APPLICATION NOTE GMSK Practical GMSK Data （应用程序注意实现GMSK实际实现GMSK数据）.pdf

APPLICATION NOTE GMSK Practical GMSK Data Transmission Practical GMSK Data Transmission Introduction The proliferation of computers in todays society has increased the demand for transmission of data over wireless links. Binary data, composed of sharp one to zero and zero to one transitions, results in a spectrum rich in harmonic content that is not well suited to RF transmission. Hence, the field of digital modulation has been flourishing. Recent standards such as Cellular Digital Packet Data (CDPD) and Mobitex* specify Gaussian filtered Minimum Shift Keying (GMSK) for their modulation method. GMSK is a simple yet effective approach to digital modulation for wireless data transmission. To provide a good understanding of GMSK, we will review the basics of MSK and GMSK, as well as how GMSK is implemented in CDPD and Mobitex systems. GMSK modems reduce system complexity, and in turn lower system cost. There are, however, some important implementation details to be considered. This paper will cover some of these details, focusing on interfacing a single chip baseband modem to the IF/RF section of a typical FM radio topology. Background If we look at a Fourier series expansion of a data signal we see harmonics extending to infinity. When these harmonics are summed, they give the data signal its sharp transitions. Hence, an unfiltered NRZ data stream used to modulate an RF carrier will produce an RF spectrum of considerable bandwidth. Of course, the FCC has strict regulations about spectrum usage and such a system is generally considered impractical. But if we start to remove the high frequency harmonics from the Fourier series (i.e. pass the data signal through a lowpass filter), the transitions in the data will become progressively less sharp. Th