Cellphone RF interference

[Arnold]

Hello All,

I have an environmental project using a cellphone to send data from a remote
location.
I'm using a LT1495 op-amp in a non-inverting mode with gain at 100 on part
of the circuits.
Unfortunately everytime the cell transmits it puts a spike on the input and
100 times that on the output!
Can anyone recommend a simple fix or is it a matter of heavy shielding etc?
Thanks in advance.
Arn

 

[Arnold]

Forgot to add that the amp is in a DC arrangement and I probably only need
BW of about 200Hz.

 

[Joerg]

Hello All,

I have an environmental project using a cellphone to send data from a remote
location.
I'm using a LT1495 op-amp in a non-inverting mode with gain at 100 on part
of the circuits.
Unfortunately everytime the cell transmits it puts a spike on the input and
100 times that on the output!
Can anyone recommend a simple fix or is it a matter of heavy shielding etc?
Thanks in advance.
Arn

Been there myself, or rather, clients of mine have. No fun. Take a look
at figure 3 in the datasheet:

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C1099,P1566,D2696

The B-E junctions of Q1 and Q2 will rectify any ever so slight RF they
find. Now if the unwanted antennas (traces) connected to IN+ and IN- are
ever so slightly different, and they always are, you'll get a huge
demodulated diff mode signal. It might be worst with GSM cell phone
where the pulses are smack dab in your 200Hz range. But CDMA phones
aren't much better either.

Suggestions: Try a CMOS opamp if you can. No diodes in there unless you
hit the supply rails. Make sure both of your inputs are away from either
rail. If you must have a bipolar opamp use strong RF filters with good
performance up to >2GHz, very close to the opamp pins. Murata and others
make then. Ferrite beads (they come in SMT) might also work but not
quite as well.

Shield the whole chebang as well. And make sure the circuit board has
one full ground plane, without it all bets are off.

If you can't do a shield at least try some metal or metallized stuff
between this board and the cell phone antenna. But this part must be
connected to the ground plane very well, no wires, pigtails and stuff.
Remember, at cell phone frequencies anything is an inductor.

 

[Arnold]

Thanks to all for the replies...I had another thought...looking at the
problem from a different direction.
If I could detect the RF using a single spare op-amp then maybe I could
pause the monitoring until the RF had gone.
So given an ordinary bipolar op-amp what would be the best way to provide an
'RF present' indication?
Thanks again
Arn

 

[Joerg]

Thanks to all for the replies...I had another thought...looking at the
problem from a different direction.
If I could detect the RF using a single spare op-amp then maybe I could
pause the monitoring until the RF had gone.
So given an ordinary bipolar op-amp what would be the best way to provide an
'RF present' indication?
Thanks again
Arn

Probably a diode detector would suffice, followed by a comparator (or
opamp).

But that sounds like putting oil on squealing brakes ...

 

[mpm]

Hello All,

I have an environmental project using a cellphone to send data from a remote
location.
I'm using a LT1495 op-amp in a non-inverting mode with gain at 100 on part
of the circuits.
Unfortunately everytime the cell transmits it puts a spike on the input and
100 times that on the output!
Can anyone recommend a simple fix or is it a matter of heavy shielding etc?
Thanks in advance.
Arn

Is this real time data, or can you store & forward?
You can probably also just detect power supply current on the cell
phone.
It will spike when the phone is transmitting.
No need for esoteric RF detectors.

If the data is clearly out of range (but still within your ability to
measure) you could just discard the readings on the receiving end and
not even worry about it??

Or course, I have no real idea what you're trying to do.....
Good luck.

-mpm

 

[[email protected]]

Hello All,

I have an environmental project using a cellphone to send data from a remote
location.
I'm using a LT1495 op-amp in a non-inverting mode with gain at 100 on part
of the circuits.
Unfortunately everytime the cell transmits it puts a spike on the input and
100 times that on the output!
Can anyone recommend a simple fix or is it a matter of heavy shielding etc?
Thanks in advance.
Arn

Is the amplifer (resistor input) connected to a sensor? if so, perhaps
that is where the offending signal is entering the system. You could
disconnect the sensor and ground it, then see if the noise goes away.

 

[Frank Raffaeli]

Thanks to all for the replies...I had another thought...looking at the
problem from a different direction.
If I could detect theRFusing a single spare op-amp then maybe I could
pause the monitoring until theRFhad gone.
So given an ordinary bipolar op-amp what would be the best way to provide an
'RFpresent' indication?
Thanks again
Arn

No one mentioned reducing the power of the source.
Consider using a different transmitter with less power. Why are you
using a cellphone? Depending upon the location, and application, you
may be able to use a lower frequency, like 912 MHz, narrow-band FM.
This will go to the horizon with enough antenna height / gain.

If you must use a cellphone, you may be able to get a reliable
cellphone connection with a lot less power. Try putting it in a metal
box with 1/4 wave antenna inside (coupler), attached to a (SMA)
connector that goes through the box. Then you can add an attenuator
and external antenna. Or move the antenna / cellphone away from the
opamp circuit.

Frank

 

[mpm]

No one mentioned reducing the power of the source.
Consider using a different transmitter with less power. Why are you
using a cellphone? Depending upon the location, and application, you
may be able to use a lower frequency, like 912 MHz, narrow-band FM.
This will go to the horizon with enough antenna height / gain.

If you must use a cellphone, you may be able to get a reliable
cellphone connection with a lot less power. Try putting it in a metal
box with 1/4 wave antenna inside (coupler), attached to a (SMA)
connector that goes through the box. Then you can add an attenuator
and external antenna. Or move the antenna / cellphone away from the
opamp circuit.

Frank

If in the US, hacking an antenna onto the cell phone will violate its
FCC Equipment Certification.
Plus, it probably won't work very well. (Matching, etc...) For most
modern cell phone networks, you cannot control the cell phone power
output because this function is handled by the network (not the
phone). If you do something to attenuate the phone, the network is
just going to tell it to power up, and this should make the problem
worse. (sort of like a complex AGC).

Inherent in your question is something that puzzles me.
If you have the ability to "wait" until the RF is gone, then you must
be storing the readings.(?)
If that's the case, what's the problem? Store the readings first,
then fire up the transmitter (cell phone). And,of course, don't take
measurements while the phone is powered.

Or, you could try to make friends with your local cell site engineer,
who can almost certainly program their network so your phone stays
locked to a particular cell (though that approach seems ripe for long
term reliability issues...)

I guess I just don't fully understand the problem.
What are you measuring? How real-time does it have to be?
Is the cellphone continuously powered up? What happens to the
measurements taken when the cell phone is not transmitting??

 

[Frank Raffaeli]

If in the US, hacking an antenna onto the cell phone will violate its
FCC Equipment Certification.
Plus, it probably won't work very well. (Matching, etc...) For most
modern cell phone networks, you cannot control the cell phone power
output because this function is handled by the network (not the
phone). If you do something to attenuate the phone, the network is
just going to tell it to power up, and this should make the problem
worse. (sort of like a complex AGC).

Inherent in your question is something that puzzles me.
If you have the ability to "wait" until the RF is gone, then you must
be storing the readings.(?)
If that's the case, what's the problem? Store the readings first,
then fire up the transmitter (cell phone). And,of course, don't take
measurements while the phone is powered.

Or, you could try to make friends with your local cell site engineer,
who can almost certainly program their network so your phone stays
locked to a particular cell (though that approach seems ripe for long
term reliability issues...)

I guess I just don't fully understand the problem.
What are you measuring? How real-time does it have to be?
Is the cellphone continuously powered up? What happens to the
measurements taken when the cell phone is not transmitting??

The cell phone is in the box with another antenna - coupling to the
outside of the box. It doesn't break any laws .... passive re-
radiation is allowed.

 

[Arnold]

Forgot to add that the amp is in a DC arrangement and I probably only need
BW of about 200Hz.

Thanks for replying.

Yes I'm storing all sorts of data like wind speed, rainfall, pressure,
radiation etc but the phone needs to stay live to accept remote commands
which can come at anytime.

As the phone needs to be live all the time I have no way of knowing when the
provider 'checking in' signals are being sent or received.

It is these provider initiated signals as well as my remote commands which
are affecting the readings.

It just seemed to me to be easier to detect the RF, discard the readings
during RF.

I have a spare op-amp to detect RF but have not found a reliable way of
arranging it.

Thanks

Arn

 

[mpm]

Thanks for replying.

Yes I'm storing all sorts of data like wind speed, rainfall, pressure,
radiation etc but the phone needs to stay live to accept remote commands
which can come at anytime.

As the phone needs to be live all the time I have no way of knowing when the
provider 'checking in' signals are being sent or received.

It is these provider initiated signals as well as my remote commands which
are affecting the readings.

It just seemed to me to be easier to detect the RF, discard the readings
during RF.

I have a spare op-amp to detect RF but have not found a reliable way of
arranging it.

Thanks

Arn- Hide quoted text -

- Show quoted text -

If the cellphone is live all the time, why do you need an RF detector?
The RF will always be there. I'm still confused.

IF -- the phone is live all the time, maybe what you claim to be RF
interference is actually the cell phone hopping to another cell site -
i.e., possible power adjustment. (This occurs even in stationary,
fixed, applications). Or, path losses or other factors are causing
the cell site to intemittently adjust the cellphone output power, and
this uptick in power is what's causing your problem? Total guess, by
the way, but seems reasonable...

If the latter, one thing I would definitely check is the coupling
efficiency of your passive antenna setup. It could be a lot worse
than you think (and this could be causing additional cell hopping -
again, assuming that is the problem).

I agree a passive antenna system would not violate the equipment
cert. I could not tell from your earlier post if you were trying to
hack into the cell phone antenna jack/PCB, etc...

Either way, I guess you're still going to need your circuit, because
even if you get near-perfect radiation off your antenna systems,
there's still no guarantee the cell phone power output won't fluctuate
over time (and I'm assuming that's what your problem is as you state
previously the cellphone is powered up all the time..)

An op-amp RF detector shouldn't be too difficult to construct, but it
sounds to me that you're going to need one with an adjustable
threshold (as RF is always present). Good luck.

BTW, you must have a really expensive phone bill??

-mpm

 

[mpm]

Thanks for replying.

Yes I'm storing all sorts of data like wind speed, rainfall, pressure,
radiation etc but the phone needs to stay live to accept remote commands
which can come at anytime.

As the phone needs to be live all the time I have no way of knowing when the
provider 'checking in' signals are being sent or received.

It is these provider initiated signals as well as my remote commands which
are affecting the readings.

It just seemed to me to be easier to detect the RF, discard the readings
during RF.

I have a spare op-amp to detect RF but have not found a reliable way of
arranging it.

Thanks

Arn- Hide quoted text -

- Show quoted text -

I just had another thought....

I wonder if you can SMS message the phone with the commands instead of
keeping it up all the time.?? I wonder if some cell phones provide
SMS text out the multi-function / charging connections provided on so
many phones...??

That would be pretty cool. Just text the phone, receive the command,
wait a couple seconds for the phone to power-down, then process the
command.

I also just figured out what you meant by cellphone powered all the
time. You mean "idle", (on but not necessarily transmitting). When I
first read that I was thinking you had an active call going on
continuously. Not that it really changes anything I said......