Annoying Speaker sound when a GSM phone is placed near

[alimani]

Hello y'all!
I am currently doing some research regarding the EMC issue of a speaker making that annoying sound "dih dih dih dih dih dih dah dih dih daaahhh" when a GSM phone is placed near it. Now, I know it does that due to the fast switching nature of TDMA close to around 217 Hz. But I wanted to know more on how and where RF energy gets coupled into the speakers? I mean GSM phones transmit at 800, 900, 1800 or 1900 MHz, but the switching frequency is just 217Hz. How does that work out?
Looking forward to hearing from you guys!

Kind Regards

[w2aew]

The cell phone sends bursts of modulated RF signals.  The speaker voice coil, speaker wiring, etc. are acting as an antenna.  The output stage isn't designed to deal with this signal, and it will basically, crudely, rectify the signal, resulting in a nasty, bursty, noisy signal that somewhat follows the RF envelope of the signal and that's what you hear.  I really poor AM detection...

[EEVblog]

[alimani]

Thankyou both of you...

One more question, Can you provide me a equivalent circuit model for this interference? I have checked all over the internet but I couldn't understand anything.. I am not that good with all these Wireless mathematics..

[Stonent]

iDEN, IS-136 and GSM based systems all do this. iDEN is particularly bad around CRT causing bad screen shaking.  More efficient systems based on CDMA which was primarily developed by Qualcomm and used mostly in the Americas and the Asia/Pacific region do not have regularly repeating cycles and do not interfere with audio. I think EDGE may have been the last phone standard based on Time Division Multiple Access. 3G technologies are I think exclusively CDMA based. (UMTS, HSDPA etc)

One of the reasons for the switch is CDMA offers more efficient use of the spectrum because there's "always room for one more" rather than the capacity being limited to a fixed time cycle.

I think in general the reason why originally CDMA did not make much progress in Europe was that it was tied to the phone ESN rather a removable card. It was fine for the US since phones were tied to contracts and there wasn't much incentive to switch phones.

[alimani]

What I mean is, how to mathematically calculate the amount of radiation induced voltage in the speaker cables, for example, that is a function of the distance, power of the GSM phone and may be orientation of the waves hitting the speaker wires.

[Stonent]

What I mean is, how to mathematically calculate the amount of radiation induced voltage in the speaker cables, for example, that is a function of the distance, power of the GSM phone and may be orientation of the waves hitting the speaker wires.

The phone also only puts out as much wattage as required to maintain connectivity with the cell tower. It won't be a constant.

[jahonen]

What I mean is, how to mathematically calculate the amount of radiation induced voltage in the speaker cables, for example, that is a function of the distance, power of the GSM phone and may be orientation of the waves hitting the speaker wires.

If you could reliably do such a thing, you would be a millionaire. Think about all those saved EMC tests and board revisions.

Short answer is that there are far too many variables involved to do it in any reasonably accuracy. Yes you can do calculation of very simplified model but that's about it. It might be realistic or usually not. Near field of an antenna is very complex thing to model accurately. Simple antenna calculations usually assume far-field conditions since far-field is much more simple.

Regards,
Janne

[BravoV]

What I mean is, how to mathematically calculate the amount of radiation induced voltage in the speaker cables, for example, that is a function of the distance, power of the GSM phone and may be orientation of the waves hitting the speaker wires.

Would you please tell a bit about your self ? I mean just a little bit about your background and experience electronics world ? RF background  and etc ?

Hopefully this will help people to help you back by finding the "right" level in explaining thing. 

[alimani]

I am in my 2nd year doing undergrad.. have studied about electronics and RF but just the basic stuff... I have this coursework due on EMC issues between GSM phones and Speakers. I have done everything except the modelling bit.. I am not that good in antennas. We have to make assumption to make the calculations of the EMI as simple as possible. and I am having a hard time figuring out how to do that.

[KJDS]

I am in my 2nd year doing undergrad.. have studied about electronics and RF but just the basic stuff... I have this coursework due on EMC issues between GSM phones and Speakers. I have done everything except the modelling bit.. I am not that good in antennas. We have to make assumption to make the calculations of the EMI as simple as possible. and I am having a hard time figuring out how to do that.

All you need are Maxwell's equations and a little time to run the math.