基于非对称滤波的解调频法的原理研究

从调频信号边频分量的幅值相位特征出发，分析了平方解调法不能对调频信号进行解调的原因。分析过程中发现由于调频信号栽波频率左右两侧的边频分量的相位具有一种特殊的对称性，和调幅信号完全不同，使得常用的解调方法不再适用于调频信号。针对上述问题，从打破调频信号边频分量的特殊对称性出发，利用非对称滤波法（滤波器的中心频率远离载波频率）对调频信号进行滤波，用常规解调法就可以进行解调。通过对调频信号边频相位的非对称特性的分析和研究，为寻找更好的解调频方法提供了新的思路。

Frequency Demodulation Based on Asymmetrical Filtering

Based on the Bessel function of the first kind,the FM signal can be expressed as the sum of infinite sinusoid terms.That means FM signals have the same frequency features as AM signals,however the test results show that AM demodulation methods are invalid for FM signals.In order to find out what reason induces these results,the amplitude and phase of the each FM signal's sidebands was analyzed carefully.The frequency difference of two adjacent sidebands of FM signal can be demodulated based on the square demodulation principle,but the sum of all difference components is equal to zeros as a result of dissymmetry of the sidebands at the different sides of carrier frequency.A new method is adopted in this paper to overcome this shortcoming.First,FM signal is passed through an asymmetrical band-pass filter,which has a center frequency is not equal to the carrier frequency.Then the filtered FM signals can be demodulated by the normal demodulation methods.