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I am having trouble with interference from a GSM modem and my switch mode PSU.

I do not know much about interference and how the radiation patterns works for GSM. Below is an image of the circuit which we build for a client. The unit worked perfectly when they previously had an external antenna installed, however it was getting vandalized, so we then installed this anti-vandalism antenna. Since then, when we close the box, the switch mode PSU dies. I assume it goes into a protection mode. And you have to cycle the power in order to get it to recover.

Installation

PSU: Meanwell MDR-10-12

Now the antenna and PSU are fairly far from one another. I can simulate the PSU behavior on another unit of mine, but putting the antenna right up against the PSU. (however it doesn't immediately die, it seems random).

Questions that come to mind:

  • Where is the interference coming from?
    • the antenna,
    • the cable,
    • the modem?
  • The PSU is connected to earth, would this not help ground the EMI?
  • I could swap the blue device & the modem around so that the PSU & Modem are not so close to one another, however my gut feel is that it is the antenna causing the issues.
  • What measures can be taken to prevent the antennas EMI from turning the PSU off?
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  • \$\begingroup\$ You wrote "when we close the box, the switch mode PSU dies" - do you mean that when you put the plastic lid on the box, the PSU dies? If so, does flexing the box kill the PSU? If it does, that suggests a loose connection somewhere. \$\endgroup\$
    – Andrew Morton
    Aug 30, 2018 at 13:10
  • \$\begingroup\$ @AndrewMorton I am referring to when we close the outer metal enclosure. This doesn't happen like clockwork. However I haven't had the PSU die when the lid was opening and we where debugging, But when ever we lockup and leave it tends to die. \$\endgroup\$
    – Zapnologica
    Aug 30, 2018 at 15:44
  • \$\begingroup\$ Is that bundled-up black wire for the antenna? If so, can you move the bundle well away from the other items, or get a shorter lead? \$\endgroup\$
    – Andrew Morton
    Sep 1, 2018 at 17:26

1 Answer 1

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You need to first measure if the PSU is indeed the problem. I would suggest looking at the spectrum on the output to see what frequency it is switching at, and to verify that this could be an issue. Even then, it is hard to really 'prove' as you don't know what IF frequencies are used in your modem.

If it is indeed the cause, through what means is it doing so: Radiated? Through the input power supply? On the output? Only then can you start working on a remedy - worst case, get a different model of PSU that has different switching frequencies and/or better EMI performance.

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