Microwave Engineering Europe - October 2007 - (Page 28)

28 OPTIMISING THE HANDSET RF FRONTEND Optimisation of GSM handsets Modern GSM RF front-end circuits used in mobile cellular handsets, such as that depicted in ﬁgure 3, typically comprise a low-band PA, a high-band PA, an LPF at the output of each PA, an SP6T RF switch which can connect an antenna port to one of: a low-band TX output, a high-band TX output, an RX1 band-pass ﬁlter input, an RX2 band-pass ﬁlter input, an RX3 bandpass ﬁlter input or an RX4 band-pass ﬁlter input. Now consider the differences between design-stage evaluation of an RF front-end Figure 3: Typical GSM RF front-end circuit. Figure 4: Outline drawing and pin conﬁguration of TDK 1.0 x 0.5 x 0.5 mm3 range of chip phase shifters. and the testing of the end-product: designstage evaluation is typically carried out by examining the data for each of the circuit blocks separately with 50 Ω terminations at the I/O ports of the separate blocks; on the other hand, once a handset reaches production, the performance of the entire RF front-end is evaluated as part of a system test. These two separate evaluation paradigms can give rise to signiﬁcant discrepancies between the calculated performance and the real performance of a handset. To illustrate this point, a typical GSM antenna is speciﬁed to provide a return loss of -6 dB over its operating bands; hence, the termination presented by the antenna can range from 16.6 Ω to 150 Ω across the four operating bands. The variation in the impedance presented by the antenna causes the performance of the handset to differ from that calculated at the design stage. In particular, the impedance variation of the antenna affects the effective radiated power (ERP) of the handset. One solution to this problem for the designer is to experiment with number of alternative matching circuits at the PA output in order to optimise the ERP of the handset. The drawback with this method is that for a two-component matching network N2 permutations are required to ﬁnd the best combination of N component values; for a three component matching solution N3 permutations are required and so on. A single-chip matching component incorporating the phase-shifting circuits of ﬁgure 1 or ﬁgure 2 can accelerate the process of optimisation by trial and error. Chip phase shifters for the RF front-end TDK have developed a line-up of low temperature co-ﬁred ceramic (LTCC) chip phase shifters for RF front-end circuits in 1.0 x 0.5 x 0.5 mm3 packages. The package dimensions of the current line up of chip phase shifters are given in ﬁgure 4, and a photograph of the chip phase shifters is given in ﬁgure 5. These components have I/O pads on the short sides of the package, and ground pads on each of the long sides of the package. These chip phase shifters are designed to save board space and simplify the optimisation of RF front-end circuits. Figure 6 shows the electrical characteristics of a TDK chip phase shifter designed to provide a 55 degree phase shift between the input and output ports in the frequency range from 1710 to 1910 MHz (GSM1800 and GSM1900 TX bands). At a frequency of 1810 MHz, a phase shift of 54 ● Figure 5: TDK chip phase shifters. Figure 6: Electrical characteristics of a TDK chip phase shifter designed to produce a phase shift of 55 degrees in the GSM1800 and GSM1900 TX bands. Microwave Engineering Europe ● October 2007 www.mwee.com 026-028-030-032_MWEE.indd 28 20/09/07 15:37:56 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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