This guy is making arbitrary waveforms via the R Pi PIO. Probably easily extended to an FM transmitter.
Turn Raspberry Pi’s GPIO into an FM Transmitter (2012)
51–60 of 89 posts
Re: Turn Raspberry Pi’s GPIO into an FM Transmitter (2012)
#52Earlier quoted context omitted.
I know nothing about how this all works, how do people know who did it / where it came from? I presume its possible to get a rough location but in a built up area I'd imagine its pretty much impossible to find the person that did it unless it's left running right?
You can use radio triangulation to determine where the signal source is while it is transmitting
Unless the transmitter is moving.
Steal The Air!! -Pump Up The Volume
Re: Turn Raspberry Pi’s GPIO into an FM Transmitter (2012)
#53Earlier quoted context omitted.
Perhaps public safety shouldn't be entirely beholden to Motorola rebranding old garbage (ancient TDMA networks being sold as new P25 compliant hardware for tens of millions of dollars) that is then sold as new. Modern protocols like 5G New Radio that have better propogation at the fringes of connectivity would serve life critical applications with improved coverage and significantly more intelligible voice quality th…
It has nothing to do with Motoroa, TDMA, or 5G. If the wanted signal is very weak (eg from a distant aircraft using VHF AM), and there is an interfering signal on the same channel, the emergency communications will be distrupted.
Aircraft using the VHF band with amplitude modulation is going to perform much differently than a police repeater and it's local upstream only amplifiers interacting with low wattage fixed handhelds and medium wattage car mounted mobile radios.
Pivoting topics is a bad faith conversational tactic FYI, and doesn't follow HN's guidelines. I encourage you to fully read the guidelines linked at the bottom of the page.
Re: Turn Raspberry Pi’s GPIO into an FM Transmitter (2012)
#54Earlier quoted context omitted.
Radio Amateurs routinely communicate around the world with just a few milli-watts. And even a few micro-watts is sufficient to communicate across town. Many years ago the record was "1000 miles per milliwatt" using CW.
They do that with very carefully optimised equipment.
And when certain atmospheric conditions exist.
Re: Turn Raspberry Pi’s GPIO into an FM Transmitter (2012)
#55Earlier quoted context omitted.
It'd have to sit down and do some math, but I'm kinda skeptical that an FM Broadcast band harmonic would land on the air and right next to it.
It's not so much "harmonics of the FM band", but the wide band of spurs either side of the transmission. The problem is that the unfiltered output of this device contains a wide range of spurs and harmonics which will radiate right across the VHF and UHF bands. Plus the antenna (a few feet of wire) will have multiple resonances across the same range. It will radiate hundreds of signals, and the odds of them interferi…
UHF and VHF? with an RPI?
Re: Turn Raspberry Pi’s GPIO into an FM Transmitter (2012)
#56Earlier quoted context omitted.
> "using some filters on the output" While it's relatively easy to remove harmonics, it is pretty much impossible to remove the broad comb of spurious signals either side of the carrier. Which why transmitters are not designed this way in the real world.
Which spurious signals do you mean? I'm assuming you're talking about stuff well outside the intended sidebands that result from the modulation. If we were talking about AM I would assume you mean the audio signal isn't bandlimited and so high-frequency audio components get upconverted by the carrier frequency, but I don't know what the spectrum of an FM signal looks like. Are we talking about a broad comb that resul…
I'm talking about the broad band of spectral components (both close in and further out) which are generated by the synthesizer. These are a separate issue to any sidebands generated by the modulation process.
see https://en.wikipedia.org/wiki/Phase_noise
It's surprisingly difficult to generate a clean carrier which doesn't have a broad band of spectral components, but this is essential for the design of effective Receivers and Transmitters.
These sidebands are generated by noise in the oscillator, spurs caused by mixing products, jitter in the synthesiser, and non-linearities in the modulation process.
The bottom line is to look at the output with a high-quality spectrum analyser and compare the result with the spectral mask which is published by the FCC.
Re: Turn Raspberry Pi’s GPIO into an FM Transmitter (2012)
#57One needs to be careful with such home made devices as they may clash with the officially assigned frequencies. We had in France a similar story where a village was losing 3G/4G connectivity almost every day around 22:00. It seemed that there are interferences with something. A special car was sent by the national regulation entity, it was trying to triangulate the source. They found a house where the owner bought a…
In my small town, local TV reception was being completely wiped out by interference. When the Radio Inspector came to check, he found a rechargeable torch sitting in it's charger bracket, which generated horrendous interference whenever the batteries reached full.
I know the reverse of your story. And OLD TV from a grandma jamming an entire town:
https://www.popularmechanics.com/science/a34229436/old-tv-br...
Re: Turn Raspberry Pi’s GPIO into an FM Transmitter (2012)
#58Author here. This code no longer works on new Pi's (it does a lot of low level very hardware stuff from userspace without a proper kernel driver). The code was written in a 24 hour Hackathon, so is very far from maintainable. However there are other projects to do the same which do work on new Pi's. There are ways to do a full software defined radio from a pi by using the I2S hardware, and that can transmit GPS, 433…
> There are ways to do a full software defined radio from a pi by using the I2S hardware, and that can transmit GPS, 433 MHz garage door openers, and various other things.
Wait, we can do sine wave up to ~1.5 GHz using the built-in PCM/I2S peripheral?
Re: Turn Raspberry Pi’s GPIO into an FM Transmitter (2012)
#59Earlier quoted context omitted.
It's not so much "harmonics of the FM band", but the wide band of spurs either side of the transmission. The problem is that the unfiltered output of this device contains a wide range of spurs and harmonics which will radiate right across the VHF and UHF bands. Plus the antenna (a few feet of wire) will have multiple resonances across the same range. It will radiate hundreds of signals, and the odds of them interferi…
UHF and VHF? with an RPI?
Any square-wave signal (eg PWM) will have spurs going up to UHF and possibly higher.
Re: Turn Raspberry Pi’s GPIO into an FM Transmitter (2012)
#60Earlier quoted context omitted.
Radio Amateurs routinely communicate around the world with just a few milli-watts. And even a few micro-watts is sufficient to communicate across town. Many years ago the record was "1000 miles per milliwatt" using CW.
They do that with very carefully optimised equipment.
They also do it with very simple transmitters using perhaps just one transistor. All it takes is a few mill-watts and a reasonably efficient antenna.