A great presentation on field probes can be found on the web. The author is Zhong Chen, an engineer with one of the probe manufacturers. I was lucky enough last year to be in the audience for a live presentation of this at our local EMC Chapter.
I found it by searching: "ets lindgren e field probe theory" It states, among other things, that frequency response correction is applied during end use! Unfortunately, it doesn't state the amount of correction needed, nor the frequency resolution needed (doh!). But, it does answer questions on whether modern probes are frequency dependent devices. It is an interesting presentation, and I highly recommend it for anyone that wants to learn more about their test equipment. On Mon, Mar 5, 2018 at 1:08 PM, Ken Javor <ken.ja...@emccompliance.com> wrote: > In turn: > > Everyone does not know that MIL-STD-461 requires RS103 pre-test > verification. I don’t. Please explain further. > > RTCA/DO-160 requires pre-calibration of the field in a manner similar to > that in the exercise that started this thread. Note at one point only, so > that the point about field variations down the length of the test set-up as > for RS103 also applies here. > > My use of the 61000-4-3 example was a pedagogical tool to illustrate that > the OP test results looked good relative to a very stringent industry > standard. > > When you ask whether some tolerance is insignificant, that begs the > question as to what is the required tolerance. In MIL-STD-461, the effect > of the ground plane and immediate vicinity of the test sample and the lack > of good quality absorption below around 80 MHz are way bigger factors than > non-idealities in probe calibration. MIL-STD-461 requires only one probe > (more are allowed) so that even if the field intensity is within the > correct tolerance at that point, there is no control over what the fields > are down the length and breadth of the test set-up. And MIL-STD-461 allows > for 3 dB variation in instrumentation tolerances, so there you go on your > field probes. > > And I don’t know what all sorts of control software is envisioned out > there, but an off-the-shelf field probe and control-display unit at least > at the time when RS103 was written (1989-1993) was not a frequency > sensitive device. The unit reported a field intensity only. The frequency > information came from the transmit side. Leveling was performed on the > field intensity reported by the probe electronics unit. There was no > correction envisioned for probe factors at each tuned frequency. Not > saying that couldn’t be done in a controller external to the probe > electronics given detailed probe calibration info, just that wasn’t part of > the plan back in the day. I would be interested to hear from readers as to > whether that is now common. > > As Mr. Woodgate notes and I affirm, a 20% variation in probe calibration > is way down in the noise even in a 61000-4-3 UFA calibration, not to > mention the OP set-up with probes 10 cm over a ground plane, which to my > knowledge is never the requirement. > > Which all leads up to an answer to this question: > > “Would you agree, as an engineer, its always better to have the data, then > to operate in the blind?” > > The answer is, “Not only no, but hell no, if I know beforehand that the > numbers don’t matter.” > > > Ken Javor > Phone: (256) 650-5261 > > > ------------------------------ > *From: *Patrick <conwa...@gmail.com> > *Date: *Mon, 5 Mar 2018 11:10:52 -0700 > *To: *Ken Javor <ken.ja...@emccompliance.com> > *Cc: *<EMC-PSTC@listserv.ieee.org> > *Subject: *Re: [PSES] Field probe calibration > > Well, some of us have to consider other requirements, not just EN's. > > What if the test is -461, or DO-160 ? > And what if the requirement is 200 V/m ? > > As everyone knows, those require a pre-test verification, not a > requirement for "uniform field"? > > So a 20% error gives anywhere from 160 V/m to 240 V/m. > A 600 V/m target is anywhere from 480 V/m to 720 V/m. > > Is that insignificant? > Maybe, maybe not. > Depends on you and your customer. > > I recommend to always check a test labs calibration factors. > If the factors don't show the resonances, then there are built in errors. > An informed decision can be made whether to accept, or move to another lab. > > Would you agree, as an engineer, its always better to have the data, then > to operate in the blind? > > > On Mon, Mar 5, 2018 at 10:21 AM, Ken Javor <ken.ja...@emccompliance.com> > wrote: > > I am totally with John Woodgate on this – 20% deltas are insignificant in > the larger picture. I only used 61000-4-3 to demonstrate the fallacy of > worrying about such small deviations when the overall requirement in a > really well-designed and expensive chamber is +6, - 0 dB. Consider the type > of absorber used, the limited frequency range (80 – 1000 MHz), the total > lack of conducting material, and the fact that 25% of the UFA may be > excluded from the 6 dB variation. > > Your variations are well within that tolerance, and your room and set-up > nowhere near that good. If you want better than what you have, you will > need a chamber and set-up much better than that for 61000-4-3 – > impractical, to say the least. > > The point is, make sure you have enough amplifier to do the job with some > margin. That’s as good as you are going to get. > > Ken Javor > Phone: (256) 650-5261 <tel:(256)%20650-5261> > > > ------------------------------ > *From: *John Woodgate <j...@woodjohn.uk> > *Reply-To: *John Woodgate <j...@woodjohn.uk> > *Date: *Mon, 5 Mar 2018 16:19:10 +0000 > *To: *<EMC-PSTC@LISTSERV.IEEE.ORG> > *Subject: *Re: [PSES] Field probe calibration > > > > +/- 20% doesn't seem to be enough to explain the reported result. After > all, assuming the +/- 20% is off the spectrum analyser, 1.2 is +1.6 dB and > 0.8 is -1.9 dB. These are small, but not negligible. If the +/-20% relates > to power, they are even smaller in dB, of course. > > > John Woodgate OOO-Own Opinions Only > J M Woodgate and Associates www.woodjohn.uk <http://www.woodjohn.uk> > <http://www.woodjohn.uk> <http://www.woodjohn.uk> > <http://www.woodjohn.uk> > Rayleigh, Essex UK > > On 2018-03-05 15:57, Patrick wrote: > > > > > Hi David - > > > > I had this same problem. > > I'm actually glad to see another engineer looking into this! > > The answer is not test setup, > > > not ground plane, > > not distance to tabletop, etc. > > > Each of those can be significant, and should be controlled, but... > > > > > > > > > The problem you describe sounds exactly like one I had two years ago. > > If it is the same problem, the root cause is the *probe calibration*. > > The "normal" calibration data that every cal lab provides is too course. > > It hides (skips over) resonances in the probes. > > Only a finer calibration step size will resolve this problem. > > > > > > > > Here is my abbreviated story... > > > > > > I started with data just like yours. > > I had 6 probes: they were two model FP4000, and four of the HI6053. > > > > > ... <skip the description of months long investigation of setup, antennas, > amplifiers, chambers, height above table, orientation, etc....> > > > > > The root cause is the "normal" cal. > > It uses a course step size in frequency. > > When it is too course, it misses resonances in the probe. > > Those resonances are significant, as much as +/- 20% in my equipment. > > > > > To prove this, I sent a probe out for "enhanced" cal. > > I requested both a "normal" cal and a higher resolution cal. > > I asked for 5% steps below 1 GHz and 100 MHz steps above 1 GHz. > > > > > When the data came back I plotted the cal factors on top of each other. > > It was obvious. > > The course cal points of a "normal" calibration will hide resonances that > are +/- 20% deviations. > > > (above sentence should be BOLD, UNDERLINE, asterisks) > > > > > > > > My conclusion: Any probe used for accredited test must have calibration > data showing the resonances. > > If it doesn't, then the lab is guaranteed to be over-testing and > under-testing. > > > > > DM me and I'll be glad to discuss. > > > > > Patrick > > > > > > > > > > > On Sun, Mar 4, 2018 at 8:33 PM, Schaefer, David <dschae...@tuvam.com> > wrote: > > > Ken, > > This data was not taken with 61000-4-3 primarily in mind. We do -4-3, but > also MIL, RTCA, and ISO testing. I should have had the probe at least 15 cm > for ISO or 30 cm for MIL like you said, but 10cm is how I took the data. > > Uniform field calibrations will be a concern eventually, but the variance > is my problem. This was not four probes set up on a bench next to each > other. This was data with one probe on the bench, centered in front of the > antenna, then removed and replaced as precisely as possible with the next > probe. > > So if I do a single point cal for ISO 11452-2, one probe might tell me > 100 V/m and another 140 V/m. I'll get questioned by customers if they fail > one day and pass another. This also runs into another issue - purchasing > amplifiers. If I specify an amp to reach a desired field strength but when > it shows up we can't hit levels due to using a different field probe, there > will be hell to pay. > > Standards are silent on probe orientation as well. Do you position the > probe to maximize field strength? If I can get an extra half a dB of power > by having it an angle instead of straight on, why not do that? I can save > that amplifier cost - at least until I get a new probe. The calibrations > don't seem to mean that much based on my data, so with a composite reading > whichever probe orientation gives me the highest field should be ok. > > Also, any replies I make may be delayed. It seems like I usually see a 4+ > hour delay between when I email the listserve, and when it is delivered. > > Thanks, > > David Schaefer > > > From: Ken Javor [mailto:ken.ja...@emccompliance.com] > <ken.ja...@emccompliance.com]> > Sent: Sunday, March 04, 2018 12:17 PM > To: EMC-PSTC@LISTSERV.IEEE.ORG > Subject: Re: [PSES] Field probe calibration > > In turn: > > It is not surprising at all that it takes less power to generate the > vertical field than the horizontal field. That's the effect of the > conducting ground plane. The OP doesn't say what spec they are working to, > but that is why MIL-STD-461 below 1 GHz has the probe 30 cm above the > ground plane, to limit that effect. > > Comments, such as Gert Gremmen's, that measurements in the presence of a > ground plane (or any conducting structure) are useless, are themselves > useless. The comment reflects a difference in standards of value. If one > is starving, food is the most important priority. If one is asphyxiating, > oxygen is the primary need. It is logically incorrect for two people > suffering these two conditions to point at each other and say the other one > is wrong about his priorities: they are both correct within the scope of > their individual circumstances. The only logical observation that can be > made is that oxygen needs to be supplied sooner than food, if the standard > of value is immediate survival. > > In the world of goods slated for use in home, office and factory, the > coin of the realm is accuracy and minimum uncertainty, so that > qualifications everywhere result in a level economic playing field. > Required field intensities (1/3/10 V/m) are very low compared to the world > of vehicle EMI testing (as high as 200 V/m, sometimes beyond), so that > (again relative) low power amplifiers may be used with antennas separated > from the test area by three meters instead of one, facilitating the > calibration of a quiet zone in the complete and utter absence of any > conducting surfaces, because the end-item use does not include installation > on or near a conducting ground plane. > > This is all in sharp contrast to the qualification of equipment slated > for use on a metal or partially metallic vehicle. The ground plane is of > material use in reducing the intensity of the horizontal field near it, as > noted in the OP and taken advantage of by the very standards that deal with > such qualifications: the ground plane is our friend. Let us count the ways: > > As mentioned above, the ground plane reduces the intensity of a > horizontally polarized field in its immediate vicinity. > > The presence of a ground plane causes cables in its vicinity to react to > fields not as an antenna as in the 61000-4-3 and 61000-4-6 paradigm, but as > a transmission line. Transmission lines radiate less per unit of current > they carry, and couple less power from an incident field, than do antennas. > Given the very stringent RE and RS requirements in vehicle standards, we > need all the help we can get. > > And finally, metallic equipment enclosures bonded to a ground plane > allows filters to efficiently shunt incoming noise to the ground plane and > away from internal circuitry, and perform that same function for noise > currents coupled to shields that terminate in a low impedance manner to the > exterior of such metallic equipment enclosures. > > Now having dealt with Gert - his recurring comment about the futility of > vehicle EMI testing re ground planes consistently eliciting the above > response from me, ad infinitum and ad nauseum, lets look at the OP provided > test data, especially in light of the 61000-4-3 required UFA (uniform field > area) uniformity requirement of +6 dB, -0 dB for 75% of the required > sixteen measurement points. > > What I see is that for a given polarization and frequency every single > position measured is within 6 dB of the others. In the immediate presence > of a ground plane, no less! The cup is not half full - the cornucopia is > overflowing. This performance greatly exceeds the MIL-STD-461 requirement: > there is no requirement for multiple measurement points, and if such are > used, the only requirement is to use the average of the measured points as > the leveling field intensity. In other such standards, such as RTCA/DO-160, > there is a requirement to precalibrate the field in the absence of the test > sample; I would say that the OP test data is evidence of an excellent > chamber. > > If I wanted minimal variation between various positions down the length > of a ground plane, I would not use an aperture antenna such as a DRG horn, > but rather one with constant beamwidth vs. frequency, such as a log > periodic. Assuming I could get the required field intensity with the > amplifiers at hand. > > Finally, while all the standards of which I am aware allow leveling on > the composite output of field probes looking in three orthogonal direction > at once, it is unsurprising that this results in significant variations in > required power level. Better testing results when the test equipment > allows leveling on the polarization of interest. Although, as I said above, > I consider the cited test data below to be exemplary. > > I would have been bragging to my colleagues, not complaining! > > Ken Javor > Phone: (256) 650-5261 <tel:(256)%20650-5261> <tel:%28256%29%20650-5261> > > ________________________________ > From: Cortland Richmond <k...@earthlink.net<mailto:k...@earthlink.net> > <k...@earthlink.net>> > Reply-To: Cortland Richmond <k...@earthlink.net<mailto: > k...@earthlink.net> <k...@earthlink.net>> > Date: Sun, 4 Mar 2018 08:01:31 -0500 > To: <EMC-PSTC@LISTSERV.IEEE.ORG<mailto:EMC-PSTC@LISTSERV.IEEE.ORG> > <EMC-PSTC@LISTSERV.IEEE.ORG>> > Subject: Re: [PSES] Field probe calibration > > > > I'm with Gert. > > > Anything "antennas" is checked in the far-field -- especially if > testing for accuracy. > > I'm a BIG fan of near-field probing for relative measurements and > localizing emissions, but we use probes appropriate to what we are looking > for; if I wanted to "calibrate" one there, I'd use a known current on a > wire/trace or a known voltage on a small plate -- and not trust *that* > much. > > > > > > > > Cortland Richmond > > > > On 3/4/18 5:35 AM, Gert Gremmen; ce-test wrote: > > > > > IMHO all probes are calibrated under far field conditions. > > > In general: Using probes in the proximity (< lambda) of anything > conductive (including ground planes at 10 cm and including EUT) makes the > measurement data useless. > > > > As James correctly states, the construction of the probe makes this > effect different per type of probe, be it the construction, the size of > battery or electronics on board or the lead (fiber or copper) , as long a > other conductors are in proximity the read out has no relation to > calibration data anymore. > > > Using a probe near a ground plane, such as usual in automotive test set > ups, indeed says not much about the test level of the EUT. > > > > > > > Repeating this test under far field conditions, preferable on an antenna > calibration facility, might give you much better results. (not that you are > allowed to generate this much of power on air ;<) > > > > Gert Gremmen > > > > On 4-3-2018 11:06, James Pawson (U3C) wrote: > > > > > > Hi David, > > > > An interesting set of results! I'm going to ask some questions that I'm > sure you've already considered so please bear with me being Captain B. > Obvious. > > > > Do your field probes use frequency correction? I'm not familiar with a > wide range of probes but my Narda PMM field probe has an internal > calibration table; you tell it what the field frequency you are applying is > and it makes the appropriate correction. However, looking at the typical > correction data from the manual (see PDF page 12 of this doc: > https://www.emctest.it/public/pages/strumentazione/elenco/ > Narda/EP%20600/Manuali/EP600-EP601EN-90302-2.02.pdf) it doesn't look like > a large difference. > > > > Is there a difference in the probe construction between the probes used? > Some probes like the Narda one above have two antenna per axis whereas ones > like this Amplifier Research probe - https://www.arworld.us/html/ > 18200.asp?id=636 only have one antenna per axis. Perhaps the proximity of > copper plate makes a difference. > > > > On the subject of copper plate, what are the differences without this > present? What are the dimensions of it and are they significant at the > frequencies selected? > > > > Have you acquired just spot readings or a full frequency sweep? There may > be some patterns in the frequency sweep data that give you more of a clue > as to what's happening. > > > > An interesting puzzle and I look forward to hearing about your results > further! > > All the best > > James > > > > > > > > > > > From: Schaefer, David [mailto:dschae...@tuvam.com] <dschae...@tuvam.com]> > > - > ---------------------------------------------------------------- > This message is from the IEEE Product Safety Engineering Society emc-pstc > discussion list. To post a message to the list, send your e-mail to < > emc-p...@ieee.org> > > All emc-pstc postings are archived and searchable on the web at: > http://www.ieee-pses.org/emc-pstc.html > > Attachments are not permitted but the IEEE PSES Online Communities site > at http://product-compliance.oc.ieee.org/ can be used for graphics (in > well-used formats), large files, etc. > > Website: http://www.ieee-pses.org/ > Instructions: http://www.ieee-pses.org/list.html (including how to > unsubscribe) > > > List rules: http://www.ieee-pses.org/listrules.html > > For help, send mail to the list administrators: > Scott Douglas <sdoug...@ieee.org> > Mike Cantwell <mcantw...@ieee.org> > > For policy questions, send mail to: > Jim Bacher: <j.bac...@ieee.org> > David Heald: <dhe...@gmail.com> > > > > > > > > > > -- > > > // > Patrick > > > > - > ---------------------------------------------------------------- > > > This message is from the IEEE Product Safety Engineering Society emc-pstc > discussion list. To post a message to the list, send your e-mail to < > emc-p...@ieee.org> > > > All emc-pstc postings are archived and searchable on the web at: > http://www.ieee-pses.org/emc-pstc.html > > > Attachments are not permitted but the IEEE PSES Online Communities site at > http://product-compliance.oc.ieee.org/ can be used for graphics (in > well-used formats), large files, etc. > > > Website: http://www.ieee-pses.org/ > Instructions: http://www.ieee-pses.org/list.html (including how to > unsubscribe) <http://www.ieee-pses.org/list.html> > <http://www.ieee-pses.org/list.html> > List rules: http://www.ieee-pses.org/listrules.html > > > For help, send mail to the list administrators: > Scott Douglas <sdoug...@ieee.org> > Mike Cantwell <mcantw...@ieee.org> > > > For policy questions, send mail to: > Jim Bacher <j.bac...@ieee.org> > David Heald <dhe...@gmail.com> > > > > - > ---------------------------------------------------------------- > > This message is from the IEEE Product Safety Engineering Society emc-pstc > discussion list. To post a message to the list, send your e-mail to < > emc-p...@ieee.org> > > All emc-pstc postings are archived and searchable on the web at: > http://www.ieee-pses.org/emc-pstc.html > > Attachments are not permitted but the IEEE PSES Online Communities site at > http://product-compliance.oc.ieee.org/ can be used for graphics (in > well-used formats), large files, etc. > > Website: http://www.ieee-pses.org/ > Instructions: http://www.ieee-pses.org/list.html (including how to > unsubscribe) <http://www.ieee-pses.org/list.html> > <http://www.ieee-pses.org/list.html> > List rules: http://www.ieee-pses.org/listrules.html > > For help, send mail to the list administrators: > Scott Douglas <sdoug...@ieee.org> > Mike Cantwell <mcantw...@ieee.org> > > For policy questions, send mail to: > Jim Bacher <j.bac...@ieee.org> > David Heald <dhe...@gmail.com> > > - > ---------------------------------------------------------------- > > This message is from the IEEE Product Safety Engineering Society emc-pstc > discussion list. To post a message to the list, send your e-mail to & > LT;emc-p...@ieee.org> > > All emc-pstc postings are archived and searchable on the web at: > http://www.ieee-pses.org/emc-pstc.html > > Attachments are not permitted but the IEEE PSES Online Communities site at > http://product-compliance.oc.ieee.org/ can be used for graphics (in > well-used formats), large files, etc. > > Website: http://www.ieee-pses.org/ > Instructions: http://www.ieee-pses.org/list.html (including how to > unsubscribe) <http://www.ieee-pses.org/list.html> > <http://www.ieee-pses.org/list.html> > List rules: http://www.ieee-pses.org/listrules.html > > For help, send mail to the list administrators: > Scott Douglas <sdoug...@ieee.org> > Mike Cantwell <mcantw...@ieee.org> > > For policy questions, send mail to: > Jim Bacher <j.bac...@ieee.org> > David Heald <dhe...@gmail.com> > > > > > -- > // > Patrick > > - > ---------------------------------------------------------------- > > This message is from the IEEE Product Safety Engineering Society emc-pstc > discussion list. To post a message to the list, send your e-mail to < > emc-p...@ieee.org> > > All emc-pstc postings are archived and searchable on the web at: > http://www.ieee-pses.org/emc-pstc.html > > Attachments are not permitted but the IEEE PSES Online Communities site at > http://product-compliance.oc.ieee.org/ can be used for graphics (in > well-used formats), large files, etc. > > Website: http://www.ieee-pses.org/ > Instructions: http://www.ieee-pses.org/list.html (including how to > unsubscribe) <http://www.ieee-pses.org/list.html> > List rules: http://www.ieee-pses.org/listrules.html > > For help, send mail to the list administrators: > Scott Douglas <sdoug...@ieee.org> > Mike Cantwell <mcantw...@ieee.org> > > For policy questions, send mail to: > Jim Bacher <j.bac...@ieee.org> > David Heald <dhe...@gmail.com> > -- // Patrick - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <sdoug...@ieee.org> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
