What Is a Demodulator?

A device that recovers the original modulated signal from the oscillations or waves generated by the modulation. Applied disciplines: communication technology (level one discipline); communication principles and basic technology (level two disciplines).

A device that recovers the original modulated signal from the oscillations or waves generated by the modulation. Applied disciplines: communication technology (level one discipline); communication principles and basic technology (level two disciplines).

A demodulator is a device that restores low-frequency digital signals modulated in high-frequency digital signals through digital signal processing technology. The demodulator is widely used in the transmission and restoration of information such as broadcasting (audio signals) and television (video signals). The demodulator is usually used in pairs with the modulator. The modulator is used to process digital signals to high-frequency signals for transmission, and the demodulator restores the digital signals to the original signals.

Chinese name

Demodulator

Foreign name

Demodulator

Applied discipline

Communication technology, communication principle and basic technology

working principle

Important part of modulation DC amplifier circuit

Function

Device for recovering original modulation signal

Including

Phase sensitive rectifier demodulator, phase sensitive amplifier demodulator

Demodulator works

It reduces the amplified AC voltage to DC voltage, and its size and polarity should correspond to the amplitude and phase of the AC voltage. The figure below shows 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 and the input AC signal have the same frequency. 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, the output terminal gets a negative pulsating DC voltage UO.

Phase-sensitive rectifier demodulator

1. Half-wave phase-sensitive rectifier The figure below shows a half-wave phase-sensitive rectifier. The crystal transistor BG reversely connected (inverted) is used as a demodulation switch. The working process is divided into two cases: Ua is positive left and negative, then BG is saturated (when the transistor is inverted, the PN junction of the C and b poles plus the forward voltage can also reach the saturation state), the output is equal to the positive half-wave voltage of the input, so the output is positive Voltage, but when Ui is opposite to Ua, that is, when Ua is positive left and right negative to saturate BG, Ui is just up negative and down positive, so the output is a negative DC voltage. Because 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 size and polarity of the input.

2. Full-wave phase-sensitive rectifier and demodulator The half-wave rectifier can only use the input half-wave voltage, so the demodulation efficiency is low. The figure below shows a full-wave phase-sensitive rectifier circuit. BG1 and BG2 both use ce reverse-connected triode, which are controlled by two equal-sized voltages Ua1 and Ua2, which are opposite to the phase plate. Suppose Ui and Ua are in the same phase. Ua1 is positive left and right negative when the half wave is positive, so that BG1 is saturated, but Ua2 is left and right but BG2 is cut off. It is a positive half wave. Conversely, in the case of a negative half wave, Ua1 and Ua2 are both left-negative and right-positive. After GB1 is cut off and BG2 is smoothed by C, a smooth and correct output voltage can be obtained. Similarly, when Ui and Ua are inverted, a negative output voltage is obtained.

Phase-sensitive amplifier demodulator

1. Half-wave phase-sensitive amplifier circuit The above-mentioned phase-sensitive rectifier and demodulator only plays a demodulation effect on the input signal and is resistant to no amplification. The figure below is a half-wave phase-sensitive amplifier circuit. Set the input signal es as a sinusoidal voltage and have a phase and frequency with the control voltage Ua.

When both es and Ua are positive half-waves, and BG and D are both on, the collector current iC flows through the load RL, and the output voltage UO is a negative half-wave. The smoothing effect of C gives the DC voltage of the solid line. Due to the amplification of BG, the power supplied to the load RL is much greater than the base input power, and the demodulated voltage Ua is used to supply the load energy. In the figure above, if es and Ua are input, es is positive, negative and positive, but Ua is positive, negative and negative. Although D is still positively biased, but BG is cut off, so ic is zero and the output is zero. Therefore, the output signal is sensitive to and amplifies the phase of the input signal, so it is called a phase-sensitive amplifier.

The following figure is a practical phase-sensitive amplifier in the DDZ-11 instrument. In the following figure A, the input AC signal is Ui, and the transformer B3 outputs an AC feedback signal. The demodulated voltage Ua is input by the transformer B2 and passed through the regulators DW1 and DW2. Amplitude, so that the peak-to-peak value of the square wave between the two points A and B is set at 18 volts, and R2 is the current limiting resistance of the zener tube. When Ua and Ui are reverse biased, ib and ic are both zero. With Ui, Ua change periodically. iC is a pulsating DC, which can be decomposed into a constant DC IC and an AC iC-, where the IC flows through RL to become a demodulated DC voltage and the AC component iC- passes the transformer B3 to obtain the feedback voltage] UF. When Ui, Ua are inverted, io and ic are equal to zero, indicating that the amplification circuit has a sensitive effect on the phase

2. Full-wave phase-sensitive amplifier The figure below shows a full-wave phase-sensitive amplifier. M in the figure is the stator winding of a DC motor. The DC signal obtained by demodulation drives the motor 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 is passed by A D1 BG Re D3 M B, and iL is a positive half-wave. The motor M rotates forward; conversely, when Ui and Ua are negative half-waves, BG reverses and ends, so iL is zero.