Which ferrite cores are the best ones to stop GSM buzz in my speakers?
I could be wrong, but I don’t think this approach will work. I think that what a ferrite bead does is to attenuate (“block” to a certain degree) the transmission of signals higher than a given frequency through the wire around which it is wrapped. The frequency at and above which the bead begins to significantly attenuate signals is probably what is meant by its rated frequency. The higher the bead’s impedance, the more it will attenuate the signal. Cell phones in the US operate in frequency bands around 800 (sometimes called 850 for political reasons) and 1900 MHz. These are obviously higher than the frequencies being sent from your sound card to your speakers, which, like the human hearing ability, range from approximately 16 Hz to 20 KHz. This would be all well and good, except for the fact that it is not the 800 or 1900 MHz carrier from the phone that you are hearing in your speakers. You couldn’t hear anything at these frequencies anyway. What you are hearing are pulses caused by
Honestly, twiggy, I don’t know anything about GSM noise specifics. Generally, however, if you are going to solve an electromagnetic interference (EMI) problem, it is awfully helpful to have the noise source characterized with some sort of visualization hardware. In this case, a spectrum analyzer would be best, followed by a really good scope if you can’t get one of those. Either is a helluva lot more costly than a bag full of crap from Digikey. (My spectrum analyzer cost $75,000 new.) The noise that you hear from RF can be an artifact of the actual interference but you can effectively treat it by addressing the RF frequencies or the audible noise frequencies. If you are dealing with this ‘blind’, hacking is as good a method as any. If you can get a spectrum analyzer that will show you the details of the noise you are seeing (either in the RF or the audio range), then you can start being more scientific. Until then, most anything you do will be trial and error. (That’s OK, by the way.)
