Microwave Engineering Europe - January/February 2008 - (Page 33)

DSP AND OFDM 33 in the last stages of transmit power ampliﬁcation, incoming low noise receive ampliﬁcation, and signal up/down block conversion. For this last gap between the CMOS DSP and the antenna, Avago Technologies offers a variety of low cost solutions that complement CMOS in the quest to effortlessly move large amounts of wireless data. Challenges with OFDM power transmission The OFDM modulation scheme works by splitting the radio signal into multiple smaller sub-signals that are transmitted simultaneously at different frequencies to the receiver. OFDM reduces the amount of crosstalk in the system by the arrangement of the magnitude and phases into an optimum bit pattern. An example of a measured OFDM signal for WiFi is shown in Figure 1. The fundamental challenge with an OFDM signal is the PAR (Peak to Average Ratio). In an ideal data frame, the PAR is about 10.8 dB. When the average transmit power is 0.1 W (typical for WiFi) the resulting peak power is 1 W, which poses an incredible challenge on the ﬁnal transmit power ampliﬁer. A simple CMOS class A ampliﬁer would consume over 8 W of DC power (and dissipate resultant heat) to serve as the ﬁnal transmit stage. This would be very impractical. In most applications, GaAs-based power ampliﬁers working in Class AB mode with distortion compensation are used. Here the PAR can often be reduced to a theoretical 6.6 dB if a base level of 2.6 dB EVM is acceptable through peak amplitude clipping. An example is shown in Figure 2. Generally, GaAs ampliﬁers can provide enough gain in 2 or 3 stages while CMOS takes 6 to 8 stages. The distortion compensation uses the natural AM-AM and AM-PM distortion of one stage and forces the following stage to distort in reverse. This technique allows the same average transmit power of 0.1 W in WiFi to be transmitted at 0.4 W peak power. Combined with class AB operation and distortion compensation, a modern WiFi power ampliﬁer can accomplish this with only 0.66 W of DC power. An example of an Avago Technologies’ class AB WiFi power ampliﬁer with distortion compensation is shown in Figure 3. In this case, the ampliﬁer is implemented in two stages of enhancement-mode PHEMT but the same techniques could be used in other technologies. The main feature is that the quiescent current is set as low as possible, allowing the current to increase naturally as power level increases. As the ﬁrst stage goes into AM-AM and AM-PM distortion, the second stage goes into reverse distortion. The result is a distortion compensation scheme that works over a limited frequency and power transmit range. This technique is used in all Avago Technologies’ WiFi and WiMAX power ampliﬁers and FEMs and the advantages are shown in Figure 4. For frequencies above 6 GHz, distortion compensation can be very hard to design and manufacture. Consequentially the typical offering is class A power ampliﬁers with their associated high current. Since nearly all OFDM applications above 6 GHz (LMDS, Point-to-Point Radio, etc.) use a ﬁxed-point base station, thermal and DC power management is easier. Avago Technologies has an entire family of class A power ampliﬁers that cover 6 GHz The Skyworks Advantage Extremely high stop-band isolation while maximizing pass-band ﬂatness and minimizing group delay distortion. Programmable in 1 MHz steps. ■ SKY73201-364LF Single Channel Differential I/O SKY73202-364LF Dual Channel Differential I/O O ■ NEW Programmable 1–28 MHz 6th Order Low Pass Butterworth Filters Power Ampliﬁers • Switches • Transceivers • Front End Modules • RF Subsystems Single Package Radios • Mixers • Synthesizers • Switches • Attenuators • Diodes • Technical Ceramics USA: 781-376-3000 • Asia: 886-2-2735 0399 Ext. 990 • Europe: 33 (0)1 41443660 • Email: sales@skyworksinc.com • www.skyworksinc.com Microwave Engineering ● January/February 2008 ● www.mwee.com 032-033-034-035-036_MWEE.indd 33 25/01/08 13:39:49 http://www.skyworksinc.com http://www.skyworksinc.com http://www.mwee.com

Table of Contents Feed for the Digital Edition of Microwave Engineering Europe - January/February 2008

Microwave Engineering Europe - January/February 2008 Contents News Comment Radio: Raising the Bar for the Radio: Making 802.11n Work Cover Feature: The RF-System-In-Package Trend - Efficient Design with Advanced Design System 2008 Wireless Sensor Networks: The Zigbee PRO Feature Set: More of a Good Thing Very Fast Measurements of Wireless Devices with Small Antennas in Reverberation Chambers WiMAX Update 2008 Bridging the Gap from the CMOS DSP to the Antenna in OFDM Systems Products Calendar

Microwave Engineering Europe - January/February 2008

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