MIT scientists have crafted a compact, energy-efficient receiver for 5G-compatible smart gadgets that exhibits approximately 30 times greater resistance to a specific form of interference than some conventional wireless receivers.

The affordable receiver would be perfect for battery-operated internet of things (IoT) devices such as environmental sensors, smart thermostats, or other appliances that must operate continuously for extended periods, including health wearables, smart cameras, or industrial monitoring sensors.

The researchers’ chip employs a passive filtering system that consumes under a milliwatt of static energy while safeguarding both the input and output of the receiver’s amplifier from undesired wireless signals that could disrupt the device.

Central to this innovative method is a unique arrangement of precharged, stacked capacitors, interconnected by a network of tiny switches. These miniature switches require significantly less power to activate and deactivate compared to those typically found in IoT receivers.

The arrangement of the receiver’s capacitor network and amplifier is meticulously designed to take advantage of a phenomenon in amplification that permits the chip to utilize much smaller capacitors than would normally be necessary.

“This receiver could enhance the functionality of IoT devices. Smart gadgets like health monitors or industrial sensors could shrink in size and enjoy prolonged battery life. They would also be more dependable in dense radio environments, such as factory floors or smart city networks,” states Soroush Araei, an electrical engineering and computer science (EECS) graduate student at MIT and lead author of a paper concerning the receiver.

He is accompanied on the paper by Mohammad Barzgari, a postdoc at the MIT Research Laboratory of Electronics (RLE); Haibo Yang, an EECS graduate student; and senior author Negar Reiskarimian, the X-Window Consortium Career Development Assistant Professor in EECS at MIT and a member of the Microsystems Technology Laboratories and RLE. The findings were recently showcased at the IEEE Radio Frequency Integrated Circuits Symposium.

A new benchmark

A receiver serves as the medium between an IoT device and its surroundings. Its role is to detect and amplify a wireless signal, eliminate any interference, and then convert it into digital data for further processing.

Traditionally, IoT receivers function on fixed frequencies and mitigate interference using a singular narrow-band filter, which is straightforward and cost-effective.

However, the new specifications of the 5G mobile network enable devices with lower capabilities that are more affordable and energy-efficient. This broadens the spectrum of IoT applications to the quicker data speeds and enhanced network capabilities of 5G. These next-gen IoT devices necessitate receivers that can tune across a wide array of frequencies while remaining economical and energy-efficient.

“This is exceptionally challenging as we now need to consider not just the power and cost of the receiver but also its flexibility to address numerous interferers present in the environment,” Araei remarks.

To minimize the size, cost, and energy consumption of an IoT device, engineers cannot depend on the bulky off-chip filters usually employed in devices that function over a wide frequency range.

One approach is to employ a network of on-chip capacitors that can filter out undesired signals. Yet, these capacitor networks are susceptible to a specific type of signal noise known as harmonic interference.

In previous research, the MIT team devised an innovative switch-capacitor network that targets these harmonic signals as early as possible in the receiver chain, filtering out unwanted signals before they are amplified and transformed into digital bits for processing.

Miniaturizing the circuit

Here, they enhanced that strategy by utilizing the novel switch-capacitor network as the feedback pathway in an amplifier with negative gain. This setup exploits the Miller effect, a phenomenon that allows small capacitors to function like much larger ones.

“This technique enables us to satisfy the filtering requirements for narrow-band IoT without necessitating physically large components, significantly reducing the circuit’s size,” Araei states.

Their receiver features an active area of less than 0.05 square millimeters.

A challenge the researchers faced was figuring out how to apply sufficient voltage to activate the switches while keeping the overall power supply of the chip to just 0.6 volts.

In the presence of interfering signals, such tiny switches can mistakenly turn on and off, especially if the voltage required for switching is extremely low.

To resolve this, the researchers devised a novel solution using a specialized circuit technique called bootstrap clocking. This methodology elevates the control voltage just enough to ensure reliable switch operation while consuming less power and fewer components than conventional clock boosting techniques.

Collectively, these innovations enable the new receiver to draw less than a milliwatt of power while blocking around 30 times more harmonic interference than traditional IoT receivers.

“Our chip is also exceptionally quiet, in terms of not corrupting the airwaves. This is due to the fact that our switches are quite small, which minimizes the amount of signal that can escape through the antenna,” Araei adds.

As their receiver is smaller than conventional devices and utilizes switches and precharged capacitors instead of more intricate electronics, it could be more economically feasible to produce. Additionally, since the receiver design can accommodate a broad spectrum of signal frequencies, it could be applied to various current and future IoT devices.

Now that they have created this prototype, the researchers aim to allow the receiver to function without a dedicated power source, potentially by harvesting Wi-Fi or Bluetooth signals from the environment to energize the chip.

This research is partially funded by the National Science Foundation.