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I have found this awesome Raspberry PI FM transmitter project. It uses RPi GPIO pin to simulate FM signal. After some filtering to smoothen it, this could act like a real transmitter.

But at this moment, I've problems with signal quality. I'm in a big city where lot of wandering signals are present. As well, the range seems not to be as large as anticipated.

I was wondering if I could, right now, create a simple amplifier using just a transistor. I have several random transistors of unknown brand at home - most of them residing in old PCB boards.

Could I use normal transistor (well, a fast one) to amplify such high-frequency signal? Wouldn't the signal penetrate through transistor (or the supply into my Raspberry)?

Tomáš Zato
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    If you're going to amplify it, make sure you check into the legal implications before doing so. – Matt Young Jun 19 '14 at 18:54
  • There is information on how to do this fairly simply, but that is likely to be a VERY nasty signal that has the potential to interfere with a lot radio bandwidth. If you're interested in radio, I would recommend checking out the ham radio community nearby. They can help guide you to experiments that are a little less hackish and a little more considerate (and legal). – Daniel Jun 19 '14 at 19:01
  • I see I should've added that note that I don't care about *legal* warnings in my question. If I'm transmitting something for two minutes nobody is gonna care - espetially on an empty frequency. – Tomáš Zato Jun 19 '14 at 19:05
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    @TomášZato you are right (I deleted my comment). Lets talk about electronics, not law. – Kamil Jun 19 '14 at 19:29
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    I cannot disagree more: as scientists and engineers, we have a moral obligation to look out for the nefarious applications of our inventions. We should not sensor technologies because they *could* be abused, but we should not encourage or develop that usage. What you're talking about doing is specifically illegal, for a good reason. You might take down your neighbor's radio, but to everyone else YOU'RE the asshole messing with their radio. – Daniel Jun 19 '14 at 19:52
  • So far, I have used empty frequencies (there's a list of used frequencies in my location) so you can calm yourself down. The thing this question asks has many completely legal uses. – Tomáš Zato Jun 19 '14 at 19:57
  • Sure, it could. Except that you explicitly said that you want to use it to interfere with your neighbor's stereo. – Daniel Jun 19 '14 at 20:45
  • That was *after* the comments I referred to. And this is rather a funny achievement than a serious reason to try to make a strong signal - I don't even know what station he listens to. You're just taking the thing unnecessary seriously and on top of it you seem to be that kind of a guy who seeks things to be taken seriously. – Tomáš Zato Jun 19 '14 at 20:53

2 Answers2

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If you don't mind being tuned to a single FM frequency, you can use a Class C stage which has a tuned resonant circuit as its load.

A class C stage is very simple because it is not concerned with linearity in the time domain. In fact, the transistor only conducts 50% of the time.

In an instrumentation amplifier or audio, or other such applications, the distortion would be unacceptable, but it doesn't matter in RF, because the encoded signal is robust against distortions of the carrier wave.

This is why the approach of using a digital pin is possible in the first place; it doesn't matter that the carrier being modulated is a square wave.

So the answer is yes; you can build an efficient amplifier which amplifies the RF signal and smooths it at the same time.

Look for schematics depicting a "tuned class C RF amplifier stage", or look up the topic in an introductory electronics text.

Kaz
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  • As of the basic [C-class amplifier without the LC filter](http://upload.wikimedia.org/wikipedia/commons/thumb/f/fa/Electronic_Amplifier_Class_C_-_bipolar.svg/364px-Electronic_Amplifier_Class_C_-_bipolar.svg.png): do I understand it correctly that Raspberry shares ground with the stronger power source? – Tomáš Zato Jun 20 '14 at 17:38
  • [Correct link to the image.](http://commons.wikimedia.org/wiki/File:Electronic_Amplifier_Class_C_-_bipolar.svg) – Tomáš Zato Jun 20 '14 at 17:44
  • @TomášZato That is up to you. Any higher-current circuit should probably have a dedicated return ground connection all the way back to some "Kelvin ground" near the master power supply, rather than being grounded through the same circuit board where you have sensitive small-signal circuits. – Kaz Jun 20 '14 at 19:52
  • I thought, that one I need to connect both - wires to the emitter so that the the signal from raspberry can open the transistor. I have used [this one](http://processmodeling.org/theory/electronics/ref/transistors/2N4401.PDF), by the way. The range improved from 3 to 4.5 metters. It might seem not worth it, but for me it's proof of concept. – Tomáš Zato Jun 20 '14 at 21:35
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You can amplify this signal to get more range, maybe just better antenna will be enough, but don't expect high audio quality because Raspberry FM "hack" uses simple digital I/O signal, you can't modulate frequency precisely.

There are many tested schematics, just search for FM transmitter circuit.

To get reasonable audio quality you should use some real FM transmitter connected to audio output.

Kamil
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  • How much bigger antenna can I use? My table is particulary made from metal... Would it make sense to screw the wire to it? – Tomáš Zato Jun 19 '14 at 19:47
  • @TomášZato You can use a simple [dipole antenna](http://en.wikipedia.org/wiki/Dipole_antenna): a piece of wire whose length depends on the frequency. The table being made of metal doesn't necessarily make it a suitable antenna. – Kaz Jun 19 '14 at 19:49