Design an all-digital modulator with an RF output

Zhuan Ye and John Grosspietsch, Motorola Labs
Apr 10, 2006 (5:00 AM), CommsDesign

A software-defined radio (SDR) terminal promotes programmable realizations of the physical layer functionalities. A lot of research work has been done in applying DSPs and FPGAs to implement the baseband functionalities of the physical layer.

An SDR is defined as a radio in which the digitization is performed at some stage downstream from the antenna. Then the radio can use flexible and reconfigurable functional blocks to implement the DSP algorithms. As technology advances, the digitization might be at, or very close to the antenna, such that almost all the radio functionalities are realized through software using a high-speed, reprogrammable DSP engine.

Current radios combine analog and digital building blocks. The RF functions are most likely being implemented using analog circuits, while the baseband functions are more suitable for DSP implementations. A digital front end (DFE) is often used to bridge between the RF and baseband processing. The DFEs are normally capable of processing signals with frequencies at about tens of megahertz, often called digital intermediate frequencies (IF). Hence, significant analog blocks remain between the RF and digital IF.

Digital RF transceivers can extend the software defined functions into radio frequencies. The key advantages of all digital transmitters are high-efficiency power amplification; digitally combining signals from multiple channels; and software programmability or reconfigurability.

Using an all digital transmitter, the entire transmitter can be realized using a DSP or FPGA, which can take advantage of the increasing performance of CMOS technology. Besides its compatibility with SDR, DSP-based RF systems can be made to compensate for the RF channel's impairments. Therefore, digital signal generation directly at radio frequency has drawn a lot of interests among researchers and engineers.

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