If signal modulation was like this back then, would you still not be able to learn it?

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If signal modulation was like this back then, would you still not be able to learn it? [Copy link]

In previous articles, we have talked about the beauty of electromagnetic waves many times, but can we communicate with electromagnetic waves? No, we need to load information onto electromagnetic waves, and then the electromagnetic waves can work as a carrier of information. How are signals loaded onto electromagnetic waves? Today we will introduce several basic signal debugging methods. Although these modulation methods have many shortcomings in the current communication standards, as the basis for getting started with signals, we might as well spend some time reviewing them.

AM：Amplitude Modulation, as the name implies, this modulation method changes the amplitude or strength of the signal. Amplitude modulation was the first type of modulation used for broadcast sound. Today, other forms of modulation are being used more and more, but amplitude modulation is still widely used.

FM : Frequency modulation , in which the frequency of the signal is changed. The advantage of frequency modulation is that the amplitude noise on the signal can be limited, since only the frequency changes carry the desired information. This is achieved by passing the signal through a stage that goes into limiting, thereby eliminating amplitude variations that may be the result of noise and general signal variations. If there is enough signal to drive a stage into limiting, any signal strength variations will not change the level of the demodulated audio, assuming that audio is being transmitted. For this reason, this form of modulation has been used in many applications, including high-quality analog sound broadcasting.

PM：Phase modulation , Phase modulation changes the phase of the carrier according to the modulating signal. Phase modulation and frequency modulation have many similarities and are related to each other - one is the difference of the other. However, phase modulation is suitable for data transmission, so its use has grown rapidly in recent years.

Each modulation method has its own advantages and disadvantages, and therefore they are all used in different radio communication applications.

In addition to the three main basic forms of modulation or modulation techniques, there are many variations of each type. Again, these modulation techniques are used in various applications, some for analog applications and others for digital applications.

Angle modulation

Angle modulation is the name given to a form of modulation that is based on changing the angle or phase of a sinusoidal carrier wave. With angle modulation, there is no change in the amplitude of the carrier wave. The two forms of modulation that fall under the category of angle modulation are frequency modulation and phase modulation.

The two types of angle modulation, frequency modulation and phase modulation, are related because frequency is the derivative of phase, i.e., frequency is the rate of change of phase.

Another way to look at the connection between these two types of modulation is that a frequency modulated signal can be generated by first integrating the modulating waveform and then using the result as the input to a phase modulator. Conversely, a phase modulated signal can be generated by first differentiating the modulating signal and then using the result as the input to a frequency modulator.

Modulation combination

Modulation forms combining amplitude and angle modulation components may be used. In this way performance enhancements may be obtained.

• Quadrature Amplitude Modulation, QAM: Using this form of information both amplitude and phase information are used to carry the signal. The data is modulated onto the in-phase and quadrature elements of the signal: I & Q and the constellation forms multiple points on two planes.

• Amplitude and Phase Filtered Keying, APSK: With APSK, the constellation can be arranged to optimize the peak-to-average power ratio and fewer amplitude levels can be set compared to QAM. This enables the RF power amplifier to operate more efficiently.

Signal bandwidth

A key element of any signal is the bandwidth it occupies. This is important because it defines the required channel bandwidth and therefore the number of channels that can be accommodated within a given segment of radio spectrum. With increasing pressure on the radio spectrum, radio signal bandwidth is an important characteristic of any type of radio emission or transmission.

Bandwidth is controlled by two main characteristics:

• Modulation Types Some forms of modulation use their bandwidth more efficiently than others. Therefore, where spectrum usage is important, this alone may dictate the choice of modulation.

• Bandwidth of the modulating signal: A law called Shannon's law determines the minimum bandwidth a signal can be transmitted in. Generally speaking, the wider the bandwidth of the modulating signal, the wider the bandwidth required.

Modulation signal type

When choosing the type of modulation to use, it is necessary to look at the pros and cons of each type of modulation. AM and FM are widely used for analog sound transmission, while phase shift keying and quadrature amplitude modulation are commonly used to transmit digital data.

Let's put amplitude modulation and frequency modulation together and observe the difference between them.

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