Tiny Core Base > Raspberry Pi

Turning the Raspberry Pi Into an FM Transmitter

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Paulo:
Hi bmarkus

1-250MHz is a very useful range indeed.
I'm not a HAM myself but do tinker with RF projects and there are a few free-bands
in that range I could experiment on.
You have convinced me to order my Raspberry Pi as projects like this are lots of fun as you rightly said
and Linux is a perfect O.S. for this kind of experimentation.

I listened to the recording and must say that the quality is pretty good.
What did you use as the antennae? Simple quarter wave ground plane, yagis ?
I assume you didn't use an amp or did you buffer the logic level to a lower impedance to better match the antenna?

It's a pity that the Raspberry Pi does not have audio in otherwise this technique could be used as a L.O. feeding a mixer for a SDR and the
resulting low I.F. further processed by the Pi itself.

bmarkus:

--- Quote ---I listened to the recording and must say that the quality is pretty good.
--- End quote ---

Yes, I was supprized too.


--- Quote ---What did you use as the antennae? Simple quarter wave ground plane, yagis ?
--- End quote ---

Just 20cm wire :) Station I talked to equipped with professianal radios, rotating yagis, etc. It was just an experiment to see how it works. Of cource you may need some L-C filters and and probably an RF power amp just to protect Pi. However it is a 3.3V output pin which means a high level. Impedance matching is not so important at the moment :)


--- Quote ---It's a pity that the Raspberry Pi does not have audio in otherwise this technique could be used as a L.O. feeding a mixer for a SDR and the resulting low I.F. further processed by the Pi itself.
--- End quote ---

Yes, it is one of the most important limitation. You need audio input working with SDR radios, communication decoders or VoIP. There are USB sound cards you can use or an USB WEB cam with microphone if stereo is not neeed.

Anyhow, Pi is a nice to play with.

Rich:
Hi Paulo

--- Quote ---It's a pity that the Raspberry Pi does not have audio in ...
--- End quote ---
If you really want to get down and dirty, you could build a pulse width modulator and connect it to one of the GPIO
pins. Then use the measured duty cycle to control the frequency deviation of the carrier. That presumes you can
get reasonable timing measurements.

Paulo:
Hi all

I got pretty excited about using the RPI as a transmitter and decided to design and simulate a buffer and filter.
The design is for the FM broadcast band (88-108MHz) as per the original project.
If anyone needs a redesign for other frequencies, let me know.

I must stress that this is only a preliminary design and simulation and I have not physically built the circuit but it can serve as a starting point.
I have created two zip files (due to size limitations on this forum for attachments) which contain the circuit diagram, the simulation response (narrow band 60-130MHz),
another simulation result (wide band 10-400MHz) all in jpeg format and also a list of the S-parameters in S2p format (plain text).

You will notice that the filter calls for some non standard capacitor and inductor values.
This is not a problem as the inductors can be made variable then adjusted in-circuit for best response and the cap values can be made up with combinations of caps.

It is highly recommended the circuit be built on a proper PCB employing good RF practices.

Finally, if anyone decides to build and use the circuit, please respect your local laws concerning transmitting/broadcasting and you use the circuit entirely at your own risk.

Happy experimenting.

Paulo:
Hi Rich


--- Quote ---If you really want to get down and dirty, you could build a pulse width modulator and connect it to one of the GPIO
pins. Then use the measured duty cycle to control the frequency deviation of the carrier. That presumes you can
get reasonable timing measurements.
--- End quote ---

Not a bad idea that, just wondering what timing accuracies one could get as the kernel is not real time so results
might vary depending on what it's doing at a given time.

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