USB serial dongle? pah, use the system-bus on the motherboard ;) https://github.com/fulldecent/system-bus-radio
Now that is rad!
Serial Port SDR
61–70 of 91 posts
Re: Serial Port SDR
#62> 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. i.e., blast noise all over the spectrum. There's a reason why real transmitters are more expensive than a serial port adapter. This kind of RF pollution is illegal for obvious reasons and if…
But he doesn't have any amplification. Nothing is happening here that doesn't happen already with normal use of the UART. If what he is doing here is illegal then the RF pollution from using the UART normally would be too.
It's an "intentional radiator", in radio terms.[1]
It's very easy to build something that blithers all over the RF spectrum by connecting a digital signal to an antenna. Hook a square wave to an antenna and you're going to get lots of harmonics. This is generally considered a bad thing.
In the early days of digital electronics, a TRS-80 Model I home computer and a Milton Bradley Big Trak toy car would both crash if near each other. Strict regulation of RF emissions, and good protection on the input side, is why we can have so many digital devices without everything randomly crashing.
[1] https://www.ecfr.gov/cgi-bin/text-idx?SID=56381cb4f24a491e7d...
Re: Serial Port SDR
#63As mentioned in the first sentence, the RTL-SDR project is out there. You can easily buy a $20 USB Software Defined Radio with software that will handle everything for you and lets you play around with an SDR: https://www.rtl-sdr.com/buy-rtl-sdr-dvb-t-dongles/
For roughly the same price there are some DVR boards that could be converted to SDR operation once they're hacked. They already use embedded Linux although no source is provided.
https://www.aliexpress.com/item/OUERTECH-5-in-1-AHD-CVI-TVI-...
Re: Serial Port SDR
#64Not being deeply into signals geekery, I had to lookup SDR. I believe the article is referring to a Software-Defined Radio. https://en.wikipedia.org/wiki/Software-defined_radio
(Not a Surveillance Detection Route.)
Actually, this is a class D transmitter. Minus the usual output filter to eliminate the higher harmonics. Look at that spectrum chart. The second and third harmonics are huge!
It's quite possible to build something that will turn a square wave into a clean sine wave, and you need that. You can also generate waveforms digitally that have weaker higher harmonics, which means the output filter is simpler. There's something called a class E amplifier, which is like a class D but with an unusual analog tank circuit on the output end to clean things up.[1]
A device like this with no output filter is an annoyance, not a communications device.
[1] https://www.crarc.ampr.org/uploads/Main/Archives/July%20Clas...
Re: Serial Port SDR
#65Well now we know what radio waves are produced by 0x55. How about other characters? I'm willing to bet that everyone's wired keyboard is an SDR so to speak and a well placed receiver can keylog you.
Most keyboards don't have an antenna attached. While you can keylog a keyboard via emissions like this, you have to have physical access to the office to do so - at that point putting a usb keylogger inline with the keyboard is easier and more reliable. (or substitution your own keyboard with a real radio)
Why office? Imagine such keyloggers under tables in public spaces where people can put their laptops.
Re: Serial Port SDR
#66Earlier quoted context omitted.
He connected an antenna to it. That's what makes it illegal.
Ugh. It's not by default "illegal", unless it provably violates Part 15. Has a Part 15 assessment been done yet? Have we analyzed power emission levels to assert that this thing is indeed 'breaking the law'? Has this device been checked by the FCC for spurious emissions during normal operation? Did it pass, or is it chinese jank uncertifiable garbage? Edit: C'mon downvoters. If you make a claim that it's "Illegal", y…
If it breaches the power limits I think it is illegal - same as if you used a high power wifi access point with a high gain antenna.
Re: Serial Port SDR
#67> 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. i.e., blast noise all over the spectrum. There's a reason why real transmitters are more expensive than a serial port adapter. This kind of RF pollution is illegal for obvious reasons and if…
But he doesn't have any amplification. Nothing is happening here that doesn't happen already with normal use of the UART. If what he is doing here is illegal then the RF pollution from using the UART normally would be too.
there is an antenna - just a wire - but that changes the equation a lot relative to what a normal UART does has part of a system that already meets FCC (or other) regulatory standards.
I'm curious about the gain of the antenna and what consideration has been made re trasmit wavelength and the antenna wire length.
Re: Serial Port SDR
#68Earlier quoted context omitted.
And it's nothing new. Same is possible with Raspberry Pi GPIO: https://github.com/F5OEO/rpitx
We measured the output of that exact experiment at our local ham radio club and it was horrible, harmonics and noise all over the spectrum.
THIS! -- I was hoping a ham would chime in here :)
For anyone looking to do experiments like this - which are really cool to do - getting the ham technician licenses is pretty easy and very educational!
Re: Serial Port SDR
#69[deleted]
Re: Serial Port SDR
#70Earlier quoted context omitted.
He didn't say this, but I suspect that his antenna was that specific length for rf reasons - wires work best as antennas when their length is some multiple of the wavelength. (ask an rf engineer if you want details, I'm close enough for discussion but all rf engineers will cry when they read that)
Multiples of 1/4 wavelengths, and if built properly 5/8 wave.
... but 5/8 isn't a multiple of 1/4, Or is that not what you meant?