Hi Everone,
Here is an interesting case where a ferrite core can actually perform
an impedance matching function and increase emissions on a cable:
http://emcesd.com/tt120199.htm
Doug
On Fri, 15 Jan 2016 09:40:22 -0800, Ed Price <edpr...@cox.net> wrote:
I would add a caution to Ken's comment about common mode cable currents
creating RE. Yes, the CM currents certainly do create RE, but you need to
probe the cables at several intervals to understand those current paths. It
is not immediately obvious that all CM current flowing on a cable at one end
of a cable does not necessarily flow at the other end of that cable.
Especially where cables are bundled, or where they pass closely along a
chassis or structural member, there are possibilities for that current to
couple off of the cable. The current flow will follow the impedances, both
at the ends and at other fortuitous nodes. This is one of the reasons that a
ferrite absorber might work much better at one position along a cable than
at another position and also why RE might be dependent on something as
obscure as cable bundle tightness.
Ed Price
WB6WSN
Chula Vista, CA USA
From: Ken Javor [mailto:ken.ja...@emccompliance.com] Sent: Friday,
January 15, 2016 8:53 AM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] Fwd: [PSES] Current probe for CM currents
I think it is important to not lose sight of the original query that started
this thread. The query was about whether placing a current probe around a
cable perturbed the current to be measured. There is no doubt that
radiated emissions can originate within an equipment
enclosure separately from driving common mode currents on a cable, but that
wasn't the query. In fact, the poster was probing cables within a large rack
(enclosure) looking for a source within an enclosure.
Ken Javor
Phone: (256) 650-5261
_____ From: Bill Owsley <000000f5a03f18eb-dmarc-requ...@ieee.org>
Reply-To: Bill Owsley <wdows...@yahoo.com>
Date: Fri, 15 Jan 2016 07:26:08 +0000
To: <EMC-PSTC@LISTSERV.IEEE.ORG>
Subject: Re: [PSES] Fwd: [PSES] Current probe for CM currents
If you can measure common mode noise on a cable, you have a problem from the
port !!
Note the world famous Ott's math on this effect in his 1st edition. Might
be in his 2nd too.
I have used both e-field and h-field (current clamp) at the same time.
We are engineers so figure out how I did that!
And since some of the work is below 30 MHz, I have also added a loop antenna
for a 3rd measurement.
My approach is if I find any emission, locally, near field, bench stuff,
that varies by position over the area of the product, then I have a problem.
E-field scan,using a o'scope probe. H-field scan usually using a personally
built small loop, and any other sort of scan, conducted or radiated, that I
can make up at the moment.
I work for a homogeneous field in the scans over the area of the product.
My assumption is that if I find a homogeneous field, then there are no or
low emission gradients which can equate to a field at a distance. So get
creative, and redundant, by different methods for measuring the emissions.
Ironic, I am good at mashing all emissions, and then they hand me an
intentional radiator and ask that I don't kill the fundamental. What ? You
mean I have to pick what to mash, and what not to mash? Ok, so I
caught on quick enough to keep the job. ps. I suffer from not being
able to use a leaky enclosure. I don't get any
shielding for the products.
Cable shielding that is bogus terminated, but at the low frequencies of
interests, it works. Then I have to deal with the higher frequencies,
the harmonics !!!
Plastic covers and pcb and cables up to 15 KW or more of digital BS to make
an analog signal.
And then 'normal' digital signals for the ADC circuits all in the middle of
this.
Management is like, we have done it this way for over 25 years and so we are
not changing it now. It works (I have to make it work) so don't
change anything. Sucks to be me
- but I do like a challenge.
_____ From: Ken Wyatt <k...@emc-seminars.com>
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