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Author Topic: PC speaker buzz from Cingular GSM phones  (Read 1866 times)
W2DUG
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Posts: 3




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« on: April 24, 2007, 06:05:22 PM »

In my office we often experience instances of a "galloping" sort of buzz periodically coming from the PC speakers near co-workers who have Cingular GSM cell phones.  A preliminary Web search turns up many instances of this problem.  My understanding is that these phones periodically communicate with the network, and during these brief bursts the carrier is modulated at around 200 Hz resulting is a clear, repeatable buzz pattern.  How is this buzz getting into the PC speaker amplifier, and, more importantly, how can it be prevented?

Thanks,
Doug, W2DUG
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AA4PB
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Posts: 14382




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« Reply #1 on: April 25, 2007, 05:21:20 AM »

The phones transmit a digital signal that is very rich in harmonics close by. The RF couples into the speaker wires and is rectified by the semi-conductor devices in the speaker amplifier.

The phones register with the tower periodically any time they are turned on. They also handshake with the tower upon receiving an incoming call.

I can tell when I'm going to receive a phone call because the buzz in the speakers appears about a second before the ring tone on the phone.

You might be able to eliminate or reduce the problem by adding ferrite beads around the speaker and power supply wires close to where they enter the speaker.
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Bob  AA4PB
Garrisonville, VA
KF7CG
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Posts: 1199




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« Reply #2 on: April 25, 2007, 05:48:19 AM »

Good analysis! The signal doesn't have to be full of harmonics though to cause this problem, they just add to the annoying nature of the resulting noise.

This interference is treated in the same manner as any other case of "fundamental overload" interference. Any noise reduction technique that will remove cell-phone frequencies from the speaker loops will be helpful.

By the way, the GSM cell-phones can interfere with ordinary landline phones, CD players, and all manner of consumer electronics. This is just like the interference caused by Ham stations to consumer electronics.

Use the methods you would use to keep your Ham signals out of the family music center and you will do well.

KF7CG
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LA4RT
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Posts: 35




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« Reply #3 on: April 26, 2007, 11:52:34 AM »

GSM uses time division multiplexing, which means that when transmitting, the transmitter is turned on for bursts of app. 5 ms every 40 ms. This rapid keying probably contributes to the RFI, rather like key clicks.
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KB1THH
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Posts: 46




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« Reply #4 on: June 02, 2007, 11:00:44 AM »

Hi Doug,
  To add to the other replies here..I'm lucky that my Cingular phone doesn't effect my PC speakers but does so with a few sensitive scanners I have if the phone is within a foot or so of them.
  I did notice that my phone time, synced with the network hourly and I disabled that feature. That cut down on the noise. Now I only hear it when I have an incomming call. ( I can tolerate that and is a nice 'heads up' )
  For s#its and giggles, leave your cell phone next to a field strength meter sometime and watch the needle.
Compare that to a trx on a 70cm HT on low power.

73  Joe
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