Small-Signal Polar Transmitters Prove Cellular Worth

TO ENABLE LOW-COST and low-power radios for cellular phones, both optimal RF transceiver architectures and innovative circuits are needed. For such transceiver architectures, challenges must first be overcome in the design of the transmitter design. At Broadcom Corp., several tradeoffs in that design process were investigated by Hooman Darabi, Henrik Jensen, and Alireza Zolfaghari. Their focus was on an analog open-loop, small-signal polar architecture. If designed properly, the researchers say that this architecture compares favorably to its closed-loop and digital-phase-locked-loop (DPLL) counterparts for power consumption.

Specifically, they performed a detailed analysis of the direct-modulated and small-signal polar transmitters for GSM/EDGE/ WCDMA applications. Polar transmitters offer several advantages, such as low power and small silicon device area. Yet the researchers emphasized that satisfactory performance will only be achieved if the impact of low-cost CMOS circuit imperfections is understood. In both the phase and amplitude paths, non-idealities contribute to performance degradation. It is therefore necessary to both characterize and compensate for these non-idealities. The researchers summarized that the requirements for 3GPP GSM and EDGE transmission can only be met if various, non-ideal factors are well comprehended, such as VCO gain nonlinearity, phase feed-through, and AM-AM and AM-PM distortions.

The researchers examined several circuit-level and architectural techniques to overcome these effects. Next, they developed a prototype GSM/EDGE polar transmitter that meets the 3GPP GSM/EDGE requirements with wide margin. The transmitter has an 8PSK 400-kHz modulation spectrum of -64/63 dBc for high-low bands. It boasts a root-mean-square (RMS) error vector magnitude (EVM) of 2.5% or better. The prototype measures just 0.9 mm2 in 65-nm CMOS. In EDGE mode, the battery-referred current drain is 77 mA including the receive PLL for the reference frequency. See "Analysis and Design of Small-Signal Polar Transmitters for Cellular Applications," IEEE Journal Of Solid- State Circuits, June 2011, p. 1237.

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