Due to the amplification effect of BG, the power supplied to the load RL is much greater than the base input power, and the demodulated voltage Ua supplies energy to the load. In the above figure, if the input es and Ua, that is, es is up negative and down positive, but Ua is up positive and down negative, although D is still positive biased, BG is cut off, so ic is zero and the output is also zero. Therefore, the output signal is sensitive to the phase of the input signal and amplifies it, so it is called a phase-sensitive amplifier. The figure below is a practical phase-sensitive amplifier in the DDZ-11 instrument. In figure A below, the input AC signal is Ui, transformer B3 outputs an AC feedback signal, and the demodulation voltage Ua is input by transformer B2 and limited by voltage regulators DW1 and DW2. The amplitude is set so that the peak-peak value of the square wave between points A and B is set at 18 volts. R2 is the current limiting resistor of the voltage regulator tube. When Ua and Ui are reverse biased, ib and ic are both zero. It changes periodically with Ui and Ua. iC is a pulsating DC, which can be decomposed into constant DC IC and AC iC-, in which IC flows through RL and becomes a demodulated DC voltage, while the AC component iC- obtains the feedback voltage]UF after passing through the transformer B3. When Ui and Ua advance in anti-phase, io and ic are equal to zero, indicating that the amplifier circuit is sensitive to the phase. 2. Full-wave phase-sensitive amplifier The following figure shows the full-wave phase-sensitive amplifier circuit. In the figure, M is the stator winding of the DC motor. The demodulated DC signal drives the motor to rotate to perform terminal display tasks. Under the working principle: If Ui and Ua are in the same phase, when Ui and Ua are positive half waves, BG is turned on, and the current iL passes from A→D1→BG→Re→D3→M→B, iL→ is a positive half wave, let The motor M rotates forward; on the contrary, when Ui and Ua are negative half-waves, BG is reverse biased and cut off, so iL is zero (see figure b below)