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DZ198PID控制器的设计和研究,DZ198PID,控制器,设计,研究
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装订线毕业设计（英文翻译）二 八 届 毕 业 设 计英文翻译学 院：信息工程学院专 业：电子信息工程姓 名：王硕学 号：2403040241指导教师：钟晓荣完成时间：2008年6月二八年六月Single-SidebandCommunication SystemsINTORDUCTIONConventional AM double-sideband communications systems, such as those discussed in Chapters 3 and 4, have two inherent disadvantages. First, with conventional AM, carrier power constitutes two-thirds or more of the total transmitted power. This is a major drawback because the carrier contains no information; the sidebands contain the information. Second, conventional AM systems utilize twice as much bandwidth as needed with single sideband system. With double-sideband transmission, the information contained in the upper sideband is identical to the information contained in the lower sideband. Therefore, transmitting both sidebands is redundant. Consequently, conventional AM is both power and bandwidth inefficient, which are the two most predominant considerations when designing modern electronic communications systems.SINGLE-SIDEBAND SYSTEMSSingle sideband was mathematically recognized and understood as early as 1914; however, not until 1923 was the first patent granted and a successful communications link established between England and the United States. There are many different types of sideband communications systems. Some of them conserve bandwidth, some conserve power, and some conserve both. Figure 5-1compares the frequency spectra and relative power distributions for conventional AM and several of the more common single-sideband (SSB) systems.AM Single-Sideband Full CarrierAM single-sideband full carrier(SSBFC) is a form of amplitude modulation in which the carrier is transmitted at full power, but only one of the sidebands is transmitted. Therefore, SSBFC transmissions require only half as much bandwidth as conventional double-sideband AM. The frequency spectrum and relative power distribution for SSBFC are shown in Figure 5-1b. Note that with 100% modulation the carrier power (Pc) constitutes four-fifths (80%) of the total transmitted power (Pt), and only one-fifth (20%) of the total power is in the sideband. For conventional double-sideband AM with 100% modulation, two-thirds (67%) of the total transmitted power for the information power is in the carrier and one-third (33%) is in the sideband. Therefore, although SSBFC requires less total power, it actually utilizes a smaller percentage of that power for the information-carrying portion of the signal.Figure 5-2shows the waveform for a 100%-modualted SSBFC wave with a single-frequency modulating signal. The 100%-modulated single-sideband, full-carrier envelope. Recall form Chapter 3 that the maximum positive and negative peaks of an AM DSBFC wave occur when the carrier and both sidebands reach their respective peaks at the same time, and the peak change in the envelope is equal to the sum of the amplitudes of the upper and lower side frequencies. With single-sideband transmission, there is only one sideband (rather the upper or lower) to add to the carrier. Therefore, the peak change in the envelope is only half carrier transmission, the demodulated signals have only half the amplitude of a double-sideband demodulated wave. Thus, a trade-off is made. SSBFC require less bandwidth than DSBFC but also produces a demodulated signal with a lower amplitude. However, when the bandwidth is halved, the total noise power is also halved (i.e., reduced by 3dB); and if one sideband is removed, the power in the information portion of the wave is also halved. Consequently the signal-to-noise ratios for single and double sideband are the same.With SSBFC, the repetition rate of the envelope is equal to the frequency of the modulating signal, and the depth of modulation is proportional to the amplitude of the modulating signal. Therefore, as with double-sideband transmission, the information is contained in the envelope of the full-carrier modulate signal.AM Single-Sideband Suppressed CarrierAM single-sideband suppressed carrier (SSBSC) is a form of amplitude modulation in which the carrier is totally suppressed and one of the sidebands removed. Therefore, SSBSC requires half as much bandwidth as conventional double-sideband AM and considerably less transmitted power. The frequency spectrum and relative power distribution for SSBSC with upper sideband transmission are shown in Figure 5-1c. It can be seen that the sideband power make up 100% of the total transmitted power. Figure 5-3 shows a SSBSC wave form for a single-frequency equal to the carrier frequency plus the modulating-signal frequency or the carrier frequency minus the modulating-signal frequency, depending on which sideband is transmitted.AM Single-Sideband Reduced CarrierAM single-sideband reduced carrier (SSBRC) is a form of amplitude modulation in which one sideband is totally removed and the carrier voltage is reduced to approximately 10% of its unmodulated amplitude. Consequently, as much as 96% of the total power transmitted is in the unsuppressed sideband. To produce produced carrier component, the carrier is totally suppressed during modulation and then reinserted at reduced amplitude. Therefore, SSBRC is sometimes called single-sideband reinserted carrier. The reinserted carrier is often called a pilot carrier and is reinserted for demodulation purposes, which is explained later in this chapter. The frequency spectrum and relative power distribution for SSBRC are shown in Figure 5-1d. The figure shows that the sideband power constitutes almost 100% of the transmitted power. As with double-sideband, full-carrier AM, the repetition rate of the envelope is equal to the frequency of the modulating signal. To demodulate a reduced carrier waveform with a conventional peak detector, the carrier must be separated, amplified, and then reinserted at a higher level in the receiver. Therefore, reduced-carrier transmission is sometimes called exalted carrier because the carrier is elevated in the receiver prior to demodulation. With exalted-carrier detection, the amplification of the carrier in the receiver must be sufficient to raise the level of the carrier to a value greater than that of the sideband signal. SSBRC requires half as much bandwidth as conventional AM and, because the carrier is transmitted at a reduced level, also conserves considerable power.AM Independent SidebandAM independent sideband (ISB) is a form of amplitude modulation in which a single carrier frequency is independently modulated by two different modulating signals. In essence, ISB is a form of double-sideband transmission in which the transmitter consists of two independent single-sideband suppressed-carrier modulators. One modulator produces only the upper sideband and the other produces only the lower sideband. The single-sideband output signals from the two modulators are combined to form a double-sideband signal in which the two sidebands are totally independent of each other except that they are symmetrical about a common carrier frequency. One sideband is positioned above the carrier in the frequency spectrum and one below. For demodulation purposes, the carrier is generally reinserted at a reduced level as with SSBRC transmission. Figure 5-1e shows the frequency spectrum and power distribution for ISB, and Figure 5-4 shows the transmitted waveform for two independent single-frequency information signal (fm1 and fm2). The two information signals are equal in frequency; therefore, the waveform is identical to a double-sideband suppressed-carrier waveform except with a repetition rate equal to twice the modulating signal frequency. ISB conserves both transmit power and bandwidth as two information sources are transmitted within the same frequency spectrum as would be required nation sources are transmitted within the same frequency spectrum as would be required by a single source using conventional double-sideband transmission. ISB is one technique that is used in the United States for stereo AM transmission. One channel (the left) is transmitted in the lower sideband, and the other channel (the right) is transmitted in the upper sideband.AM Vestigial SidebandAM vestigial sideband (VSB) is a form of amplitude modulation in which the carrier and one complete sideband are transmitted, but only part of the second sideband is transmitted. The carrier is transmitted at full power. In VSB, the lower modulating-signal frequencies are transmitted single sideband. Consequently, the lower frequencies can appreciate the benefit of 100% modulation, whereas the higher frequencies cannot achieve more than the high frequencies. The frequency spectrum and relative power distribution for VSB are shown in Figure 5-1f. Probably the most widely known VSB system is the picture portion of a commercial television broadcasting signal which is designated A5C by the FCC.Comparison of Single-Sideband Transmission to Conventional AMForm the preceding discussion and Figure 5-1, it can be seen that bandwidth conservation and power deficiency are obvious advantages of single-sideband full-carrier transmission (i.e., conventional AM). Single-sideband transmission requires only half as much bandwidth as double sideband, and suppressed- and reduced-carrier transmissions require considerably less total transmitted power than full-carrier AM.The total power transmitted necessary to produce a given signal-to noise ratio at the output of a receiver is a convenient and useful means of comparing the power requirement and relative performance of single-sideband to conventional AM systems. The signal-to-noise ratio determines the degree of intelligibility of a received signal.Figure 5-5 summarizes the waveforms produced for a given modulating signal for three of the more common AM transmission systems: double-sideband suppressed (DSBFC,) double-sideband suppressed carrier (DSBFC), and single-sideband suppressed carrier (SSBSC). As the figure shows, the repetition rate of the DSBFC envelope is equal to the modulating signal frequency, the repetition rate of the DSBSC envelope is equal to twice the modulating signal frequency, and the SSBSC waveform is not an envelope at all but rather a single-frequency to the unsuppressed sideband frequency (i.e., either the upper or lower side frequency).A conventional AM wave with 100% modulation contains 1 unit of carrier power of carrier power and 0.25 unit of power in each sideband for a total transmitted peak power of 1.5 units. A single-sideband transmitter rated at 0.5 unit of power will produce the same S/N ratio at the output of a receiver as 1.5 units of carrier plus sideband power from a double-sideband full-carrier signal. In other words, the same performance is achieved with SSBSC using only one-third as much transmitted power and half the bandwidth. Table 5-1 compares conventional AM to single-sideband suppressed carrier for a single-frequency modulating signal. Peak envelope power (PEP) is the rms power developed at the crest of the modulation envelope (i.e., when the modulating-signal frequency components are at their maximum amplitudes).The voltage vectors for the power requirements stated are also shown. It can be seen that it requires 0.5 unit of voltage per sideband and 1 unit for the carrier with conventional AM for a total of 2 PEV (peak envelope volts) and only 0.707 PEV for single sideband. The RF envelopes are also shown, which correspond to the voltage and power relationships previously outlined. The demodulated signal at the output from a conventional AM receiver is proportional to the quadratic sum of the voltages from the upper and lower sideband signals, which equals 1 PEV unit. For single-sideband reception, the demodulated signal is 0.7071=0.707 PEV. If the noise voltage for conventional AM is arbitrarily chosen as 0.1V/kHz, the noise voltage for single-sideband signal with half the bandwidth is 0.0707V/kHz, Consequently, the S/N performance for SSBSC is equal to that of conventional AM.Advantages of single-sideband transmission. Following are four predominant advantages of single-sideband suppressed- or reduced-carrier transmission over conventional double-sideband full-carrier transmission.Power conservation. Normally, with single-sideband transmission, only one sideband is transmitted and the carrier is either suppressed or reduced significantly. As a result, much less total transmitted power is necessary to produce essentially the same-quality signal in the receiver as is achieved with double-sideband, full-carrier transmission. At least two-thirds of the power in a standard double-sideband, full-carrier AM signal is contained in the carrier, and the maximum power contained in either sideband is only one-sixth of the total power. Thus, eliminating the carrier would increase the power available for the sidebands by at least a factor of three providing a signal power advantage of 10 log (3) or approximately a 4.8 dB improvement in the signal-to-noise ratio.Bandwidth Conservation. Single-sideband transmission requires half as much bandwidth as conventional AM double-sideband transmission. This advantage is especially important actually reduces the required bandwidth by more than a factor of two, because most modulating signals, including audio signals, rarely extend all the way down to 0 Hz (dc). A more practical lower frequency limit for audio signals is 300 Hz; thus, a 3-kHz audio channel actually has a bandwidth of approximately 2700Hz (300Hz to 3000Hz). Consequently, a 2700-Hz audio channel transmitted over a double-sideband AM system would require 6 kHz of bandwidth whereas the same audio information would require only 2700 Hz of bandwidth using a single-sideband system. Hence, the single-sideband system described appreciates a bandwidth improvement of 10 log (6000/2700) or a 3.5-dB reduction in the noise power. A safe, general approximation is a 50% reduction in bandwidth for single sideband compared to double sideband which equates to an improvement in the signal-to-noise ratio of 3dB.Combining the bandwidth improvement achieved by transmitting only one sideband and the power advantage of removing the carrier, the overall improvement in the signal-to-noise ratio using single-sideband suppressed carrier is approximately 7.8dB (3+4.8) better than double-sideband full carrier.Selective Fading. With double-sideband transmission, the two sidebands and carrier may propagate through the transmission media by different paths and, therefore, experience different transmission impairments. This condition is called selective fading. One type of selective fading is called sideband fading. With sideband fading, one sideband is significantly attenuated. This loss results in a reduced signal amplitude at the output of the receiver demodulator and, consequently, a 3-dB reduced signal-to-noise ratio. This loss causes some distortion but is not entirely detrimental to the signal because the two sidebands contain the same information.The most common and most serious form of selective fading is carrier-amplitude fading. Reduction of the carrier level of a 100%-modulated wave will make the carrier voltage less than the vector sum of the two sidebands. Consequently, the envelope resembles an over modulated envelope, causing severe distortion to the demodulated signal.A third cause of selective fading is carrier or sideband phase shift. When the relative positions of the carrier and sideband vectors of the received signal change, a decided change in the shape of the envelope will occur, causing a severely distorted demodulated signal.When only one sideband and either a reduced or totally suppressed carrier are transmitted, carrier phase shift and carrier fading cannot occur, and sideband fading only changes the amplitude and frequency response of the demodulated signal. These changes do not generally produce enough distortion to cause loss of intelligibility in the received signal. With single-sideband, transmission, it is not necessary to maintain specific amplitude or phase relationship between the carrier and sideband signals.Noise Reduction. Because a single-sideband system utilizes half as much bandwidth as conventional AM, the thermal noise power is reduced to half that of a double-sideband system. Taking into consideration both the bandwidth reduction and the immunity to selective fading, SSB systems enjoy approximately a 12-dB S/N ratio advantage over conventional AM (i.e., a conventional AM system must transmit a 12-dB more powerful signal to achieve the same performance as a comparable single-sideband system).Disadvantages of single-sideband transmission. Following are two major disadvantages of single-sideband reduced- or suppressed-carrier transmission as compared to conventional double-sideband, full-carrier transmission.Complex Receivers. Single-sideband systems require more complex and expensive receivers than conventional AM transmission, because most single-sideband transmissions include either a reduced or suppressed carrier; thus, envelope detection cannot be used unless the carrier is regenerated at an exalted level. Single-sideband receivers require a carrier recovery and synchronization circuit, such as a PLL frequency synthesizer, which adds to their cost, complexity ,and size.Tuning Difficulties. Single-sideband receivers require more complex and precise tuning than conventional AM receivers. This is undesirable for the average user. This disadvantage can be overcome by using more accurate, complex, and expensive tuning circuits.第五章 单边带通信系统5.1 引言第3章和第4张所讨论的是普通AM双边带通信系统有两个缺点：第一，在普通AM中，载波功率总发射功率的2/3以上，这是一个主要的缺点，因为包含信息的是边带而不是载波。第二，普通AM系统的带宽是单边带系统的两倍，并且上、下边带包含的信息相同，因此，两个边带都发射是多余的，而且造成功率和带宽等的效率低。在现代电子通信系统设计中，主要要考虑这两点。本章主要介绍集中单边带AM系统，并将其与普通AM比较，说明选择他们所带来的好处和坏处。当多信道通信系统采用频分复用（FDM）时，常使用抑制载波的单边带系统，如在长话系统中。本章后面将介绍频分复用，并将在第16章对其进行详细讨论。5.2 单边带系统早在1914年，就在数学上验证了单边带，但直到1923年，才第一次承认这个专利，并在英国和美国间成功建立了通信链接。目前的单边带系统很多，有些能节约频带，有些能节约功率，有些能同时节约这两者。图5.1比较了普通AM和集中常见单边带（SSB）系统的频谱和功率分配。5.2.1 AM全载波单边带AM全载波单边带（SSBFC）是一种调幅形式。在此调制过程中，载波全功率发射，但边带只发射一个。因此，在SSBFC传输中，带宽是普通双边带AM带宽的一半。SSBFC的频谱和功率分配关系如图5.1（b）所示。注意，当100%调制时，载波功率占总发射功率的2/3（67%），边带功率占1/3（33%）。因此，银冠SSBFC需要的总功率少，但有用信息所占的功率百分比也很低。当对单频调制信号采用100%调制时，其SSBFC波形如图5.2所示。100%的全载波单边带包络和50%的全载波双边带包络相同。在第3章讲过，当载波和边带同时到达各自的峰值时，AMDSBFC波出现最大的正向和负向峰值，包络的峰值变化量等于上边频和瞎编频振幅之和。在单边带传输中，只有一个带（上边带或下边带）叠加在载波上，包络的峰值变化量仅是双边带变化量的一帮。因此在全载波单边带传输中，解调信号的振幅是双边带解调的一半，这样产生一个折中，即SSBFC需要的带宽比DSBFC小，但解调信号的振幅也比较低。当带宽减半时，噪声总功率也减半（即减少了3dB），而滤掉一个边带也将信息功率减半。一次单边带和双边带的信噪比相同。在SSBFC中，包络的重复率等于调制信号的功率，调制的深度也调职信号振幅成正比。因此， 和双边带传输一样，信息包好在全载波已调信号的包络中。5.2.2 抑制载波的AM单边带抑制载波的AM单边带（SSBSC）是一种调制形式，在此调制过程中，载波被完全抑制，并龙滤掉一个边带，因此，SSBSC需要的是带宽的普通双边带AM的一半，需要的发射功率很低。SSBSC（如上边带传输）的频谱和功率分配关系如图5.1（c）所示。可以看出，边带功率就是发射的总功率。图5.3是一个单频调制信号的SSBSC波形。可以看出，波形不是一个包络，而是一个单频正弦波。取决于要发送的是上边带还是下边带，其频率是载波频率加上调制信号频率或载波频率减去调制信号频率。5.2.3 载波减小的AM单边带载波减小的AM单边带（SSBRC）是一种调幅形式，在这种调制过程中，一个边带被完全过滤掉，而载波电压减小到未调制载波的10%。因此，边带功率占总发射功率的96%。为了产生振幅减小的载波成分，在调制时，先要将载波完全抑制掉，然后再将一个振幅减小的载波插入。因此SSBRC有时又称为插入载波的单边带。插入载波通常称为导频，是为了解调而插入的，本章后面将对此进行详细的说明。SSBRC的频谱功率分配如图5.1（d）所示，可以看出边带功率几乎占发射功率的100%。对于全载波双边带，包络的重复率等于调制信号的频率。若用普通峰值检波器来解调载波减小的波形，必须将载波分离出，然后对其放大，当振幅增大到一定程度时，再插到接收机中。由于在解调前先要放大接收机中的载波，使其振幅大于边带信号，因此有时也将这种载波减小的传输成为激励载波传输。SSBRC的带宽是普通AM的一半，由于发射的载波振幅小，也节省了相当多的功率。5.2.4 AM独立边带AM独立边带（ISB）是一种调幅形式。在此调制过程中，两个不同的调制信号互不相干地调制同一个载波频率。实际上，ISB是双边带传输的一种形式，其发射机有两个独立的一直载波的单边带调制器。一个调制器产生上边带，另一个调制器产生下边带。两个调制器单边带传输喜好结合在一起，形成双边带信号，这两个边带信号的相互独立的，但关于同一个载波对称。在频谱中，一个边带比载波高，另一个比载波低。和SSBRC一样，为了解调，要重新插入一个振幅减小的载波。ISB的频谱和功率分配如图5.1（e）所示。图5.4是独立的两个单边带信号（fm1和fm2）的发射波形，这两个信号的频率也相同，因此，波形与抑制载波的双边带波形相同，只是重复率是调制信号的两倍。在ISB中，两个信息所占用的频带宽度和一个信息的普通双边带所占用的频带宽度相同，因此，ISB能节省发射功率和带宽。在美国，ISB技术用于立体声AM传输。一个频道（左边）发射下边带，另一个频道（右边）发射上边带。5.2.5 AM残留边带AM残留便带（VSB）是一种调幅形式。在此调制过程中，载波和一个完整边带被发送，但另一个边带只发送了一部分。载波以全功率发射。在VSB中，调制信号低频部分的双边带都被发送，而高频部分之发送了一个边带。因此低频部分富有100%调制的优点，而高频部分连50%的调制效果都达不到。由于调制信号的低频部分被加强，因此在解调器中产生的低频信号振幅比高频成分大。VSB的频谱和功率分配如图5.1（f）所示。广为所
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