CMOS RF Transmitters with On-Chip Antenna for Passive RFID and IoT Nodes

Abstract

The performances of two RF transmitters, monolithically integrated with their antennas on a single CMOS microchip fabricated in a standard 0.35 µm process, are presented. The usage of these architectures in the Internet of Things (IoT) paradigm is envisioned, as part of a custom conceived data transmission system. The implemented circuits use two different directly on-off keying (OOK) modulated oscillator topologies whose outputs are employed to feed two loop antennas. The powering of both transmitters is duty-cycled for reducing the average power consumption to a few tenths of a microwatt, allowing the usage as low-power transmitters for IoT nodes. The integrated loop antennas radiate sufficient power for a few meters' communication range. The OOK transmitted signal can be easily detected using a commercial receiver.2 of 12 device, communication is the most power consuming function, thus requiring low power and highly efficient transmitters [16][17][18].A few of the systems already presented in the state-of-art literature exploit different Integrated Circuit (IC) production technologies, such as SiGe BiCMOS [4] and silicon on insulator (SOI) CMOS [12] to boost the radiating properties of the substrate and of the devices, or more performing CMOS processes with higher transistor cutoff frequencies, thus requiring higher costs compared to the 0.35 µm CMOS process used in this work. Several studies presented different antenna topologies, such as single-ended circuit-to-antenna, as is the case for patch antennas [13], or differential feeding, as is the case for dipoles [3][4][5][6][7][8] or for the loop antenna used in this work and in [9]. In other cases, the antenna is micromachined after the IC fabrication [2,11,13], resulting in an increase in the realization cost of the single IC. The systems that can be found in literature exploit antennas, showing good performances at frequencies well above those used in this work (1.0-2.4 GHz) and in [16], but this is obtained at the expense of much more complex and costly fabrication processes and receiving apparatus.In each transmitter topology, the RF oscillator is the critical building block which mainly determines the overall performances of the transmitting architecture. LC oscillators offer high frequency stability over temperature, voltage, and process variations and low phase noise [19]. These characteristics allow the use of these oscillator architectures in gigahertz applications. Among the different LC circuits typologies, Colpitts oscillators and cross-coupled topologies are the most frequently implemented in CMOS technologies. In Colpitts oscillators, the loop voltage gain must be very high to sustain oscillation, especially when low quality factor integrated inductors are employed in the resonator tanks, which implies a higher power consumption with respect to cross-coupled architecture [20]. Finally, although ring oscillators (RO) have poor phase noise characteristics compared to LC oscillators, they have the advantages of a wider oscillation frequency range a...

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