科技英语入门

作者简介

《科技英语入门》共分10单元，内容包含数学、物理学、生物学、化学、通信、电子、计算机及机械等理工科题材。 本教材最大的特色就是在口语方面做了较多尝试，编者在每章都配备了针对本单元主题和文章内容的口语题目，供老师在上课时选择性进行师生、生生等口语交流练习。

书籍目录

Unit One Mathematics and PhysicsUnit Two Computer SoftwareUnit Three Computer NetworksUnit Four Analog ElectronicsUnit Five Digital Logic CircuitsUnit Six An Important Communication TooI--MATLABUnit Seven Communication ModulationUnit Eight MechanicsUnit Nine ChemistryUnit Ten Biology

章节摘录

　　The modulation is the trapezoidal wave shown in Fig. 7-2 （a）, while thecarrier is the cosine wave shown in Fig. 7-2 （b）. In the amplitude modulatedcase, Fig. 7-2 （c）, the modulation forms the envelope of the modulatedcarrier. That is, both the positive and negative excursions of the modulatedcarrier are determined by the magnitude of the modulation waveform.Notice that in the AM case neither the frequency nor the phase of the wavechange with time.　　In the frequency-modulated wave,　Fig. 7-2（d）,　the amplitude isconstant but the frequency varies.　The frequency is lowest when themodulation is least positive. One can view the frequency-modulated wave asone in which the sinusoids bunch up or spread out according to themagnitude of the modulating wave.　　The phase-modulated wave, Fig. 7-2 （e）, also exhibits bunching andspreading of sinusoids, but in a different way than for frequency modulation.During the flat portions of the modulation （constant phase）, the PM wavelooks just like the carrier except that its phase is different.

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