带通欠采样技术及在数字中频软件接收机中的工程应用

Uniform undersampling technique of bandpass signal and its engineering application in software digital IF receiver

对无混叠带通信号的均匀采样定理进行了讨论,分别指出了经典的低通信号均匀采样定理和最小无混叠带通信号采样定理均是该定理的特例,整数频带位时两倍带宽采样率的潜在危险,大于四倍带宽的采样率并不能保证带通信号的无混叠采样,无混叠采样率的降低与对信号保护带和采样时钟精度要求之间的矛盾,以及欠采样区间的选取与欠采样后原信号正负频谱错位之间的关系。之后,结合作者的工程实践,论文给出了该技术在数字中频软件接收机中的具体应用实例。根据实际中需要考虑的几个制约因素对采用的欠采样率进行了精确选择,使得系统可以采用一款廉价的DSP芯片,并成功地使用了该芯片所集成的低速ADC和有限的DSP运算资源,大幅降低了接收机的生产成本。其中还对带通信号欠采样后的频谱结构进行了讨论,指出利用欠采样实现免混频解调法存在的不安全性,为此,论文提出了在欠采样后再进行低频混频和低通滤波的解决办法。

Several conclusions are derived from a discussion about the bandpass signal＇s alias-free uniform undersampling theorem. For example, the classical lowpass signal＇s uniform sampling theorem and the minimum alias-free bandpass signal＇s sampling theorem can be both treated as the special cases of the above theorem, risks are lurking among the integer band position case, the sampling frequency more than quadruple bandwidth does not promise the alias-free sampling with bandpass signal, conflicts exist between the decrease of alias-free sampling frequency and the requirements with the width of signal＇s guard-band and the accuracy of sampling clock, the relationship as well as is given between the selection of undersampling frequency regions and the spectrum misplacement with the positive and negative spectrum for undersampled signal, and so on And then a concrete application sample for soRware digital IF （Intermediate Frequency） receiver is presented in combination with the authors＇ engineering practice. In consideration of several limited factors in practice, the undersampling frequency is accurately selected. Consequently, a low-cost DSP （Digital Signal Processor） may be used in the above system. In addition, the ADC （Analog to Digital Converter） and the limited processing resources integrated in the DSP are put into operation successfully. As a result, the production cost of the receiver is reduced notably. Above all, the undersampled bandpass signal＇s spectrum structure is investigated, from which, the unreliability comes to light in the mixer-free demodulation by undersampling method. For this purpose, the paper brings forward a way out by low-frequency mixing with lowpass filtering after the bandpass signal＇s undersampling.