EMC filtering for real situations?....not the usual ideal setup 1

[waveboy]

Hi,
We have got to pass radiated EMC (30MHz to 1GHz), but in a “real” situation where we cannot afford “proper” RF EMC techniques…and in truth…”real” EMC techniques just wouldn’t be mandated, or realistic.
Our setup is as attached.
The following doc calls for a “RF Reference plane” for use with EMC filtering and shielding….

https://interferencetechnology.com/wp-content/uploads/2016/10/2016_IT_EMC_Filters_Guide.pdf

…That sinks us from the start. We have two panels, whose metal enclosure is earthed, but that earth comes from the earth wire in the long 3-core power cord…..so that connection is so high Z to RF that its NOT a connection at all. So we have no RF reference plane!!!…and our metal chassis is basically floating at RF !!!
So straight away no official EMC literature is of any help to us!!!

Does anyone have an “EMC guide for real-life, cost constrained situations” document, instead of these “perfect EMC” guides which tell how to do EMC for super-expensive medical devices or military devices with RF gasketted connectors etc? I’ve read tons of them and they are no use.

I mean, Feedthrough capacitors, etc etc aren’t much use without an RF reference plane.
Take the blue cable between the two enclosures in the attached.
What are the options?

(a)…Two core cable unscreened?
(b)…Three core cable with third connector connected to chassis at each end?
(c)…Two core cable with screen…screen left floating…it may as well float…both our chassis’s are already floating at RF?
(d)…Two core cable with screen connected to chassis at “A”?
(e)…Two core cable with screen connected to chassis at “B”?

…and what if we just don’t bother connecting our chassis’s (A and B) to earth at all?....after all, the power cord earth connection is NOT a connection at RF…its too high Z. And then what should we connect our Y capacitors to?....there is no Earth. How are the common mode chokes working at all?
No EMC literature discusses these real options. Do you know the real answers?

 

[Comcokid]

The proper lay to address EMC begins as you create the schematic and as you layout the PCB. I's the sixth-sense of knowing where your high DV/DT edges are, your fast processor clocks, your power devices in heatsinks that will create common mode noise, etc. Trace length and lead length on capacitors is inductance and can nullify the presence of a bypass capacitor. Sometimes less capacitance is better.

All of those filter kits, clamp-on ferrites, etc are really only useful as a troubleshooting aid once you've failed at the lab and you begin to try things to get an idea of where the emissions are coming from or how they escape from the case.

You've really got to know your design. You need your knowledgeable hardware design engineer at the lab when the product is tested. I've solved EMC problems on-site by changing the value of a resistor, or capacitor. I recently had a situation with a failing UUT wrapped in foil and numerous ferrites added on cables with no help. But once the case was opened and several 100pf and 1000pf m capacitors added at the right spots all the issues went away.

 

[FacEngrPE]

Some times a choke works of you need to block RF. At RF frequencies an excited ground plane can become a radiator rather than an RF trap.

Start with the wiring diagram. Then dig deeper.

This will sound strange - if the noise is in the shield of the connecting cable, try winding the cable around a wooden doughnut (torridal choke) I did this once with a welding work lead to solve a noisy welding machine that was interfering with a radio.

Something like this

 

[zappedagain]

Good questions. Again -

https://www.emiguru.com/books/edn-designers-guide-paypal/

(I am not affiliated with emiguru.com, they just taught me a lot of valuable knowledge that I've used for my entire career)

Z