Cell Phone Interference

[mpm]

Thanks for your cellphone behaviour analysis.

Interesting.
Yes. It is possible the TV power supply is being interferred with.
However, if this is caused by the cell phone in close proximity, I
would certainly expect the behavior to exhibit while the cell phone is
ringing.

It is unlikely that there is a significant change in cell phone RF
output power, regardless of signalling vs. voice channel. It may
change some, but not enough to matter. (i.e., the difference in
antenna response vs. frequency, etc.. Really, not enough to be
concerned with.)

Perhaps the TV is at fault and it has nothing to do with the cell
phone?
Could be a problem in the TV's power supply, or the power quality of
the branch circuit it is using has a problem. (such as: abnormal
line harmonics, bad neutral, bad grounding or bonding, etc..)

You "might" be able to deduce this by trying to replicate the TV
problem by selectivly turning ON and OFF certain high-drain appliances
in the home, especially those with motors. Air Conditioner, Clothes
Dryer (on high heat), Refrigerator, Hot Water Heater, and/or Hot Tubs,
Pool Pumps, well water pumps, etc... if you happen to have those.

Based on your last reply, I am beginning to think the problem is not
with the cell phone proximity. It just sounds like a coincidence.
(??)

To answer one other question, cell phones do not normally "collect and
periodically retransmit" data back to the cell site. Some do, but
they are very specialized devices. Not your run-of-the-mill consumer
handsets.

-mpm

 

[GregS]

Typical cell phones are in communication with the base station even
when you are not using the phone for conversation. This is how the
cell phone company "knows" which cell you're on in case you are
moving. (They do not ring every cell tower in the world. That would
be a waste of spectrum.)

There are "channels" for signalling, and "channels" for voice
communications.
If nothing changes, your cell phone might stay on a single signalling
"channel" for weeks.
But...

If your phone experiences a fade, or loss of signal, the reacquisition
may occur on another signalling "channel". (This can, and does, often
happen many times a day.) It can also happen if the cell site
transmitter drops the channel, or if they dynamically reallocate
spectrum to deal with cell site congestion issues, etc...

The point is, when your cell phone (lying peacefully on the TV table)
needs to get updated, it will broadcast a short burst of data back to
the serving cell site. This is probably what you are experiencing (TV
Interference) when the phone is not in Communication.

To extend this reasoning, when you are talking on the phone, First,
you are using a different "channel". One other than the Signalling
"channel", but very likely in the same general band. But many things
have changed. The orientation to the TV for one thing. (Presumably
you are further away unless you put your ear on the TV table?) Also,
the cell site may have instructed the cell phone to reduce output
power on the voice channel, etc...

Unintentional receivers in close proxmity to cell phones and the like
are usually bombarded with fairly high RF signal strengths, which
their front ends were not designed to deal with. While it is
certainly possible to make receivers that would be immune to this
proximity effect, it is not often economical, and may impose physical
size restrictions (for the front end filtering) which consumers could
find objectionable. In short, you just have to live with it.

BTW, I offset the word "channels" in quotes because in the strictest
sense, not all cellular systems (air interface standards) use discrete
channalized bandwidth, but the intent should be clear from the above.

I had a flashing back on one of my older phones. It sensed any RF and flashed,
thus i knew when it was transmitting..

My current motor Razr seems to run the battery down real fast at times
when I enter the place where i am currently at, where there is no reception.
It seems to get in some mode where it must be doing a lot of transmitting
where there is no reception. Sounds dumb.

Of course teh cell phone will use full power when its in a poor location, and
can't back off power like in a good location.

greg

 

[Barry Lennox]

Scam? I doubt it. I can pick up GSM cell phones signaling over FM radios
from several yards distance.

And I can easily hear GSM phones signalling if I'm on a normal phone.
Or on the car radio as the GSM phone switches between cell-sites

There's an interesting collection of real-world EMC problems at
http://www.compliance-club.com/banana_skins.asp

Barry

 

[Didi]

However, if this is caused by the cell phone in close proximity, I

would certainly expect the behavior to exhibit while the cell phone is
ringing.

It does, actually. Caught it yesterday, was checking if the caller
ID worked and went to the TV to check that as well, it was
messing up the TVs SMPS allright.

Perhaps the TV is at fault and it has nothing to do with the cell
phone?

Hey, I would know that.

Would you believe I have twice opened and fixed this (cheap) TV,
once it had a dried electrolythic cap which was mounted too
close to a heating 7805 regulator and once I don't remember
what, after a lightning which had killed the Ethernet controller
(and not the coax PHY buffer...) of my Nukeman, the TV and
the cable modem, got the latter replaced and the other two fixed.

But the case for the cellphone is proved now, anyway. It messes
up the power supply (IIRC the one which makes the 5V stays on
all the time, not sure about the others, I think the sweeps supplies
were off but I am no more than 60% sure of that, lightning event
was a year ago). About 30 cm (1ft) from the board is close enough,
I am not sure the missus will allow me to try any closer (she keeps
the phone in the other corner of the room since it got under
suspicion .....

Dimiter

 

[mpm]

My current motor Razr seems to run the battery down real fast at times
when I enter the place where i am currently at, where there is no reception....

Sounds right.
When far from the tower, the tower will instruct the cell phone to
increase it's output power to improve the signal. (And lower when
you're in close to avoid overloading the cell site's receiver
multicouplers.) So, if you're in an area of poor coverage, you have
at least 2 things eating away at battery life:

Temporary signal fades, which drive the need for more frequent
updates, and,
High RF power ouput instructions from the cell site.

-mpm

 

[Paul Hovnanian P.E.]

Then it is, by definition, a poorly designed radio receiver. Being
disrupted by relatively moderate emissions at 8 or 19 times the normal
receiver operating frequency is clearly an inferior design.

What do you mean "it"? I've observed the effect on numerous receivers,
some quite expensive (which isn't a guarantee of quality) and several
quite sensitive.

As someone suggested, its probably overloading the receivers' front end
or IF sections. But that qualifies as interference.

 

[Paul Hovnanian P.E.]

Now, picture this kind of interference mesing up a pacemaker or other
medical electronics.

 

[Michael A. Terrell]

What do you mean "it"? I've observed the effect on numerous receivers,
some quite expensive (which isn't a guarantee of quality) and several
quite sensitive.

As someone suggested, its probably overloading the receivers' front end
or IF sections. But that qualifies as interference.

It could also be caused by an external device overloading and
generating spurious signals.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida