Common linear modulation methods based on MATLAB

Common linear modulation methods

One 、 The principle of analog modulation

in application , In order to meet the radiation efficiency , Realize the multiplexing of channels , Improve the anti-interference ability of the system , To modulate the signal , Convert the original signal into a signal suitable for transmission in the channel .

Analog modulation refers to the use of baseband analog signals from the source to modulate a carrier , Common analog modulation methods include amplitude （ linear ） Modulation and angle （ nonlinear ） modulation , Including amplitude modulation （AM）、 Bilateral belt （DSB）、 Single sideband （SSB） And residual sidebands （VSB） Equal modulation mode belongs to amplitude modulation .

Amplitude modulation is to control the amplitude of high frequency carrier wave by modulated signal , The process of making it change linearly with the modulated signal . The carrier wave is an exact periodic waveform , It can be expressed in the following form

among A Is the carrier amplitude ,${\omega }_c$ Is the carrier angular frequency ,${\omega }_0$ Is the initial phase of the carrier .

The model of the modulator can be shown in the figure below .

It can be seen from the formula that , The waveform amplitude of the modulated signal changes in direct proportion to the baseband signal , After the multiplier , The output signal is an amplitude and m(t) Proportional cosine wave , The amplitude of the carrier waveform is modulated . Its modulated signal spectrum and modulated signal spectrum are linear , However, the modulated signal and the modulated signal do not directly conform to the linear relationship .

Two 、 Common modulation methods

2.1 amplitude modulation （AM）

Standard amplitude modulation is the most common double sideband modulation , The modulation model is shown in the following figure

2.2 Suppress carrier double sideband signals （DSB-SC）

AM Although the signal modulation is simple , But the power utilization is low , To solve this problem , DC can be $A_0$ lose , Get a more efficient modulation , That is to suppress the carrier double sideband signal （DSB-SC）, The modulation process can be abstracted as the following model

Since there is no carrier component , The modulation efficiency is 100%.

3、 ... and 、 Modulation simulation

utilize Octave Do simulation experiments , More intuitive understanding of the changes before and after signal modulation .

Baseband signal is

The waveform in time domain is as follows

The waveform diagram in frequency domain is as follows

Four 、 summary

Through the learning of analog adjustment system , Know the different classification standards of modulated signals , Master common signal modulation methods and demodulation methods , We know the transformation process from time domain to frequency domain and the reason why it is linear , Understand the advantages and disadvantages of different signal modulation methods .