Serial Port SDR • Hacker News Board

Re: Serial Port SDR

#51

Post by lima » Thu, Dec 06, 2018, 4:55 PM UTC

xtf wrote: post #35

lima wrote: post #10

> 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…

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.

Re: Serial Port SDR

#52

Post by lima » Thu, Dec 06, 2018, 5:07 PM UTC

bluGill wrote: post #22

poizan42 wrote: post #17

Earlier quoted context omitted.

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.

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)

Also, there's a big difference between random noise and a modulated signal on a specific frequency, with a matching antenna.

One of those is noise, the other one is a transmission.

Re: Serial Port SDR

#53

Post by lima » Thu, Dec 06, 2018, 5:12 PM UTC

wglb wrote: post #46

jimnotgym wrote: post #29

Earlier quoted context omitted.

The harmonics are above the transmission frequency, so a low pass filter is normally all that is needed to clean this up. There is a project that uses a Raspberry pi to transmit from is GPIO To use a home built transmitter you need amateur radio license, no matter what.

Not true. Low power FM and AM stations are allowed. https://www.fcc.gov/media/radio/low-power-radio-general-info...

And there's ISM bands as well. However, all of these unlicensed bands have strict limits.

For any of the fun stuff, you need an amateur radio license :-)

Re: Serial Port SDR

#54

Post by jimnotgym » Thu, Dec 06, 2018, 5:32 PM UTC

jimnotgym wrote: post #29

lima wrote: post #10

> 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…

The harmonics are above the transmission frequency, so a low pass filter is normally all that is needed to clean this up. There is a project that uses a Raspberry pi to transmit from is GPIO To use a home built transmitter you need amateur radio license, no matter what.

I got called away while writing this earlier and rushed my last point which is clearly wrong. It is too late to edit.

Instead: To use a home built transmitter you need amateur radio license, except in certain specific low power situations. To comply with those situations would be difficult without specialised test gear.

Re: Serial Port SDR

#55

Post by veli_joza » Thu, Dec 06, 2018, 5:34 PM UTC

codeulike wrote: post #20

cheschire wrote: post #6

Not 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

Thanks.

The SDR they talk about in article introduction is specifically using cheap USB TV tuners in combination with open source software to capture various signals. The list of applications is really impressive: https://www.rtl-sdr.com/tag/applications-2/

Re: Serial Port SDR

#56

Post by jimnotgym » Thu, Dec 06, 2018, 5:36 PM UTC

jimmytucson wrote: post #16

lima wrote: post #10

> 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…

I don't think transmitters are made artificially expensive to keep out the newbs. As you said, it's illegal. And if you're a newb (like me) interested enough to try this at home, you might have read to the end of the article where it states: > DISCLAIMER: It should go without saying that you should never transmit on any frequencies or at any power levels for which you are not authorized by the local governing body, b…

Transmitters are not even expensive. A Baofeng is around $20. Some CW kits are $5.

Re: Serial Port SDR

#57

Post by madengr » Thu, Dec 06, 2018, 6:02 PM UTC

IIAOPSW wrote: post #12

Well 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.

0xAA. Maybe 0x55,0xAA, ... for PSK.

Re: Serial Port SDR

#58

Post by StillBored » Thu, Dec 06, 2018, 6:53 PM UTC

tjohns wrote: post #50

EarthIsHome wrote: post #38

Earlier quoted context omitted.

The wire is being used as an antenna. When you use UART, you have the other end of the wire terminated with another circuit. (i.e. The energy goes to into the connecting circuit.) Because the wire ends at an open circuit, the currents on the wire radiate, turning the wire into an antenna.

Wires connected to a closed circuit will radiate too, if you drive them at high frequency. Maybe not efficiently , but it'll still radiate. There's nothing special about an open circuit that makes an antenna work. Shielding (e.g. coax, or a properly grounded enclosure) is what prevents ordinary wires from becoming antennas.

Normal rs232 is run through shielded cables as well. It was standard practice to wire the d-sub connectors to the cable shields. The wires themselves may not have been shielded, but the cable was shielded against being an antenna.

Re: Serial Port SDR

#59

Post by StillBored » Thu, Dec 06, 2018, 6:55 PM UTC

This is just the RF version of the apple ][ speaker playing audio. Toggle a binary pin attached to something analog fast enough and you can approximate a signal (plus a ton of harmonics).

Re: Serial Port SDR

#60

Post by dogecoinbase » Thu, Dec 06, 2018, 7:02 PM UTC

rlonstein wrote: post #33

sorenjan wrote: post #2

Reminds me of using an output pin on Raspberry Pi to transmit FM radio: http://icrobotics.co.uk/wiki/index.php/Turning_the_Raspberry...

Reminds me of abusing the CRT on the TRS-80 to play "music" on an AM radio. Except we were kids and had very little idea what we were doing.

Tempest for Eliza is an cool old piece of code to play mp3s over your CRT via AM: http://www.erikyyy.de/tempest/