Microwave Engineering Europe - January/February 2008 - (Page 34)

34 DSP AND OFDM Figure 4: Measured effects of distortion compensation. to 42 GHz in separate bands. Figure 5 shows an example of the AMMP-6408 MMIC. For the same 0.1 W average power transmission, this ampliﬁer is backed off 10 dB from its 1 dB compression point. The resultant DC power consumption is 3 W of which 90 percent must be dissipated away as heat from the 5- x 5mm package. Challenges with OFDM signal reception Since receive signals are at such a low power level, the PAR is normally not an issue unless the transmitter is very close to the receiver. The most common problem faced in the reception of OFDM signals is the dynamic range of the receiver. The system noise ﬁgure determines the lowest signal that the OFDM system can detect and process. The ﬁltering and input of 1 dB system compression determine the largest signal that can be processed. Unfortunately the two factors can often be dynamically opposed. In the upper level of signal reception, the desired signal may include low power, interferers, self-mixing products, and outof-band transmitters (from cell phones to air trafﬁc control radars) that can saturate the receiver. To reduce the effect of these unwanted signals, the best ﬁlters and switches combined with the highest 1 dB compression/lowest gain LNA (Low noise ampliﬁer) must be used. A typical input 1 dB compression for an OFDM system is -2 dBm in band. The lowest level of signal reception is determined by the system noise ﬁgure. Unfortunately, the addition of ﬁltering and switching in front of the LNA directly increases the system noise ﬁgure. Elements after the LNA contribute less to the system since their effects are reduced by the LNA gain. If the LNA gain is more than 10 dB, then post LNA effects can be ignored. A typical goal for the system noise ﬁgure is under 4 dB. A quality LNA runs 1 to 2 dB of noise ﬁgure which translates to a maximum loss of 2 to 3 dB in the ﬁlter and switch before the LNA. To use a lower quality LNA with a higher noise ﬁgure means that higher quality switches and ﬁlters must be used to meet the same system specs and visa-versa. Avago Technologies also offers a variety of LNAs and discrete FETs that can be used to build LNAs for OFDM systems. The ATF-54xxx and ATF-55xxx ePHEMT discrete FET series are popular with many custom 0.1-18 GHz systems designed for high performance and low cost. These Figure 5: Typical ﬁxed point class A power ampliﬁers (AMMP-6408 6-18 GHz PA). Figure 6: Typical application of an ATF series ePHEMT FET for customer OFDM DC-18 GHz custom low noise applications. Figure 7: Typical MGA series product for 2-6 GHz WiFi and WiMax applications. Microwave Engineering Europe ● January/February 2008 ● www.mwee.com 032-033-034-035-036_MWEE.indd 34 25/01/08 13:40:05 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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