×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

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

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

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

(OP)
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/uplo...

…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?

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

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.

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

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now
The Great Project Profitability Debate
A/E firms have a great opportunity to lead the world into the future, but the industry’s greatest asset—real-time data—is sitting wasted in clunky, archaic ERP platforms. Learn how real-time, fully interactive dashboards in a modern ERP allow you to unlock data that will shape the future of the world. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close