My first EMC webinar hosted by Rohde & Schwarz went off pretty well last Thursday, with 276 attendees from around the world checking in. Thanks to all for listening to some of the most common EMC issues I deal with during my consulting.
There were a number of questions asked, that I wasn’t able to get to due to lack of time, however, Rohde & Schwarz will be providing a list of the unanswered questions early next week and I intend to group like ones together and answer these periodically via this blog. Some of the more interesting questions will be answered on The EMC Blog, hosted by Test & Measurement World.
Thanks again for all who attended. I suspect we’ll do this again some time.
design-4-emc.com 
Concerning the Webinar on June 20th, were you able to “capture” the questions asked, that your addressed during the Webinar? I was the one that asked about a shielded cable between 2 PCBs & which end of the shield to GND – both? – or if only 1 end which end? I would like to see your response to this & other questions that were asked. Also, I was told not to use the drain wire as the GND connection between 2 PCBs (there is only 1 GND pin on the connector) & I disagree. This is the return path for all signals in the cable.
Hi William, I’ve not yet received the list of questions for R&S. I’ll check with them today.
William…to answer your question on cable shield termination, you should ground the shield at both ends for proper magnetic field shielding. This is the typical case for most (99%) of shielded cables. If all you’re concerned with is shielding from electric fields, then terminating the shield to ground at one end is sufficient.
Thanks Ken for your previous answer. But the 2nd part of the question, “Also, I was told not to use the drain wire as the GND connection between 2 PCBs (there is only 1 GND pin on the connector) & I disagree. This is the return path for all signals in the cable.”, is now a concern. Unfortunately, the 2 connectors contain only 1 GND pin. So it is either using the shield drain wire to connect the GND pins, or use 1 wire of a twisted pair to connect GNDs & leaving the shield & drain wire totally unconnected at either end!
Hi William,
There’s a number of issues at play here when using cables with drain wires and (I assume) a foil shield. I’m also presuming from your hint, that all the internal wires are twisted pairs, which is good.
Therefore, the signal returns for each wire should be part of the twisted pair. This will define a path for the signal return currents.
The outer foil shield is really just extra protection against outside fields and should not be utilized for signal or power return currents. The drain wire will have a certain inductance (about 20 nH per inch) that, at high frequencies, will compromise the shielding effectiveness of the foil shield, if that drain wire is the only connection to chassis ground.
Ideally, the foil and drain wire should be clamped to the metal body of the connector plug, which then connects tightly to chassis ground. If the connector you’re using does not have a clamping feature, then the shielding effectiveness of the foil shield is compromised. Not all connectors – even MIL connectors – have this clamping feature.
Hi All,
I just posted the first set of questions and answers regarding cables and cable terminations on The EMC Blog hosted by Test & Measurement World. I’ll be posting additional question grouped by topic through July. Stay tuned. Here’s the link: http://www.tmworld.com/electronics-blogs/the-emc-blog/4417778/Questions-on-cables-for-EMC-mitigation