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I've got a tracking device that uses a Telit GE864-GPS combined GSM/GPS module. The GPS antenna is connected to the Telit module via a bias tee circuit, as shown below:

http://upload.wikimedia.org/wikipedia/commons/6/6c/Bias_t.png

The RF only port is connected directly to the antenna input on the Telit module. the RF+DC port is connected to the GPS antenna itself (A Siretta Alpha 7 -> Datasheet: here as you need to login to download it from their site.), and the DC port is connected directly to the board's 5V PSU (a 5V, 3A switching regulator). The capacitor is a 22pF 50V NP0 cap (Farnell) and the inductor is a 47nH, 400mA inductor (Farnell).

I'm running into a problem on several units where the inductors are failing due to overcurrent and burning out. Unfortunately, I'm having trouble determining what the cause of the overcurrent it. The antenna itself only draws 22mA max at 5V according to the datasheet so in theory, the inductor should be more than capable of handling that.

I've had a couple of units running continuously on the bench and out of 9 units, one failed after a week of continuous running. I then transferred that antenna to another working board, which worked fine for a few weeks. Then the system was powered off for a few days, then turned back on and the inductor failed a few minutes after powering on (glowed red for a few seconds then died). I've since transferred that antenna to a third board and the antenna continues to function. The remaining boards have all worked fine during this period.

I'm struggling the figure out what is causing the inductor to fail. It could be the antenna, but it's odd that I get this momentary power surge (which lasts for a few seconds until the inductor burns out) but the antenna then continues to work fine.

I could potentially prevent the problem by sticking a resettable fuse in front of the inductor to prevent any surges from blowing it, but I'd love to find out what the cause of the problem is.

Any thoughts?

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You should always use a current limiter (typically about 50 mA) to feed a bias tee for a GPS antenna. There are just too many ways to create a temporary short when connecting cables, etc.

It sounds like your antenna has a faulty cable or some other internal fault that causes intermittent shorts.

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My guess is that your 5V power supply is poorly designed or possibly has a manufacturing fault such as an open MLCC output capacitor and has a voltage surge or very high ripple voltage that is causing the antenna circuitry to latch up and draw excessive current.

Suggest you look carefully at the quality of the 5V supply.

Edit: Of course it could also be an intermittent short or other fault within the antenna itself (assuming only one antenna does it) or the cable and connections, as Dave Tweed suggests. It's a good idea to put a current limiter (eg. a small LDO) to prevent any accidental short from causing problems, but if you've got a systemic problem you could still experience down time as a result.

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