Microwave Engineering Europe - October 2007 - (Page 32)

32 OPTIMISING THE HANDSET RF FRONTEND Figure 10: Typical dual-mode RF switching circuit for 3G applications employing an SP7T RF switch, a pair of LPFs and a UMTS duplexer. magnitude of close to unity; and a phase shifter can be used to rotate the phase of this reﬂection co-efﬁcient so that it presents the optimum value of zero Ohms to the RF switch at one or more of the harmonic frequencies, thereby further reducing the harmonic generation of the switch. Phase shifters in 3G handsets The switching or multiplexing section of a typical 3G or dualmode RF front-end circuit is shown in ﬁgure 10. In addition to the need to match the PA to the switching circuit, and the requirements of low harmonic generation by the RF switch, a dual-mode RF front-end circuit has extra linearity constraints due to the fact that simultaneous TX and RX are required. UMTS TX signals generated by the RF front-end circuit in the frequency range from 1.92 to 1.98 GHz, and blocker signals at the antenna port in the frequency range from 1.73 to 1.79 GHz, mix in the RF switch producing third order intermodulation products which fall in the frequency range from 2.11 to 2.17 GHz. It should be noted that the GSM1800 TX band extends from 1.71 to 1.785 GHz, so blocker signals in the frequency range from 1.73 to 1.785 GHz represent a real rather than a theoretical concern. Fortunately, intermodulation products can be minimised by reducing the magnitude of the blocker signal entering the RF switch, and this can be achieved if the reﬂection co-efﬁcient presented to the blocker signal at the input of the RF switch has a magnitude of -1. Figure 11 shows the dual-mode RF switching circuit of ﬁgure 10 where a phase shifter is placed between the UMTS duplexer and the RF switch, with the phase shifter selected to rotate the reﬂection coefﬁcient to approximately -1 in the 1.73 to 1.785 GHz frequency range. Figure 12 shows the effect of adjusting the phase of the reﬂection co-efﬁcient presented by the UMTS duplexer of a dual mode RF switching circuit in the frequency range from 1.73 to 1.785 GHz as described above. It can be seen that the third order intermodulation product varies by approximately 8 dB depending on the phase of the reﬂection co-efﬁcient in the GSM1800 TX band. Conclusion The potential to maximise the power output from the TX chain of a GSM handset, to minimise harmonic generation by the PA or the RF switch in an RF front-end circuit, or to reduce the intermodulation products that arise in a 3G RF front-end circuit, offered by chip phase shifters presents the handset designer with a new tool for use when facing the task of optimising handset performance. Using the latest multilayer technology, TDK chip phase shifters can be deployed with the least number of concessions in terms of board space and design complexity. More information on the latest chip phaseshifters can be found at: http://www.tdkcomponents.de References [1] David M Pozar, “Microwave Engineering”; Wiley and Son; ISBN 0471-17096-8; pp 208-212. [2] ETSI EN 300 607-1 “Digital cellular telecommunications system (Phase 2+); Mobile Station (MS) conformance speciﬁcation; Part 1: Conformance speciﬁcation.” Figure 11: Typical dual-mode RF switching circuit employing SP7T RF switch and featuring a phase shifter connected between the SP7T RF switch and the UMTS duplexer. Figure 12: Variation of 3rd order intermodulation product generated in dual mode RF switching circuit of ﬁgure 11 arising from the mixing of TX signals in the UMTS TX band and DCS TX signals at the antenna port as a function of the phase of the reﬂection co-efﬁcient presented by the UMTS duplexer in the DCS TX band. Company Information TDK Components www.tdk-components.de Microwave Engineering Europe ● October 2007 ● www.mwee.com 026-028-030-032_MWEE.indd 32 20/09/07 15:38:38 http://www.tdk-components.de http://www.tdk-components.de http://www.tdk-components.de http://www.mwee.com

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Microwave Engineering Europe - October 2007 Contents Comment News CMOS RF: Si-On-Sapphire Goes Mainstream Cover Feature: New Data Protection Concept for UHF RFID Tags CMOS RF: RF Design Team Touts CMOS Spin for 3G PAs Wireless HID – Are You Following the Standard to Another “Average” Product Development? Phase Optimisation of the RF Front-End Direct Synthesis of UWB-WiMedia Signal Generation 4G Chips to Target 700 MHz Applications Femtocells Mobilize to Fight Wi-Fi in the Home Products Product Feature: AXIEM Pioneers the Future of EM Technology Calendar

Microwave Engineering Europe - October 2007

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