Headword : Modulation
Pinyin ：
Basic explanation
To transmit information (e.g telegraph , Phone radio broadcast Or TV) Periodicity Or intermittent carrier Or some characteristic of the signal (such as amplitude, frequency or phase ）Changes made. explicate
1. Deployment and manufacturing. For example, prepare pills according to the doctor's prescription.
2. Survey preparation. Liang Qichao The Congress Meeting Period and the Beginning Period of Accounting: "The budget proposals of ministers must be prepared before April." Zhang Yukun Literature Society Wuchang First Uprising Documentary · Joint Meeting of Literature Society and Progressive Association:“ military The preparatory staff prepared the Wuchang street map, scheduled the distribution of the team and the overall plan of the first event. " Modulation is to process the information of the signal source and add it to the carrier to make it suitable for channel The process of transmission form is the technology that makes the carrier change with the signal. Generally speaking, the information of the signal source (also known as the source) contains DC components and frequency The lower frequency component is called baseband signal 。 The baseband signal can not be used as the transmission signal, so the baseband signal must be converted into a signal with a very high frequency relative to the baseband frequency to be suitable for channel transmission. This signal is called modulated signal, while baseband signal is called Modulated signal 。 Modulation is achieved by changing the high frequency carrier That is, the amplitude of the carrier signal of the message phase Or frequency, which changes with the baseband signal amplitude. Demodulation is the process of extracting the baseband signal from the carrier for processing and understanding by the intended receiver (also called the sink). There are many kinds of modulation, classification method It is also inconsistent. According to the form of modulation signal, it can be divided into analog modulation and digital modulation. use analog signal Modulation is called analog modulation; Modulation with data or digital signal is called digital modulation 。 According to the type of modulated signal, it can be divided into pulse Modulation, sine wave modulation, and intensity modulation (e.g., for incoherent Light modulation ）Etc. The modulated carriers are pulse, sine wave and light wave. Sine wave modulation includes amplitude modulation, frequency modulation and phase modulation The last two are collectively referred to as Angular modulation 。 In addition, there are some variant modulation, such as single sideband AM, vestigial sideband AM, etc. Pulse modulation can also be classified in a similar way. In addition, there are compound modulation and multiple modulation. Different modulation modes have different characteristics and performances. Sine wave amplitude modulation
sine Modulation in which the carrier amplitude varies with the modulation signal amplitude modulation （AM）。 Digital amplitude modulation is also called amplitude keying（ ASK ）。 AM technology and equipment are relatively simple, spectrum Narrow, but anti-interference Poor performance, widely used in long, medium and short waves radio broadcast , small wireless telephone, telegraph and other electronic equipment. Early radio machines used spark arresters to generate high-frequency oscillations. During signal transmission, the spark type transmitter emits high-frequency oscillation waves, and the transmitter does not output when the number is empty. The carrier wave of this kind of telegraph signal is not pure sine wave. It contains many harmonic components, which will seriously interfere with other signals.
modulation Where U C is the carrier amplitude; Ω =2π F , is the angular frequency of the modulated signal, where F Is the frequency of the modulated signal; m A is a constant proportional to the amplitude Ua of the modulated signal, called the amplitude modulation coefficient, and the value should be between 0 and 1. Figure 1a, Figure b and Figure c are single frequency modulation signals ua (t) carrier signal Uc (t) and amplitude modulated wave u A( t ）Of wave form 。 Instantaneous amplitude change of amplitude modulated wave curve be known as Envelope 。 Amplitude modulation coefficient m A cannot be greater than 1, otherwise Envelope And modulation signal cannot be maintained linear relationship , distortion will occur. This condition is called overmodulation. 
Spectrum of AM signal Sine wave frequency modulation
Sinusoidal carrier instantaneous frequency Modulation that changes with the instantaneous value of the modulated signal is called frequency modulation (FM) for short. Digital frequency modulation is also called frequency shift keying (FSK). FM was in 1933 E. H. Armstrong Invented. This modulation has good anti-interference performance and is widely used in high-quality radio, TV sound Multiplex communication And frequency sweeper. The ideal frequency modulation is the instantaneous angular frequency of the carrier ω It is equal to the instantaneous value ua (t) of the modulated signal linear relationship , while the amplitude U C does not change. Instantaneous angular frequency in case of single frequency modulation ω The expression of is ω=ω C+墹 ω cos Ω t (2)
Where: ω = k f Ua， Is a constant proportional to the amplitude Ua of the modulated signal, called the maximum angular frequency offset. Figure 3 is the waveform of FM wave. The expression of FM wave is
（3） After sine wave frequency modulation, new frequency components will also be generated. These components and frequency modulation index m F Relevant. In theory, when single frequency FM is used, the FM wave has an infinite number of side frequency components, and the distance between adjacent side frequencies is equal to F 。 Usually, the side frequency component whose amplitude is less than 1/100 of the original amplitude of the carrier is ignored, and the frequency band occupied by the effective side frequency component is the bandwidth of the FM wave.
Figure 4 is the frequency spectrum of the frequency modulation wave when single frequency modulation is used. m When f is small, the carrier frequency component is large and the side frequency amplitude is small; m When f increases, the carrier frequency amplitude decreases, and the side frequency amplitude increases frequency Also increased; m When f continues to increase, the amplitude of the carrier frequency and each side frequency increases or decreases alternately frequency The amplitude of the component is m F is the order of the number Bessel function 。 
Waveform of FM wave BWf≈2(mf+1）F （4）
Where F = Ω /2 π is the frequency of the modulated signal. When m When f is large, the bandwidth of the FM wave is approximately equal to the maximum frequency offset 墹 f Twice as much.
Broadband FM has strong anti-interference performance. Armstrong certification in 1933: when input Signal-to-noise ratio S i/ N When i is large, the output signal-to-noise ratio of FM receiver S O/ N O and maximum frequency shift 墹 f Is proportional to the square of; Increasing the bandwidth of FM wave can improve the quality of communication system. However, this improvement is limited, because when the bandwidth is too large, the internal noise of FM receiver N I increase, S i/ N I decreases; When S i/ N I is reduced to a certain threshold When, S O/ N O has deteriorated sharply. Figure 6 is the output signal to noise ratio of FM receiver S O/ N O and input signal-to-noise ratio S i/ N I, on the left of the curve inflection point, FM's anti-interference performance is worse than AM's. The method of pre emphasis and feedback FM reception can make S O/ N O has been improved. 
modulation Sine wave phase modulation
Modulation in which the instantaneous phase of the sinusoidal carrier varies with the modulation signal, referred to as phase modulation (PM). Digital phase modulation is also called phase shift keying (PSK).
Waveform of frequency modulation wave in single frequency modulation U φ (t)=UCcos (ω Ct+墹 φ cos Ω t+φ 0) (5)
Where: φ by carrier phase The maximum phase shift that varies with the modulation signal is called the phase modulation index. It is proportional to the amplitude Ua of the modulation signal, but it is proportional to the modulation angular frequency Ω irrelevant. This is the difference between phase modulation and frequency modulation. The frequency spectrum of the phase modulation wave is similar to that of the frequency modulation wave, but when φ When it is a fixed value, its spectrum width BW φ with Ω And change, Ω Large time spectrum width, Ω Narrow hourly spectrum. therefore frequency band Not fully utilized. Digital phase modulation has excellent anti-interference performance and narrow frequency band. It is an ideal modulation mode and is widely used in various data transmission and digital communication systems. 
modulation Pulse modulation signal has a wide spectrum, but it has good anti-interference performance except for pulse amplitude modulation, especially pulse code modulation, which is an ideal modulation method. Digital telephone telemetering 、 remote control As well as the rapidly developing integrated communication network, most of them use this modulation. Modulation is very important in electronics. The striking development trends are: ① Due to the continuous increase of digital services, the capacity of digital communication systems needs to be expanded, which requires the development of ultra-high rate digital modulation technology; ② In order to make full use of Radio spectrum Resources, further research required Spectral efficiency High and low bit error rate modulation; ③ In coherent optical communication and CD In terms of storage devices, optical phase modulation, frequency modulation and polarization modulation are also important research topics. Modulation plays a very important role in communication systems. Through modulation, not only can spectrum shifting Move the spectrum of the modulated signal to the desired position, so as to convert the modulated signal into a modulated signal suitable for propagation, and it has a great impact on the transmission effectiveness and reliability of the system, modulation mode It often determines the performance of a communication system. In communication, we often adopt the following modulation modes:
（1） Analog modulation: modulate a high-frequency sine wave with a continuously changing signal
It mainly includes: 1. amplitude modulation（ amplitude modulation AM， Double sideband modulation DSBSC， SSBSC, Vestigial sideband modulation VSB and independent sideband ISB); 2. Angle modulation (FM, Phase modulation PM). Because the rate of change of phase is the frequency, the phase modulation wave and Frequency modulated wave Is closely related; （2） Digital modulation: using digital signal pairs sine Or cosine high frequency oscillation Perform modulation It mainly includes: 1 amplitude Keying ASK ； 2. Frequency Keying FSK ； （3） Pulse modulation: use pulse sequence as carrier
It mainly includes: 1. Pulse Amplitude Modulation (PAM);
2. Pulse duration modulation (PDM);
three Pulse position modulation （PPM：Pulse Position Modulation）； 4. PCM (Pulse Code Modulation);
With the increase of communication traffic, spectrum resources are increasingly strained. In order to improve the system capacity, the channel spacing has been reduced from the initial 100 kHz to 25 kHz, and will be further reduced to 12.5 kHz, or even smaller Digital communication It is flexible in network construction, easy to use digital error control technology and digital encryption, easy to integrate, and able to access ISDN network, so the communication system is being simulated System Transition to digital system. Therefore, numbers must be used in the system modulation technique However, general digital modulation technologies, such as ASK, PSK and FSK, cannot meet the requirements of mobile communication due to their low transmission efficiency. Therefore, it is necessary to specially study some digital modulation technologies with strong anti-interference, good bit error performance and high spectrum utilization to maximize the Bit rate To meet the requirements of narrowband data transmission for mobile communication. as minimum Frequency shift keying （MSK－Minimum Shift Keying）； Gaussian filter minimum frequency shift keying （GMSK－Gaussian Filtered Minimum Shift Keying）； QPSK Quadrature Reference Phase Shift Keying;
OQPSK － Offset Quadrature Reference Phase Shift Keying;
DQPSK － Differential Quadrature Reference Phase Shift Keying;
π/4 Quadrature phase shift keying （π/4－DQPSK－Differential Quadrature Reference Phase Shift Keying）； Already in digital cell Mobile communication system It is widely used in. 
