Why is There No More Interference When on a Cell Phone Call?

[whozum]

This is a 'why' question:

When my phone (GSM, Tmobile in the US) is sitting next to my monitor, or by my radio in my room and is about to receive a call/msg, and while sending a call/msg I get a lot of interference on my radio and monitor. But as soon as the call is in progress, the interference goes away. Now the message is obvious, since there's no more signal being transfered, but with a call, I'm still holding a signal to the tower, why is there no more interference?

The radio station was at 101.5Mhz, and from a GSM info site:

"GSM 900 uses the circa 900Mhz band
The frequency band used is 890-915MHz (mobile transmit) and 935-960MHz (base transmit). "

 

[Antiphon]

The interference to your radio is probably from a very low frequency signal
(a few hundred KHz to few hundred MHz) coming from the activation of the computer in the phone
when the call comes in. When the phone is operating at 900MHz on the
RF link, the frequency
is too high to cause interference to your particular radio.

 

[waht]

re

looks like your house isn't properly grounded. The interference is trasmited through the power cord of both your radio and monitor. Basically alll the residual energy has no place to go.

 

[chroot]

waht,

I don't think you're correct at all; it has nothing to do with proper grounding.

Antiphon is correct; it's due partly to a particular pulsing sequence used when negotiating with the tower, which includes frequencies to which your monitor and speakers are sensitive.

Another factor is that your phone uses its maximum transmission power at first to make sure the tower hears it; it then negotiates with the tower to lower its transmission power to maximize battery life. If you travel a lot and pay close attention, you'll notice that your phone dies a lot faster (even if it's not being used) when it has only a single bar of signal strength versus a strong signal.

- Warren

 

[waht]

re

I agree, during start up the cpu eminates low frequency pulses whose harmonics can be picked up in the 100 MHz band. That's probably due to bad shielding. I take the grounding issue back, but I have seen interference being trasmitted through power cords. Maybe this is not the case. The FCC makes pretty tight regulation ragarding interference, so it could be a bad phone.

Art

 

[chingkui]

I have some different experience when I use my phone in my car, no matter whether it is during call setup or call is in progress, my car radio is strongly interfere by the signal (doesn't matter if I am listening to CD or radio in any channel). During call setup, the interference is stronger, but it is still very annoying after call setup. Just wonder if anything can be done. (Right now, I solve the problem with the simplest solution: turn off the radio, is there anything smarter than that?)

 

[antiver]

I experience this type of interference on a daily basis.

In addition to hearing interference on speakers (etc) immediately before a call, I also hear interference when my phone checks my email - via GPRS. So, I'm fairly confident that you shouldn't be looking at teh GSM spectrum, you should be paying attention to the GPRS spectrum. I wouldn't doubt that the phone uses GPRS, when possible, to (at least) help negotiate a call (and find the phone, etc) before the call is connected.

On top of that, the interference sounds indubitably digital and repetitive in nature (a recognizable string of sharp clicks and pops.. not new data everytime a call comes in). In fact, I've learned to reach for my phone up to about 2 seconds before it actually rings or shows a call on the screen based on the interference I hear from nearby speakers.

 

[whozum]

Would a GSM phone call at 900Mhz have any interaction with a 900Mhz CPU, or are those different things, one a wave and one is a frequency of repitition?

 

[Francis M]

I've never had this through my car radio, or radios at my house, but I've heard it come through the speakers on my computer and the speakers of a couple of PA systems. It's consistent for me though not just at the start of a call. It's a rapid popping static sound. It's the cell phone sending out it's digital pulse signals. Could the speaker wires be acting as antenna and picking up the signal to drive the speakers?