New RF Transmitter Techniques for RFID, Cellular, and mmW Applications
- Author(s): Kuo, Nai-Chung
- Advisor(s): Niknejad, Ali M
- et al.
Reader/Transmitter (Tx) designs for inductive-power-transfer (IPT) systems have to power up the tag and must be able to listen to it. The low power transfer efficiency (PTE) and the high Tx-to-Rx interference are the main challenges when a tiny tag is involved. To improve PTE, an analytical approach for optimizing the Tx coil and the miniature rectenna has been developed. The equation-based approach optimizes the IPT design rapidly, locating the optimal IPT frequency and the coil geometries. An optimized 2.2-mm IPT uses a RF power of 33.1 dBm, and the designed 0.01mm2 CMOS rectenna harvests a dc power of 0.1 mW. To suppress the Tx-to-Rx leakage, a new T/Rx architecture has been invented with a two-tone Tx that simultaneously charges the tag and excites a third-order intermodulation (IM3) frequency from the tag nonlinearity. The IM3 tone is modulated by the tag data and transmitted back to the reader Rx via the same coupled-coils. The Tx/Rx frequency separation allows significant filtering on the Tx-to-Rx leakage. This technique was adopted in IPT systems and far-field systems with a custom-designed tag and even demonstrated with a commercial UHF Gen2 tag. Multi-tone Tx has also been studied for further performance improvement.
The other key application of RF transmitters is cellular communication. We have fabricated an all-digital CMOS Tx core on three interposers with high output power, >50% efficiency, and a collective bandwidth from 0.4 to 4 GHz. To further achieve frequency reconfiguration with a single package, a band-selecting interposer was designed to combine three identical CMOS Tx cores. The band selection is carried out by reconfiguring the switching devices in the CMOS PAs. Peak power higher than 23 dBm and efficiency better than 25\% are achieved from 0.4 to 4 GHz, by rotating the three sub-Txs. Finally, an E-band OOK-QPSK Tx element in 28nm bulk CMOS is presented. The Tx element is a suitable building block for digitally-modulated phased arrays and high-speed communications. Employing the Tx elements, the implemented Watt-level-EIRP mmW digital array demonstrates good efficiency and the capability of synthesizing a pattern null at a given direction. The leakage-suppression technique exploits the combination redundancy as the array synthesizes the desired spatial symbols, so the conventional high-resolution elements are not required.