This is probably a fairly elementary question but... I need to test an automotive tracking device that uses gprs to send packets to our servers. It's a 3rd party device but I've done some programming using their provided interface internal to the unit. I can trace my code using a serial cable connected to the device. My problem is that I need to simulate a dropped cell signal. I bought a "faraday cage" pouch that works well to kill the signal when the serial cable isn't connected but when it is connected the pouch will only degrade the signal somewhat. I've tried stuffing as much of the cable in the pouch as possible and while it helps a little it doesn't get to the point of dropping the signal (degrades to about -80 dBm). I'm assuming the serial cable is acting as an antenna though I'm a sw guy so that's only a guess. Anyway, I've tried a bunch of alternatives I've seen on youtube (involving mostly a bunch of tin foil) but nothing really has worked. Anyway... anyone have any suggestions or is it just not doable short of taking all my gear to some remote site (the serial cable also requires an external power source)? Thanks for any input.... chris

 

This is probably a fairly elementary question but... I need to test an automotive tracking device that uses gprs to send packets to our servers. It's a 3rd party device but I've done some programming using their provided interface internal to the unit. I can trace my code using a serial cable connected to the device. My problem is that I need to simulate a dropped cell signal. I bought a "faraday cage" pouch that works well to kill the signal when the serial cable isn't connected but when it is connected the pouch will only degrade the signal somewhat. I've tried stuffing as much of the cable in the pouch as possible and while it helps a little it doesn't get to the point of dropping the signal (degrades to about -80 dBm). I'm assuming the serial cable is acting as an antenna though I'm a sw guy so that's only a guess. Anyway, I've tried a bunch of alternatives I've seen on youtube (involving mostly a bunch of tin foil) but nothing really has worked. Anyway... anyone have any suggestions or is it just not doable short of taking all my gear to some remote site (the serial cable also requires an external power source)? Thanks for any input.... chris

Is the serial cable just used for programming? If so, I would remove it as it won't be there during normal operation.

 

For that to be a problem there would have to be a connection or coupling from the serial cable to the RF input of your device. Wouldn't it be easier to find a place where ther is a minimal cellphone signal. I know that with my carrier (SPRINT) those places are all over.

Do you know if your cable is shielded or unshielded? You could try a shieled cable, but I think there is something else going on. Do you have access to a spectrum analyzer?

 

You might try a small ferrite snap-on choke on the serial cable as it exits the bag but any small gap in the pouch will limit the effectiveness of the 'Faraday cage' when the frequency range is high.

 

Hi, thanks for the replies! Gerty, unfortunately I need the cable attached to see what my code is doing (no trace logging in the device unfortunately) so no way to run w/o it attached to get any good info. The code I'm trying to test is specifically the signal watchdog/recovery code so need to have it connected and simulate a dropped signal.

Papabravo, unfortunately again the serial cable requires external power so need to be near a plug. Since the device uses at&t 3g they actually have pretty good coverage where I am (s. California). Looking at their coverage map the closest weak 2g spot is a fairly good haul away. As for the coupling I'm assuming you're saying that the cable has to have been purposely used (and physically connected) as an antenna by the manufacturer? That would make sense as they definitely would want to maximize reception as the device is designed to operate in some remote and fairly harsh conditions. Not sure though how I could disconnect it though. Looking at where the wiring connects to the boards I don't see anything obvious. Anyway, thanks again for the replies....

 

nsaspook, ooops, you replied while I was typing I guess. I just had a look and can go grab one from the local radio shack here. Is the idea that I would put it on right where the cable comes out of the pouch? I did just try being sure that the opening was as tight as possible around where the cable exited and it seemed to make it a little better but just a few dBm....

 

I'm sure they did not intentionally connect any of the wires is the serial cable to the input stage of the RF deck. What I am saying is that apparently there is either electric field (capacitor) coupling between the serial cable and the RF deck or magnetic field (inductor) coupling between the serial cable and the RF deck. A spectrum analyzer can help you establish the coupling mechanism.

People are often amazed at the cost of RF instrumentation and human expertise, but in most cases there is no substitute for spending money to solve such problems.

How about a schematic of the receiver input stage and the transmitter output stage? The geometry of the antenna would also be helpful. If the GSM antenna is vertically polarized and the serial cable is horizontal there will be a substantial reduction in coupling. Even though you don't need the serial cable to be long, if you make it longer to reduce it's resonant frequency that might also help. RS-232 is supposed to be good for 25 feet.

 

nsaspook, ooops, you replied while I was typing I guess. I just had a look and can go grab one from the local radio shack here. Is the idea that I would put it on right where the cable comes out of the pouch? I did just try being sure that the opening was as tight as possible around where the cable exited and it seemed to make it a little better but just a few dBm....

There is a old trick to make a somewhat effective Faraday cage using a metal trash can (~40-80dBm depending on how well it's sealed and frequency range, up to 80dBm at 500mhz well sealed). If you can fit your entire device/controller/analyzer with a portable power source inside the can with the everything insulated from the bottom and sides by old newspaper or something non-conductive and then tape the lid lip seal with aluminum duct sealing tape it might give you sufficient isolation with the pouch for testing. The smallest gap in a seam will create a discontinuity in the field distribution for a Faraday cage causing inside and outside currents to flow at the aperture and couple energy at high frequency.
http://www.learnemc.com/tutorials/Shielding02/Practical_Shielding.html