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本文对不对称调幅所引起的失真进行了理论分析,与实验结果有较好的符合,并提出在不对称调幅幅中应引入“等效调幅度”的概念。调幅波由于占据的频带较窄,接收机较为简单,是最早被采用的调制方式之一,直到现在在中波和短波广播中仍被广泛地采用。但是由于最大调幅系数m不应大于1。平均调幅系数更小,使得它的边带波功率小,抗干扰性差,覆盖面积小。因此,如何提高平均调幅系数,提高信噪比,增加覆盖面积,是调幅制研究中的重要课题之一。采用各种语言加工技术是提高平均调幅系数的重要途径(见[1])。采用超过百分之百的上调幅系数,而下调幅系数限制在百分之百以内的不对称调制方式,是提高平均调幅系数的另一重要途径(见[5])。问题在于当上调幅系数超过百分之百,而下调幅系数限制在百分之百以内时,会带来多大的失真。本文将对此问题进行初步的探讨。近年发展起来的脉冲宽度调制调幅方法,由于它的调制是带有直流成分的,在过调幅时,边带波功率增加较多,而失真增加较少,为实现不对称调幅提供了可行的方法。
In this paper, the theoretical analysis of the distortion caused by asymmetric amplitude modulation is carried out, which is in good agreement with the experimental results. The concept of “equivalent amplitude modulation” should be introduced into the asymmetric amplitude modulation amplitude. Due to the narrower frequency band occupied by the AM wave, the receiver is relatively simple and is one of the earliest modulation schemes adopted until now, and is still widely used in medium and shortwave broadcasting. However, the maximum amplitude modulation factor m should not exceed 1. The average amplitude modulation coefficient is smaller, making its sideband wave power is small, anti-interference is poor, covering a small area. Therefore, how to improve the average modulation index, increase the signal to noise ratio and increase the coverage area is one of the important topics in AM research. The use of various language processing techniques is an important way to increase the average amplitude modulation coefficient (see [1]). The use of asymmetric modulation with a percentage increase of more than 100% and a factor of less than 100% for amplitude reduction is another important way to increase the average modulation index (see [5]). The question is, how much distortion will be caused when the rate of increase is above one hundred percent and the rate of decrease is limited to one hundred percent. This article will conduct a preliminary discussion of this issue. In recent years, the pulse width modulated AM method has been developed because of its modulation with a DC component. When the amplitude is overshooted, the power of the sideband increases more and the distortion increases less, which provides a feasible method for realizing asymmetric amplitude modulation .