Why do cell phones cause noise on my office speaker phone?

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This phones is branded technology it the aslo deziner set and also has grated funtions are available.

I think Bill is closest to the correct answer. Mobile phones emit a fraction of a Watt of RF power, and any semiconductor device with a P/N junction close to it (one-two meters apart) will behave as a RF detector.
If you have an audio amplifier close to your mobile phone, its transistors or ICs will happily detect the RF pulses from your phone, some even can amplify your voice and cause an audio feedback.
A transistor radio can also detect it, as well as a leakage from a microwave oven at 2.45 GHz.

I would be interested in reading another blog entry that elaborates on the differences between time, code, and (web?) code division multiple access, the frequencies utilized by major carriers, a global geographic overlay of the said carriers' areas of operation, how the frequencies interact with each other in areas of congestion, etc.
@bill - thanks for your comments on filtering low level amp circuits and mics in hearing aids. I am annoyed enough by this phenomenon that I may try to attempt to cushion the circuits of some of my speakers.

My 3G AT&T phone does it whether in GSM or HSDPA (3G) mode- it's a HTC Kaiser also sold as the AT&T tilt. THe difference is that the WCDMA modulation used in 3G mode has a more noiselike quality, has a higher pitch where a tonality can be discerned, and is less bursty apparently. The 217 Hz frame repetition rate of a typically bursty GSM transmission is extremely audible with a raspy tone owing to the harmonic rich pulsed waveform.

My Nokia 3120b Cingular/ATT phone would alert me to incoming calls on the Cisco VOIP phone on my desk within a few feet, but at other times as well. I assume when it didn't predict a call, there was just periodic handshaking going on with the host tower.

I think your explanation is a bit off the mark. The pulses emitted by the cell phones are not likely to directly couple into the speakers of the confrence phone but rather into the amplifier input lines. They are then ampliified and fed to the speaker. They are essentially bursts of high frequency energy at a low frequency repetition rate. We in the hearing aid industry have been plagued with cell phones since they went digital. They are detected by the low level amplifier circuits and amplified. The key to eliminating the interference is careful filtering of the low level circuits, espicially the microphones, and proper shielding of the circuitry. Undoubtly the confrence phones could be improved by similar techniques.

@Bergeron: GSM phones (and CDMA phones too) vary their transmit level based on the strength of the base-station signal they are currently receiving. If you merely rotate the phone a few degrees (blocking the antenna with the phone body or your own), the power level can change significantly. This wide variation in transmit power makes it impossible to specify an exact distance to separate cellphone from speakerphone.

I've noticed Nextel phones are by far the worst offenders. I can tell it's going to ring by the pulsing noises it makes seconds before it actually rings.

Bergeron - I've found it varies a lot by phone. My iPhone needs to sit right on top of the phone to have an impact while my former phone, an AT&T Tilt (made by HTC) would cause problems from 4 feet away.

Interesting, I always wondered what specifically caused that. Is there a specific proximity to speakers that should be avoided with GSM phones?

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