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Home > Circuit Diagram > Communication Circuit > Demodulator circuit principle

Demodulator circuit principle

Source:qjy_dali
2024-03-24 04:00:00 16

1、 Operating principle of demodulator
Demodulator is an important part of modulated DC amplifier circuit. It reduces the amplified AC voltage to DC voltage, and its size and polarity should correspond to the amplitude and phase of AC voltage.
The following figure shows the principle circuit of the demodulator. RL is the load, C is the filter capacitor, which is used to smooth the output DC voltage. 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 capacitor C, the smooth DC voltage UO should be obtained. On the contrary, if Ui is negative, the switch is turned on, and Ui is positive, the switch is turned off, and the output terminal gets a negative pulsating DC voltage UO.



2、 Phase sensitive rectifier demodulator
1. Half wave phase sensitive rectifier
The following figure shows the half wave phase sensitive rectifier, which uses C-e reverse connected (inverted) crystal triode BG as the demodulation switch. The working process can be divided into two cases: when Ui and Ua are in the same phase, if Ua is positive left and negative right, BG is saturated (when the transistor is inverted, the PN junction of C and b poles plus forward voltage can also reach the saturation state), and the output is equal to the positive half wave voltage of the input, so the output is positive voltage, but when Ui and Ua are in the opposite phase, that is, When Ua is left positive and right negative to saturate BG, Ui is just up negative and down positive, so the output is negative DC voltage. Since the rectifier output is related to the phase of Ui, it is called a phase sensitive rectifier demodulator. Its output can reflect the change of input size and polarity.


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 following figure shows the full wave phase sensitive rectifier circuit. Both BG1 and BG2 use c-e reverse connected triodes, which are respectively controlled by two equal voltage Ua1 and Ua2 with reverse phase plates. Let Ui and Ua be in phase. In case of positive half wave, Ua1 is positive on the left and negative on the right, so that BG1 is saturated and connected, but Ua2 is negative on the left and right but BG2 is off. At this time, since Ui is also a positive half wave (negative on the top and bottom), the output Umo is a positive half wave. On the contrary, in case of negative half wave, Ua1 and Ua2 are both negative on the left and positive on the right, so that after GB1 cut-off and BG2 are smoothed by C, the smooth and correct output voltage can be obtained. Similarly, when Ui and Ua are inverted, a negative output voltage is obtained.



3、 Phase sensitive amplifier demodulator
1. Half wave phase sensitive amplifier circuit
The above phase sensitive rectifier demodulator can only demodulate the input signal without amplification. The figure below is a half wave phase sensitive amplifier circuit. The input signal es is assumed to be a sinusoidal voltage and has phase and frequency with the control voltage Ua. The diode D makes the collector current iC flow in one direction Re play a negative feedback role in current series



When both es and Ua are positive half waves, BG and D are in the conduction state, the collector current iC flows through the load RL, and the output voltage UO is a negative half wave, as shown by the dotted line in the UO waveform diagram, and then the DC voltage of the solid line is obtained through the smoothing action of C. Due to the amplification effect of BG, the power supplied to load RL is much greater than the base input power, and the load energy is supplied by demodulation voltage Ua. In the figure above, if the input es and Ua, that is, es is up negative down positive, while Ua is up positive down negative, although D is still positive offset, BG is 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 has an amplification effect, so it is called a phase sensitive amplifier.

The following figure is a practical phase sensitive amplifier in the DDZ-11 instrument. In Figure A, the input AC signal is Ui, the transformer B3 outputs the AC feedback signal, and the demodulation voltage Ua is input by transformer B2 and limited by the voltage stabilizing tubes DW1 and DW2, so that the square wave peak peak value between A and B is set at 18 V, and R2 is the current limiting resistance of the voltage stabilizing tube. When Ua and Ui are reverse offset, 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. IC flows through RL to demodulate DC voltage and AC component iC passes through transformer B3 to obtain feedback voltage] UF.
When Ui and Ua are in inverse phase progression, and io and ic are equal to zero, it indicates that the amplification circuit is sensitive to phase



2. Full wave phase sensitive amplifier
The following figure shows the full wave phase sensitive amplification. M in the figure is the stator winding of the DC motor. The demodulated DC signal drives the motor to rotate to perform the terminal display task. Working principle: if Ui and Ua are in phase, when Ui and Ua are positive half wave, BG is on, current iL passes through A → D1 → BG → Re → D3 → M → B, iL → is positive half wave, and motor M is set to rotate forward; Conversely, when Ui and Ua are negative half waves, BG reverse bias is cut off, so iL is zero (see figure b below)



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