During our RF integration chat session last week, an additional design concept was brought up. Dirceu Rodrigues talked about a radio link used to bring the rest of a complex system out of its sleep state. The link uses a ultra-low-power RF receiver. When it hears the proper signal, it wakes up the rest of the system. Rodrigues used the example of a ZigBee radio.
A radio using the ZigBee protocol or specs is already considered a low-power device (as networks go), but power draw can be reduced further. In fact, some devices are powered from coin cells, other very small battery packs, or super-caps. The latter are typically used in conjunction with energy harvesting systems.
For these applications, you would want miniscule power draw all the time. The technique here is to combine the ultra-low-power receiver (always on) with burst-mode sensors, a microcontroller (MCU), and the ZigBee radio. When the system wakes up, the digital clock source is enabled, and so is the MCU. The sensors monitor their respective aspects, and the MCU processes the sensor information. The ZigBee radio is enabled, joins the network, and sends the data. Then everything goes back to sleep.
If you could combine the wakeup receiver, the ZigBee radio, the MCU, and perhaps a real-time clock, you could make a device that not only uses little power but also takes up little space. Rodrigues suggested that such a device would be suitable for inventory control. It could be attached to each piece of merchandise. You could walk into your warehouse and quickly check stock/inventory levels. Or, in a tool crib, you could quickly locate a particular tool (or see if it's been loaned out).
There is work being done along these lines. A synopsis on the University of Michigan's Association of University Technology Managers indicates that researchers are already working on such a radio design.
While consuming 98 nW of power during transmission and 11 nW while [idle], this wake-up radio has a sensitivity of -41dB at 915 MHz and has a transmission range up to 6 m. It has been developed for on-off keying (OOK) modulation, can transfer data at 100 kbps and has a chip space requirement of 0.03 mm2 .
Have you done design work with such RF devices? Let us know about frequency of operation, data rate, power draw, and range.
- RF Integration: Can We Get It All?
- Remote Meter Reader Data Transmitter Power Supply
- Integrated RF Analog, Multi-Standard, Software-Defined Radio Receivers
- The Squeaky Wheel Gets the Repair
- Digital Tuning for RF Analog Front End
- All-CMOS Bluetooth/ZigBee Transceiver for Personal Networks
- Medical Device ASIC Integration: Optimize, Don’t Maximize