Microwave Engineering Europe - October 2007 - (Page 30)

30 OPTIMISING THE HANDSET RF FRONTEND degrees is demonstrated and the return loss is shown to be 23.5 dB. The penalty for insertion loss arising from this component is only 0.17 dB. Figure 7 shows the low-band section of an RF front-end circuit, where chip phase shifters have been added between the various blocks of the circuit. As described above, phase shifter PS 1 is added at the output of the PA to maximise the ERP of the handset by ensuring that the optimum load is presented to the PA. In fact, the non-standard impedance presented to the PA via the antenna and RF front-end circuit can be of beneﬁt in maximising the ERP since the maximum output power from a typical PA is usually achieved when it is terminated by an impedance with both real and imaginary parts. Minimising the generation of harmonics in the RF frontend For GSM operation, spurious emissions of a cellular handset must be below -30 dBm Figure 7: Possible applications of phase shifters in typical GSM RF front-end circuit. Figure 8: Experimental set-up for measuring the harmonic generation of proprietary RF switch module for GSM applications as a function of the phase of a 4:1 mismatch presented at the antenna port. Figure 9: Measured power of harmonics relative to TX input generated by proprietary GSM switch module as a function of the phase of a 4:1 mismatch connected at the antenna port. RF input power 33dBm at 1785MHz, CW and GSM burst mode. at frequencies up to 12 GHz[2]. The 2nd and 3rd harmonic generation levels of a typical commercial multi-pole RF switch are in the order of -65 dBc, so that for a 35 dBm TX input signal, the 2nd and 3rd harmonics will be at the marginal level of -30 dBm. In addition, the low-pass ﬁlter at the PA output provides only a ﬁnite level of attenuation of harmonics generated by the PA itself, so that some harmonics generated by the PA will also be passed to the antenna. Harmonic generation of the RF switch degrades rapidly as the load presented to the switch diverges from 50 Ω. However, the load presented to the switch at the antenna port can be optimised in a similar manner to the optimisation of the load presented to the PA output as described in the previous section. Generally, if the phase of the mismatch presented by the antenna is within a particular range of values, the harmonic generation of the RF switch can be minimised. An experimental set-up to determine the optimum phase of a mismatch presented at the antenna port of a proprietary RF switch module is shown in ﬁgure 8. The mismatch at the antenna port is provided by a highpass ﬁlter connected in series with a 2 dB attenuator pad, with the high-pass ﬁlter presenting an almost perfect mismatch and the 2 dB attenuator reducing the return loss from zero to -4 dB, representing a mismatch of 4.42 to 1. Figure 9 shows the measured power at harmonic frequencies as a function of the phase of the mismatch for the same RF switch module. The highlighted range shows the phase of the reﬂection coefﬁcient within which the harmonic generation of the RF switch module is at a minimum. Thus it can be seen that the addition of a suitable phase shifter at the antenna port of an RF switch when employed in a RF front-end circuit, can substantially lower the unwanted harmonics emitted at the antenna port. This is illustrated as phase shifter PS 2 in ﬁgure 7. The function of phase shifter PS 2 at the antenna port of the circuit of ﬁgure 4 is to rotate the phase of the reﬂection co-efﬁcient presented by the antenna and to minimise harmonic generation by the RF switch. A further application of a chip phase shifters is shown as PS 3 connected between the SP6T RF switch and the LPF of the RF front-end circuit of ﬁgure 7. The reﬂection co-efﬁcient of the LPF at harmonic frequencies generally has a ● Microwave Engineering Europe ● October 2007 www.mwee.com 026-028-030-032_MWEE.indd 30 20/09/07 15:38:25 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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