调频调幅调相
调频 调幅 调相(FM PM)
调频调幅调相（FM PM）Amplitude modulation: a modulated signal that changes the amplitude of a carrier; FM: changes in frequency or phase. Send FM - receive - am: should be received under certain conditions, but the detection efficiency is not necessarily high. For example, the receiver (amplitude modulation) loop can receive the FM signal at a low efficiency when it is in the state of slope detection (referring to radio data).FM and phase modulation are different, phase modulation at the same time, the frequency will certainly change, but the frequency modulation phase does not necessarily change.++++++++++++++++++++++++++++++++What is the difference between amplitude and frequency modulation?1. FM, am, anti-interference abilityForeign interference, processing and atmospheric interference, influence of modulated wave mainly produce parasitic amplitude modulation, noise. FM system can use amplitude limiting method to eliminate parasitic modulation caused by interference. In amplitude modulation system, the amplitude of the amplitude modulated signal is varied, so that the amplitude limit can not be used, and it is difficult to eliminate the external interference.In addition, the greater the signal to noise ratio, the stronger the anti-interference ability. The signal-to-noise ratio ofthe demodulated signal is related to the modulation coefficient. The greater the modulation coefficient, the greater the signal-to-noise ratio. Because the FM coefficients are much larger than the amplitude modulation coefficients, the FM wave has a high signal-to-noise ratio, and the interference noise in FM broadcasting is small.2. FM Bobbi amplitude modulation wave frequency bandwidthThe band width is related to the modulation factor, that is, the modulation coefficient is large and the frequency band is wide. The frequency modulation coefficient is often greater than 1, while the amplitude modulation coefficient is less than 1. Therefore, the band width of FM wave is much larger than that of amplitude modulated wave.3. frequency modulation power utilization is greater than amplitude modulation systemIn the total power delivered, the power of the sideband frequency is the effective power of the transmitted modulation signal, while the side frequency power is related to the modulation coefficient, the modulation coefficient is large, and the side frequency power is large. Because the frequency modulation coefficient MF is greater than the amplitude modulation coefficient Ma, the power utilization ratio of the FM system is higher than the amplitude modulation system.++++++++++++++++++++++++++++++The difference between FM and am like mobile phone GSM and CDMA,is a different transmission mode, CDMA technology should I do not know how much advanced than the GSM, but not more than 133 of the mobile phone signal of mobile phone signal 139 strong, but not as good as. Why is the same 139 mobile phone, some manufacturers strong signal, and some manufacturers weak signal? That is to say, a good or bad product is not determined by the mode of transmission, but by the technical ability of the manufacturer and the degree of product completion.So, is there any difference between FM and am in wireless transmission? No FM is characterized by narrow bandwidth and long distance. Narrow bandwidth means that the penetration capability of the obstruction is weak, but the transmission distance is long. This technology is generally used in mobile phones, pagers and other needs of long distance (5 kilometers or more) transmission products. Readers should know that Mobile Corporation's signal towers are far removed from each person's mobile phones. Amplitude modulation is characterized by wide bandwidth and short distance. Wide bandwidth means that the penetration of the barrier is strong, but the transmission distance is shorter. This technology is generally used in wireless alarm, wireless security and other fields. For in a house the most important thing is not the distance, but the penetrating power.So, FM technology is advanced,Is the technique of amplitude modulation behind? No The Federal Communications Commission (FM) introduced FM and am technology since the last century and has been used in all our living areas. FM and am are just different transmission methods for a wirelesstechnician, and there is no technical difficulty.Is there any difference between FM and am? Neither！ There are many differences, one of which is the cost. Because the frequency modulation line is more complex, needs the component quantity to be many, therefore the frequency modulation product's price must compare the banner product the price to be much higher. Why do most domestic wireless call system manufacturers adopt amplitude modulation mode? First of all, the transmission in the building does not need to use high cost FM to do, as long as the scroll products to improve the degree of completion, to fully solve the transmission distance and coverage area. As an agent, the same product shape, the same distance effect, different price, which one would you choose? In fact, plainly, customers do not need to know the frequency modulation or amplitude modulation, inexpensive, economical and practical products is the truth.+++++++++++++++++++++Widely used in military, communications, radio, television, radio and other fieldsWhat are frequency modulation (FM), amplitude modulation (AM), short wave (SW) and long wave (LW)?Both AM and FM band in general radio or radiocassette, I believe we are already familiar with, the two band is used for you to listen to the radio broadcast home, if there SW band, so in addition to domestic shortwave radio, you can listen to the world broadcast radio program. In order for you to have a betterunderstanding of the use of radio, what follows is a brief description of what AM, FM, SW, and LW are.In fact, "AM" and "FM" refer to two different modulations in wireless electrical engineering. The AM: Amplitude Modulation is called amplitude modulation, while the FM: Frequency Modulation is called fm. Just MW: Medium Wave (general radio broadcasting), using the amplitude modulation (AM) way, unconsciously, between MW and AM on an equal sign. In fact, MW is just a lot of using a broadcast AM modulation, as in a high frequency (3-30MHz) modulation methods used in the International Shortwave Radio is AM, even higher frequency than the FM radio navigation communication (116-136MHz) is AM, AM band is our daily said that Poland broadcast (MW).What about FM? It is similar to the fate of MW. We used FM to refer to the general FM radio (76-108MHz, in China for87.5-108MHz, Japan 76-90MHz), the fact that FM is a kind of modulation, even in the short wave range 27-30MHz, as amateur radio, satellite communications, space applications band, but also the use of frequency modulation (FM) mode the.And what about SW? In fact, can be said to be short wave of a simple name, the correct statement should be high frequency (HF:High Frequency) more appropriate. And how does the name "shortwave" come from? At wavelengths, the medium wave (MW) is between 200-600 meters (meters), while the wavelength of HF is between 10~100 meters (meters). Compared with the wave appearance, the wavelength of HF is really shorter,因此就把hf称做短波 (sw: short wave).同样的, 比中波mw更低频率的150khz - 284khz之间的这一段频谱也是作为广播用的, 以波长而言, 它大约在1000～2000米 (公尺) 之间, 和mw的200 - 600米相比较显然 "长" 多了, 因此就把这段频谱的广播称做长波 (lw: long wave).实际上, 不论长波 (lw) 、中波 (mw) 或者是短波 (sw) 都是采用am调制方式.对一般收 (录) 音机而言, fm、mw、lw波段是提供您收听国内广播用的, 但我国目前没有设立lw电台, 而sw波段则主要供您收听国内/ 国际远距离广播..短波知识百年前, 三声短促而且微弱的讯号, 向世界宣布了无线电的诞生.一九? 一年, 扎营守候在讯号山 (signal hill位于加拿大东南角) 的意大利科学家马可尼, 终于接收到了从英格兰发出的跨过大西洋的无线电讯号, 这个实验向世人证明了无线电再也不是仅限于实验室的新奇东西, 而是一种实用的通讯媒介.此后短波用作全球性的国际通讯媒介便开始发达起来了.虽然马可尼的试验结果令人相当振奋, 可是当时一般人认为无线电传播方式类似光波, 发射之后, 绝对沿直线方向进行传播, 从英国到加拿大, 再怎么说也无法完成直线的无线电通讯 (因为地球表面是弧形的).当时的科学理论更证明, 从英国发射后的无线电波一定直驱太空, 怎么可能到达加拿大? 可是从马可尼用简陋的无线电设备征服长距离通讯的试验记录来看, 白天, 讯号可以远达700英哩, 晚间更远达2000英哩以上, 这些试验数据, 使得以往的理论所推断出来的必然结果, 开始发生动摇了.+ + + + + + + + + + + + + + + + + + + +使载波振幅按照调制信号改变的调制方式叫调幅.经过调幅的电波叫调幅波.它保持着高频载波的频率特性, 但包络线的形状则和信号波形相似.调幅波的振幅大小, 由调制信号的强度决定.调幅波用英文字母am表示.使载波频率按照调制信号改变的调制方式叫调频.已调波频率变化的大小由调制信号的大小决定, 变化的周期由调制信号的频率决定.已调波的振幅保持不变.调频波的波形, 就像是个被压缩得不均匀的弹簧, 调频波用英文字母fm表示.目前, 中波广播使用的频段大致为550khz - 1600khz, 主要靠地波传播, 也伴有部分天波; 调频制无线电广播多用超短波 (甚高频)无线电波传送信号, 使用频率约为88mhz - 108mhz, 主要靠空间波传送信号.使载波振幅按照调制信号改变的调制方式叫调幅.经过调幅的电波叫调幅波= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =调制又分为三种: 调幅, 调频和调相.The amplitude modulation is represented by AM, the FM is represented by FM, and the phase modulation is expressed by PM. This is where we can find the AM.FM sign when we listen to the broadcast.Its basic principle is that the modulated signal to betransmitted (here we use the voice signal as an example) from low frequency to high frequency shifting, so that it can be transmitted through the ionosphere reflection in the remote receiver we use proper demodulation device then restore the original signal without distortion, to transmit voice frequency signal the purpose of.For example, am, we cannot directly transfer the voice, we first use a voice signal conversion device (or say) is converted into voltage signal amplitude changes smoothly, this is our signal to be transmitted, called modulation signal, and then the modulation signal and a high frequency signal in a multiplier, after an adder circuit, get a signal of high frequency, its envelope (the so-called envelope is connected with periodic signal in each cycle peak of the imaginary line) varies with the amplitude of modulation signal, we put the high frequency signal is called the carrier, the signal has been modulated called amplitude modulation wave.That is to say, the voice signals we are transmitting are already included in the AM wave, in other words, we move the modulation signal from low frequency to high frequency to facilitate ionospheric propagation. So we will adjust the signal transmitting device through the launch at the receiver after receiving the signal, the demodulation device to recover the original signal after conversion device voltage signal recovery in adult Mandarin, is implemented between two of two people to call, which is the basic principle of shortwave radio.We look at FM, with the above knowledge base, we can understand the principle of FM, FM, is the frequency of the carrier withthe voice signal (amplitude modulated signal) changes the voice signal amplitude, the frequency of the carrier is increased, the voice signal amplitude is small, the frequency of the carrier is reduced accordingly note that, here is a change in the frequency but not amplitude, this is the difference between frequency and amplitude, we modulated, had a frequency change with the change of the modulation signal modulated signal, we call "FM signal".==========================================================================Generally speaking, FM refers to 64---108MHz, that is, ultrashort wave, suitable for city high fidelity short distance broadcast, generally within 100 kilometersGenerally speaking, FM refers to 64---108MHz, that is, ultrashort wave, suitable for city high fidelity short distance broadcast, generally within 100 kilometersAmplitude modulation is medium wave, the range is 503---1060KHz, and the distance is far away, which is greatly affected by weather factors. It is suitable for the broadcast of inter provincial radio station.Amplitude modulation is medium wave, the range is 503---1060KHz, and the distance is far away, which is greatly affected by weather factors. It is suitable for the broadcast of inter provincial radio station.。
调频与调相
1
Nout 4 2 A2
fm fm
(2
f
)2
N0df
2 3A2
N0
f
3 m
解调后，输出信噪比为
Sout
Nout
KF2M E m
t
2 3A2 2N0 fm3
3A2 KF2M
2N0
f
3 m
E m
t
2
因为
FM
K FM
| m(t) |max fm
Sout
Nout
3
2 FM
E m
no(t) 可以看成ns(t)经过微分器，而 ns (t) V (t)sin( ' (t))是一个均值为0，
功率为N0BFM的低通型窄带噪声，其带宽范围
BFM 2
, BFM 2
。
微分器的传输响应函数为
H () 1 j 2A
所以，经过微分后噪声的功率谱密度为
1
4 2 A2
(2
f
)2
N0
经过低通[-fm,fm]后，噪声概率为
2 FM
Si n0 fm
m(t) ，0
A2 m(t)2，
2
A | m(t) |max
改善门限效应的方法
加重和去加重 锁相环解调* 负反馈解调等*
加重和去加重
输出噪声呈抛物线形式
经过鉴频器后，噪声的功率谱密度变为抛物线）型， 即在信号的低频处，噪声的功率谱密度小，而在信号 的高频处，信号的功率谱密度大。由于一般信号在高 频分量处，信号的功率本身就小，因此高频分量处的 信噪比就较差。这实际上影响着调频的输出信噪比。
m t'
t m
d
则对m(t)’调相等价于对m(t)调频。
信号处理与测试技术习题及答案
第一章习题：一、填空题1、电量分为和，如电流、电压、电场强度和电功率属于；而描述电路和波形的参数，如电阻、电容、电感、频率、相位则属于。
2、传感器输出的经过加工处理后，才能进—步输送到记录装置和分析仪器中。
3、现代科学认为，、、是物质世界的三大支柱。
4、与三大支柱相对应，现代科技形成了三大基本技术，即、、。
5、传感技术是人的的扩展和延伸；通信技术是人的的扩展和延伸；计算机技术是人的的延伸。
6、、、技术构成了信息技术的核心。
二、简答题1、举例说明信号测试系统的组成结构和系统框图。
2、举例说明传感技术与信息技术的关系。
3、分析计算机技术的发展对传感测控技术发展的作用。
4、分析说明信号检测与信号处理的相互关系。
三、参考答案（－）填空题1、电能量、电参量、电能量、电参量2、电信号、信号调理电路3、物质、能量、信息4、新材料技术、新能源技术和信息技术5、感官（视觉、触觉）功能、信息传输系统(神经系统)、信息处理器官(大脑)功能6、传感、通信和计算机第二章习题：一、填空题1、确定性信号可分为和两类。
2、信号的有效值又称为，它反映信号的。
3、概率密度函数是在域，相关函数是在域，功率谱密度是在域上描述随机信号。
4、周期信号在时域上可用、和参数来描述。
5、自相关函数和互相关函数图形的主要区别是。
6、因为正弦信号的自相关函数是同频率的，因此在随机噪声中含有时，则其自相关函数中也必然含有，这是利用自相关函数检测随机噪声中含有的根据。
7、周期信号的频谱具有以下三个特点：_________、________、_________。
8、描述周期信号的数学工具是__________；描述非周期信号的数学工具是________。
9、同频的正弦信号和余弦信号,其相互相关函数是的。
10、信号经典分析方法是和。
11、均值E[x(t)]表示集合平均值或数学期望，反映了信号变化的，均方值反映信号的。
12、奇函数的傅立叶级数是，偶函数的傅立叶级数是。
第三章4-2调频
P
Vc2m 2
J n2 (m f
n
)
第一类贝塞尔函数的特性是
J
2 n
(m
f
)
1
n
所以调频波的功率为
P Vc2m 2
②从时域角度看 调频波是一个等幅波，其幅度与调制前一样，
P Vc2m 2
调频波的功率等于调制前载波的功率 与从频谱的角度计算的功率值相同
对调频波功率的理解：
调频波比调制前增加了那么多边频，为什么功率不变？
调频波的富里叶展开式为 ：
e jm f sin t
J n (m f )e jnt
n
J n (m f
)
1
2
e jm f
sin t
e jnt dt
v(t)
Vm Re
nJ n (m f
)e
j (ct nt )
Vm J n (m f ) cos(c n)t
n
分析调频波的频谱
v(t) Vm Jn (mf ) cos(c n)t n
A
.
以载频ω 为中心，有无数对边频分量 C
：
ωC，ωC±Ω，ωC±２Ω，……ωC±nΩ（n为正整数）
调制前
调制后
c
频谱的非线性搬移——与调幅不同
B. 调频波的每条谱线的幅度为 J n (m f )Vm
J n (m f ) ——宗数为 m f 的n阶第一类贝塞尔函数
J
n
(m
f
)
Jn (mf Jn (m
调相波 (t) ct k pv (t)
v(t) Vcm cos (t) Vcm cos(ct k pVm cos t)
调相指数 mp m k pVm
调频调相及其解调
展望
未来通信系统对信号传输速率和抗干扰能力的 要求越来越高，因此需要研究更加高效和可靠
的调制解调技术。
在未来，调频调相技术的研究将更加注重节能减排和 环保，以适应绿色通信的发展趋势。
随着通信技术的发展，调频调相技术将不断进 步和完善，进一步提高通信质量和可靠性。
随着物联网、智能家居等新兴领域的发展，无线 通信需求将不断增加，调频调相技术将在这些领 域得到更广泛的应用。
通过调制技术，可以将多个低频信号 调制到同一个载波频率上，从而实现 多路复用，提高通信系统的效率。
02
调频调相的基本原理
调频原理
01
02
03
调频信号的生成
通过改变振荡器的输入信 号的幅度或相位，从而改 变振荡器的频率，产生调 频信号。
调频信号的解调
通过滤波器或匹配滤波器 将调频信号还原为原始信 号。
在宽带通信中，调频调相技术可以用于高速数据 传输，提高通信速率和数据吞吐量。
雷达领域的应用
距离测量
调频调相技术可以用于雷达中，通过测量信号的往返时间来计算 目标距离。
速度测量
雷达通过多普勒效应可以测量目标的相对速度，调频调相技术可 以提高测速的精度和分辨率。
目标识别
调频调相技术可以提高雷达的目标识别能力，通过对回波信号的 分析和处理，实现对目标类型的识别和分类。
调频调相及其解调
• 引言 • 调频调相的基本原理 • 调频的实现方法 • 调相的实现方法 • 解调技术 • 调频调相的应用场景 • 总结与展望
01
引言
背景介绍
调频调相技术是通信领域中的 重要技术之一，广泛应用于广 播、电视、无线通信等领域。
调频调相技术能够实现信号的 调制和解调，从而实现对信号 的传输和接收。
调频与调相
t
2
| m(t) |max
A2 / 2 N0 fm
G
So / No Si / Ni
3
2 FM
E
m
t 2
BFM
|
m(t
)
|2
max
fm
6F2M (FM
1)
E
m
t
2
|
m(t
)
|2
max
单频情况
|
m(t)2 m(t) |2max
1/ 2
讨论
当 FM
1 时，我们可以得到G
3
3 FM
，
所以调频方式具有很好的抗噪声性能。
SFM
(t)
A cos(c t
KFM Am fm
sin mt)
A cos(ct FM sin mt)
这里
FM
KFM Am fm
称为调频指数（最大频偏/信号最高频 率）。
单频调频信号波形
SFM (t) Acosct cos( FM sinmt) Asinct sin( FM sinmt)
1
Nout 4 2 A2
fm fm
(2
f
)2
N0df
2 3A2
N0
f
3 m
解调后，输出信噪比为
Sout
Nout
KF2M E m
t
2 3A2 2N0 fm3
3A2 KF2M
2N0
f
3 m
E m
t
2
因为
FM
K FM
| m(t) |max fm
Sout
Nout
3
2 FM
E m
a1 cos1 a2 cos2 a cos
各种基本调制信号的带宽关系
各种基本调制信号的带宽关系1. 引言1.1 概述概述部分的内容可以如下所示：在通信领域中，基本调制信号是指通过改变信号的某些特性来传输信息的一种方式。
常见的基本调制信号包括调幅信号、调频信号和调相信号，它们分别通过改变信号的幅度、频率和相位来实现信息的传输。
这些调制信号在实际应用中具有不同的带宽关系，即信号占用的频带范围，对通信系统的性能和资源利用有着重要影响。
本文旨在深入探讨各种基本调制信号的带宽关系，通过对其特性和调制方式的分析，揭示它们之间的差异和联系。
理解基本调制信号的带宽关系对于设计和优化通信系统非常重要，可以提高信息传输的效率和可靠性。
在接下来的几个章节中，我们将针对每种调制信号分别进行讨论。
首先，我们将讨论调幅信号的带宽关系，即调幅信号在频域上的能量分布情况。
接着，我们将深入研究调频信号和调相信号的带宽关系，分析它们在频域上的特点以及与调幅信号的异同。
此外，我们还将探讨调幅调频信号、调幅调相信号和调频调相信号的带宽关系，探究它们在频域上的相互作用。
最后，我们将在结论部分总结各种基本调制信号的带宽关系，并展望调制信号带宽关系的应用前景。
通过对基本调制信号带宽关系的深入理解和研究，我们可以为未来通信系统的设计和优化提供更好的参考和指导，进一步提高通信技术的发展水平。
通过本文的阐述，读者将能够全面了解各种基本调制信号的带宽关系，为实际应用中的通信系统设计和优化提供指导和参考。
同时，对于相关领域的研究人员和工程师来说，本文也将是一个重要的参考资料，促进通信技术的发展和进步。
文章结构部分的内容可以参考以下编写：1.2 文章结构本文主要探讨各种基本调制信号的带宽关系。
为了便于读者理解和掌握相关概念，本文将按照以下结构进行论述：第一部分是引言部分。
在引言部分，我们将对本文的内容进行概述，包括各种基本调制信号的定义和特点，并介绍文章的目的和意义。
第二部分是正文部分。
正文部分将详细探讨各种基本调制信号的带宽关系。
信号的三种调制方式
y ( x) c1 J ( x) c 2Y ( x)
齿轮故障特征
1．在各种齿轮故障诊断方法中,以振动检测为基础的齿 轮故 障诊断方法具有反映迅速、测量简便、实时性 强等优点。 2．齿轮发生断齿情况下其振动信号冲击能量达到最大， 均方值和峰值减小,表明齿轮传动接触减少,对经过磨合 期的齿轮,接触减少只可能是齿轮断齿或磨损厉害,但因 峭度和峰值指标增大,又表明齿轮存在较强的振动冲击, 而磨损厉害并不会出现较大的冲击振动信号,所以齿轮发 生的是 x] p( x)dx
4
式中x(t)为瞬时振幅，x杠为振幅均值，p(x)为概率密度， σ为标准差
1 K N
xi x i 1 t
N
4
式中xi为瞬时振幅，x杠为振幅均值，N为采样长度， σt为标准差。 峭度（Kurtosis）K是反映振动信号分布特性的数值 统计量，是4阶中心矩，峭度指标是无量纲参数， 由于它与轴承转速、尺寸、载荷等无关，对冲击信 号特别敏感，特别适用于表面损伤类故障、尤其是 早期故障的诊断。在轴承无故障运转时，由于各种 不确定因素的影响，振动信号的幅值分布接近正态 分布，峭度指标值K≈3;随着故障的出现和发展，振 动信号中大幅值的概率密度增加，信号幅值的分布 偏离正态分布，正态曲线出现偏斜或分散，峭度值 也随之增大。峭度指标的绝对值越大，说明轴承偏 离其正常状态，故障越严重，如当其K>8时，则很 可能出现了较大的故障。
4.均方根值由于对时间取平均值，因而适用于像磨损、表面裂 痕无规则振动之类的振幅值随时间缓慢变化的故障诊断。
X 1 N
x
1
N
i
2
5.齿轮偏心是指齿轮的中心与旋转轴的中心不重合，这种故障 往往是由于加工造成的。 (1)时域特征 当一对互相啮合的齿轮中有一个齿轮存在偏心时，其振动波 形由于偏心的影响被调制，产生调幅振动，图为齿轮有偏心 时的振动波形。
4-15 调频与调相
4.5.3 调角波的频谱及频带宽度
一、频谱
vFM (t) J0 (m f )
J1(m f )[cos(0 )t cos(0 )t] J2 (m f )[cos(0 2)t cos(0 2)t] J3(m f )[cos(0 3)t cos(0 3)t]
（2） BW0.1 2(75 1) 152KHz
n0
Jn (mf ) 是以mf为宗数的n阶第一类贝赛尔函数。
4.5.3 调角波的频谱及频带宽度
一、频谱
vFM (t) J0 (m f )
J1(m f )[cos(0 )t cos(0 )t] J2 (m f )[cos(0 2)t cos(0 2)t]
1、频谱数量? 2、各分量相位关系？ 3、带宽？
fI
本 振 信 号
fL
fI
镜像 干扰
fK
f
上节内容回顾与扩展
混频器的干扰
交叉调制系数
Kf
干扰信号所转移的调制 有用信号调制
1 2
f '''VS0Vk20m2 cos 2t f 'VS0m1 cos 1t
结论：
1 m2 2 m1
f f
''' '
Vk20
(1)交叉调制是由非线性器件中的三次项或更高次
调相 瞬时相位 (t) 0t kpV cosΩt 0
mp
kpV
Dp
瞬时频率 (t) 0 kpV sin Ωt
4.5.2 数学表达式与相关参数
以单音调制波为例 调制信号 vΩ (t) VΩ cost
调相调频_精品文档
f
1
2 L(C0 Cm cos t)
1
2 L(1 Cm cos t)
C0
2001 Copyright
SCUT DT&P Labs
f
fc
1 2
fc
Cm C0
cos t
fc f
f
1 2
fc
Cm C0
cos t
频偏
2001 Copyright
SCUT DT&P Labs
3. 变容二极管调频原理电路p158 分析思路：
载波信号：
则
uc(t) Ucmcosct
FM u(t) U mcos(ct m f sin t)
PM u(t) U mcos(ct mp cost)
2001 Copyright
SCUT DT&P Labs
2. 调制指数
调频时
mf
f
k f Um
与调制信号振幅 成正比，频率成 反比。
调相时 m p k pU m 与调制信号频率无关。
第三对边频 J3(mf )cos(c 3)t J3(mf )cos(c 3)t ...]
结论：
1. 一个FM波，除有载频c 分量外，
还有无穷多个边频分量，边频之
间的间隔仍为 。
2. 边频幅度的大小为 Um Jn (，m f 由) Bessell函数决定。
2001 Copyright
SCUT DT&P Labs
电抗管：由一只晶体管或场效应管加上由 电抗和电阻元件构成的移相网络组成。
它等效为一个电抗元件（电感或电容） 且其参数可以随调制信号而变化。
原理电路：见光盘
2001 Copyright
调相调频
(t ) c k f u (t ) c k f Um cost
k f Um
c cos t
SCUT DT&P Labs
频偏 角频率偏移程度由调制信号幅度决定
2001 Copyright
（已调波）
调频波： u(t ) U cos (t ) m
Flash
2001 Copyright
——统称为角度调制
SCUT DT&P Labs
原 因：
u(t ) U m cos (t )
(t ) t dt
瞬时相角：
瞬时角频率：
d (t ) (t ) dt
t
0
为积分常数， 即初始相角
u(t ) U mcos( t dt )
2001 Copyright
SCUT DT&P Labs
结论：
1 mf相同时，贝塞尔函数阶数升高，其值变化 不一定越小。这意味着，调频波的频谱幅度不是 线性递减的。 2 mf值越大，贝塞尔函数的阶数将增多。这意 味着，调频波频谱中的边频数目增多，从而使频 带加宽。
2001 Copyright
SCUT DT&P Labs
0
2001 Copyright
t
SCUT DT&P Labs
一、调频及数学表达：
6.2 调角波的性质
u (t ) U m cos t
设调制信号为单一频率的正弦波： c为常数，不 随时间变化
பைடு நூலகம்
载波信号： uc (t ) U cm cosct
调制后瞬时角频率：（受调制信号控制，随时间变化）
本章主要内容：
描述调幅、调频、调相的区别。
描述调幅、调频、调相的区别。
调幅（Amplitude Modulation，AM）、调频（Frequency Modulation，FM）和调相（Phase Modulation，PM）是三种常见的模拟调制技术，用于在无线通信中将信息信号转换成无线信号的形式以便传输。
它们之间的区别主要体现在调制参数的不同以及对信号特性的影响上。
调幅是一种将基带信号的幅度变化转换为载波信号的幅度变化的调制技术。
在调幅过程中，信号的幅度被调制到载波上，使得载波的振幅随着信号的变化而变化。
调幅的特点是简单易实现，但对于噪声和干扰比较敏感。
调幅的解调过程是通过检测载波的幅度变化来恢复原始信号。
调频是一种将基带信号的频率变化转换为载波信号的频率变化的调制技术。
在调频过程中，信号的频率被调制到载波上，使得载波的频率随着信号的变化而变化。
调频的特点是抗干扰性能较好，信号传输质量稳定，在广播电台和移动通信等领域得到广泛应用。
调频的解调过程是通过检测载波频率的变化来恢复原始信号。
调相是一种将基带信号的相位变化转换为载波信号的相位变化的调制技术。
在调相过程中，信号的相位被调制到载波上，使得载波的相位随着信号的变化而变化。
调相的特点是对干扰和噪声比较敏感，但在一些特定的应用场景下，如雷达、无线电导航等，调相技术具有独特的优势。
调相的解调过程是通过检测载波相位的变化来恢复原始信号。
总结起来，调幅、调频和调相是三种常见的模拟调制技术，它们分别通过改变载波的幅度、频率和相位来实现对基带信号的调制。
它们的选择取决于具体的应用需求和信号特性要求。
调幅简单易实现，但对干扰和噪声敏感；调频抗干扰性能较好，传输质量稳定；调相在特定应用场景下具有优势。
了解它们的区别和特点有助于我们在实际应用中选择合适的调制技术，以实现高质量的信号传输。
高频问答题及部分答案整理
思考题与习题1.1通信系统的组成包括几部分，各部分的作用是什么？答：是由信源、输入变换器、输出变换器、发送设备、接收设备和信道组成。
信源就是信息的来源。
输入变换器的作用是将信源输入的信息变换成电信号。
发送设备用来将基带信号进行某种处理并以足够的功率送入信道，以实现信号的有效传输。
信道是信号传输的通道，又称传输媒介。
接收设备将由信道送来的已调信号取出并进行处理，还原成与发送端相对应的基带信号。
输出变换器将接收设备送来的基带信号复原成原来形式的信息。
1.2 在通信系统中为什么要采用调制技术？答：调制就是用待传输的基带信号去改变高频载波信号某一参数的过程。
采用调制技术可使低频基带信号装载到高频载波信号上，从而缩短天线尺寸，易于天线辐射，实现远距离传输；其次，采用调制技术可以进行频分多路通信，实现信道的复用，提高信道利用率。
1.3 简述超外差接收机中混频器的作用。
答：混频器是超外差接收机中的关键部件，它的作用是将接收机接收到的不同载频已调信号均变为频率较低且固定的中频已调信号。
由于中频是固定的，且频率降低了，因此，中频选频放大器可以做到增益高、选择性好且工作稳定，从而使接收机的灵敏度、选择性和稳定性得到极大的改善。
1.5 和模拟通信相比，数字通信有什么优点？答：数字通信与模拟通信相比具有明显的优点。
它抗干扰能力强，通信质量不受距离的影响，能适应各种通信业务的要求，便于采用大规模集成电路，便于实现保密通信和计算机管理。
不足之处是占用的信道频带较宽。
1.6在数字通信系统中，指出模拟信源和数字信源对系统的影响。
答：在数字通信系统中，若是数字信源，其系统组成如图1.2.4所示。
若是模拟信源，可在数字信源系统基础上，附加两个变换环节：一是在信源编码前加A/D转换，二是在信源解码后加D/A 转换。
2.1描述选频网络的性能指标有哪些？矩形系数是如何提出来的？答：常用谐振频率、通频带和选择性三个参数来描述选频网络的性能指标。
调频 调幅 调相
那么，调频和调幅在无线传输上没有区别吗？不是！调频的特点是频宽窄，距离长。频宽窄的意思是对阻碍物的穿透能力弱，但是传输距离长。这种技术一般使用在手机、寻呼机等需要长距离（5公里以上）传输的产品使用。读者应该都知道，移动公司的信号发射塔和每个人携带的手机距离是非常远的。调幅的特点是频宽宽，距离短。频宽宽的意思是对阻碍物的穿透能力强，但是传输距离较短，这种技术一般应用在楼宇内的无线报警、无线安防等领域。因为在一个楼宇里面最重要的不是距离，而是穿透能力。
同样的，比中波MW更低频率的150KHz-284KHz之间的这一段频谱也是作为广播用的，以波长而言，它大约在1000～2000米(公尺)之间，和MW的200-600米相比较显然"长"多了，因此就把这段频谱的广播称做长波(LW: Long Wave)。实际上，不论长波(LW)、中波(MW)或者是短波(SW)都是采用AM调制方式。
我们再来看调频，有了上面的知识做基础，我们就不难理解调频的原理，调频，就是载波的频率随着话音信号（调制信号）幅度的变化而变化，话音信号幅度大，载波的频率相应变大，话音信号幅度小，载波的频率相应变小，注意，这里变化的是频率，而不是幅度，这也是调频和调幅的区别，我们经过调制，就得到了一个频率随着调制信号变化而变化的已调信号，我们称之为“调频信号”。
另外，信号的信噪比愈大，抗干扰能力就愈强。而解调后获得的信号的信噪比与调制系数有关，调制系数越大，信噪比越大。由于调频系数远大于调幅系数，因此，调频波信噪比高，调频广播中干扰噪声小。
2．调频波比调幅波频带宽
频带宽度与调制系数有关，即：调制系数大，频带宽。调频中常取调频系数大于1，而调幅系数是小于1的，所以，调频波的频带宽度比调幅波的频带宽度大得多。
就是说，我们要传输的话音信号已经包含在了调幅波中，换句话，就是我们把调制信号从低频搬移到了高频，以便利用电离层传播。这样我们通过发射装置将已调信号发射出去，在接收端接收信号后，通过解调装置恢复出原信号，在经过转换装置将电压信号恢复成人的普通话音，就实现的两地之间两个人的通话目的，这也是短波通信电台的基本原理。
调频 调幅 调相(FM PM)
调频调幅调相（FM PM）Amplitude modulation: a modulated signal that changes the amplitude of a carrier; FM: changes in frequency or phase. Send FM - receive - am: should be received under certain conditions, but the detection efficiency is not necessarily high. For example, the receiver (amplitude modulation) loop can receive the FM signal at a low efficiency when it is in the state of slope detection (referring to radio data).FM and phase modulation are different, phase modulation at the same time, the frequency will certainly change, but the frequency modulation phase does not necessarily change.++++++++++++++++++++++++++++++++What is the difference between amplitude and frequency modulation?1. FM, am, anti-interference abilityForeign interference, processing and atmospheric interference, influence of modulated wave mainly produce parasitic amplitude modulation, noise. FM system can use amplitude limiting method to eliminate parasitic modulation caused by interference. In amplitude modulation system, the amplitude of the amplitude modulated signal is varied, so that the amplitude limit can not be used, and it is difficult to eliminate the external interference.In addition, the greater the signal to noise ratio, the stronger the anti-interference ability. The signal-to-noise ratio ofthe demodulated signal is related to the modulation coefficient. The greater the modulation coefficient, the greater the signal-to-noise ratio. Because the FM coefficients are much larger than the amplitude modulation coefficients, the FM wave has a high signal-to-noise ratio, and the interference noise in FM broadcasting is small.2. FM Bobbi amplitude modulation wave frequency bandwidthThe band width is related to the modulation factor, that is, the modulation coefficient is large and the frequency band is wide. The frequency modulation coefficient is often greater than 1, while the amplitude modulation coefficient is less than 1. Therefore, the band width of FM wave is much larger than that of amplitude modulated wave.3. frequency modulation power utilization is greater than amplitude modulation systemIn the total power delivered, the power of the sideband frequency is the effective power of the transmitted modulation signal, while the side frequency power is related to the modulation coefficient, the modulation coefficient is large, and the side frequency power is large. Because the frequency modulation coefficient MF is greater than the amplitude modulation coefficient Ma, the power utilization ratio of the FM system is higher than the amplitude modulation system.++++++++++++++++++++++++++++++The difference between FM and am like mobile phone GSM and CDMA,is a different transmission mode, CDMA technology should I do not know how much advanced than the GSM, but not more than 133 of the mobile phone signal of mobile phone signal 139 strong, but not as good as. Why is the same 139 mobile phone, some manufacturers strong signal, and some manufacturers weak signal? That is to say, a good or bad product is not determined by the mode of transmission, but by the technical ability of the manufacturer and the degree of product completion.So, is there any difference between FM and am in wireless transmission? No FM is characterized by narrow bandwidth and long distance. Narrow bandwidth means that the penetration capability of the obstruction is weak, but the transmission distance is long. This technology is generally used in mobile phones, pagers and other needs of long distance (5 kilometers or more) transmission products. Readers should know that Mobile Corporation's signal towers are far removed from each person's mobile phones. Amplitude modulation is characterized by wide bandwidth and short distance. Wide bandwidth means that the penetration of the barrier is strong, but the transmission distance is shorter. This technology is generally used in wireless alarm, wireless security and other fields. For in a house the most important thing is not the distance, but the penetrating power.So, FM technology is advanced,Is the technique of amplitude modulation behind? No The Federal Communications Commission (FM) introduced FM and am technology since the last century and has been used in all our living areas. FM and am are just different transmission methods for a wirelesstechnician, and there is no technical difficulty.Is there any difference between FM and am? Neither！ There are many differences, one of which is the cost. Because the frequency modulation line is more complex, needs the component quantity to be many, therefore the frequency modulation product's price must compare the banner product the price to be much higher. Why do most domestic wireless call system manufacturers adopt amplitude modulation mode? First of all, the transmission in the building does not need to use high cost FM to do, as long as the scroll products to improve the degree of completion, to fully solve the transmission distance and coverage area. As an agent, the same product shape, the same distance effect, different price, which one would you choose? In fact, plainly, customers do not need to know the frequency modulation or amplitude modulation, inexpensive, economical and practical products is the truth.+++++++++++++++++++++Widely used in military, communications, radio, television, radio and other fieldsWhat are frequency modulation (FM), amplitude modulation (AM), short wave (SW) and long wave (LW)?Both AM and FM band in general radio or radiocassette, I believe we are already familiar with, the two band is used for you to listen to the radio broadcast home, if there SW band, so in addition to domestic shortwave radio, you can listen to the world broadcast radio program. In order for you to have a betterunderstanding of the use of radio, what follows is a brief description of what AM, FM, SW, and LW are.In fact, "AM" and "FM" refer to two different modulations in wireless electrical engineering. The AM: Amplitude Modulation is called amplitude modulation, while the FM: Frequency Modulation is called fm. Just MW: Medium Wave (general radio broadcasting), using the amplitude modulation (AM) way, unconsciously, between MW and AM on an equal sign. In fact, MW is just a lot of using a broadcast AM modulation, as in a high frequency (3-30MHz) modulation methods used in the International Shortwave Radio is AM, even higher frequency than the FM radio navigation communication (116-136MHz) is AM, AM band is our daily said that Poland broadcast (MW).What about FM? It is similar to the fate of MW. We used FM to refer to the general FM radio (76-108MHz, in China for87.5-108MHz, Japan 76-90MHz), the fact that FM is a kind of modulation, even in the short wave range 27-30MHz, as amateur radio, satellite communications, space applications band, but also the use of frequency modulation (FM) mode the.And what about SW? In fact, can be said to be short wave of a simple name, the correct statement should be high frequency (HF:High Frequency) more appropriate. And how does the name "shortwave" come from? At wavelengths, the medium wave (MW) is between 200-600 meters (meters), while the wavelength of HF is between 10~100 meters (meters). Compared with the wave appearance, the wavelength of HF is really shorter,因此就把hf称做短波 (sw: short wave).同样的, 比中波mw更低频率的150khz - 284khz之间的这一段频谱也是作为广播用的, 以波长而言, 它大约在1000～2000米 (公尺) 之间, 和mw的200 - 600米相比较显然 "长" 多了, 因此就把这段频谱的广播称做长波 (lw: long wave).实际上, 不论长波 (lw) 、中波 (mw) 或者是短波 (sw) 都是采用am调制方式.对一般收 (录) 音机而言, fm、mw、lw波段是提供您收听国内广播用的, 但我国目前没有设立lw电台, 而sw波段则主要供您收听国内/ 国际远距离广播..短波知识百年前, 三声短促而且微弱的讯号, 向世界宣布了无线电的诞生.一九? 一年, 扎营守候在讯号山 (signal hill位于加拿大东南角) 的意大利科学家马可尼, 终于接收到了从英格兰发出的跨过大西洋的无线电讯号, 这个实验向世人证明了无线电再也不是仅限于实验室的新奇东西, 而是一种实用的通讯媒介.此后短波用作全球性的国际通讯媒介便开始发达起来了.虽然马可尼的试验结果令人相当振奋, 可是当时一般人认为无线电传播方式类似光波, 发射之后, 绝对沿直线方向进行传播, 从英国到加拿大, 再怎么说也无法完成直线的无线电通讯 (因为地球表面是弧形的).当时的科学理论更证明, 从英国发射后的无线电波一定直驱太空, 怎么可能到达加拿大? 可是从马可尼用简陋的无线电设备征服长距离通讯的试验记录来看, 白天, 讯号可以远达700英哩, 晚间更远达2000英哩以上, 这些试验数据, 使得以往的理论所推断出来的必然结果, 开始发生动摇了.+ + + + + + + + + + + + + + + + + + + +使载波振幅按照调制信号改变的调制方式叫调幅.经过调幅的电波叫调幅波.它保持着高频载波的频率特性, 但包络线的形状则和信号波形相似.调幅波的振幅大小, 由调制信号的强度决定.调幅波用英文字母am表示.使载波频率按照调制信号改变的调制方式叫调频.已调波频率变化的大小由调制信号的大小决定, 变化的周期由调制信号的频率决定.已调波的振幅保持不变.调频波的波形, 就像是个被压缩得不均匀的弹簧, 调频波用英文字母fm表示.目前, 中波广播使用的频段大致为550khz - 1600khz, 主要靠地波传播, 也伴有部分天波; 调频制无线电广播多用超短波 (甚高频)无线电波传送信号, 使用频率约为88mhz - 108mhz, 主要靠空间波传送信号.使载波振幅按照调制信号改变的调制方式叫调幅.经过调幅的电波叫调幅波= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =调制又分为三种: 调幅, 调频和调相.The amplitude modulation is represented by AM, the FM is represented by FM, and the phase modulation is expressed by PM. This is where we can find the AM.FM sign when we listen to the broadcast.Its basic principle is that the modulated signal to betransmitted (here we use the voice signal as an example) from low frequency to high frequency shifting, so that it can be transmitted through the ionosphere reflection in the remote receiver we use proper demodulation device then restore the original signal without distortion, to transmit voice frequency signal the purpose of.For example, am, we cannot directly transfer the voice, we first use a voice signal conversion device (or say) is converted into voltage signal amplitude changes smoothly, this is our signal to be transmitted, called modulation signal, and then the modulation signal and a high frequency signal in a multiplier, after an adder circuit, get a signal of high frequency, its envelope (the so-called envelope is connected with periodic signal in each cycle peak of the imaginary line) varies with the amplitude of modulation signal, we put the high frequency signal is called the carrier, the signal has been modulated called amplitude modulation wave.That is to say, the voice signals we are transmitting are already included in the AM wave, in other words, we move the modulation signal from low frequency to high frequency to facilitate ionospheric propagation. So we will adjust the signal transmitting device through the launch at the receiver after receiving the signal, the demodulation device to recover the original signal after conversion device voltage signal recovery in adult Mandarin, is implemented between two of two people to call, which is the basic principle of shortwave radio.We look at FM, with the above knowledge base, we can understand the principle of FM, FM, is the frequency of the carrier withthe voice signal (amplitude modulated signal) changes the voice signal amplitude, the frequency of the carrier is increased, the voice signal amplitude is small, the frequency of the carrier is reduced accordingly note that, here is a change in the frequency but not amplitude, this is the difference between frequency and amplitude, we modulated, had a frequency change with the change of the modulation signal modulated signal, we call "FM signal".==========================================================================Generally speaking, FM refers to 64---108MHz, that is, ultrashort wave, suitable for city high fidelity short distance broadcast, generally within 100 kilometersGenerally speaking, FM refers to 64---108MHz, that is, ultrashort wave, suitable for city high fidelity short distance broadcast, generally within 100 kilometersAmplitude modulation is medium wave, the range is 503---1060KHz, and the distance is far away, which is greatly affected by weather factors. It is suitable for the broadcast of inter provincial radio station.Amplitude modulation is medium wave, the range is 503---1060KHz, and the distance is far away, which is greatly affected by weather factors. It is suitable for the broadcast of inter provincial radio station.。
工业控制网络试题
一、解释以下名词：
1、工业控制网络
2、基带传输
3、调频、调幅、调相
4、现场总线
5、计算机网络的拓扑结构
二、简答题：
1、现场总线技术的特点，典型的现场总线有那些类型。
2、工业控制网络相对信息网络的特点。
3、与常规仪表相比，基金会现场总线仪表的主要特点。
4、描述CAN总线的报文结构。
5、工业以太网的主要技术及典型结构有那些
三、举例说明CAN总线位仲裁的实现方式。
四、画出01001100的NRZ编码、曼码、和差分曼码的编码波形。
五、画出OSI参考模型图。
一、解释以下名词：
1、工业控制网络
2、基带传输
3、调频、调幅、调相
4、现场总线
5、计算机网络的拓扑结构
二、简答题：
1、现场总线技术的特点，典型的现场总线有那些类型。
2、工业控制网络相对信息网络的特点。
3、与常规仪表相比，基金会现场总线仪表的主要特点。
4、描述CAN总线的报文结构。
5、工业以太网的主要技术及典型结构有那些
三、举例说明CAN总线位仲裁的实现方式。
四、画出01001100的NRZ编码、曼码、和差分曼码的编码波形。
五、画出OSI参考模型图。
调制与解调的概念
调制与解调的概念调制与解调是通信技术中重要的概念，它们是实现信息传输的关键技术。
在通信系统中，调制与解调的作用是将信息信号转换成一定的形式，以便能够在传输媒介中传输。
本文将从调制与解调的基本概念、调制与解调的分类、调制与解调的实现原理以及调制解调器的应用等方面进行介绍。
一、调制与解调的基本概念调制是指把信息信号（如语音、图像等）按照一定的规律转换成调制信号，使得信息信号能够适应传输媒介的特性，以便能够在传输媒介中传输。
调制的过程就是在信号中加入一定的高频载波信号，使得信息信号的频率被调制到高频载波信号的频率范围内，从而形成调制信号。
解调是指在接收端将调制信号还原成原始信息信号的过程。
解调的过程就是将接收到的调制信号中的高频载波信号去除，从而得到原始的信息信号。
解调是调制的逆过程，也是通信系统中非常重要的一个环节。
二、调制与解调的分类调制和解调可以根据不同的分类方式进行划分。
1. 按照信号的调制方式分类调制和解调可以按照信号的调制方式进行分类，常见的调制方式有模拟调制和数字调制。
模拟调制是指将模拟信号进行调制，将其转换成模拟调制信号。
模拟调制分为调幅、调频和调相三种方式。
调幅是指将模拟信号的幅度加到载波信号上，形成调幅信号；调频是指将模拟信号的频率加到载波信号上，形成调频信号；调相是指将模拟信号的相位加到载波信号上，形成调相信号。
数字调制是指将数字信号进行调制，将其转换成数字调制信号。
数字调制分为ASK、FSK、PSK、QAM等多种方式。
ASK是指将数字信号转换成调幅信号；FSK是指将数字信号转换成调频信号；PSK是指将数字信号转换成调相信号；QAM是指将数字信号同时转换成调幅和调相信号。
2. 按照载波信号的性质分类调制和解调可以按照载波信号的性质进行分类，常见的载波信号有连续波和脉冲波。
连续波调制是指将信息信号加到连续的正弦波或余弦波上，形成连续波调制信号。
连续波调制主要包括调幅、调频和调相三种方式。
调频调相代码
调频调相代码
信号调制技术大体上可以分为：模拟调制、数字调制，模拟调制有幅度调制AMDSBSSBVSB相位调制PM频率调制FM。
数字调制有：幅移键ASK频移键控FSK相移键控PSK。
调制解调有两种分类：①基带调制②带通调制。
基带调制常用的编码方式：
①不归零制：正电平代表1，负电平代表0。
②归零制：正脉冲代表1，负脉冲代表0。
③曼彻斯特编码：位周期中心的向上跳变代表0，位周期中心的向下跳变代表1。
也可反过来定义。
④差分曼彻斯特编码：在每一位的中心处始终都有跳变。
位开始边界有跳变代表0，而位开始边界没有跳变代表1。
（3）基本的带通调制方法
①调幅AM：载波的振幅随基带数字信号而变化。
②调频FM：载波的频率随基带数字信号而变化。
③调相PM)：载波的初始相位随基带数字信号而变化。
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调频调幅调相
调频、调幅、调相是无线电通信中常用的三种调制方式。
它们分别是通过改变载波频率、振幅和相位来传输信息信号的。
下面将分别介绍这三种调制方式的原理和应用。
调频是指通过改变载波频率来传输信息信号。
在调频调制中，信息信号被转换成一个高频信号，然后这个高频信号被调制到一个载波信号上。
调频调制的优点是抗干扰能力强，传输距离远，适用于广播、电视、卫星通信等领域。
调幅是指通过改变载波振幅来传输信息信号。
在调幅调制中，信息信号被转换成一个低频信号，然后这个低频信号被调制到一个载波信号上。
调幅调制的优点是简单易实现，适用于短距离通信和音频信号传输。
调相是指通过改变载波相位来传输信息信号。
在调相调制中，信息信号被转换成一个低频信号，然后这个低频信号被调制到一个载波信号上。
调相调制的优点是抗多径干扰能力强，适用于雷达、导航、通信等领域。
除了以上三种调制方式，还有一种常用的调制方式是脉冲调制。
脉冲调制是指通过改变脉冲的宽度、间隔和幅度来传输信息信号。
脉冲调制的优点是传输速率高，适用于数字信号传输。
调频、调幅、调相是无线电通信中常用的三种调制方式，它们各有
优点，应用范围也不同。
在实际应用中，需要根据具体情况选择合适的调制方式，以达到最佳的传输效果。