The problem of the envelope detector in the communication principle

Envelope-demodulation is a vibration signal processing method based on filter detection, which has a strong ability to identify initial faults and fault signals with low signal-to-noise ratio.

By connecting the peak points of high-frequency signals for a period of time, you can get an upper (positive) line and a lower (negative) line, which are called envelopes. The envelope is the curve that reflects the change in the amplitude of a high-frequency signal. For equal-amplitude high-frequency signals, these two envelopes are parallels.

When a low-frequency signal is amplitude modulated (i.e., amplitude modulated) to a high-frequency signal, the low-frequency signal becomes the envelope of the high-frequency signal. Such a signal is called an amplitude modulation signal. The process of demodulating a low-frequency signal from an amplitude modulated signal is called envelope detection.

That is, the envelope detection is an amplitude detection. The common method of bag detection is to use diodes for one-way filtering followed by low-pass filtering. Direct low-pass filtering without a diode cancels out the positive and negative envelopes, making low-frequency signals undetectable.

In addition to envelope detection, there are frequency offset (frequency modulation) detection, phase shift (or phase) detection, and so on.

The demodulation of an amplitude modulated wave is the process of taking a modulated signal from the amplitude modulated signal, which is often called a detector. Detection in the broad sense of the word, commonly referred to as demodulation, is the inverse process of modulation, that is, the process of extracting the modulated signal from the modulated wave. For an amplitude-modulated wave, it is the process of extracting a modulated signal from its amplitude change; For frequency modulation waves, it is the process of extracting a modulated signal from its frequency change; Modulated phase wave is the process of extracting a modulated signal from its phase change.

In engineering practice, there is a type of signal called amplitude modulation wave signal, which is a special signal that uses low-frequency signals to control the amplitude of high-frequency signals. In order to take out the low-frequency signal, a special circuit called a detector circuit is required. The use of diodes allows for the formation of the simplest amplitude modulated wave detection circuit.

Amplitude modulation wave demodulation methods include diode envelope detector and synchronous detector. At present, the most widely used is the diode envelope detector, which can be demodulated by a synchronous detector circuit composed of a multiplier and a low-pass filter regardless of the amplitude modulation signal. However, for ordinary amplitude modulation signals, the carrier component is suppressed, and the nonlinear device can be directly used to achieve multiplication to obtain the required demodulation voltage without adding a synchronous signal, which is usually called an envelope.

First of all, you have to distinguish what is the difference between the modulated signal and the modulated signal, and what is the difference between the DC signal, the low-frequency signal and the high-frequency signal. Then the process of envelope detection is understood step by step.

The modulated signal does not include the DC signal and is a high-frequency signal. After passing through the rectifier, there are DC components, low-frequency components (envelopes) and high-frequency components. A low-pass filter is used to filter out high-frequency components.

The book you read may not be well written. The term rectifier is not appropriate here. It is recommended to find other books to read.

1. The conditions of use are different.

When the synchronous detection circuit detects the wave, it is necessary to add a synchronous signal at the same frequency and in phase as the amplitude modulation signal.

Envelope detection. There is no need to add a synchronization signal.

2. The scope of application is different.

Synchronous detection circuits can be used to demodulate any amplitude modulation wave, but in order to obtain a synchronous signal at the same frequency and in phase as the amplitude modulated signal, the circuit is more complicated, so it is mainly used to demodulate unilateral sideband and bilateral amplitude modulation signals.

The envelope detection of the hidden wave of the electric touch hall is very simple, but it is only suitable for demodulating ordinary amplitude modulation waves.

The demodulation characteristics of the envelope detector depend on the parameters of the circuit and the characteristics of the input signal, mainly in the following aspects:

1. Demodulation sensitivity: Demodulation sensitivity refers to the ratio of the output voltage of the envelope detector to the input voltage, which reflects the response degree of the envelope detector to the input signal. The higher the demodulation sensitivity, the more accurate the envelope geophone can be to restore the original Xinxiao limb.

2. Demodulation distortion: Demodulation distortion refers to the deviation between the output voltage of the envelope detector and the envelope of the input signal, which reflects the degree of distortion of the envelope detector to the input signal.

3. Demodulation bandwidth: The demodulation bandwidth refers to the frequency range in which the envelope detector can effectively demodulate the input signal, which reflects the frequency response ability of the envelope detector to the input signal.

How the peak detector works:

The peak detector is a circuit that can remember the peak of the signal, and the size of its output voltage always follows the peak of the input signal, and remains at the maximum peak of the input signal.

When VI > VO: the signal is added from the (+) terminal, the output VA of OPA is a positive voltage, and the diode D is turned on, so the output current is charged by D to the capacitor C until it is charged to a voltage equal to VI. (When D conducts electricity, this circuit acts like a voltage follower).

When viWhen < VO: The output VA of the OPA is reverse biased, which is equivalent to an open circuit, so the capacitor C is neither charged nor discharged, and is maintained at the maximum voltage of the input.

Envelope detection is a type of incoherent demodulation.

Envelope detection: The vibration signal processing method based on filter detection has a strong ability to identify initial faults and fault signals with low signal-to-noise ratio.

Incoherent demodulation: A demodulation method that does not require extraction of carrier information (or recovery of coherent carriers).

It is a kind of incoherent demodulation, and other methods such as the zero-crossing detection method of 2FSK signal and the phase comparison method of 2dpsk also belong to incoherent demodulation.

In the former, coherent demodulation is the multiplication of carriers to obtain the original signal, and everything else is incoherent.

That is, the accepted signal is first filtered out of out-of-band noise and spurious signals through a band-pass filter to improve the signal-to-noise ratio of the detector input, and then detected by the Paul detector, and then through a low-pass filter to filter out the high-frequency components to obtain the demodulation signal, and finally by the sampling judgment for timing judgment to obtain a more complete digital demodulation signal.