DSP收音机学习

18 www.rfdesign.com April 2005 Software-Defined Radio New technology facilitates true software-defined radio While the article summarizes products available for software-radio application, it presents for the first time a carrier speed 5 GHz RF to a digital converter, which when coupled with state-of-the-art filtering software, can be used to meet the SDR Forum’s definition of the ideal software radio. With this technology, the article demonstrates the first true software radio. By Ronald M. Hickling adio technology as we know it is undergoing sweeping changes.RFor those of you old enough to recall, many of the earliest radio receivers were built using crystals and fine wire probes that were moved by the listener to form a diode and included a variable inductor with a slider to tune the receiver to a radio station’s frequency so one could hear music or voice. The need for precise tuning has changed little in the more than 80 years since CW transmitters replaced the very broadband spark gap approach originally used by Marconi. Today, even with advancements in RF design and the powerful digital processing available, all radio receivers still use analog parts to tune the radio to a specific carrier frequency. But this is about to change. The FCC, the SDR Forum and the radio industry are united in the pursuit of the ideal software-defined radio or SDR. According to the Federal Communications Commission (FCC), “In a software-defined radio (SDR), functions that were formerly carried out solely in hardware, such as the generation of the transmit- ted signal and the tuning and detection of the received radio signal, are performed by software that controls high-speed signal proces- sors.” The SDR Forum defines an SDR device as one that functions independently of carrier frequencies and can operate within a range of transmission protocol environments. But the SDR Forum goes a step further by defining the ideal SDR as one that has transceivers that perform upconversion and downconversion between baseband and the RF carrier itself exclusively in the digital domain, reducing the RF interface to a power amplifier in the transmit path, a low noise amplifier in the receive path, and little or no analog filtering. In this ideal radio, it is possible to upgrade or completely change the features by simply uploading new software. This ideal radio defined by the SDR Forum has, until recently, been unachievable due to the lack of very high-frequency RF to digital converters capable of converting carrier frequencies directly to digital data. Now new integrated circuit processes are offering higher speed and lower power. State-of-the art IC design is being applied to these new processes to enable RF to digital conversion directly on carrier frequencies above 5 GHz. This article describes the products available in production today and then presents the first carrier speed 5 GHz RF to digital converter to be proven in silicon. This converter, coupled with state-of-the art filtering software, can be used to meet the SDR Forum’s definition of the ideal SDR. With this technology the first true software radio is achievable. Effective radio communication SDR is of critical importance to the future of efficient and effective radio communication that must include interoperability. As we have seen, municipal services such as fire departments and police depart- ments often have radios that will not communicate between services. With the ideal SDR radio, software can be used to act as an interpreter between completely incompatible radio frequencies and modulation techniques. 900 MHz radios can talk to 2.4 GHz radios, GSM cell phones can communicate with CDMA phones. And the military can start to move to radios that allow all of the services to communicate seamlessly. The U.S. Military Joint Tactical Radio System (JTRS) program has announced the goal of supporting 33 modulation techniques on multiple carrier frequen- cies ranging from 2 MHz to 55 GHz with one radio design. The ideal SDR radio will be critically impor- tant in meeting that goal. Until now, radio design has been limited to the use of analog RF front ends (RFEs) for upconversion and downconversion to an intermediate frequency (IF) of below 100 MHz that off-the-shelf analog to digital converters (ADCs) could handle (Figure 1). The radio receiver is the toughest challenge for the ideal SDR. The ideal receiver must extract rapidly changing information from small RF signals buried within a sea of noise. Newer RFEs are using superheterodyne, direct-conversion, and hybrid techniques. But these radios do not meet the ideal radio defined by the SDR Forum. Instead, they extend the software-defined baseband processors that today’s cell phones employ to become soft- ware-defined IF processors. As an example, a GSM900 receiver with a centerTable 1. RF to digital converter target specs by process. Parameter GaAs SiGe Maximum clock frequency > 5 GHz > 15 GHz SINAD (signal to noise+distortion) > 70 dB > 110 dB Eff. resolution bits at 2.5 GHz carrier: 14 bits 18 bits 10 MHz BW 11 bits 14 bits 100 MHz BW Matching I/Q within -70 dBc Within -90 dBc RF input voltage range (differential) -1.5 V