While at McCarren waiting to fly home we talked about ways to synchronise the ranging nodes. We revisited the wake-up radio concept. Mik wondered about a crystal set that would use no power but could trigger a relay to turn on receiver. David, who spent his youth building crystal sets came up with a possibly more practical idea.
Use BLE to repeatedly transmit small packets.
Packets are received by BLE antenna network and fed through chain of notch filters and op-amps to produce a voltage that can generate an interrupt.
The packet repeat frequency is chosen to hit the sweet spot of the op amp.
Mik recalled some very low-power op-amp he had seen used to measure very low currents. the AD8428. It has a fixed gain of 2000 and a bandwidth of 3.5 MHz. described in this article.
By then we are waiting for take off, but David finds even better OpAmps with tiny power requirements.
We kicked it around and wondered if we could make some prototype to test out the concept. ——————————
Minimizing Energy Consumption in Sensor Networks Using a Wakeup Radio
It would be cool to use the charging method we saw at the Murata booth and have it do double duty as part of the wake-up circuit. Since each tag would have a wire loop for charging if a loop was energized with a series of pulses (from inside the tag that wants to wake up another tag) a nearby tag might have enough signal induced in its loop to wake it up.
But for now I think it is best to use a BLE signal with some kind of audio-frequency information on it that the nano-power op-amp can respond to (up to 9kHz). This would be 5kHz:
<–burst of 1’s->|<——–Dead time————-><—-burst––>|<——–Dead time———–>
<——————-200 uS———————–><——————-200 uS——————->
Surprise – detector diodes commonly used are made by Avago: http://www.avagotech.com/docs/AV02-1388EN. This document makes me feel kind of scared because of the engineering knowledge I don’t have, but encouraged that it might be possible to make a small, inexpensive wakeup circuit. It appears the diode needs to be biased with about 1uA, which sucks, but it’s still within reason. A voltage doubler detector can be implemented which we probably want.
Need to study this datasheet more. It would help a lot to have an RF mentor. I don’t know anybody offhand. Google will have to suffice. —————-
I ran across an excellent article from Linear Technology, written in 1998.http://cds.linear.com/docs/en/lt-journal/LTC1540_0298_Mag.pdf. I think it’s cool that it references a 1918 publication. The article is good because it talk about how to connect the detector diode to a length of transmission line to increase sensitivity and selectivity.
This article also is interesting: http://www.elucidare.co.uk/assignments/project_WUR/05992833.pdf. It has a table of different WUR methods, the power consumed, and a bibliography. This got me over to IEEExplore and a flurry of papers downloaded. See Google DriveTS06DEWBLYDewbly Shared DocsWUR. The problem is that all of the papers discuss research but nothing we can implement tomorrow.
The more sophisticated WUR’s have DSP to decode packets coming in to see if they really should turn on. I am in favor of narrow bandpass filters at audio frequencies because 1) I can understand it, 2) It could be implemented now and 3) the audio filters would consume zero power.
Since the wake-up issue is a big problem that lots of people are working on and a problem everybody will have whether or not they realize it, why don’t the radio manufacturers (like Nordic) put wake-up circuits in their products? It’s not like there is a shortage of published ideas. Maybe Nordic’s new radio will have one (probably not).