Engineers Build Teeny-Tiny Bluetooth Transmitter That Runs On Less Than 1 Milliwatt

Engineers at the University of Michigan have built the first millimeter-scale stand-alone device that meets Bluetooth Low Energy (BLE) specifications. 'Consuming just 0.6 milliwatts during transmission, it would broadcast for 11 years using a typical 5.8-mm coin battery,' reports IEEE Spectrum. 'Such a millimeter-scale BLE radio would allow these ant-sized sensors to communicate with ordinary equipment, even a smartphone.' From the report: The transmitter chip, which debuted last month at IEEE International Solid-State Circuits Conference, had to solve two problems. The first is power consumption, and the second is the size of the antenna. An ordinary transmitter circuit requires a tunable RF oscillator to generate the frequency, a power amplifier to boost its amplitude, and an antenna to radiate the signal. The Michigan team combined the oscillator and the antenna in a way that made the amplifier unnecessary. They called their invention a power oscillator. The key part of an oscillator is the resonant tank circuit: an inductor and a capacitor. Energy sloshes back and forth between the inductor's magnetic field and the capacitor's electric field at a resonant frequency determined by the capacitance and inductance. In the new circuit, the team used the antenna itself as the inductor in the resonant tank. Because it was acting as an inductor, the antenna radiated using changing magnetic field instead of an electric field; that meant it could be more compact.

However, size wasn't the only thing. Quality factor, or Q, is a dimensionless quantity that basically says how efficient your resonator is. As a 14-mm long loop of conductor, the antenna was considerably larger than an on-chip inductor for a millimeter-scale radio could be. That led to a Q was that was about five times what an on-chip inductor would deliver. Though it was a much more efficient solution, in order to meet BLE specifications, the team needed a better way to power the power oscillator. Their solution was to build an on-chip transformer into the circuit that supplies power to it. The transformer looks like two nested coils. One coil is attached to the supply voltage end of the oscillator circuit, and the other is attached to ground side. Pumping the transformer at a frequency twice that of the power amplifier wound up efficiently boosting the flow of power to the antenna.

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