AD9874_Microwave_RF_Article| Application Note
Low-Power IC Digitizes 300 MHz IF
Flexible IF digitizer has up to 270 kHz BW with >90 dB dynamic range
R. Schreier Design Engineer P. Hendriks Applications Engineer
Analog Devices, Inc., Wilmington MA 01887; (781) 937-1175, FAX: (781) 937-1027; e-mail: paul.hendriks@analog.com, Internet: http://www.analog.com
UPERHETERODYNE receivers achieve high dynamic range, but typically require more power and are more complex than lower-performance direct conversion receivers. The AD9874 IF-digitizing IC eliminates this barrier by integrating all the blocks except the VCOs needed for IF-to-digital conversion in a narrowband superheterodyne receiver. This high level of integration is provided by a small device with low power consumption, without compromising dynamic range.
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Designers of next-generation digital radio communications systems are faced with many challenges as they strive to advance the state of the art in mobile radio systems. These challenges culminate in finding unique ways to reduce power consumption, physical size and equipment cost, while increasing performance, functionality and multi-mode configurability. A key strategy for achieving these goals is smart partitioning, namely determining the best system structure given power, performance and time constraints. Should a traditional superheterodyne receiver or a direct RF or IF solution be used? Is an analog or a digital quadrature modulation scheme better? What is the distribution of analog and digital filtering in the RF strip? The answers to these questions depend on the specific application requirements and on the capabilities of available components. For example, police, fire and ambulance services rely on radio communications to coordinate their daily operations. In these applications the radio is a life-s
aving device which must meet the highest standards of reliability and performance. Private mobile radio (PMR) encompasses the above mission-critical applications as well as taxi, delivery and security services, which also demand high-performance radios in order to maximize coverage. PMR applications require components for
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three main platforms: handsets, mobile units (in vehicles) and basestations. PMR handsets require ultra low power to maximize operating time, but PMR basestations emphasize linearity and dynamic range in order to maximize network performance. In next-generation cellular basestations, smaller size, lower power and lower costs are driving reasons for redesign. GSM basestation designers are seeking ways to achieve these advancements while maintaining the overall integrity of the system. The AD9874 takes a novel approach to partitioning the IF strip of a narrowband radio receiver that helps designers meet their power, size and cost goals while achieving high dynamic range. The superheterodyne receiver, an example of which is illustrated in Figure 1, is a popular architecture known for high dynamic range. The implementation shown avoids quadrature analog downconversion to baseband by using filtering together with multiple analog downconversion operations, thereby side-stepping troublesome issues associated with q
uadrature analog downconversion and low-frequency analog signal processing. Constructing such a receiver is complicated by the need to find a good frequency-plan and the need to deal with a multitude of sub-blocks. The AD9874 simplifies the design problem by inte-
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