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Forum Index : Microcontroller and PC projects : Turn Raspberry Pi’s GPIO into an FM Transmitter
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andreas Senior Member  Joined: 07/12/2020 Location: GermanyPosts: 226 |
Just found it on mastodon. http://www.icrobotics.co.uk/wiki/index.php/Turning_the_Raspberry_Pi_Into_an_FM_Transmitter -andreas |
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Mixtel90 Guru  Joined: 05/10/2019 Location: United KingdomPosts: 7937 |
Seriously naughty! I wonder what the harmonics are like? Totally illegal in the UK by the way, we don't have band provision for any sort of low powered FM stations. 88-108MHz is particularly restricted as you need a commercial broadcasting licence. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
I expect the harmonics COULD be really horrible, as there is no LPF(low-pass filter) on the RF output. The output RF is normally filtered on most decent transmitter designs, to reduce harmonics and spurious signals. I would love to hook this up to my spectrum analyzer to see what it is actually radiating.  You'd probably find it was radiating all over the band. VERY CLEVER idea though, I will certainly give it that! Would not be illegal here as we have upper and lower LPFM(low power FM) bands that anyone can use for free up to one watt now, I seem to recall - it's been a while since I last looked at our LPFM regulations. LPFM is common in many countries - I'm surprised UK does not allow it. Possibly simply cos there are just so many stations now using up all the band? Here in NZ, there are not really THAT many FM stations in any one area, exposing several "Holes" in the band when LPFM can co-exist with commercial stations. The regs specify the frequencies you can use, and no commercial stations are given licenses in those bands, so it is a good co-existence situation.  However, the onus is on the LPFM user, to ensure their transmitter is compliant with the minimum standard, and I don't expect the Pi transmitter would comply!  If I knew enough about programming on the Pi(I don't) I would set one of these up and put it on the analyzer to see what it was doing. I'd be very interested to find out! Smoke makes things work. When the smoke gets out, it stops! |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 5091 |
Dont waste too much time on this. It is smart thinking, but a few discrete components will give you a far cleaner spectrum.. PicomiteVGA PETSCII ROBOTS |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 7937 |
This is very true. :) Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| matherp Guru Joined: 11/12/2012 Location: United KingdomPosts: 10315 |
I've got a tentative plan for using the Pico to make a "pantry" AM transmitter using an AD9833 module. These are used to allow old AM radios to be used for things like mp3 playback. The Pico will generate a 44.1KHz interrupt which will adjust the amplitude of the carrier generated by the AD9833 on a suitable AM frequency. Legality of this sort of thing varies throughout the world but if the power is low enough not to leave the house then..... |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
@ Volhout & Mixtel90 - Agreed. The PI idea, they would appear to be using it as a glorified oscillator, which, as you both point out, is much easier and cheaper done using discrete-components. But I still admire the idea and concept. Just don't put one on-air! These days, you CAN actually buy FM transmitters from eBay or AliExpress that ARE compliant. The designs have gotten better and yet, still cheaper. One unit I use that is compliant is this one here. The whip antenna and power adaptor are binned immediately. The unit is then powered from an approved 12v supply, and roof antenna with coax. They comply on my spectrum analyzer in terms of good harmonic and spurious suppression, and are only fifty bucks! A Raspberry Pi will cost you about that much. A sample of these was also tested by my local radio inspector, and he confirmed that they comply and can be legally used for LPFM here in NZ. I used one a couple of years ago to replace a Veronica kit transmitter that was pinged for not being compliant. When I was involved in the job, I told the station just to outright replace the transmitter as they are cheap enough now, and the Veronica ones have a bit of a reputation, unfortunately. They work OK, but the have horrible harmonic and spurious figures, which is what the inspector pinged it on. My guess is that one of the harmonics was upsetting something else, which caused the radio inspector to get involved, and he tracked it to the Veronica transmitter and issued the station with a notice to fix the output or take it off-air. ...but I digress... Edited 2022-11-14 11:00 by Grogster Smoke makes things work. When the smoke gets out, it stops! |
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TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6283 |
The RPi transmitter is probably closer to this: https://www.ebay.com.au/itm/172273986374 I do have one similar but most of the modules are now Bluetooth The price is about the same as a pico, much cheaper than a RPi The AM version Peter is working on will be popular for antique radio collectors. It is getting hard to find AM broadcasts to listen to so the AM modulator connected to a web radio stream brings the old radio back to life. Jim VK7JH MMedit |
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| andreas Senior Member Joined: 07/12/2020 Location: GermanyPosts: 226 |
Thank's for info! I found these details about the AD9833: https://www.analog.com/en/products/ad9833.html#product-reference -andreas |
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| matherp Guru Joined: 11/12/2012 Location: United KingdomPosts: 10315 |
See https://geoffg.net/SignalGenerator.html for the module I will use - has digital potentiometer needed to adjust the amplitude |
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| andreas Senior Member Joined: 07/12/2020 Location: GermanyPosts: 226 |
Oh that looks very good - esp. the digital input. Will look at it! Some time ago I bought a XR2206-kit from amazon (I have forgotten the price). One can tune the knobs but you don't know the frequency and the values are "jumping" due to the "quality" of the potentiometers. There are jumpers for selecting ranges of values and for the form of the signal. (see below)  poor mans signal generator -andreas |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 7937 |
I got one of those! Rather horrible, aren't they? :) Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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Nimue Guru Joined: 06/08/2020 Location: United KingdomPosts: 420 |
If we can "do" FM - can we do this? PAL TV via PWM  N Entropy is not what it used to be |
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| andreas Senior Member Joined: 07/12/2020 Location: GermanyPosts: 226 |
Yes! but better than nothing ;-) -andreas |
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| andreas Senior Member Joined: 07/12/2020 Location: GermanyPosts: 226 |
When reading this, Tempest attacks come to my mind. -andreas |
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| Pluto Guru Joined: 09/06/2017 Location: FinlandPosts: 375 |
An other option if you need higher frequency could be AD9850 with amplitude control by DAC or digital potentiometer: https://www.ic72.com/pdf_file/a/148731.pdf The AD9850 (62.5MHz) modules are however more expensive than the AD9833 (12.5MHz) modules. Looking forward to hear/see more about your project! Fred |
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