The basic components of a digital communication system include a source, source coding, channel coding, modulation, channel, demodulation, channel decoding, and a destination. The source refers to the device or system that generates information, such as a microphone or camera. Source coding is the process of converting the information generated by the source into a digital signal. Channel coding is the process of encoding the digital signal to adapt it for transmission over the channel. Modulation is the process of converting the digital signal into an analog signal suitable for transmission over the channel. The channel is the pathway or medium for transmitting signals, such as a cable or optical fiber. Demodulation is the process of converting the analog signal into a digital signal. Channel decoding is the process of decoding the digital signal to recover the original information. The destination is the device or system that receives the information, such as a speaker or display.
The advantages of digital communication systems include strong anti-interference capabilities, good confidentiality, high transmission efficiency, and high reliability. In digital communication systems, because digital signals only have two states, "0" and "1", they are less susceptible to interference during transmission and can be securely transmitted using encryption technology. Furthermore, digital communication systems offer high transmission efficiency and reliability, enabling long-distance, high-quality transmission.
Digital communication systems are widely used in telephone communications, computer networks, broadcasting, and mobile communications. In telephone communications, digital telephone systems have replaced traditional analog telephone systems, improving call quality and security. In computer networks, digital data communication systems have become one of the infrastructures of the Internet, supporting the development of various network applications. In broadcasting, digital image communication systems and digital video communication systems have become the foundation of new media such as high-definition television and digital television. In mobile communications, digital communication systems have become the foundation of 4G, 5G, and other mobile communication technologies, supporting the development of various mobile applications.
Spurious signals refer to unwanted signals that appear in a communication system besides the normally transmitted signal. These signals may be caused by factors such as nonlinear components in the system, electromagnetic interference, power supply noise, and ground interference. In digital communication systems, the generation of spurious signals can lead to problems such as increased bit error rate and degraded signal quality, thereby affecting the performance of the communication system.
In digital communication systems, the sources of spurious signals mainly include the following aspects:
1. Nonlinear distortion: Nonlinear components in the system, such as amplifiers and mixers, will cause nonlinear distortion to the transmitted signal, resulting in stray signals such as harmonics and intermodulation products in the signal.
2. Cross-modulation: When multiple signals pass through a nonlinear device at the same time, cross-modulation may occur between them, resulting in new frequency components in the signal and forming spurious signals.
3. Intermodulation distortion: In digital communication systems, intermodulation distortion between multiple signals can also generate spurious signals. For example, when two signals pass through a nonlinear device simultaneously, their combined frequency components, such as the sum and difference of their frequencies, may appear, forming spurious signals.
4. Other factors: In addition to the factors mentioned above, the causes of stray signals in digital communication systems may also include electromagnetic interference, power supply noise, ground interference, etc.
To reduce the generation of spurious signals in digital communication systems, the following measures can be taken: selecting devices with good linearity, designing circuit structures rationally, adopting good grounding and shielding measures, and using filters to filter signals. Furthermore, during the system design phase, factors such as signal characteristics, transmission medium features, and device performance should be fully considered to minimize the generation of spurious signals.
