Sensor that penetrates a shield - Page 1
Forum section: electronics / rf-microwave

author: josechow
author_rank: Contributor
timestamp: September 26, 2017, 11:15:32 pm
content: So I am wondering what should be done in this instance:I have a shielded system, which, due to switching noise I don't want to propagate out and cause a failure in EMC. I have an external sensor on the other side of the system, like that depicted below, that penetrates the shield. Where the sensor is located it wouldn't be very easy to stick a shield around it, can I stick a pi filter or a ferrite bead and a couple of wraps to prevent EMI escape through the conductors connecting to the sensor? The penetration to the shield is relatively small.Any help is appreciated.

author: Wirehead
author_rank: Regular Contributor
timestamp: September 28, 2017, 05:53:11 am
content: seeing you use an AC source - can the sensor be capacitively coupled? Passthrough capacitors might help

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 28, 2017, 12:53:50 pm
content: If you need more bandwidth on the sensor than the lowest significant frequency of the offending noise source, then use a common mode filter. Worst case, use a shielded cable, so that the CMRR can be quite high. Else, a CM filter is still acceptable, but you can save even more by using dumb resistors and capacitors on each line. In either case, the filter should be grounded to shield, not blue, and it should have an impedance (R and/or L) facing inwards. (Impedance on the outside is okay too: it'll tend to dampen resonance on the sensor cable, improving immunity. Usually only one FB is needed in that case.)Any guess as to the induced ground loop voltage? That is, how much voltage drop along the blue wire, between where it was grounded and where it leaves the shield to connect to the sensor.Tim

author: josechow
author_rank: Contributor
timestamp: September 28, 2017, 10:03:48 pm
content: Quote from: T3sl4co1l on September 28, 2017, 12:53:50 pmThanks for the reply Tim. As far as the ground loop voltage I do not know or wouldn't know how to guess at it, as I am still learning EMC.I know the diagram I gave looked very simplistic; but between the two "R" 's I used to indicate loads, are actually microcontrollers. The predominate offending frequency I am trying to get rid of is from an RS-485 line switching at 1.4MHz, and some LEDS's being switched at 600khz.Quote from: T3sl4co1l on September 28, 2017, 12:53:50 pmIf I understand you correctly, a shielded cable leading up to the sensor will still help me even though the sensor itself isn't covered in shielding tape? And in addition, I can use a CM filter on the leads leaving the enclosure to further reduce any potential problems? EDIT: added diagram of my thoughts to see if I understand you correctly. I want to use RC filter so I can change the values and adjust as needed.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 28, 2017, 10:43:48 pm
content: If you don't know the inside ground loop voltage, then better to nix the inside C-to-GND. Since, that puts ground loop voltage directly on the yellow line. Or, the C-to-GND shorts out the ground loop voltage, giving you ground loop current instead, and now your current paths matter (more to analyze).A little bit of sensor peeking out from a shielded cable, where the sensor wires are already bypassed to the shield anyway, won't be a problem.Tim