Cellphone RF interference

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

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

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.

Shielding, filtering, and distance... the last of the three perhaps being the cheapest.

Cellphones (TDMA-type in particular) are notorious for interfering with electronic devices that aren't well filtered and shielded... monitors, speakers, etc. In most cases, this is considered to be a defect in the affected devices... "undesired operation".

There are a lot of articles pertaining to proper RF bypassing of op-amps (try google). You might try a 100pf chip cap across the differential inputs so that the RF presented is balanced and not amplified.

One thing that is unfortunate about CDMA and TDMA phones is the constant switching of the RF carrier. It may be best to physically isolate the cellphone antenna from your circuit with either space or a mettalic enclosure for your circuit.

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 ... :-)

I doubt you would be happy with a work around like that. Better to learn how to fix RFI and EMI problems at the design. More below:

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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

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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.

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

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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??

an

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.

MPM wrote:

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

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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

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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......