{"id":1009,"date":"2018-03-25T11:12:53","date_gmt":"2018-03-25T11:12:53","guid":{"rendered":"http:\/\/naich.net\/wordpress\/?p=1009"},"modified":"2018-03-26T12:09:15","modified_gmt":"2018-03-26T12:09:15","slug":"taming-the-pifm-transmitter-part-2","status":"publish","type":"post","link":"https:\/\/naich.net\/wordpress\/index.php\/taming-the-pifm-transmitter-part-2\/","title":{"rendered":"Taming the PiFM Transmitter (Part 2)"},"content":{"rendered":"

In part one of this guide<\/a> it became clear that a Raspberry Pi with a 700 mm long wire on pin 7, running a variant of the PiFM software is an easy way to make a nuisance of yourself. We might not be broadcasting kilowatts of power and realistically, you are not going to be knocking planes out of the sky, but the Pi is a dirty old man when it comes to broadcasting and we need to clean up its act.<\/p>\n

The obvious way to do that is to put a filter between the Pi’s output and the aerial. If the design considerations and analysis of the filter’s performance don’t interest you, skip to the end for circuit diagrams, construction instruction and purchase info (possibly).<\/p>\n

As always, these posts are for educational use only. Do not use your Pi as a transmitter unless it is legal for you to do so, which is highly unlikely. Using a filter will not make it any less illegal for you to use your Pi as a transmitter.<\/strong>\u00c2\u00a0 Always brush your teeth before bedtime and be nice to people.<\/em><\/p>\n

To recap, this is typically the sort of thing that comes out of your Pi when you use it as an FM transmitter:<\/p>\n

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There’s a lovely spike around the 144 MHz mark, which is the amateur radio 2m band. There are probably not many radio hams near me that like the sort of music I listen to. Come to think of it, some times I’m not sure I do either. In general, it’s a broad splattering of crap all over the spectrum. And the Pi’s transmitted output is just as bad, ho ho ho. Ahem. This is the sort of thing we need:<\/p>\n

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You might notice R1 there. The GPIO pins are not designed to drive inductive or capacitive loads, so we need to make the filter input a bit more friendly. The easiest way is to put a resistor between the Pi and the filter’s inductor. I’ve tried it and it works, but it does reduce the range of the transmission. If you want to try it without R1, don’t blame me if you fry your Pi. There’s about 5 dB loss with this design, which might be fine for you. For me, it reduced the range just enough that the signal was fading out if I stood in the wrong part of the kitchen. The solution was either to avoid using the fridge or to amplify the output a bit.<\/p>\n

I’m not an expert with RF circuits (although I probably know more than you), so I used the interwebs to find a design that would<\/p>\n