Mobile & Telephone ??

[ahmed osama]

Dear all

why when there is a mobile near an ordinary telephone we hear some noise in it (while a call is coming to the mobile) although the mobile works in >800Mhz and telephone <<20khz !!!

thanks alll

bye

 

[cohe30ve]

becuse the phone play the freq that make with data in other hand you cant hear 800mhz with long 01010101010101 but yo can hear 0000000111111____111111000000________1111000000000

 

[Z-match]

i think MP not play frequency, but it use QPSK. not FM modulation. the interference are produced by IF (intermediate Frequency) such as 38MHZ osccilator, and or IO frequency of communication between IC's.

B.R.

 

[moustafa.abdelhady]

What you hear is the rate of the bursts of data.
The burst time is 8 time slots.
The time slot is 0.557 ms
so the burst time is 4.456 ms, so the rate of bursts is in the audio range of frequencies, so we can hear it. Also the signaling of the call setup is on the same range of audio frequencies.

 

[ahmed osama]

What you hear is the rate of the bursts of data.
The burst time is 8 time slots.
The time slot is 0.557 ms
so the burst time is 4.456 ms, so the rate of bursts is in the audio range of frequencies, so we can hear it. Also the signaling of the call setup is on the same range of audio frequencies.

this is so nice but all that is on carrier 800-900 Mhz !!! not in the base band

 

[Rossano]

yep, but one non-linearity (and PA are non linear) spreads harmonics all over the spectrum, thus in base-band too.

 

[pravardhan]

I think the problem is with the Audio Amplifier used in the Telephone.

I have made an Power Audio Amplifier using National Semiconductor's IC, it makes a hell lot of noise during an SMS or Call, that too with a minimum of 3 feet distance from the amplifier.

But, i still don't understand how can these Audio Amplifiers get (receive) those signals Wirelessly from the Mobile Phones....? :?

 

[Rossano]

Hi

yes they can, from a dirty ground or from radiated field inside the mobile phone (each element acts as an antenna) are typical examples. This is the audio buzz !

 

[DBurg]

I guess it is because of the mutual coupling between the inductors in the cellphone and the fixed phone. where the current in an inductor can generate flux that intern produce a current in a mutual inductors in the fixed phone.
for more details about this see Electromagnetic text book.
-DBurg

 

[aravindbprasad]

I think its the interference between the line frequency and the mobile frequency.....

 

[betwixt]

Everyone is wrong so far....

It is nothing more than the bursts of RF from the phone. The RF is being picked up by the wiring and rectified by non-linearities in the telephone semiconductor devices. It isn't the phone DATA causing the interference, it's the absence or presence of the carrier. GSM and similar use sequential bursts from different phones on one frequency so each has an on and off time slot. As the carrier goes on and off, the rectification causes small but rapid offsets in the amplifier biasing which you hear as the buzz,buzz,buzz,buzz, buzzzzzzz noise.

Brian.

 

[fouwad]

well as u sed RF i being picked up by wiring, i have a cordless landline phone at my home n it still sounds tht buz buz buz wen a sms or call comes on my cell phone?
thr must b some other explanation