Microwave Engineering Europe - November 2008 - (Page 26) 26 MMICs — RF FRONT END frequency range and across multiple modulation modes. Instead of optimising a Þnal RF stage power transistor supplied by constant voltage, the supply is changed dynamically, modulated in synchronism with the envelope of the incoming RF signal (see panel). Doing this ensures that the output device stays in saturation Ð its most efÞcient operating region. As a result, handsets incorporating HAT technology can cover the complete range of 3G-plus modes and frequencies with just two wideband ampliÞers instead of seven (see Figure 3) without compromising efÞciency. Figure 3: RF sub system with wideband enveloping tracking. Figure 4: Efficiency locus when using Nujira’s HAT. Figure 5: Estimated average RF power output versus battery power requirements for envelope-tracking power amplifier compared with conventional MMICs. Adopting envelope-tracking technology allows handset designers to use far fewer, less efÞcient wideband PAs, since the HAT technology more than compensates for the lost efÞciency. The HAT based RF front end design in Figure 3 uses just two PAs but covers the same frequency spectrum as the seven PA design shown in Figure 2, matching or exceeding its performance and efÞciency, and saving an estimated 30 percent the BOM cost of multi-banding the RF circuit. The diagram shows clearly that HAT implementation in the handset has none of the scaling issues associated with other efÞciency enhancing techniques used in the network. The DC-DC converter powering the RF subsystem is simply replaced with a power modulator. Of course some subsystems are powered direct from the battery without a converter Ð but the introduction of one additional component is more than justiÞed by the removal of Þve PAs. In these designs, the HAT power modulator offers the additional beneÞt of providing the subsystem with a controlled operating point regardless of the state of the battery, simplifying the design of the wide bandwidth matching network. Figure 4 demonstrates the high efÞciency of an envelope-tracking ampliÞer throughout the high-probability region of continuous output power. It is essentially a superposition of the previous Figure 1, showing a series of drain efÞciency versus RF power output curves as the supply voltage is varied. The locus of these curves represents the efÞciency of a power ampliÞer driven by variable voltage. Reducing RF power consumption simpliÞes the thermal design of the terminal, easing heat ßow through the handset itself and reducing PA packaging costs. This will lessen the tendency for the phone to warm up in use, which users can Þnd disconcerting. To retain compliance with regulatory requirements for low noise and spurious emissions the power modulator tracks the RF signal envelope with utmost accuracy in both timing and amplitude. It does so by calculating the amplitude from the digital signal (√I² + Q²) and applying a shaping function to compensate for known nonlinearities. In parallel, a delay is calculated and applied to the RF signal before it is input to the ampliÞer, thus reducing timing errors The envelope control signal can be derived digitally or by detecting the amplitude of the RF signal. High Accuracy Tracking can be used with Microwave Engineering Europe ● November 2008 ● www.mwee.com http://www.mwee.com
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