Pretty much everything by Gert looks incorrect or possibly a some typos.
More
turns on the core gives you less output over most of the frequency range
but
more output at the very low end of the frequency.
Zt = M/L * Zo in the flat region of the frequency response where Zo is the
50
Ohm load on the probe, M the mutual inductance between the probe and
measured
circuit an L is the inductance of the probe coil. M goes up as the number
of
turns but L goes up as the square of the turns. This is the result of the
low
pass filter formed by the probe coil and 50 Ohm load.
Doug Smith Sent from my iPhone IPhone: 408-858-4528 [tel:408-858-4528]
Office: 702-570-6108 [tel:702-570-6108] Email: d...@dsmith.org Website:
http://dsmith.org
On Sun, Jan 31, 2016 at 23:05, ce-test, qualified testing bv - Gert Gremmen
<g.grem...@cetest.nl> wrote:
As a rule of thumb the transfer impedance (Zt) is the load impedance of
your SA
( = 50 Ohm) multiplied
by the square of the applied windings on your probes core. You should load
the
probe with
a low impedance (attenuation) network (1 Ohm ->50 Ohm) to reduce Zt and
limit
the number of windings.
The latter gives you better control on frequency response, while more
windings
give more
output signal and extends the lowest usable frequency . So it's a trade off
between sensitivity, and frequency response.
It's not so different from classical signal transformer analysis, you
should be
familiar with.
A Zt of less than 10 Ohm will give good accuracy for common CM currents.
If you try to shield your home-made probe, make sure the shield is
discontinuous
with low capacitance, otherwise
it will short the magnetic field in the core.
As usual with rule of thumbs, this is not an exhaustive analysis of Zt, but
very
usable in terms of
learning what's going on.
Regards,
Ing. Gert Gremmen
Approvals manager
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-----Original Message-----
From: Ken Javor [mailto:ken.ja...@emccompliance.com]
Sent: Monday 1 February 2016 03:41
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] SV: [PSES] Fwd: [PSES] Current probe for CM currents
Doug's point is well taken. If you know the probe's transfer impedance, and
you
know it works properly (is well-shielded) then instead of just "getting a
number" and trying to lower it you can work against a computed current
limit
that radiates at or below the RE limit which is the actual goal.
BTW, SAE ARP 6236 shows how you can measure your probe's transfer
impedance.
Ken Javor
Phone: (256) 650-5261
> From: Doug Smith <d...@emcesd.com>
> Reply-To: <d...@emcesd.com>
> Date: Sun, 31 Jan 2016 16:21:19 -0800
> To: <EMC-PSTC@LISTSERV.IEEE.ORG>
> Subject: Re: [PSES] SV: [PSES] Fwd: [PSES] Current probe for CM
> currents
>
> That is why it is very difficult to make current probes well shielded
> enough and certainly with a flat transfer impedance that covers a few
> decades of frequency. The cost of a good current probe, for instance
> an
> F-33-1 or F-61 is less than what it has already cost your company in
> time lost.
>
> Doug
>
> Douglas C. Smith
> University of Oxford Course Tutor
> D C Smith Consultants
> PO Box 60941
> Boulder City, NV 89006
> Email: d...@dsmith.org
> Web: http://www.dsmith.org
> Web: http://emcesd.com
> Tel: 702-570-6108
> Mobile: 408-858-4528
>
> On Sun, 31 Jan 2016 17:49:08 -0600, Ken Javor
> <ken.ja...@emccompliance.com> wrote:
> You needed a metal shield to prevent capacitive coupling.
>>
>> Ken Javor
>> Phone: (256) 650-5261
>>
>>
>>> From: Amund Westin <am...@westin-emission.no>
>>> Reply-To: Amund Westin <am...@westin-emission.no>
>>> Date: Sun, 31 Jan 2016 20:21:48 +0100
>>> To: <EMC-PSTC@LISTSERV.IEEE.ORG>
>>> Subject: [PSES] SV: [PSES] Fwd: [PSES] Current probe for CM currents
>>>> A clamp-on ferrite with a few turns of wire and connected to a
>>>> spectrum
>>> analyzer, worked out to be a good tool for measuring CM currents on
>>> single cables. It gave me some measured numbers [dBuV], and then I
>>> worked
>> on trying
>>> to get the numbers down :)
>>>> But when I placed the home-made probe on wires / cables inside a
>> noisy rack,
>>> problems started. The probe picked up almost all kinds of
>>> frequencies, even when no cables or wires where going through the
probe
(ferrite).
>>>>>> This lesson told me that a simple home-made current probe
>> works good on a
>>> stand-alone cable, but it does not work that good then measurements
>>> are carried out closed to other noisy sources. Then you might need a
>>> more professional current clamp.
>>>>>> #Amund
>>>>>>>>>>>> Fra: Ken Javor [mailto:ken.ja...@emccompliance.com]
>>> Sendt: 15. januar 2016 17:53
>>> Til: EMC-PSTC@LISTSERV.IEEE.ORG
>>> Emne: 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
>>> <mailto:000000f5a03f18eb-dmarc-requ...@ieee.org> >
>>> Reply-To: Bill Owsley <wdows...@yahoo.com
>>> <mailto:wdows...@yahoo.com> >
>>> Date: Fri, 15 Jan 2016 07:26:08 +0000
>>> To: <EMC-PSTC@LISTSERV.IEEE.ORG <mailto: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 <mailto:k...@emc-seminars.com> >
>>>> -
>>> ----------------------------------------------------------------
>>> This message is from the IEEE Product Safety Engineering Society
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