[Computer Network] The reason why many channels cannot transmit low-frequency and DC components

1 Introduction

        Today, when I was studying teacher Xie Xiren’s network planning, I saw this sentence:
        Baseband signals often contain more low-frequency components and even DC components, and many channels cannot transmit such low-frequency components or DC components . In order to solve this problem, the baseband signal must be modulated.

        As a software engineering student, I don’t understand low frequency and DC very well, but I can’t find the corresponding entries in Baidu Encyclopedia, so I have compiled answers from experts for everyone’s reference. If I make a mistake somewhere, please let me know.

2. Low frequency component

2.1 What is low frequency component

Low frequency (LF, Low frequency) refers to the radio waves in the frequency band from 30 KHz to 300 KHz.
Low frequency components refer to the signal components in the frequency band from 30 KHz to 300 KHz.

2.2 Why the channel cannot transmit "low frequency components"

1. The lower the frequency of the signal, the larger the radius of the device emitting the signal must be.
   Radius of transmitting device >= wavelength/4.
   Wavelength = speed of light/frequency.
   The speed of light is fixed, the smaller the frequency, the larger the wavelength; the larger the wavelength, the larger the radius of the transmitting device.

2. The frequency range of low-frequency signals is relatively small. If they are transmitted all over the world, interference will result in incomplete content being received.

3. DC component

3.1 What is the DC component?

"DC component": DC component refers to the signal component with a frequency of 0 . For example, for a signal, by performing Fourier transform on it, you can obtain its components at each frequency; the component corresponding to zero frequency f=0 (including the amplitude and phase of the signal) is its DC component. From another perspective, the DC component can also be regarded as the time average of the signal.

3.2 Why the channel cannot transmit "DC component"

The DC component (generally a steady current) does not have changes in current (voltage) , so it cannot carry information.

Reference links
https://zhidao.baidu.com/question/143697796.html
https://zhidao.baidu.com/question/125651224.html
https://zhidao.baidu.com/question/159865169.html
https:/ /blog.csdn.net/Dallin0408/article/details/52522776
https://baike.baidu.com/item/%E6%B3%A2%E7%9A%84%E8%A1%8D%E5%B0%84 /9589084?fr=aladdin