The small receiver that keeps a wireless sensor asleep
A wireless sensor can spend most of its life waiting. Its processor sleeps, measurements are infrequent and its main radio is switched off. Somewhere, though, a circuit still has to notice when another device wants its attention.
A wake-up receiver does that listening at very low power. The difficulty is sensitivity: making the listening circuit economical can also make a faint incoming signal harder to recognise. Fraunhofer IPMS and Hamburg University of Technology have been investigating a MEMS amplifier placed ahead of the receiver to improve that part of the link.
Their MEMS-ParAmps project combines the wake-up receiver with a parametric amplifier built around a mechanically tunable capacitor, or varactor. Instead of treating capacitance as a fixed property, a drive circuit changes it in a controlled way. In parametric amplification, that changing circuit parameter provides a route for transferring energy from a pump into the signal.
The device still needs drive energy. The project reports a voltage-pulse generator capable of exciting the MEMS varactor with approximately 3.3 microjoules per pulse. Energy per excitation is only one part of the budget: repetition rate and the behaviour of the rest of the receiver determine the continuing demand on the battery.
A component result and a radio-system target
The consortium set targets of up to ten years of battery life and a kilometre of open-space range. Those are not reported field results. Fraunhofer describes laboratory and simulated-scenario validation with industrial partner Actemium BEA, including adaptation of the communication protocol for greater distances between nodes.
Range depends on more than receiver gain. Transmit power, antenna performance, interference, the propagation path and the message format all enter the link budget. A receiver that wakes reliably in a quiet laboratory can still face a different task beside machinery or inside a metal enclosure.
The measurement work also became part of the project. The partners developed dynamic capacitance measurement with bandwidth above 10MHz because existing commercial systems did not meet their requirements. Fraunhofer’s broader MEMS-varactor platform uses electrostatic actuation to vary capacitance, enabling impedance and frequency adjustment as well as signal compensation.
The project finished in January, with the results now detailed in a new release. Its proposed next uses include low-noise mixers, sensitive wake-up receivers and sensor nodes that obtain energy from their surroundings. The work remains a research platform with laboratory validation, rather than a catalogue radio module with a guaranteed kilometre link.
