Photo source: University of Michigan
Bluetooth transmitter chip (lavender, second on the right) is connected to a magnetic monopole antenna (green, far right) as an integral part of the transmitter resonator circuit. The left half of the circuit board is for testing purposes only.
Teeny-Tiny Bluetooth transmitter runs less than 1 milliwatt
Bluetooth low-power packets can now be transmitted through millimeter-level IoT transmitters
Author: Samuel K. Moore
You may be wandering in Bluetooth radio signals now. But none of these comes from the smallest and lowest power consumption end of the Internet of Things. These battery-powered and energy-gasping millimeter-level sensors can be used for years without replacement, but their transmitters cannot concentrate on the energy needed to communicate using even the lowest-energy version of Bluetooth, called Bluetooth Low Energy (BLE).
Engineers at the University of Michigan have now designed and produced ’s first millimeter-level independent device called BLE . Only 0.6 milliwatts of are consumed during the transmission process, it will use a typical 5.8 mm button battery to continuously work for 11 years. This millimeter-level BLE transmitter will allow these ants-sized sensors to communicate with ordinary devices, or even smartphones.
The transmitter chip that debuted last month at the IEEE International Solid State Circuit Conference must solve two problems, explains David Wenzlov, an associate professor at the University of Michigan who is responsible for the study. The first one is power consumption, and the second one is the size of the antenna. "The size of the antenna is usually based on physics, and you can't fool physics," Wentzloff said. The team's solution involves two issues.
Ordinary transmitter circuits require a tunable RF oscillator to generate frequency, a power amplifier to enhance its amplitude, and an antenna to radiate signals. Michigan team combines oscillator and antenna to make the amplifier unnecessary. They call their invention a power oscillator. The key part of the
oscillator is the resonant loop circuit: inductor and capacitors. energy sways back and forth between the magnetic field of the inductor and the electric field of the capacitor, and the resonance frequency of is determined by the capacitor and inductor. In the new circuit, the team uses the antenna itself as an inductor in the resonant loop. Because it acts as an inductor, the antenna uses a changing magnetic field instead of an electric field radiation; this means it can be more compact.
Image from: University of Michigan
Integrated transformer [octagonal, lower left corner] in Bluetooth transmitter power supply improves efficiency. The effective area of the chip is only 0.5 square millimeters.
However, the size is not unique. "The biggest advantage is that the antenna is an inductor with a higher quality factor than on-chip inductors," said Wentzloff. The Q value basically indicates the efficiency of the resonator. As a 14mm-long conductor ring, the antenna is much larger than the on-chip inductor of a millimeter-scale transmitter. This results in a Q value of about five times that of the inductor on chip.
Although it is a more efficient solution, to meet the BLE specification, the team needs a better way to power the power oscillator. Their solution is to build an on-chip transformer in the circuit that powers it. The transformer looks like two nested coils. One coil is connected to the power supply voltage terminal of the oscillator circuit and the other is connected to the ground side. Wentzloff said pumping the transformer at a frequency twice as high as the power amplifier effectively enhances the power flow of the antenna.
tests the new transmitter by broadcasting BLE "advertising" packets - a set of bits tells the receiving device where the transmitter is. “If you want to build a millimeter-level tracker device, that’s all you need,” Wentzloff said. But the payload part of the ad package can reach up to 31 bytes, which is very suitable for packaging sensor data.
This study is part of the University of Michigan M3 project, which is developing modular, millimeter-level sensors. Wentzloff said the next step is to integrate the BLE radio into one of the sensors. “We used multiple transmitters in the M3 project,” he said."BLE will be another option for this modular platform."
source: spectrum.ieee.org
