📚 Communications Systems in A-Level Physics | A-Level物理中的通信系统
Communication systems form the backbone of the modern world, enabling the transfer of information over vast distances. In CIE A-Level Physics, this topic explores how signals are transmitted, the challenges posed by noise and attenuation, and the fundamental principles behind technologies like radio, television, and the internet. Understanding modulation, bandwidth, and digital sampling is essential for mastering this section.
通信系统构成了现代世界的骨干，使得信息能够在广阔的距离之间传输。在 CIE A-Level 物理中，本主题探讨信号如何传输、噪声和衰减带来的挑战，以及广播、电视和互联网等技术背后的基本原理。理解调制、带宽和数字采样对于掌握这一部分至关重要。
1. The Basic Model of a Communication System | 通信系统的基本模型
Every communication system can be broken down into three essential components: the transmitter, the channel (or transmission medium), and the receiver. The transmitter converts a message into a signal suitable for transmission, the channel carries the signal, and the receiver processes the signal to reconstruct the original message.
每个通信系统都可以分解为三个基本组成部分：发射器、信道（或传输介质）和接收器。发射器将信息转换为适合传输的信号，信道承载信号，接收器处理信号以重建原始信息。
In CIE examinations, you often need to identify these blocks in specific contexts, such as radio broadcasting, where a microphone (transducer) feeds an audio signal to a modulator (transmitter), which sends electromagnetic waves through the atmosphere (channel) to an antenna and demodulator (receiver).
在 CIE 考试中，你通常需要识别特定情境下的这些模块，例如无线电广播，麦克风（换能器）将音频信号馈送到调制器（发射器），后者通过大气（信道）将电磁波发送到天线和解调器（接收器）。
2. Carrier Waves and Modulation | 载波与调制
In long-distance communication, raw audio signals (typically 20 Hz to 20 kHz) cannot travel far without significant attenuation because their frequencies are too low for efficient radiation. To solve this, a high-frequency electromagnetic wave, known as the carrier wave, is used. The process of encoding the information signal onto the carrier wave is called modulation.
在远距离通信中，原始音频信号（通常为 20 赫兹至 20 千赫）频率太低，无法有效辐射，因此没有显著衰减则无法远距离传输。为了解决这个问题，需要使用一种高频电磁波，称为载波。将信息信号加载到载波上的过程称为调制。
kHz audio signal directly, the wavelength λ = c/f is 20 km, requiring a 5 km tall antenna! using a 1 MHz carrier, the wavelength reduces to 300 m, making the antenna size practical.
此外，所需发射天线的长度约为 λ/4。如果我们尝试直接传输 15 千赫的音频信号，波长 λ = c/f 为 20 公里，就需要一个 5 公里高的天线！通过使用 1 兆赫的载波，波长减小到 300 米，使得天线尺寸变得实用。
3. Amplitude Modulation (AM) | 调幅 (AM)
Amplitude Modulation (AM) involves varying the amplitude of the high-frequency carrier wave in accordance with the instantaneous amplitude of the information signal. The frequency and phase of the carrier remain constant.
调幅 (AM) 涉及根据信息信号的瞬时幅度来改变高频载波的幅度。载波的频率和相位保持不变。
When a single-tone modulating signal is used, the modulated wave produces sidebands. The system effectively shifts the baseband signal to a higher frequency. The bandwidth of an AM wave is twice the maximum modulating frequency:
当使用单音调制信号时，已调波会产生边带。系统有效地将基带信号上变换到更高的频率。调幅波的带宽是最大调制频率的两倍：
Bandwidth = 2fm
The sidebands are the upper sideband (fc + fm) and the lower sideband (fc - fm). The carrier frequency component (fc) transmits no unique information but consumes a significant amount of power.
边带是上边带 (fc + fm) 和下边带 (fc - fm)。载波频率分量 (fc) 不传输特有的信息，但消耗了大量功率。
4. Power and Efficiency in AM | 调幅的功率与效率
In AM, the total transmitted power depends on the modulation index, m. A higher modulation index means more power is allocated to the information-carrying sidebands. The total power relation is given by:
在调幅中，总传输功率取决于调制指数 m。较高的调制指数意味着更多的功率被分配给携带信息的边带。总功率关系由下式给出：
Ptotal = Pc (1 + m²/2)
For 100% the sidebands, and since information is identical in both sidebands, only one-sixth (≈16.7% que information. This low power efficiency is a major disadvantage of standard AM systems.
对于 100% 调制 (m = 1)，只有总功率的三分之一位于边带中，并且由于两个边带中的信息相同，只有总功率的六分之一（≈16.7%）用于承载特有的信息。这种低功率效率是标准调幅系统的一个主要缺点。
5. Frequency Modulation (FM) | 调频 (FM)
Frequency Modulation (FM) varies the instantaneous frequency of the carrier in accordance with the amplitude of the information signal, while keeping the carrier amplitude constant. The amount by which the frequency changes is called the frequency deviation (Δf).
调频 (FM) 根据信息信号的幅度改变载波的瞬时频率，同时保持载波幅度恒定。频率变化的大小称为频偏 (Δf)。
Unlike AM, the amplitude of an FM signal remains constant. Therefore, non-linear amplitude distortions caused by the channel or the receiver's amplifiers do not affect the quality of the recovered information. The bandwidth requirements for FM are approximated by Carson's Rule:
与调幅不同，调频信号的幅度保持恒定。因此，由信道或接收器放大器引起的非线性幅度失真不会影响恢复信息的质量。调频的带宽需求可近似通过卡森准则计算：
Bandwidth ≈ 2(Δf + fm)
This larger bandwidth requirement is the primary trade-off for the improved signal quality provided by FM over AM.
这种更大的带宽需求是调频相对于调幅提供更好信号质量的主要代价。
6. AM vs FM: Noise and Bandwidth Comparison | AM 与 FM 的比较：噪声与带宽
AM is highly susceptible to noise. Most natural and man-made electrical noise affects the amplitude of a signal (e.g., lightning interference). Since information in AM is encoded in amplitude variations, these disturbances become part of the audio output.
调幅极易受到噪声的影响。大多数自然和人为的电气噪声会影响信号的幅度（例如雷电干扰）。由于调幅中的信息被编码在幅度变化中，这些扰动就成为音频输出的一部分。
FM, on the other hand, encodes information in the carrier frequency. Amplitude spikes can be clipped off by limiters in the FM receiver without losing the information. This property, known as the capture effect, makes FM highly superior for high-fidelity music broadcasting. The comparison can be summarised as follows:
另一方面，调频将信息编码在载波频率中。幅度尖峰可以通过调频接收器中的限幅器切除而不会丢失信息。这种特性被称为捕获效应，使得调频在高保真音乐广播中具有极高的优越性。对比如下：
| Feature (特性) | AM (调幅) | FM (调频) |
|---|---|---|
| Bandwidth | Narrow (2fm) | Wide (2(Δf + fm)) |
| Susceptibility to Noise | High | Low |
| Transmitting Power | Moderately efficient | High (constant amplitude) |
| Range | Long (sky-wave propagation) | Line-of-sight typically |
7. Signal Attenuation and Channel Effects | 信号衰减与信道效应
As a signal travels through a medium, its power decreases. This reduction is known as attenuation. In practical systems, attenuation is compensated for by using repeaters or amplifiers spaced along the channel. Excessive attenuation can cause the signal power to drop below the noise floor, making recovery impossible.
当信号在介质中传播时，其功率会下降。这种减小被称为衰减。在实际系统中，通过沿信道设置中继器或放大器来补偿衰减。过度衰减会导致信号功率下降到噪声底限以下，使得信号无法恢复。
The attenuation is often measured in decibels (dB). If the transmitted power is Pin and the received power is Pout, the attenuation is:
衰减通常以分贝 (dB) 为单位。如果发射功率为 Pin，接收功率为 Pout，则衰减为：
Attenuation (dB) = 10 log10 (Pin / Pout)
A common example in CIE problems involves coaxial cables, where a specific attenuation per meter is given, and students must calculate the maximum distance before a repeater is needed to maintain the SNR above a certain threshold.
在 CIE 问题中，一个常见的例子是同轴电缆，题目会给出每米特定的衰减值，学生必须计算在所需中继器之前能够传输的最大距离，以便将信噪比保持在某个阈值以上。
8. Signal-to-Noise Ratio (SNR) | 信噪比 (SNR)
The Signal-to-Noise Ratio (SNR) is a critical measure defining the clarity of a received signal. It is defined as the ratio of the signal power to the noise power at a given point in the system. A high SNR indicates that the signal is much stronger than the background noise.
信噪比 (SNR) 是定义接收信号清晰度的关键指标。它定义为系统某一点上信号功率与噪声功率的比值。高信噪比表明信号强度远高于背景噪声。
The ratio is usually expressed in decibels because it can cover a vast dynamic range:
由于信噪比可以覆盖很大的动态范围，因此通常用分贝表示：
SNR (dB) = 10 log10 (Ps / Pn)
If operating with voltages, the formula becomes 20 log10 (Vs / Vn). For high-quality voice communication, an SNR of at least 30 dB is generally required, while high-quality music reproduction demands 60 dB or more.
如果电压工作，公式变为 20 log10 (Vs / Vn)。对于高质量的语音通信，通常需要至少 30 分贝的信噪比，而高质量的音乐还原则需要 60 分贝或更高。
9. Analogue to Digital Conversion (ADC) | 模数转换 (ADC)
Analogue signals vary continuously in time and amplitude. In modern communication and computing, these signals are converted to digital binary data (0s and 1s). The conversion involves two key processes: sampling and quantisation.
模拟信号在时间和幅度上连续变化。在现代通信和计算中，这些信号被转换为数字二进制数据（0 和 1）。转换涉及两个关键过程：采样和量化。
Sampling involves taking the amplitude of the analogue signal at regular time intervals. To faithfully regenerate the original signal, the sampling frequency (fs) must satisfy the Nyquist criterion, which states that the sampling rate must be at least twice the highest frequency component of the analogue signal.
采样涉及以固定的时间间隔获取模拟信号的幅度。为了忠实地还原原始信号，采样频率 (fs) 必须满足奈奎斯特准则，即采样率必须至少是模拟信号最高频率分量的两倍。
fs ≥ 2fmax
Quantisation involves assigning the sampled amplitude to the nearest discrete level. The difference between the true analogue value and the quantised value introduces a small error known as quantisation noise or error.
量化涉及将采样幅度分配到最近的离散电平。真实模拟值与量化值之间的差异引入了一个小型误差，称为量化噪声或误差。
10. Digital Communication: Advantages and Regeneration | 数字通信的优势与再生
Digital data transmission offers profound advantages over analogue systems. The primary advantage is regeneration. Unlike an analogue signal, where amplification amplifies both the signal and accumulated noise, a digital signal in a system with regenerative repeaters can be perfectly recreated at each station.
数字数据传输相对于模拟系统具有深远的优势。主要优势是再生。与模拟信号不同，模拟信号的放大过程会同时放大信号和累积的噪声，而带有再生中继器的数字信号可以在每个站点被完美重建。
Only a '1' or '0' needs to be detected. Even if noise is present, as long as the decision threshold is not crossed, the clean binary sequence can be reconstructed and retransmitted. This ensures high fidelity over global distances. Additionally, digital signals allow for error detection and correction coding.
只需要检测到 '1' 或 '0'。即使存在噪声，只要不超过判决阈值，就可以重建干净的二进制序列并重新传输。这确保了全球远距离的高保真度。此外，数字信号可以进行误码检测和纠错编码。
Other benefits include the ability to blend different types of information (data, voice, video) into a single bit stream, and the use of advanced compression algorithms to reduce bandwidth requirements.
其他好处包括能够将不同类型的信息（数据、语音、视频）融合到单一比特流中，以及使用先进的压缩算法来降低带宽需求。
11. Multiplexing: FDM and TDM | 多路复用：频分复用与时分复用
Multiplexing is the technique of combining multiple independent signals into one composite signal for transmission over a common channel. This maximises the efficient use of expensive physical resources like cables or satellite links.
多路复用是将多个独立信号合成为一个复合信号在公共信道上传输的技术。这有助于最大化利用昂贵的物理资源，如电缆或卫星链路。
Frequency Division Multiplexing (FDM) divides the available channel bandwidth into smaller, non-overlapping frequency bands, each assigned to a specific user or information channel. A guard band is left between slots to prevent interference. Analogue radio and television broadcasting rely on FDM.
频分复用 (FDM) 将可用的信道带宽划分为更小的、不重叠的频率段，每个频率段分配给特定的用户或信息信道。各频段之间留有保护频带以防止干扰。模拟广播和电视依赖于频分复用。
Time Division Multiplexing (TDM) allocates the entire channel bandwidth to each user for a short, specific time slot. The process repeats cyclically. Digital systems primarily use TDM because it interfaces seamlessly with the output of ADC circuits. Modern telephone networks integrate TDM structures.
时分复用 (TDM) 在短暂且特定的时隙内，将整个信道带宽分配给每个用户。此过程循环重复。数字系统主要使用时分复用，因为它能够与模数转换电路的输出无缝对接。现代电话网络集成了时分复用结构。
12. The Decibel System and Power Gain in Communications | 通信中的分贝系统与功率增益
In communication systems, power levels can vary by several orders of magnitude. The decibel (dB) is a logarithmic unit used to express the ratio between two power values. This logarithmic scale allows engineers to handle massive differences in magnitude simply using addition and subtraction.
在通信系统中，功率电平可能相差几个数量级。分贝 (dB) 是一种对数单位，用于表示两个功率值之间的比值。这种对数尺度允许工程师通过简单的加减法来处理巨大的数量级差异。
If the output power is greater than the input power, the device provides a gain. If it is less, it introduces a loss (or negative gain). The formulas are consistently applied:
如果输出功率大于输入功率，则设备提供增益。如果小于，则引入损耗（或负增益）。公式统一适用：
Gain (dB) = 10 log10 (Pout / Pin)
A reference level often used in CIE questions is dBm, which is the power ratio relative to a 1 mW standard. This helps in calculating absolute power in communication links. For example, if a received signal is -30 dBm, the power is 10⁻³ times 1 mW (or 1 μW).
CIE 问题中常用的一个参考电平是 dBm，即相对于 1 毫瓦标准的功率比。这有助于计算通信链路中的绝对功率。例如，如果接收信号为 -30 dBm，则功率为 10⁻³ 乘以 1 毫瓦（即 1 微瓦）。
Published by TutorHao | Physics Revision Series | aleveler.com
