Microwave Engineering Europe - April 2008 - (Page 19)

MILITARY/AEROSPACE FOCUS 19 accurately converts it to an analog signal of sufﬁcient magnitude (power) for transmission by means of an antenna. One problem with this architecture is that software processing must be carried out at a frequency at least twice the carrier frequency. For example, to implement an IEEE 802.11a compliant device, the required processing speed will exceed 11 Gsamples/second! Today, and in any foreseeable future, the digital processing of signals is carried out at baseband or at IF (intermediate frequency), and the processed signal is then upconverted to the higher RF frequency with the help of a frequency synthesizer. Even if hardware (including DACs) fast enough to allow purely digital implementation of the signal processing functions at the ﬁnal RF frequency will be available in the far future, we will still have great difﬁculties in connecting the DAC directly to the antenna. Obviously, in any practical implementation, the DAC backend must possess the characteristics of an RF power ampliﬁer (PA). Namely, it is expected to provide the necessary performance (output power, harmonics and spurious levels, noise ﬂoor, etc.) over the whole frequency range of interest for the ISWR. The PA fulﬁlls a very important function: it converts the input DC power provided by a battery or other DC power source, into an accurate replica of the signal to be transmitted. Integrated circuits (IC) have on chip power transistors capable to provide a few tens of milliwatts: if higher power is required, external transistors are used. Anyway, additional functions are always needed in any practical implementation: these include smoothing (ﬁltering out the unavoidable spectral artifacts that are a result of discrete signal representation), impedance matching, power combining, harmonics ﬁltering, RF power control (ALC), DC power regulation, and other analog circuits. And almost all of these functions are either frequency dependent or bandwidth limited. The inescapable conclusion is that our ideal transmitter DAC must be replaced by the more practical equivalent shown in Figure 2. Thus, we will have to design a software transmitter, without getting rid of any current aspect of analog transmitter hardware design. Unfortunately, now we must also solve new problems, especially with respect to the higher noise ﬂoor level, which is due to the poor spurious free dynamic range (SFDR) performance of high speed DACs. The major RF power ampliﬁer design challenge is heat dissipation (heat is due to losses inherent in the DC to RF power conversion process). Current PA efﬁciency ranges from 40 to 10 percent when linear ampliﬁcation is required, even using different linearization techniques. The required linearity depends on the modulation scheme employed in a radio system. The OFDM scheme, regarded as a critical component of the JTRS wideband networking waveform (WNW), requires very high PA linearity, even when clipping and forward error correction (FEC) are used to optimize overall system performance. The need for an appropriately dimensioned heatsink, capable of dissipating the power generated by internal losses within the radio system, greatly affects radio set size. As a general statement, radio set size depends to a great extent on its RF power output. This statement stems from the very basic fact that the whole physical envelope of a radio set is used to dissipate power (heat pipe technology just helps transport the heat to the envelope). The statement, which holds true for any radio, including a software radio, may easily prove a major limiting factor in multi channel radio [2]. • The receive path Many suggest that the real opportunity for software to contribute to a radio system’s RF testing out of control? Only Keithley gives you the RF test tools you need to rein in today’s devices and tame tomorrow’s challenges. MODEL 2920 RF SIGNAL GENERATOR MODEL 2820 RF SIGNAL ANALYZER ■ Test the most complex signal structures, including 802.11n WLAN MIMO and 802.16e Wave 2 WiMAX. Conﬁgure a 4x4 MIMO test system costeffectively. Generate and analyze signals up to 6 GHz repeatybly and accurately with our instruments’ software-deﬁned radio architecture. Reduce your time to market and cost of test with MIMO systems optimized for R&D and production test. ■ ■ ■ Go to www.keithley.com/tame and try a demo. www.keithley.com/contact info@keithley.de Microwave Engineering ● April 2008 ● www.mwee.com 018-019-020-021-022_MWEE.indd 19 26/03/08 18:07:08 http://www.keithley.com/tame http://www.keithley.com/tame http://www.keithley.com/contact http://www.mwee.com

Table of Contents Feed for the Digital Edition of Microwave Engineering Europe - April 2008

Microwave Engineering Europe - April 2008 News Contents Comment Test and Measurement: Comprehensive WiMAX and Wi-Fi Product Design Demands Effective Channel Emulation Military/Aerospace Focus: Hardware Needs Limit Software Radio Interview — Mitsubishi Electric Europe: GaAs Technologies Spanning High-End Space and Radar Through to Cost-Sensitive Handset and LNB Applications How Do You Test ZigBee Transmitters? Advanced Receiver Design Boosts Performance CMOS PAs Pave the Way for One-Chip Phones Products Calendar

Microwave Engineering Europe - April 2008

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