Hybrid Technique Breaks Backscatter Distance Barrier

Low cost, long range, or low power — when it comes to wireless connectivity, historically you’ve only been able to pick two. But a group at the University of Washington appears to have made a breakthrough in backscatter communications that allows reliable data transfer over 2.8 kilometers using only microwatts, and for pennies apiece.

For those unfamiliar with backscatter, it’s a very cool technology that modulates data onto RF energy incident from some local source, like an FM broadcast station or nearby WiFi router. Since the backscatter device doesn’t need to power local oscillators or other hungry components, it has negligible power requirements. Traditionally, though, that has given backscatter devices a range of a few hundred meters at most. The UW team, led by [Shyamnath Gollokota], describe a new backscatter technique (PDF link) that blows away previous records. By combining the spread-spectrum modulation of LoRa with the switched attenuation of incident RF energy that forms the basis for backscatter, the UW team was able to cover 2800 meters for under 10 microwatts. What’s more, with printable batteries or cheap button cells, the backscatter tags can be made for as little as 10 cents a piece. The possibilities for cheap agricultural sensors, ultracompact and low power wearable sensors, or even just deploy-and-forget IoT devices are endless.

We’ve covered backscatter before, both for agricultural uses and for pirate broadcasting stations. Backscatter also has also seen more cloak and dagger duty.

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Automatic Audio Leveling Circuit Makes Scanning More Fun


[Alan’s] friend came to him with a problem. He loved listening to his scanner, but hated the volume differences between stations. Some transmitters would be very low volume, others would nearly blow his speakers. To solve the problem, [Alan] built up a quick automatic leveling circuit (YouTube link) from parts he had around the lab.

[Alan’s] calan-scope2ircuit isn’t new, he states right in the video that various audio limiting, compressing, and automatic gain control circuits have been passed around the internet for years. What he’s brought to the table is his usual flair for explaining the circuits’ operation, with plenty of examples using the oscilloscope. (For those that don’t know, when [Alan] isn’t building circuits for fun, he’s an RF applications engineer at Tektronix).

Alan’s circuit is essentially an attenuator. It takes speaker level audio in (exactly what you’d have in a desktop scanner) and outputs a limited signal at about 50mv peak to peak, which is enough to drive an auxiliary amplifier. The attenuator is made up of a resistor and a pair of 1N34A Germanium diodes. The more bias current applied to the diodes, the more they will attenuate the main audio signal. The diode bias current is created by a transistor-based peak detector circuit driven off the main audio signal.
But don’t just take our word for it, watch the video after the break.

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The pi pad

In the world of electronics we have impedance; the combination of all forces which oppose the flow of electric current. Often times we have circuits with different impedances, 50 ohms for RF, or 75 for cable TV. It’s pretty important to use the right coax in these circuits, else you’ll be wondering why your RG-58 antenna feed line doesn’t give you anything good to watch.

It’s pretty important to match impedances when connecting different circuits. Apart from the obvious flaws such as a 50 ohm load blowing up a 300 ohm amplifier, there are subtler things such as signal reflection and destructive interference which might just be enough to break whatever it is your playing with. RF mosfets are not cheap! But how could we match impedances? Well we could always use a transformer, but those are rather expensive and bulky. What if we only have a box of resistors to play with? Continue reading “The pi pad”