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74017 Datasheet(PDF) 4 Page - Skyworks Solutions Inc. |
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74017 Datasheet(HTML) 4 Page - Skyworks Solutions Inc. |
4 / 10 page CX74017 On the Direct Conversion Receiver 4 Skyworks Solutions, Inc., Proprietary and Confidential 101735A Preliminary Data Subject to Change July 20, 2001 101735A 6_071801 RF band-select filter LNA Wideband filter RF IF RF DC 0f f Wideband IF f 0 LPF LPF Image-reject filter Figure 6. Wideband IF with Double Conversion. Direct Conversion Receivers Direct conversion reception, also referred to as homodyne, or zero-IF, shown in Figure 7, is the most natural solution to receiving information vehicled by a carrier. However, it has only been a decade or so that this type of reception has found applications other than pagers, for example [13]. For it has several qualities which make it very suitable for integration as well as multi-band, multi-standard operation, but severe inherent obstacles that had for long kept it in the shadow of the superheterodyne. RF band-select filter LNA RF DC LPF RF f DC 0f LPF 101735A 7_071801 Figure 7. The Direct Conversion Receiver First, the image problem is no longer present, since the IF is zero and the image to the desired channel, for all but single- sideband signals, is the channel itself. Then, only one LO is needed, which means only one phase noise contribution. The need for the bulky, off-chip filters is consequently removed. Filtering now only occurs at low, that is, baseband, frequencies with some amplification. This means less current consumption than at higher frequencies (to drive device parasitics), fewer components, lower cost, etc. Practically, however, strong out-of- band interferers or blockers may need to be removed prior to downconversion in order to avoid desensitizing the receiver by saturating subsequent stages, as well as producing harmonics and intermodulation terms, which then appear in baseband. Such a filter may be placed after the LNA, for example. DCR, however, brings its own set of issues. The following paragraphs describe those in more detail. DC offsets In direct conversion, as the signal of interest is converted to baseband very early in the receive chain, without any filtering other than RF band-selection, various phenomena contribute to the creation of DC signals, which are directly appearing as interfering signals in the band of interest. The LO may be conducted or radiated through an unintended path to the mixer’s RF input port, thus effectively mixing with itself, producing an unwanted DC component at the mixer output, see Figure 8. Worse still, this LO leakage may reach the LNA input, producing an even stronger result. This effect presents a high barrier against the integration of LO, mixer, and LNA on a single silicon substrate, where numerous mechanisms can contribute to poor isolation. These include substrate coupling, ground bounce, bond wire radiation, and capacitive and magnetic coupling. |
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