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 –