This SDR Uses A Tube

March 7, 2019 by Al Williams 17 Comments

When you think of a software defined radio (SDR) setup, maybe you imagine an IC or two, maybe feeding a computer. You probably don’t think of a vacuum tube. [Mirko Pavleski] built a one-tube shortwave SDR using some instructions from [Burkhard Kainka] which are in German, but Google Translate is good enough if you want to duplicate his feat. You can see a video of [Mirko’s] creation, below.

The build was an experiment to see if a tube receiver could be stable enough to receive digital shortwave radio broadcasts. To avoid AC line hum, the radio is battery operated and while the original uses an EL95 tube, [Mirko] used an EF80.

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The Cat, The Aircraft, And The Tiny Computer

March 4, 2019 by Jenny List 29 Comments

Sharing your life with a cat is a wonderful and fulfilling experience. Sharing your life with an awake, alert, and bored cat in the early hours when you are trying to sleep, is not. [Simon Aubury] has just this problem, as his cat [Snowy] is woken each morning by a jet passing over. In an attempt to identify the offending aircraft, he’s taken a Raspberry Pi and a software-defined radio, and attempted to isolate it by spotting its ADS-B beacon.

The SDR was the ubiquitous RTL chipset model, and it provided a continuous stream of aircraft data. To process this data he used an Apache Kafka stream processing server into which he also retrieved aircraft identifying data from an online service. Kafka’s SQL interface for interrogating multiple streams allowed him to untangle the mess of ADS-B returns and generate a meaningful feed of aircraft. This in turn was piped into an elasticsearch search engine database, upon which he built a Kibana visualisation.

The result was that any aircraft could be identified at a glance, and potential noise hotspots forecast. Whether all this heavy lifting was worth the end result is for you to decide, however it does provide an interesting introduction to the technologies and software involved. It is however possible to monitor ADS-B traffic considerably more simply.

Thanks [Oleg Anashkin] for the tip.

MIT IAP Tackles Radio

February 18, 2019 by Al Williams 2 Comments

MIT is well known for rigorous courses, but they also have a special four-week term at the start of each year called the IAP — Independent Activities Period. This year, the MIT Radio Society had several interesting presentations on both the history and application of radio. You weren’t there? No problem, as the nine lecture were all recorded for you to watch at your leisure. You can see one of the nine, below.

These aren’t some five minute quicky videos, either. They are basically live captures that run anywhere from an hour to almost two hours in length. The topics are a great mix including radio history, software-defined radio, propagation, radio astronomy, RADAR, and even 5G.

You might have to pick and choose. Some of the lectures are suitable for just about anyone. Some assume a bit more radio expertise in electronics or math. Still, they are all worth at least a cursory skim to see if you want to really sit and watch in detail. The only nitpick is that some presenters used a laser pointer that doesn’t show up on the inset slide graphics in the video. That makes sense because the inset slides are not really in the room, but it can make it a little difficult to understand what the speaker is pointing to on a crowded slide.

Of course, if you want to dive deep and you need more background, MIT — along with many other institutions — will let you use their learning material for free. We were especially fans of the circuits class but there are many others including just raw materials from OCW.

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Radio Telescopes Horn In With GNU Radio

January 20, 2019 by Al Williams 26 Comments

Who doesn’t like to look up at the night sky? But if you are into radio, there’s a whole different way to look using radio telescopes. [John Makous] spoke at the GNU Radio Conference about how he’s worked to make a radio telescope (PDF) that is practical for even younger students to build and operate.

The only real high tech part of this build is the low noise amplifier (LNA) and the project is in reach of a typical teacher who might not be an expert on electronics. It uses things like paint thinner cans and lumber. [John] also built some blocks in GNU Radio that made it easy for other teachers to process the data from a telescope. As he put it, “This is the kind of nerdy stuff I like to do.” We can relate.

The telescope is made to pick up the 21 cm band to detect neutral hydrogen from the Milky Way. It can map the hydrogen in the galaxy and also measure the rotational speed of the galaxy using Doppler shift. Not bad for an upcycled paint thinner can. These are cheap enough, you can even build a fleet of them.

This would be a great project for anyone interested in radio telescopes or space. However, it is particularly set up for classroom use. Students can flex their skills in math, engineering, programming, and — of course — astronomy and physics.

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Underclocking The ESP8266 Leads To WiFi Weirdness

January 4, 2019 by Tom Nardi 39 Comments

Sometimes the best hacks come from the most basic of questions. In this case, [CNLohr] was wondering what would happen if he started to reduce the clock speed of the ESP8266’s Baseband PLL (BBPLL) while still trying to communicate with it. You know, as one does. The results ended up being fairly surprising, and while it’s not immediately clear if there’s a practical application for this particular trick, it’s certainly worth some additional research.

The idea here is that the BBPLL is the reference clock for the entire system, including all of the peripherals. So underclocking it doesn’t just slow down code execution as you might expect, but it also slows down the chip’s interactions with the outside world. [CNLohr] demonstrates this concept in the video below, showing how the baud rate used to view the serial output from the ESP8266 needs to be adjusted to match the chip’s frequency or else you’ll only get garbage on the line.

But what happens to the WiFi? As [CNLohr] discovered, while the center frequency itself doesn’t change, the channel width gets narrower as the clock rate is lowered. When viewed on the waterfall display of a software defined radio (SDR), the transmission can be seen “compressing” in a step pattern as the clock rate is reduced. As one might expect, the 802.11 packets become indecipherable to a normal WiFi device running in monitor mode. The signal is still at the correct frequency, but the devices can no longer understand each other.

Now it was time for another of those basic questions. What would happen if you did the same thing to a second ESP8266? Much to his surprise, [CNLohr] discovered that the two devices could still communicate successfully as long as their BBPLL clock speed was the same. From an outsider’s perspective it looked like gibberish, but to the two ESPs which had been slowed by the same amount, everything worked as expected even though the 802.11 standards say it shouldn’t.

So what can you do with this? The most obvious application is a “stealth” WiFi connection between ESP8266s which wouldn’t show up to normal devices, a communications channel invisible to all but the most astute eavesdropper. [CNLohr] has made all the source code to pull this trick off public on GitHub, and it should be interesting to see what kind of applications (if any) hackers find for this standards-breaking behavior.

If your thing is devices being forced into operations they were never intended to by particularly twisted hackers, check out our recent coverage of the USB serial adapter turned SDR by [Ted Yapo].

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Radio Gets Ridiculous

December 21, 2018 by Al Williams 16 Comments

There were plenty of great talks at this year’s Supercon, but we really liked the title of Dominic Spill’s talk: Ridiculous Radios. Let’s face it, it is one thing to make a radio or a computer or a drone the way you are supposed to. It is another thing altogether to make one out of things you shouldn’t be using. That’s [Dominic’s] approach. In a quick 30 minutes, he shows you two receivers and two transmitters. What makes them ridiculous? Consider one of the receivers. It is a software defined radio (SDR). How many bits should an SDR have? How about one bit? Ridiculous? Then you are getting the idea.

Dominic is pretty adept at taking a normal microcontroller and bending it to do strange RF things and the results are really entertaining. The breadboard SDR, for example, is a microcontroller with three components: an antenna, a diode, and a resistor. That’s it. If you missed the talk at Supercon, you can see the newly published video below, along with more highlights from Dominic’s talk.

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Your USB Serial Adapter Just Became A SDR

December 6, 2018 by Tom Nardi 40 Comments

To say that the RTL-SDR project was revolutionary might be something of an understatement. Taking a cheap little USB gadget and using it as a Software Defined Radio (SDR) to explore the radio spectrum from the tens of megahertz all the way into gigahertz frequencies with the addition of nothing more than some open source tools may go down as one of the greatest hacks of the decade. But even in the era of RTL-SDR, what [Ted Yapo] has manged to pull off is still pretty incredible.

With a Python script, a length of wire attached to the TX pin, and a mastery of the electron that we mere mortals can only hope to achieve, [Ted] has demonstrated using a common USB to serial adapter as an SDR transmitter. That’s right, using the cheap little UART adapter you’ve almost certainly got sitting in your parts bin right now and his software, you can transmit in the low megahertz frequencies and even up into VHF with some trickery. The project is still very much experimental, and though this may be the first time, we’re willing to bet this isn’t the last time you’ll be hearing about it.

The basic idea is that when sending certain characters over the UART serial line, they can combine with the start and stop bits to produce a square wave burst at half the baud rate. [Ted] found that sending a string of 0x55 at 19200 baud would generate a continuous square wave at 9600 Hz, and if he turned the baud rate all the way up to 2,000,000 where these USB adapters top out, that signal was transmitted at 1 MHz, right in the middle of the AM dial.

A neat trick to be sure, but alone not terribly useful. The next step was to modulate that signal by sending different characters over UART. [Ted] explains at great length his experiments with multi-level quantization and delta-sigma schemes, and each step of the way shows the improvement of the transmitted audio signal. Ultimately he comes up with a modulation scheme that produces a impressively clean signal, all things considered.

This alone is impressive, but [Ted] isn’t done yet. He realized that this method of transmission was generating some strong frequency harmonics which extended far beyond the theoretical maximum 1 MHz frequency of his UART SDR. In his experimentation he found he was able to pick up a signal from all the way out to 151 MHz, though it was too poor to be of any practical use. Dialing back the expectations a bit, he was able to successfully control a cheap 27 MHz RC toy using the 43rd harmonic of a 631 kHz signal at a range of about 10 feet with a FT232RL adapter, which he notes produces the cleanest signals in his testing.

[Ted] is still working on making transmissions cleaner and stronger by adding filters and amplifiers, but these early accomplishments are already very promising. His work reminds us of a low frequency version of the USB to VGA adapter turned GHz SDR transmitter, and we’re very eager to see where it goes from here.

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