# Cell Phones - Technical Question...



## Greg Alsobrook (Apr 2, 2007)

Can anyone explain to me why AT&T (GSM) phones make those weird interference sounds in nearby electronics and Verizon (CDMA) phones don't?


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## Steve Mehs (Mar 21, 2002)

I can't explain it, but it also happens with Nextel (IDEN). It's kind of cool actually, I get the interference in my PC or car speakers before the phone rings, so I know I'm getting a call before I actually do.


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## phat78boy (Sep 12, 2007)

Its all about the frequency being used. It seems speakers react to lower frequencies, like 700 - 800. CDMA, unless a dual band analag and digital, use 1800mhz for normal operation. GSM also uses several frequencies, but for the most part is in the 700 -800 range. I believie AT&T and Nextel are both around 800mhz for normal operating. 

It should be noted that very good speaker systems which are sheilded correctly will not do this. Unfortunately, even reputable speaker makers will sometimes not sheild properly in order to reduce cost. You can even see the interference in some monitors and TV's.


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## jacksonm30354 (Mar 29, 2007)

phat78boy said:


> Its all about the frequency being used. It seems speakers react to lower frequencies, like 700 - 800. CDMA, unless a dual band analag and digital, use 1800mhz for normal operation. GSM also uses several frequencies, but for the most part is in the 700 -800 range. I believie AT&T and Nextel are both around 800mhz for normal operating.
> 
> It should be noted that very good speaker systems which are sheilded correctly will not do this. Unfortunately, even reputable speaker makers will sometimes not sheild properly in order to reduce cost. You can even see the interference in some monitors and TV's.


The above is not quite correct. AT&T(GSM) and Verizon(CDMA) are legacy cellular providors and both utilize the 800-850MHz extensively. They also both have licenses in some areas in the newer "PCS" bands of 1900MHz. (European GSM carriers are 900MHz and 1800MHz).

Sprint and T-Mobile had to start from scratch and are all in the 1900MHz range (unless they acquired a provider that is in the 800MHz range).

Cellular providers in the US do not use the 1800MHz range and I do not believe the 700MHz range has been auctioned off as of yet. I think it's still used for Television ;-).


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## phat78boy (Sep 12, 2007)

jacksonm30354 said:


> The above is not quite correct. AT&T(GSM) and Verizon(CDMA) are legacy cellular providors and both utilize the 800-850MHz extensively. They also both have licenses in some areas in the newer "PCS" bands of 1900MHz. (European GSM carriers are 900MHz and 1800MHz).
> 
> Sprint and T-Mobile had to start from scratch and are all in the 1900MHz range (unless they acquired a provider that is in the 800MHz range).
> 
> Cellular providers in the US do not use the 1800MHz range and I do not believe the 700MHz range has been auctioned off as of yet. I think it's still used for Television ;-).


You are correct with CDMA range, my apologies. I do know that Sprint/Nextel utilitze 900 also. I listed 700-800 as AT&T just purchased 700, althought is not officially used as of yet.


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## Greg Alsobrook (Apr 2, 2007)

jacksonm30354 said:


> AT&T(GSM) and Verizon(CDMA) are legacy cellular providors and both utilize the 800-850MHz extensively. They also both have licenses in some areas in the newer "PCS" bands of 1900MHz.


So if they each use virtually the same frequencies... any idea why one causes interference and the other doesn't??


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## phat78boy (Sep 12, 2007)

AirRocker said:


> So if they each use virtually the same frequencies... any idea why one causes interference and the other doesn't??


The only way a Sprint or Verizon phone would use the lower frequency is if it was in analog roaming. If in that state, it would have the same effect. Normal operating of a Sprint/Verizon phone is 1900mhz, which will not have that effect. Most AT&T phone still operate in the 800 range. There are a few newer models from AT&T that utilize the higher frequency for normal operation, but the majority of their phones are in 800 range.


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## Greg Alsobrook (Apr 2, 2007)

phat78boy said:


> The only way a Sprint or Verizon phone would use the lower frequency is if it was in analog roaming. If in that state, it would have the same effect. Normal operating of a Sprint/Verizon phone is 1900mhz, which will not have that effect. Most AT&T phone still operate in the 800 range. There are a few newer models from AT&T that utilize the higher frequency for normal operation, but the majority of their phones are in 800 range.


ahh.. gotcha..

it's not really that big of a deal... i've just kinda always wondered... i've had verizon mostly.. but just recently got an iphone... and if it's sitting on my desk near my computer speakers... it always messes with them right before a call or text.. i'm guessing the interference is caused by the phone talking back to the tower??


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## phat78boy (Sep 12, 2007)

AirRocker said:


> ahh.. gotcha..
> 
> it's not really that big of a deal... i've just kinda always wondered... i've had verizon mostly.. but just recently got an iphone... and if it's sitting on my desk near my computer speakers... it always messes with them right before a call or text.. i'm guessing the interference is caused by the phone talking back to the tower??


Yup, when the tower initiates a text or phone call with your phone, the phone responds back with increased power to retrieve it. That is when your speakers will react.


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## Steve Mehs (Mar 21, 2002)

Does Nextel still operate at 900Mhz? I thought they had to relocate to a different frequency due to interference with emergency broadcast bands.


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## phat78boy (Sep 12, 2007)

Steve Mehs said:


> Does Nextel still operate at 900Mhz? I thought they had to relocate to a different frequency due to interference with emergency broadcast bands.


Yea, I think the military took it back in the last couple years. In the late 90's Nextel was using it, but I believe you are correct and they have since been told to stop using it.


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## Steve Mehs (Mar 21, 2002)

Was it 800Mhz or 900Mhz? I just checked the hidden status menu on my phones, my personal phone is locked in at 856.16Mhz and my work phone at 864.54Mhz.


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## HIPAR (May 15, 2005)

I'm thinking it has nothing to do with the operating frequency of the phone but it's radiation from the phone digital processors that's causing the interference. Shielding might be more effective in the Verizon phone.

--- CHAS


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## phat78boy (Sep 12, 2007)

Steve Mehs said:


> Was it 800Mhz or 900Mhz? I just checked the hidden status menu on my phones, my personal phone is locked in at 856.16Mhz and my work phone at 864.54Mhz.


They have set ranges which they can operate. Thats why I gave a range. I believe the military has taken everything commercial in the 900 range. You phone are locked onto different channels within the providers range. Thats how they try to keep interference at a minimum with other devices in the same range.


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## phat78boy (Sep 12, 2007)

HIPAR said:


> I'm thinking it has nothing to do with the operating frequency of the phone but it's radiation from the phone digital processors that's causing the interference. Shielding might be more effective in the Verizon phone.
> 
> --- CHAS


It actually is the RF given off by the cell phone which is causing the interference. Commercial frequencies are much stronger then personal frequencies. A personal frequency would be an 800mhz home phone or 2.4ghz wifi. A cell phone on 800mhz will put out much more juice and basically drown a personal device on 800mhz out. Cell phones are not the only things to do this. A big one is a microwave. Try operating wifi or home phone near a microwave. Its not the radiation from the microwave, its the massive RF its giving off.


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## orinth (Aug 5, 2007)

phat78boy said:


> You are correct with CDMA range, my apologies. I do know that Sprint/Nextel utilitze 900 also. I listed 700-800 as AT&T just purchased 700, althought is not officially used as of yet.


the 700mhz auction hasn't taken place yet


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## lostman72 (Nov 28, 2005)

Verizon in most sates uses 800mhz for Voice. Also in some states they use both 1900mhz and 800mhz. In major citys lots of times Verizon will use 800mhz for voice and 1900 for Evdo. Nextel used 800mhz only. Sprint uses 1900mhz. ATT uses 800mhz on there old network (TDMA network) and 1900 on there GSM network. 

When a call comes to your phone from the cell tower or towers it pages your phone and your phone responds at full power (200 to 500 Milliwatts). Then the cell tower will tell your phone to keep that power or lower it depending on the RF power level it sees your mobile. This is part of the battery saving feature. 

Also the 900mhz is used for paging and gov. 929-930 931-932 one way paging & 940-941 2way paging.

Verizon 800mhz/1900mhz CDMA & EVDO 3G (Analog going way in feb of 08) 
ATT 800mhz/1900mhz GSM GPRS UMTS HSDPA TDAM Edge almost 3G
Sprint 1900mhz CDMA & EVDO 3G
Nextel 800mhz Iden
T-Moble 1900mhz GSM GPRS UMTS HSDPA EDGE again almost 3G


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## orinth (Aug 5, 2007)

lostman72 said:


> Verizon in most sates uses 800mhz for Voice. Also in some states they use both 1900mhz and 800mhz. In major citys lots of times Verizon will use 800mhz for voice and 1900 for Evdo. Nextel used 800mhz only. Sprint uses 1900mhz. ATT uses 800mhz on there old network (TDMA network) and 1900 on there GSM network.
> 
> When a call comes to your phone from the cell tower or towers it pages your phone and your phone responds at full power (200 to 500 Milliwatts). Then the cell tower will tell your phone to keep that power or lower it depending on the RF power level it sees your mobile. This is part of the battery saving feature.
> 
> ...


Why is everyone calling it 800, it's usually referred to as 850. And second, ATT uses 850 for most of it's GSM. They don't hold that many 1900 licenses.


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## lostman72 (Nov 28, 2005)

orinth said:


> Why is everyone calling it 800, it's usually referred to as 850. And second, ATT uses 850 for most of it's GSM. They don't hold that many 1900 licenses.


You are right the Base station uses 850 to TX but the mobile uses 824. Were I come from you call 1900 PCS and we call 850 800. You have two main bands in the 800 range. A band and B band. Even the antennas used are 806.000-896.000 Mhz. The filters in the base station are specific for B-band or A-band. Also ATT does now have a bunch of PCS bands they took over Cingular.


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## orinth (Aug 5, 2007)

lostman72 said:


> You are right the Base station uses 850 to TX but the mobile uses 824. Were I come from you call 1900 PCS and we call 850 800. You have two main bands in the 800 range. A band and B band. Even the antennas used are 806.000-896.000 Mhz. The filters in the base station are specific for B-band or A-band. Also ATT does now have a bunch of PCS bands they took over Cingular.


I'm very aware of the paired frequency bands, but I've never heard of it being termed as "800". Even the phone manufacturers say 850. The PCS band which is termed as 1900 starts starts at 1850.


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