Odd one. Roadside cabinet for a stormwater pumping station: a small controller reads a 4-20 mA ultrasonic level transmitter through a 250 ohm shunt into a 0-5 V ADC input, and a cellular router in the same cabinet sends readings to Hermetry. The level chart has spikes, +15 to +40 cm, in bursts of a few seconds. They line up with the router’s traffic LED. When our technician stands next to the cabinet on a call, it gets worse. The router antenna is the stub one screwed straight onto the router, inside the steel cabinet. Wiring is ordinary unshielded two-core, about 1 m from the transmitter gland to the controller.
We have something similar with a speaker in our hotel lobby, it buzzes when someone’s phone rings near it. Same thing?
Same physics as David’s speaker, yes. The RF burst gets rectified by the input stage of the ADC (or any semiconductor junction on the line), and that shows up as a DC offset for the length of the burst. 2G-style bursts at 217 Hz are the worst, but LTE uplink bursts do it too. A cabinet I look after at work had exactly your picture, spikes during uploads only.
What fixed ours, cheapest first:
- Get the antenna out of the cabinet. A short-cable antenna on the cabinet roof. Inside a steel box the router is shouting at your wiring from 30 cm.
- Shielded twisted pair for the 4-20 mA run, shield grounded at the controller end only.
- An RC filter right at the ADC pin, 1 k and 100 nF, plus a 100 pF cap close to the pin for the RF itself. Your level doesn’t change in milliseconds, you can afford it.
- A clip-on ferrite on the signal pair where it enters the controller.
- If the router has a setting to lock it to LTE and not fall back to 2G, use it.
After the first two the spikes were gone for us. Old spikes stay in your history, so if anyone set an alarm on that level, check whether they were tripping it. A readings-in-a-row debounce of 2 or 3 on the high-level rule hides any single leftover.
Antenna on the roof with a 1 m cable and the RC filter on the input. Two days, not one spike, even with the tech calling from right beside it. The high-level alarm had fired 11 times last month on these, which nobody had linked to the router.
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