Principle of detector diode

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Principle of detector diode

The detector diode has the characteristics of low junction capacitance, high working frequency and small reverse current, which is traditionally used for amplitude modulation signal detection. The working principle is as follows:

Amplitude modulation signal is a high-frequency signal carrying a low-frequency signal. The envelope of amplitude modulation signal is the baseband low-frequency signal. If the average value is taken in each signal period, it is always zero.

If the amplitude modulation signal is passed through the detector diode, due to the unidirectional conductivity of the detector diode, the negative part of the amplitude modulation signal is cut off, leaving only the positive part. At this time, if the average value (low-pass filtering) is taken in each signal cycle, the resulting envelope of the amplitude modulation signal is the baseband low-frequency signal, realizing the demodulation (detection) function.

The germanium material is point contact type, the working frequency can reach 400MHz, the forward voltage drop is small, the junction capacitance is small, the detection efficiency is high, the frequency characteristic is good, it is 2AP type. In addition to the detection, the diode similar to the point contact type can also be used in limiting, clipping, modulation, mixing, switching and other circuits. There are also two diode assemblies with good consistency for frequency modulation and detection.

Detection is generally called demodulation, which is the inverse process of modulation, that is, the process of extracting modulated signal from modulated wave. Amplitude modulation wave is the process of extracting modulation signal from its amplitude change; frequency modulation wave is the process of extracting modulation signal from its frequency change; phase modulation wave is the process of extracting modulation signal from its phase change.

Narrow sense detection refers to the process of extracting modulation signal from the envelope of amplitude modulation wave. It is sometimes called envelope detection or amplitude detection.

The demodulation of AM wave is the process of taking out the modulation signal from AM signal, which is usually called detection. The demodulation methods of AM wave include diode envelope detector and synchronous detector. No matter what kind of amplitude modulation signal, it can be demodulated by the synchronous detection circuit composed of multiplier and low-pass filter. The carrier component of the common AM signal is suppressed, and the direct nonlinear device can realize the multiplication and get the required demodulation voltage without additional synchronous signal. This kind of amplitude detector is usually called envelope detector. At present, diode envelope detector is the most widely used, and in the integrated circuit, triode emitter envelope detector is mainly used. Synchronous detection, also known as coherent detection, is mainly used to demodulate the modulation signals of double side band and single side band. It has two implementation circuits. One is composed of a multiplier and a low-pass filter, and the other directly uses diode envelope detection.

In engineering, there is a kind of signal called AM signal, which is a special signal that uses low frequency signal to control the amplitude of high frequency signal. The circuit that takes out the low frequency signal is called the detection circuit. Diodes are used to form the simplest AM detection circuit.

Principle of diode detection: amplitude modulation signal is the input of diode detection circuit, and the diode only allows unidirectional conduction. If silicon tube is used, only the part with voltage higher than 0.7V passes through the diode. The output terminal of the diode is connected with a capacitor. The capacitor and resistance cooperate to short circuit the high frequency signal in the diode output to the ground, so that the output signal is basically the signal envelope. The circuit function of capacitance and resistance is called filtering.