To say that the RTL-SDR project is revolutionary might be a bit of an understatement. Taking a cheap little USB gadget and using it as a Software Defined Radio (SDR) to explore the radio spectrum from tens of megahertz all the way up to gigahertz frequencies with the addition of nothing more than some open source tools can become one of the biggest hacks of the decade. But even in the era of RTL-SDR what [Ted Yapo] has managed to pull it off is still pretty amazing.
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] demonstrates the use of a simple USB to serial adapter as an SDR transmitter. That’s right, using the cheap little UART adapter you almost certainly have in your parts bin right now and its software, you can transmit in the low megahertz frequencies and even up into the VHF with some trickery. The project is still very experimental, and while this might be the first time, we’re willing to bet that it won’t be the last time you hear about it.
The basic idea is that when sending certain characters over the UART serial line, they can be combined with the start and stop bits to produce a square wave packet 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 increased the baud rate all the way to 2,000,000, where these USB adapters peak, that signal was being transmitted at 1 MHz, right in the middle on the AM dial.
A nice trick for sure, but not very useful on its own. The next step was to modulate this signal by sending different characters over the UART. [Ted] explains at length his experiments with multilevel quantization and delta-sigma schemes, and every step of the way shows the improvement of the transmitted audio signal. He eventually devised a modulation scheme that produced an impressively clean signal, all things considered.
That alone is impressive, but [Ted] it’s not ready yet. He realized that this transmission method generated some strong frequency harmonics that extended well beyond the theoretical maximum frequency of 1 MHz of his UART SDR. In his experiments, he found that he was able to pick up a signal all the way up to 151 MHz, although it was too weak to be of practical use. Lowering expectations a bit, he was able to successfully drive an inexpensive 27 MHz RC toy using the 43rd harmonic of a 631 kHz signal at a distance of about 10 feet with an FT232RL adapter, which he noted produced the cleanest signals in his testing.
[Ted] is still working on making the transmissions cleaner and louder by adding filters and amplifiers, but these early advances are already very promising. His work reminds us of a low frequency version of USB to VGA adapter turned GHz SDR transmitterand we are very eager to see where it goes from here.
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