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CX74017 Datasheet(PDF) 1 Page - Skyworks Solutions Inc. |
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CX74017 Datasheet(HTML) 1 Page - Skyworks Solutions Inc. |
1 / 10 page Application Note Skyworks Solutions, Inc., Proprietary and Confidential 101735A Preliminary Data Subject to Change July 20, 2001 CX74017 On the Direct Conversion Receiver Abstract Increased pressure for low power, small form factor, low cost, and reduced bill of materials in such radio applications as mobile communications has driven academia and industry to resurrect the Direct Conversion Receiver (DCR). Long abandoned in favor of the mature superheterodyne receiver, direct conversion has emerged over the last decade or so thanks to improved semiconductor process technologies and astute design techniques. This paper describes the characteristics of the DCR and the issues it raises. Introduction Very much like its well-established superheterodyne receiver counterpart, introduced in 1918 by Armstrong [1], the origins of the DCR date back to the first half of last century when a single down-conversion receiver was first described by F.M. Colebrook in 1924 [2], and the term homodyne was applied. Additional developments led to the publication in 1947 of an article by D. G. Tucker [3], which first coined the term synchrodyne, in a receiver, which was designed as a precision demodulator for measurement equipment rather than a radio. Another paper by the latter in 1954 [4] reports the various single down-conversion receivers published at the time, and clarifies the difference between the homodyne (sometimes referred to as coherent detector) and the synchrodyne receivers: the former obtains the Local Oscillator (LO) directly, for example, from the transmitter, whereas the latter synchronizes a free-running LO to the incoming carrier. Over the last decade or so, the drive of the wireless market and enabling monolithic integration technology have triggered research activities on DCRs, which integrated with the remaining analog and digital sections of the transceiver, has the potential to reach the “one-chip radio”. Besides, it favors multi-mode, multi-standard applications and constitutes thereby another step towards software radio. The present article often refers to several recent publications [5-6], providing a thorough survey and insight, and displaying the renewed interest for DCRs. Overcoming some of the problems associated with the traditional superheterodyne and being more prone to integration, DCR has nevertheless an array of inherent challenges. After a brief description of alternative and well-established receiver architectures, this paper presents the direct conversion reception technique and highlights some of the system-level issues associated with DCR. Traditional Reception Techniques The Superheterodyne The superheterodyne, or more generally heterodyne1, receiver is the most widely used reception technique. This technique finds numerous applications from personal communication devices to radio and TV tuners, and has been tried inside out and is therefore well understood. It comes in a variety of combinations [7-9], but essentially relies on the same idea: the RF signal is first amplified in a frequency selective low-noise stage, then translated to a lower intermediate frequency (IF), with significant amplification and additional filtering, and finally downconverted to baseband either with a phase discriminatory or straight mixer, depending on the modulation format. This is illustrated in the generic line-up of Figure 1. RF band-select filter LNA Channel-select filter RF IF RF IF f f Image-reject filter 101735A 1_071801 Figure 1. The Superheterodyne Receiver Superheterodyning entails several trade-offs. Image rejection is a prevailing concern in this architecture. During the first 1 Homo: Greek from “homos” - same; Hetero: Greek from “heteros” – other; Synchro: Greek from “sunkhronos” – same time; Dyne: Greek from “dunamis” – power. |
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