Microwave Engineering Europe - October 2007 - (Page 26)

26 OPTIMISING THE HANDSET RF FRONTEND Phase optimisation of the RF front-end By Brian Kearns, Manager of Intellectual Property, TDK Electronics Ireland Ltd. Figure 1: PI-type phase shifter — LPF type. Figure 2: PI-type phase shifter — HPF type. E nsuring that each block in the frontend circuit of a cellular handset presents the optimum impedance to the adjacent blocks can give rise to a dramatic improvement in the effective radiated power (ERP) of a handset and can also signiﬁcantly reduce the harmonics generated by the various components in the front-end and emitted at the antenna. For UMTS handsets, matching between the front-end blocks can also reduce intermodulation distortion products, which are generated in the transmit chain, and which are fed to the receive chain of the handset. Phase shifter networks Phase-shifting circuits are widely used to maximise efﬁciency of RF front-end circuits as described above. The PI-type phase-shifting network shown in ﬁgure 1, can be used to provide a range of phase-shift angles and offers a solution for performance optimisation of RF front-end circuits when the component values are appropriately chosen. The circuit of ﬁgure 1 alters the phase of a signal incident on I/O port 1, so that it is rotated by a given angle when emitted at I/O port 2. The circuit of ﬁgure 1 also produces increasing attenuation between the I/O ports as the frequency increases above the passband of the phase shifter. An alternative phase-shift circuit is shown in ﬁgure 2, the electrical properties being similar to the circuit of ﬁgure 1 except that the phase shift produced on a signal entering port 1 and emitted from port 2 of the circuit is positive rather than negative and the circuit of ﬁgure 2 produces increasing attenuation between the I/O ports as the frequency is decreased below the pass band of the phase shifter. The s-parameters of the circuit of ﬁgure 1 can be derived from the ABCD matrix of a PI-type network, which is given in a number of texts dealing with RF networks[1]. The s-parameter, S21, is given by equation 1 for the case where the capacitors C1 and C2 both have a capacitance of C, and where the circuit is terminated by an impedance of Z0 at both I/O ports. The tangent of the phase shift between I/O ports 1 and 2 is given by the complex part of equation 1 divided by the real part. Thus, the phase shift of the PI-type network of ﬁgure 1 is given by equation 2. It can be seen that a wide range of negative phase-shift angles are available at a given frequency with suitable choices of the values of the capacitance C the inductance L and the ratio Z. Equation 1 where: Equation 2 The phase-shifting circuits, shown in ﬁgure 1 and ﬁgure 2 offer a range of matching solutions by virtue of the fact that these phase shifters rotate the phase of a non-zero reﬂection co-efﬁcient presented at one of the ports. Microwave Engineering Europe ● October 2007 ● www.mwee.com 026-028-030-032_MWEE.indd 26 20/09/07 15:37:38 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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