Compared with positive realness, finitefrequency positive realness is a new notion, and it also finds some important applications in control theory.
相比于正实性，有限频率正实性则是较近提出的概念，并且亦在控制理论中找到了大量应用。
2
In STFT is window is of finite length, and we no longer have perfect frequency resolution.
快速傅立叶变换用到的窗宽度长度有限，因此不能获得完美的频率分辨率。
3
Due to the quantization of electromagnetic fields there is an upper frequency limit in the spectrum of any finite energy transmitted by an electromagnetic system.