Your USB Serial Adapter Just Became a SDR

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.

36 thoughts on “Your USB Serial Adapter Just Became a SDR

    1. Good idea. But you can do the same thing driving dtr and/or rts directly from code in a controlled loop. And more power, if you want, because you can tie them together. Did that in vb6 long time ago. Though I used it to get higher dc voltage via charge pump. I also use DTR and/or RTS direct control from, to control directly small DIP reed relay in simple PC automation project.

  1. “To say that the RTL-SDR project was revolutionary might be something of an understatement”
    Why does anything here to do with SDR include this sort of thing? I’m sure I can’t be the only one who finds it all a bit tedious.

    1. Probably because few people even knew what SDR was before then? And since RTL-SDR hit the scene we’ve seen such an explosion in interest that we now have stuff like the LimeSDR and HackRF?

      RTL-SDR is like MakerBot. 3D printing existed before MakerBot, but it wasn’t practical for the masses. Once it broke that limitation, the market was flooded with new options.

      Honestly if you don’t know how important RTL-SDR was to putting this kind of tech in the hands of the average hacker, you probably don’t know enough about the situation to even be commenting.

      1. I know what SDR is. I just wanted to point out that perhaps not everyone is as excited about it as you are. It’s like someone continually raving on about their favourite band without realising that maybe not everyone is into them. Be enthusiastic sure, but calm down with the “everyone knows this is the best thing ever invented”.

        1. What would you say is a larger accomplishment/event in the world of affordable SDR then? If you don’t think it’s rtl, then you must think it’s something else

          If on the other hand your saying SDR in general doesn’t interest you then OK, but why did you click on an SDR article and feel the need to comment then? Would you have done the same to somebody posting about a band you didn’t like to use you’re own example?

      2. Makerbot? Really? They helped but I think you are giving them WAY too much credit.

        If by masses you mean the maker/hacker sub-culture then Makerbot was second to the party. There were a lot of people building RepRaps while Makerbot was just a gleam in Bre Petis’s eye.

        If by the masses you truly mean all the every-day average people then no way! 3d printers STILL haven’t reached most households even now! They may feel like old news to us but they are nowhere near commonplace even today. Frankly, they aren’t ready to be either. There is still too much tweaking and maintenance involved in their use for that to be a thing. Your typical non-techincal person, plucked randomly from the masses will probably be stressed out just over adjusting the tram of the bed. Imagine how they would handle their first clogged hotend!

        But… people are starting to THINK that 3d printers are mainstream. I wonder how many will be gifted this Christmas. I predict a whole lot of almost-new printers on Craigslist/Ebay this coming February all in need of just a little “basic” maintenance.

        Come to think of it.. if anyone has a used printer they might want to sell… they had better do so NOW before this years gifts hit the resale market!

        Anyway.. Makerbot. Yah, they were first to sell kits and pre-built printers to the maker community (not the masses). That was big but I’m sure it was bound to happen sooner rather than later with or without them. These days their products are mediocre at best compared to the rest of what is out there and they are no longer their own company. Instead they are an arm of Stratasys, the company that sat on 3d printing technology for 30 years and never bothered to market it to anyone with less than a 6 digit budget.

        That’s really not something worth romanticizing.

    2. Your absolutely right. its only really much of a thing in the “maker” communities, and not nearly as much as some make it seem. Get out in the real world of electronics/radio development, and people could give a rats a$$ less really. You see this sort of thing a lot in the “maker” communities. Even products that have been used for many years outside of “maker” communities, get similar treatment once somebody attaches it to an Arduino or Raspberry Pi. Its very odd, and has a tendency to set these folks back some actually. I mean, all we need is an article on balancing robots or printed CNC machines to take the next step in the maker revolution.

      1. Of course it’s only revolutionary to hobbyists, commercial products 1) have little use for SDR to begin with since most radio implementations are specific to that application and 2) can afford real hardware.

        RTLSDRs are huge because for $10 anyone can access a whole world of radio receiving that was previously inaccessible. Remember when the RTLSDRs were first discovered the cheapest SDR you could buy was the USRP which at its cheapest configuration was several hundred dollars.

  2. I’ll bet you [Ted] is not a licensed amateur radio operator. If he was, I don’t think he’d be transmitting a 631kHz fundamental just to control an RC vehicle on its 43rd harmonic. While his technical knowledge appears to be sufficient, he needs a handle on the legal and compliance portion. I would highly recommend he pursue a license before continuing on with these experiments.

    While I think this type of research is valuable and interesting, the fundamentals and harmonics of the square waves he’s producing are trampling all over existing licensed frequencies and running afoul of FCC rules. He needs to be performing these experiments in a faraday cage or via a closed RF system where he can control or prevent OTA emissions.

    The last thing we need is for the FCC to knock on his door, confiscate his equipment, and fine the bejeezus out of him. A bandpass filter should be an absolute requirement for proceeding.

    1. IMO as long as he uses the device without an amplifier he will not do something illegal. He just transmit some characters over a UART. Normal FTDI cables also have 1m of wire connected to the output of the chip.

      Of course filters are good and an absolute must if he uses any kind of amplifier and he should confine his activities to ISM frequencies without a license. And it’s good practice to reduce unnecessary emissions as far as possible, e.g. do measurements with a closed circuit at a dummy load or attenuator.

    2. This reminds me of an article, I think in Dr. Dobbs, about using a UART to output audio. A simple d/a, the pulse width varies by changing the byte sent to the UART.

      It was a neat trick then, and appropriate for audio. This is stretching things.

      You’re right, there are solutions that “work” but which have other problems. Forty year ago, people would describe well thought out “rf modulators” to get your video signal into a TV set. Then someone showed a ttl Schmitt trigger as an oscillator, modulated by the video signal. It relied on harmonics to get the right radio frequency. The author at least recognized the problem, but it was “cheap and easy”.

      Michael

    3. “The last thing we need is for the FCC to knock on his door, confiscate his equipment, and fine the bejeezus out of him.” Fortunately for him, FCC enforcement these days is almost nonexistent. One has to really create quite a ruckus before the FCC will even notice. Even when the FCC decides to take action, it often takes a decade or more for something to actually happen, if at all.

    1. Ah, you’re right! Sometimes acronyms feels so common we forget to explicitly detail their meaning at least once. SDR is Software Defined Radio: a modern method of configuring a radio transmitter or receiver using software, rather than needed to swap out hardware to make the same changes. I’ve added an explanation of the acronym in the preamble of the article. Thanks!

      1. I guess this happens to everyone sometimes.
        To prevent/circumvent this HaD could implement a list of words/phrases/abbreviations that are automatically linked to explanations (Wikipedia and so on). But if I remember correctly this was already proposed several years ago in similar comments….

          1. you don’t say?
            Either I’d need to teach such a script myself which makes it mostly useless OR it uses the first Wikipedia hit or meta page which is often just as useless especially for abbreviations.

            And the general concept of applying something repeatedly thousands times over at many clients instead of once at the source is energetically and ecological not a good idea IMHO (unless the amount of transmitted information/data increase unreasonably).

            And if everyone who doesn’t know such abbreviations searches for it manually it’s even worse because it’s time consuming on top of the other reasons.

        1. We really insist on the expansion of uncommon acronyms, so no glossary should be necessary.

          But there’s a grey zone a mile wide between what everyone knows and what nobody knows. For instance, we’ll leave RAM up to you. And we don”t even capitalize scuba or laser anymore. :)

          SDR is somewhere in-between — must be known by all radio freaks, but probably not common in the general public. Who is the target audience of this story?

  3. I find SDR interesting, because there is a lot of wireless devices out there, and there are safety and security concerns. Seems like SDR would make it possible for anyone with a laptop and a dongle, could pretty much intercept any signal, any band, and use what he finds as he pleases. Remote entry in to cars, or garages, homes they don’t own? Hijack someones drone, or other RC ‘toy’. How about block, or over-ride a weaker signal. Sure it’s a tool, but there are a lot of ways to abuse it, and radio seems to have relied on hardware to keep it secure enough. Now the hardware is the software on a laptop, and just needs to be written to exploit the weakness. Laws are like locks, only stop honest people.

    It is pretty cool, to think one device could eventually handle your every wireless need.

  4. There’s a slew of projects out there that generate a radio signal by bitbanging a digital output. It’s a horrible idea because of the harmonics and interference it causes, but there’s a ton of them.

  5. Any switching output generates RF. Any. This has been known for a really long time, and at least as long ago as the 1960’s people developed programs to play music when an AM radio was placed near a mainframe computer.

    There are some elements of basic civility which people engaging in experiments with this should engage in, because yes, you really can be heard at great distance in some cases, and you interfere with other people’s communications.. Constructing a filter on your output to restrict it, as much as possible, to your desired frequency is one. Just read about LC circuit design. Wires are antennas. Yours should be resonant on your desired frequency, so make it about 1/4 wavelength long and give it a ground plane with 3 or 4 elements of the same length. The web will teach you of these things, or what, you don’t have “The Radio Amateur’s Handbook”? It’s a hacker essential. Do not put a very long wire on your output, or you will spew interference on many frequencies and disgruntled hams may make you a target of direction-finding, etc.

  6. He needs to eat a lot of crow pie. Pi filters! This is as bad as a spark gap transmitter, like the ones hanging on power poles all over town. I get a lot of RFI from them all the way up on the FM broadcast band.

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