Forums

EMC - RF IMMUNITY

Started by Unknown November 3, 2017
Hello everyone.

1.) I have an electronics board that has 10 wires coming out that gets connected to an end users board with an Ethernet type cable.   During our Electromagnetic Compatibility (EMC) Testing for the RF Immunity Testing there is a disturbance of our system around 100-120 MHz and 1 GHz –2.7 GHz.  (Some of the values returned from the Analog to Digital converter of our microcontroller show a noise disturbance.)

2.) When a ferrite core clamp is connected over the entire cable the problem goes away.

3.) When the grounds and shield wires of the cable are pulled out of the cable and the ferrite core clamp is only applied to these wire (just 4 wires out of the 10) the problem also goes away.

4.) There seems to be something going on when we apply a common-mode chock to these ground / return paths.

5.) What is the best solution for something like this if we want on on-board solution on the PCB?

Thanks.

Mike
Sounds like you have a resonance at those 2 frequencies 

For the lower one and possibly also the higher one, add ferrite beads to each line:

http://katalog.we-online.de/pbs/datasheet/74279266.pdf

Cheers

Klaus 
On 2017-11-03 13:50, eeglory1234@gmail.com wrote:
> Hello everyone. > > 1.) I have an electronics board that has 10 wires coming out that > gets connected to an end users board with an Ethernet type cable. > During our Electromagnetic Compatibility (EMC) Testing for the RF > Immunity Testing there is a disturbance of our system around 100-120 > MHz and 1 GHz –2.7 GHz. (Some of the values returned from the Analog > to Digital converter of our microcontroller show a noise > disturbance.) > > 2.) When a ferrite core clamp is connected over the entire cable the > problem goes away. > > 3.) When the grounds and shield wires of the cable are pulled out of > the cable and the ferrite core clamp is only applied to these wire > (just 4 wires out of the 10) the problem also goes away. > > 4.) There seems to be something going on when we apply a common-mode > chock to these ground / return paths. > > 5.) What is the best solution for something like this if we want on > on-board solution on the PCB? >
If the BOM cost is very critical you have to take the time and find the sources, the root causes, and fix those. If cost is not so critical but time is pressing there are common mode choke arrays you can place on the board such as this: http://www.onsemi.com/pub/Collateral/EMI4184-D.PDF It depends on your signals and how much you were over the EMC limits for each peak. Got to make sure you aren't coupling lines that aren't supposed to and also not dampen diff-mode too much if you have more than two lines for a signal connection. If unsure you could post a schematic. Last but not least lots of those problems happen when a board does not have a proper and complete gound plane or when bypassing isn't done shortest path. I have had cases where introducing such a complete plane almost wiped out all EMC problems at a client. Mostly the radiated stuff. -- Regards, Joerg http://www.analogconsultants.com/
PCB mounted CM Choke on the lines at the point where they come onto the PCB.  Wide clearance area on the EMI dirty side of the CM choke to any other traces.  Keep things symmetrical as far as trace/space & lengths on the EMI dirty side, and as short as possibly to the wires or connectors involved.  Avoid pathways for the RF to jump around the CM choke because other components or wires are nearby.  Cable shields go directly to metal chassis ground at the point of entry into the box.  Never bundle EMI dirty wires with other wires, until AFTER they have passed through or EMI filter.  If the lines involved are DC or low frequency, you can add 100nF bypass capacitors.  If lines involved are high speed signal the values used for bypassing will have to remain very small.  Differential signals can get their own two-line CH choke in the 50-500uH range.  DC lines can get CM chokes with a higher inductance, although testing in your case did not reveal and issues below 100Mhz.  For extreme EMI robustness, DC signals can be given two stages of CM choke.  The first stage being low inductance and knocks out VHF/UHF and above, while the second stage being high inductance attenuates HF and lower.  

Many times I see products being EMI tested, where the set-up is ill conceived.  You can't ignore the need to apply extensive EMI filtering in situ, to any/all cables that are not normally part of the product, but which you are using to monitor or power the DUT (device under test). Simple things often trip up most engineers. A battery source is a far better choice than a lab power supply for example.      
OP
sounds like you have, what audio engineers call  the "Pin 1 problem'

Google that.

m


On Wednesday, November 8, 2017 at 9:17:16 AM UTC-8, mako...@yahoo.com wrote:
> OP > sounds like you have, what audio engineers call the "Pin 1 problem' > > Google that. > > m
Thanks for all the replies guys.