RF integration: another point of view from Analog Devices
[2008-7-21]
Tag : Antenna RF
In a recent feature , EDN explored the integration of RF circuitry into SoCs forwireless mobile devices. There is a trend, certainly, toward thesingle-chip radio, excluding the power amplifier (PA) and antennaswitch that are for most air interfaces just too taxing for today'sdigital CMOS processes. But despite the trend, there is still abooming market in small-signal RF components. One example comesfrom Analog Devices, which recently introduced a pair of powerdetector ICs: one for base stations and the other for handsets.Each illustrates a good argument for keeping this function outsideof both the PA and the baseband SoC.
First, the function itself. In wireless applications, powerdetectors form the feedback loop between the amplifier output andthe baseband processor. Feedback control of power is necessary forseveral reasons. First, process, voltage, and especiallytemperature variations make open-loop control of output powerimprecise without a lot of software compensation. Second, in thehandset, harmless-looking actions like changing the orientation ofthe phone near a large metal surface, or—RF desingers'favorite--putting your hand on the antenna can make a significantchange in VSWR, changing the entire power picture independent ofthe gain setting of the PA.
Why is precise power control important? In most air interfacesexcept GSM, precise control over output power turns out to be veryimportant, according to Analog Devices applications engineer CarlosCalvo. OFDM radios, and even to some extent HSDPA systems, exhibithigh peak-to-RMS ratios, and hence operate their PAs in theirlinear region. But at lower gain, the PA is less efficient. So forbest efficiency, and hence best battery life, you want the PA tunedas close to saturation as possible. Yet not too close: if you startclipping those very high wave crests, you will waste energy, impairsignal quality and violate the regulations for spurious radiation.So getting the PA output power just right is a big deal.
In a recent feature , EDN explored the integration of RF circuitry into SoCs forwireless mobile devices. There is a trend, certainly, toward thesingle-chip radio, excluding the power amplifier (PA) and antennaswitch that are for most air interfaces just too taxing for today'sdigital CMOS processes. But despite the trend, there is still abooming market in small-signal RF components. One example comesfrom Analog Devices, which recently introduced a pair of powerdetector ICs: one for base stations and the other for handsets.Each illustrates a good argument for keeping this function outsideof both the PA and the baseband SoC.
First, the function itself. In wireless applications, powerdetectors form the feedback loop between the amplifier output andthe baseband processor. Feedback control of power is necessary forseveral reasons. First, process, voltage, and especiallytemperature variations make open-loop control of output powerimprecise without a lot of software compensation. Second, in thehandset, harmless-looking actions like changing the orientation ofthe phone near a large metal surface, or—RF desingers'favorite--putting your hand on the antenna can make a significantchange in VSWR, changing the entire power picture independent ofthe gain setting of the PA.
Why is precise power control important? In most air interfacesexcept GSM, precise control over output power turns out to be veryimportant, according to Analog Devices applications engineer CarlosCalvo. OFDM radios, and even to some extent HSDPA systems, exhibithigh peak-to-RMS ratios, and hence operate their PAs in theirlinear region. But at lower gain, the PA is less efficient. So forbest efficiency, and hence best battery life, you want the PA tunedas close to saturation as possible. Yet not too close: if you startclipping those very high wave crests, you will waste energy, impairsignal quality and violate the regulations for spurious radiation.So getting the PA output power just right is a big deal.
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