Wireless Transceiver RF Front-Ends An overview of the main architectures in RF front-end design Fraidun Akhi April 1, 2003 Electrical and Computer Engineering Auburn University

Contents  The complete transceiver system model - What role does the RF front-end play?  An overview of RF front-end architectures - Advantages and disadvantages  Industry trends  Design example - RFMD’s b WLAN chipset  Conclusion

Transceiver System Model

Typical RF Front-End Design

Superheterodyne Architecture  Advantages - High performance - High performance - Low power - Low power - Avoid DC offset - Avoid DC offset - Low Design Risk - Low Design Risk - Easier to design LNA and Mixer - Easier to design LNA and Mixer  Disadvantages - High cost due to large component quantities - Not as compact as other designs

Direct Conversion Architecture  Advantages - Low Cost - Eliminates IF SAW filter, IF PLL, image filter, and mixer - 30% less parts than superheterodyne  Disadvantages - Hard to achieve I/Q quadrature balance at RF - LO self mixing causes DC offset - 10% more power consumption than superheterodyne

Low IF Architecture  Advantages - Low cost - Eliminates IF SAW filter, IF PLL, image filter - No DC offset due to LO self mixing  Disadvantages - Hard to achieve I/Q quadrature balance at RF - Requires LPF’s with higher passbands, and higher performing ADC’s

A GSM Phone RF Front-End

Industry Trends  More integration and fewer components - Direct conversion favored - Power consumption gap is closing  Single chip systems - Analog/digital baseband, RF, codecs, power management, everything included! - TI has promissed a single chip GSM/GPRS phone by 2004  SiGe gaining popularity due to advantages such as higher speed and lower cost

Design Example

Transmitter

Transmitter System Parameters  Transmitter input level = 100 mVpp  PA output ~ 25 dBm (300 mW) - Can be as high as 27 dBm (500 mW) - Up to 1W allowable in the ISM band  Filter insertion loss (S21) < 1 dB

Receiver

Receiver System Parameters  Filter insertion loss (S21) < 1 dB  LNA/Mixer cascaded gain = 35 dB - Cascaded IP3 = -25 dBm - GSM requires IP3 < -19 dBm - Cascaded NF = 4.1 dB  Receiver cascaded gain = 70 dB - Cascaded IP3 = -100 dBuV - 5 to 35 dB depending on gain

Conclusions  There are practical advantages to each front-end architecture  Compactness, integration, and economics provide practical and aesthetic advantages that give direct conversion systems the upper hand

References  Dr. Foster Dai’s ELEC 6970 notes  Texas Instruments  RF Micro Devices –