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Mobile phone rings, Speakers buzz, Why?

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Is there possibly any danger in placing one of these phones near a home PC? Mine causes wierd patterns on the CRT monitor, that I'm sure aren't stopping it from aging.
 
No but its realy anoying (especialy wen the speakers pick it up)

Im conatantly charging my phone over its USB data cable (used to transfer files betwean the phone an the PC)ans its only 10cm from the PC. (nice thing is that the USB sitll has power wen the PC if off so it can charge the baterys)
 
The noise is called "GSM Breakthrough", it is very annoying and you cannot really do much about it.

GSM transievers are rated at 1-2W for 2.5G and 2W for 2G. By law thay are not allowed to be any more powerful than that. However, the GSM works using time slots, and this coding system means that the data is transmitted in is bursts. There are 217 timeslots in any one second for any one device on a particular channel, each burst is approx 1 milli second long. There are up to 4 devices on each channel. During a burst at full power, GSM modules can draw anywhere upto 8 amps peak. The peak power is usually well over 10W, and it is this peak power output that causes such a big problem.

At idle the phone sends bursts at 2Hz, with a series of 8 bursts at 217Hz after every 8 bursts at 2Hz. When a call is established the power is defaulted to full, this is why you get very loud breakthrough at the beginning of calls (making that classic dud dud dud, dud dud dud noise if you can imagine that!!! :D ), and the power is then adjusted back down after about one to three seconds. When a call is active and a constant streame to data is being sent all available time slots are used, that's 217 bursts a second, this is why you get a burring noise during a call.

The noise is caused in speakers because the amplifier circuit amplifies the noise that is injected onto the signal line, which typically will carry a signal much lower than line level. From experience, a GSM device can inject upto about 200mV common mode noise when reasonably close. That is typically a fifth of the full line level, this is why it is so loud.

Another reason it is so bloody loud is that 217Hz is smack in the middle of the frequency range that humans use to to power speach. The power in someones voice when they are talking is about 150Hz to 300Hz, the intellegability is formed using frequencies from about 700Hz to about 3kHz (this is the lower level noises that define the word). Human ears are very sensitive to thes frequencies.

There are ways to combat breakthrough, screening analogue circuitry is good, as we are dealing with rf. Using differential driving techniques for low level signals is probably the best way, as you can achieve near 100% common mode noise rejection. Speaker lines are ok and don't need any special attention, it's the low level signal lines you need to worry about. The best way of all is to keep mobiles away from analogue equipment.
 
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