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MIT showcases picowatt radio at Internet of Things Conference

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February 24, 2015

MIT scientists and researchers working on low-power radio technology have given the International Solid State Circuits Conference (ISSCC) an insight into how simple standby power can be cut in Internet of Things devices.

And MIT should know-- it's a delicate issue. If for example, a sensor is waiting to be polled so it can answer with tidbits of data, the radio has to be in a state where it can be woken up since it's currently in a standby power mode.

On the other hand, as MIT electrical engineering professor Anantha Chandrakasan says, when the device is off, you want the least possible leakage currents in order for that standby reserve of power to last as long as possible.

As Chandrakasan notes, even in the off-state, semiconductors almost always allow minute currents to flow across the gates, even if we don't want that, and it's that leakage he's looking to eliminate for good.

In an effort to accomplish that feat, his group has experimented with using charge pumps to provide a small reverse-polarity charge to the transistors in the radio.

For example, in a NPN transistor, a positive voltage on the gate (base) is what allows current to flow across the semiconductor. To block the leaky current, the MIT group's charge pump applies a small negative charge to the base when the device is idle.

This, the university's release explains, drives electrons away from the transistor and makes it a better insulator when it's idling, since the transistor is basically turned off completely in such a state, maximizing the power source's reserve capacity.

The Massachusetts university describes the operation of the charge pump-- “When the charge pump is exposed to the voltage that drives the transistor, charge builds up in one of the capacitors. Throwing one of the switches connects the positive end of the capacitor to the ground, causing a current to flow out the other end. This process is repeated over and over. The only real power drain comes from throwing the switch, which happens about 15 times a second.”

Even with the power needed for switching, the university says tests on a prototype made by TSMC found the design “spent only 20 picowatts of power to save 10,000 picowatts in leakage”.

The other key part of the work described in the snappily-titled A +10dBm 2.4GHz Transmitter with Sub-400pW Leakage and 43.7 percent System Efficiency was to break down the radio system design so that most of the circuit can run at the lowest possible voltage.

But in this instance, the issue is different-- the higher the radio frequency you want to drive the transmitter at, the higher the voltage you need and hence more power it's going to consume. So it becomes a catch 22 situation.

It's cheaper to build a radio chip as a single unit, meaning that the operating voltage of the whole module is a function of what the transmitter needs.

Instead, to drive down power, the Chanrakesan team used voltage doubler circuits of capacitors and inductors around the transmitter to give it a higher operating voltage, while leaving the rest of the device at a lower voltage.

MIT says the device is suitable for Bluetooth and 802.15.4 transmissions. Texas Instruments and Shell Oil contributed in funding the research, along with MIT.

Source: The Massachusetts Institute of Technology.

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