Power/Performance Bits: Dec. 3

Waking up IoT devices
Researchers at UC San Diego developed an ultra-low power wake-up receiver chip that aims to reduce the power consumption of sensors, wearables, and Internet of Things devices that only need to communicate information periodically.

“The problem now is that these devices do not know exactly when to synchronize with the network, so they periodically wake up to do this even when there’s nothing to communicate. This ends up costing a lot of power,” said Patrick Mercier, a professor of electrical and computer engineering at UC San Diego. “By adding a wake-up receiver, we could improve the battery life of small IoT devices from months to years.”

The wake-up receiver measures 4.55 cm² and only uses 22.3 nanowatts to continuously look for a wake-up signature, a specific radio signal that tells the chip when to wake up the main device. It also can perform over a wide temperature range, with consistent performance from -10 C up to 40 C (14 F to 104 F).

By only waking up a wireless device when necessary, the wake-up receiver (chip stack to the left of the penny) can cut down on power use and extend battery life. The system includes a miniaturized antenna (gold-colored plate below the receiver). (Credit: David Baillot/UC San Diego Jacobs School of Engineering)

The new device targets higher frequency radio signals, 9 gigahertz, than most wake-up receivers. This allowed the team to shrink everything, including the antenna, transformer and other off-chip components down into a package at least 20 times smaller than prior nanowatt-level work.

There is a small tradeoff in latency. There is a 540-millisecond delay between when the receiver detects the wake-up signature and when it wakes up the device. But for the intended applications, the researchers say this amount of delay is not a problem.

“You don’t need high-throughput, high-bandwidth communication when sending commands to your smart home or wearables devices, for example, so the trade-off of waiting for a mere half a second to get a 100,000x improvement in power is worth it,” Mercier said.

Finding causes of EM noise
Researchers at Osaka University determined equations to quantify and determine the source of electromagnetic (EM) noise, finding that EM noise was caused not only by the interference between transmission lines, but also by conditions of elements connected to the electric circuit.

To describe EM noise, the researchers used a three-line (multi-conductor transmission line (MTL)) circuit, to which lumped-parameter circuits were connected. In addition to a conventional two-line circuit configuration, another conductor line was connected on the source side as the ground.

From this, the team derived telegraphic equations, wave equations, and reflection coefficients in the normal mode (NM) that represents circuit signals and the common mode (CM) that is generated by interaction with the environment and causes various noise. Considering the noise mode conversion in the MTL, they derived equations that describe behaviors of the NM and CM.

They found that the CM converted to the NM due to both the geometrical relation between the circuit and the environment and the electrical connections between the MTL and the elements connected to the MTL, generating EM noise.

Based on this, the researchers say a symmetrical configuration of three transmission lines together with lumped circuits was the only solution to eliminate EM noise.

Masayuki Abe, a professor at Osaka, said, “In addition to the improvement of device performances, we aim to develop an ‘EM noise-less infrastructure’ to create a society in which people can use high value-added devices, devices with ultra-low power consumption and ultra-low waste heat.”

More stable perovskites
Researchers from Rice University and Fudan University developed a new perovskite solar cell that, while having less efficiency, lasts much longer. Using indium to replace some of the lead in perovskites enabled the team to better engineer the defects in cesium-lead-iodide solar cells that affect the compound’s band gap, a critical property in solar cell efficiency.

The new cells can be made in the open air and last for months, compared to standard perovskite solar cells.

“This is different from the traditional, mainstream perovskites people have been talking about for 10 years — the inorganic-organic hybrids that give you the highest efficiency so far recorded, about 25%. But the issue with that type of material is its instability,” said Jun Lou, a materials scientist at Rice. “Engineers are developing capping layers and things to protect those precious, sensitive materials from the environment. But it’s hard to make a difference with the intrinsically unstable materials themselves. That’s why we set out to do something different.”

The team built and tested perovskite solar cells of inorganic cesium, lead and iodide, which tend to fail quickly due to defects. By adding bromine and indium, defects in the material were reduced, raising the efficiency above 12% and the voltage to 1.20 volts.

The new cells can be manufactured in open air conditions, including the humid ambient environment of Rice University in Houston, Texas. Stability was increased significantly with encapsulated cells remaining stable in air for more than two months, far better than the few days that plain cesium-lead-iodide cells lasted.

“The highest efficiency for this material may be about 20%, and if we can get there, this can be a commercial product,” said Jia Liang, a postdoctoral researcher at Rice. “It has advantages over silicon-based solar cells because synthesis is very cheap, it’s solution-based and easy to scale up. Basically, you just spread it on a substrate, let it dry out, and you have your solar cell.”