Preferred Transmission Frequency for Size-Constrained Ultralow-Power Short-Range CMOS Oscillator Transmitters

A method is presented for minimizing the power consumption of size-constrained oscillator transmitters by selecting the preferred carrier frequency from among the standard ISM bands. The method has been applied to CMOS oscillator transmitters in which a single-turn loop antenna doubles as the inductor in the frequency-defining LC tank. A detailed model of the transmitter circuit, including the antenna, is combined with standard assumptions about the link and receiver to determine the minimum transmitter bias current for successful demodulation as a function of antenna size and transmission frequency. From this, the optimal operating frequency in terms of transmitter power budget, and the minimum transmitter power consumption at that optimal frequency, are determined for a given antenna size constraint. Two common oscillator topologies are studied, both implemented in 0.18 mum CMOS: the Colpitts oscillator and the complimentary cross-coupled oscillator. A combination of the EKV and BSIM models is used for MOS transistor modelling, while a novel energy conservation method is used to determine the oscillator bias current as a function of transmit power. The results show that, with the correct choice of operating frequency, transmitter power budgets of the order of 10 muW should be achievable for very short-range (ca 1 m) radio links with data rates up to 1 Mb/s and antenna sizes down to several mm radius.