Audioguru's FM Transmitter Questions

takao21203

Joined Apr 28, 2012
3,702
I spent quite a lot of time trying to tune the transmiter, but was foiled by its sensitivity to stray capacitance. Just placing my hand near the PCB, the power wires, or especially the MP3 player caused the frequency to shift. Of course the power wires are too long waving in the breeze, but the cable from the MP3 player is shielded.

What sort of assembly error might I have made that would cause this frequency instability? And are the two variable capacitors normally interactive?

Thanks.
A EMI ferrite can help. Pull all the wires a few times through it.

When I just take the USB cable in my hand, the radio starts to hiss with white noise. Approaching the circuit does not change so much, since the coils have ferrites. This makes them insensible to hand waiving.
 

Audioguru

Joined Dec 20, 2007
11,249
Too sensitive to stray capacitance?
Simply mount the circuit in a metal box connected to 0V with the pcb spaced away from the box.

The output LC is a high impedance that is loaded down by the low impedance antenna so its tuning is very broad. If you tune it for a peak in the middle (98MHz) then it is still pretty good at 88MHz and at 108MHz. Therefore it is not sensitive to something changing the capacitance of the antenna. It should have an air core.

A coil with a ferrite core needs a very small value capacitor to tune it so it is more sensitive to stray capacitance than a coil with an air core.
 

takao21203

Joined Apr 28, 2012
3,702
Depends. If you insert the core just a bit, the frequency is only adjusted a little. It is more stable than pulling apart or compression air coils. The flux is also more contained inside the coil, since into air the same field does not induce much.

I was wondering if the field over the coil is all the same, or if is different in the area where the core is inserted.
 

Thread Starter

tracecom

Joined Apr 16, 2010
3,944
Hello,

If you are using canned coils, like the one in the link, you will have less effects of nearing the coils:
http://www.qsl.net/yo5ofh/doc/slug_tuned_inductors/slug_tuned_inductor.htm

Bertus
In my case, it's not the coils that are most sensitive; in fact, it's not even the variable capacitor unless I use a metal screwdriver. It's the peripheral wires, especially the ground side of the batteries and the audio in lead from the MP3 player. Of course, the original circuit used an electret mic mounted to the stripboard, and there were no long power leads, so perhaps it's my application that exacerbates the problem.
 
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