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 <[email protected]>
To: [email protected]
Sent: Thursday, January 14, 2016 8:24 PM
Subject: [PSES] Fwd: [PSES] Current probe for CM currents
Ken is correct. Near field probes are good for finding sources, but current
probes are better for characterizing the emissions from cables. However, if the
EUT has a leaky enclosure, or other radiating structures besides cables, you'll
really need to monitor the emissions from a distance while you're
troubleshooting. I've found a distance of 1m works well. For small EUTs, I
place them at one end of a bench and set up a small monitoring antenna and
spectrum analyzer at the other. I've written many articles on this technique.
Take a look at The EMC Blog in EDN.com. I also wrote an article on how to make
and use current probes for Interference Technology.
Kenneth WyattWyatt Technical Services
Woodland Park, [email protected] from my iPhone.
Begin forwarded message:
From: Ken Javor <[email protected]>
Date: January 14, 2016 at 5:14:36 PM MST
To: [email protected]
Subject: Re: [PSES] Current probe for CM currents
Reply-To: Ken Javor <[email protected]>
Re: [PSES] Current probe for CM currentsIMO, a near field probe gets you back
to where Bill Owlsley was at: you are X dB over the limit on the test site, so
you wave a probe around and look for X dB reduction in signal from whatever
change was made, hoping the probe picks up something proportional to the RE
field at three or ten meters.
Whereas with a current probe, even without any site testing, a common mode
conducted emission (CMCE) limit can be determined based on the three or ten
meter RE limit, and then the design work is done on the cable before any
testing at any site.
Ken Javor
Phone: (256) 650-5261
From: "McDiarmid, Ralph" <[email protected]>
Reply-To: "[email protected]"
<[email protected]>
Date: Thu, 14 Jan 2016 14:20:33 -0800
To: <[email protected]>
Subject: Re: [PSES] Current probe for CM currents
Would a Near Field probe be a better choice?
_______________________________________________________________________________
Ralph McDiarmid | Schneider Electric | Solar Business | CANADA |
Regulatory Compliance Engineering
From: Ken Javor <[email protected]>
To: [email protected],
Date: 01/13/2016 01:22 PM
Subject: Re: [PSES] Current probe for CM currents
A current probe measures the net current on the conductor within its opening.
The impedance of the circuits attached to that conductor may affect how much
current flows through the conductor, but not the measurement thereof.
Ken Javor
Phone: (256) 650-5261
From: Amund Westin <[email protected]>
Reply-To: Amund Westin <[email protected]>
Date: Wed, 13 Jan 2016 18:48:23 +0100
To: <[email protected]>
Subject: [PSES] Current probe for CM currents
Planning to do some EMI troubleshooting with a «homemade» current probe.
Probing a lot of cables inside a rack and try to find the source.
Will make a current probe by a ferrite core (two halves, a few turns wire and
coax plug) as many EMI experts have posted on the web and on this forum.
Then find the Zt[dBohm], so make a measurement on a cable and verify the
readings on the spectrum.
But when clamping on the current probe, which is a ferrite, does that change
the cable impedance and therefore actually changes the real current flow, so my
reading will not be true? ... or am I missing some fundamental theory here? ...
#Amund
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