Tuesday, November 18, 2014

The Diagnosis of Under-Performing Semi-Anechoic Chamber Designs


As mentioned in the upcoming article ‘Applying Stealth Technology to EMC Chamber Designs’, and also mentioned during the recent EMC-LIVE webinar ‘Elephants in the TestRoom,’ the intention is to use 3D EM simulation software to explore possible design improvements to today’s under- performing chambers.

The article itself is not out yet, but we can make useful progress in the meantime by exploring the performance of the present ‘hot-wall’ arrangement. The hot-wall is the absorber clad wall behind the RF immunity calibration plane, and currently test labs are forced to point the antenna at a corner of the chamber to achieve test field compliance. Later on we will explore hot-wall wave-deflection (angled faces) as a possible design improvement.

Thanks are due to CST for the loan of their CST Studio Suite® 2014 Software, and particular thanks are due to Dr. Andreas Barchanski of CST for helping with the modelling and analysis.

IN THE BEGINNING

As with any engineering project, before any engineering time is invested, it pays to be sure of the required outcome, and have a direction of attack.

Well, the first objective is to establish why it is so difficult to obtain field uniformity at the calibration plane with the present hot-wall design, so I propose our direction of attack is to gather information on the performance of a standard flat absorber clad wall. I further propose we establish the individual performance of each type of absorber (pyramidal and ferrite tile).

GETTING STARTED

We will start with an analysis of the pyramidal absorber performance. As stated above, we first need to be clear on what we want the software to do and how we want it to do it.


Figure 1 shows a side view of a single pyramidal absorber. It is made from carbon loaded dissipative material shaped to form an impedance taper. We want to examine it’s two-way wave attenuation performance, so there is a fully reflective PEC (perfect electrical conductor) sheet covering the surface of the pyramid base. The pyramid is designed to face the wave it is intended to attenuate directly, so we will get the software to strike it with a plane wave at normal incidence and make a relative measurement of the ‘echo’.

A field probe positioned to the left of the pyramid will see both the incident and reflected waves, so I suggest we use a trick of the trade. In RF engineering, if you want to establish the loss (unwanted copper and dielectric loss) in say a four-way combiner, one approach is to manufacture two of them, connect them back-to-back (connect four connectors to the other four connectors), and  measure the overall loss. Half that loss and you have the loss of one combiner.

We can use a similar approach with the pyramid absorber by placing two of them back-to-back as shown in Figure 2. We then place one field probe on the left and one on the right and measure the fields in isolation. We lose the 180 degree phase change caused by the reflective sheet (we will need to remember this), but we gain data on the reduction of the magnitude of the ‘echo’ wave.

 









Figure 3 and Figure 4 show the same approach used for a non-normal angle of incidence.


 



















ANTICIPATED RESULTS














It is good practice to anticipate the results so you can use them in a sanity check later.

Figure 5 shows the plane wave about to strike the absorber. The vertically polarized RF waveform is represented by colored arrows showing the direction and strength of the field along one wavelength. Starting from the right and working left, the arrows show the upward pointing field of the first half-cycle gaining in strength to its peak (red), then falling in strength for the remainder of the half cycle, and then repeating this variation in strength for the downward pointing field of the second half-cycle. The upward pointing arrow on the right will impinge on the absorber first.



 










Figure 6 shows the anticipated results. For clarity only the effect on the peaks of the wave are shown. The right hand arrow in the pyramid represents the first half-cycle peak as it travels though the absorber. The center of the arrow has passed furthest through the pyramid lossy material and so that segment is attenuated the most (now green). Segments above and below the center have travelled different distances and so display different levels of attenuation. The left hand arrow has not travelled as far and only the center segment (orange) has dropped in field strength up to that point in the pyramid.

EARLY RESULTS

Well, the anticipated results went straight out the window because the actual results were a complete and utter surprise.

Figure 7 shows a still shot of the animated simulation results. When the animation is run, blocks of concentric field-rings march left to right in much the same way as the satellite view of a hurricane making its way West to East over the Gulf of Mexico.
It is mesmerizing to watch. Hopefully the EMC-Zone editorial staff can arrange for readers to witness the animation on-line.

They say there is nothing new under the sun and the effect may have been predicted/observed by some great scientist (Fresnel or another), but until we know what they are, and for the purposes of this EMC-Zone discussion alone, we will christen the blocks of concentric rings ‘Eddy Field Blocks’ (EFBs).

Even if turns they only occur over a comparatively narrow frequency band, or due to slowing of the wave (dielectric constant of pyramid material holding carbon in suspension not equal to 1), they are absolutely fascinating to observe. Anyone out there know a meteorologist or someone familiar with fluid dynamics that has seen these type of circular fields before? I know vortices form in fast moving rivers etc, but these seem quite different (no spiral swirl to the center).














ROUGH DESCRIPTION

Initial observation suggests:-

An EFB begins to form inside and outside the material at the apex of the pyramid.

The EFBs are created at the apex every half-cycle of the incident wave, and dissipate at the base of the pyramid at the same rate.

EFBs are made up of concentric field rings, each ring pointing in the same direction, each ring remaining separate from the others within the eddy block.

Adjacent EFBs have field rings pointing in the opposite direction (clockwise or anti-clockwise).

The diameter of the EFBs is about ¼ wavelength of the incident wave.

The created EFB moves to the right and then pauses for the time it takes ¼ wavelength of the incident wave to pass. The EFB creation rate and dissipation rate are still the same (one every half cycle of the incident wave).

The EFB seems to be an entity in its own right, evidenced by continuing to exist while an incident wave minimum passes past it during the time the EFB pauses.

To be continued............

-Tom Mullineaux

Monday, October 27, 2014

Elephants in the Test Room Roundtable Commentary 1


On Thursday, Oct. 16, Interference Technology hosted a roundtable during EMC Live - 'Elephants in the Test Room' based on our blog series. Below is additional commentary about testing issues, from our expert panel.

Panel
Tom Mullineaux - Moderator, Consultant and Author
Patrick Andre - President, André Consulting, Inc.
Fin O'Connor - Defense and Space Consultant and Contractor, Alion Science and Technology
Adiseshu Nyshadham - Senior Consultant, DVT Solutions Inc.
Steve Koster - Vice President, Washington Labs






Elephants discussion points:

Elephant Discussion #1 – Poor EMC Measurement Consistency
No one is surprised when a round robin test shows multiple EMC testhouse measurements taken under supposedly identical test conditions are up to 10dB apart. By its nature, the ISO17025 laboratory accreditation standard covers a very broad church of test situations. However, the EMC industry is a distinct, identifiable niche. The various compliance groups providing the audit-service should be able to work together to improve inter-EMC laboratory measurement accuracy.

Can the EMC industry work with ISO17025 laboratory accreditation teams to improve measurement consistency?


Elephant Discussion #2 – Underperforming EMC Chambers
When calibrating a test field to 6GHz for commercial RF immunity testing, to obtain a compliant test field many test houses are finding they are forced to point the antenna at one corner of the room. All standard 3 meter semi-anechoic test chambers are cuboids with flat walls, ceiling and floor. The four walls and the ceiling are clad in RF absorber. The ‘hot’ wall (the one behind the calibration plane) performs the same as the other three walls.

Is a flat absorber lined ‘hot’ wall truly the only possibility?


Elephant Discussion #3 – Automotive Tests that Put the Car Audio System Performance First
Until fairly recently it seemed the RF immunity tests conducted inside the car cabin space were purely to ensure good sound system performance.

What is the future of RF immunity tests that actually check for electronic sub assembly compatibility inside the cabin space of a car?

DISCUSSION:

Patrick: 
Q. Can the EMC industry work with ISO17025 laboratory accreditation teams to improve measurement consistency?
A. Yes, improvements can always be made. However the challenge will be to first establish repeatability in a single test setup for each chamber. Often, changes of ±4 dB is seen with the most minor of test setup changes. The movement of a cable or the position of the support equipment can radically alter the results. Added to this the difficulty of different laboratory layouts, with different sized rooms, anechoic material differences, antenna configurations (both type and location), power line and signal line routings into and out of the room, to name a few, and soon you have so many variables between two so-called “identical setups” that getting only 10 dB differences is actually remarkably good.
Q. It might be necessary to introduce the use of a fully anechoic chamber where the floor is now covered with absorber, reducing at least one variable in the measurements.
A. All standard test chambers are cuboids with flat walls, ceiling and floor. Is this shape truly the only possibility?
Of course not. Reverberant chambers are already being made with non-parallel walls. Also, simply adding a metal or anechoic panel at a 45 degree angle in a corner can alter the fields significantly. However, I am somewhat surprised by the difficulties the labs have, since the cone style of anechoic material should work remarkably well at these frequencies.

Fin:
Boy, the #1 could go on for days!! I’m on the 461 committee and we will not have any uncertainties in the spec (ever as some say) the uncertainties are built into the limits. Now the test problem differences are very difficult to get a handle on. I travel to many independent and gov labs witnessing testing for NAVAIR and the differences in the way these test houses INTERPRET the standard is surprising. I am trying to get more description for each test into the ‘G’ version of the standard (which we are working on now) but I am getting resistance because they feel the standard has enough description for the competent engineer/tech. I was also involved, years ago, with the round robin testing for the NVLAP Mutual recognition project and this was a for the very simple open filed testing with specific parameters. And even with this test control, the differences between the test houses was 10 + db at least. I would love to see some way to get the testing differences smaller so anything that can be suggested and proven would be welcome but it’s a uphill road.

Patrick:
I think Fin and I are in violent agreement. As a fan of MIL-STD 461 and DO-160 (mainly due to my aerospace background), I love the fact that cable length and positions are defined and controlled (1 meter, 2 meters, 3.3 meters, and so forth, depending on the test and standard). Cables are placed on 5 cm standoffs, 10 cm from the front of the ground plane. And even then, even with all this control, we get these variations.
I am working on a piece of medical equipment, with five transducers, five monitors, plus peripheral and support equipment. How do I ever lay out the cables and support equipment the same so I can get the same results twice? I have seen 5 dB swings just moving one cable.
I think the only science less accurate than ours is Astronomy, where they state distances of remote objects within a couple decades, e.g. 10^6 to 10^8 light-years away. And I wanted to be an Astronomer.
Then again, I have had no success convincing the police officer of the accuracy of my speedometer. He just doesn’t buy the idea that as long as I am between 6 MPH and 600 MPH I am within the range of accuracy.

Tom:
Just to make sure you are up to speed with where the elephants are history wise, you should read the article I wrote recently for ITEM.
http://www.interferencetechnology.com/a-design-review-of-the-automotive-radiated-emissions-test-fixture/
In the article I am very critical of the design of the emissions test fixture. I have just received 3D EM software from CST so I can come up with a superior arrangement. Might have preliminary results in time for our webinar. But you should know that if you praise the 5cm high cable over a ground plane I am likely to lambast you without mercy, and have supporting data to do it with.

Fin:
Oh no, not a 5 cm debate. I’m always interested in why the specs are the way they are. I will be real interested in your research, and will pass on to the committee,if it will make the readings better. But being honest, we have been using this method for the longest and our planes fly, missiles find there mark and radios work, in theater/hostile environments and we have feedback from the fleet and we track problems and why they occur (NAVAIR has the ASEMICAP group which tracks fleet problems and finds the reason and solutions) so to get the community to change, you will have to have a really good reason. But sometimes change is good!! Just hard to get through.

Tom:
Yes, the long history and the fact that planes are not falling out of the sky speaks volumes. Hard to argue with physics though, particularly the possible mismatch between the noise source (EUT) and the test fixture.

Fin:
Tom, ‘test fixture’?? Not sure what you mean? The 5 cm is to closely resemble the common actual installation (Military environment, loop impedances), to get some consistency and to work with the LISN (below 10 MHz). Unfortunately though, as Patrick had, most of the time, the set-up is a mess and standardization is mostly impossible so these controls are the best comprise we have. But, like I’ve said before, suggestions are welcome, at least by me!

Patrick:
I agree with Fin completely.
A story I was told about 5 cm standoffs: I was doing testing at Celect, a division of Cincinnati Electronics, in the early ‘90’s with a man named John Day. When I made a comment about 5 cm standoffs, he stated that he may have had something to do with the fact they are a bit inconvenient. He told me that in the early days of military EMC emissions, someone thought that it would be a good idea make sure the cables are not laying directly on the copper bench, so they grabbed 2x4’s and put the cables onto them (which are 4 cm high). However, once things needed to be documented, and they were converting to metric, John was asked to go measure the height of the cables off the ground plane in centimeters. When he reported the measurement, he reported the height of the cable, which was a large cable bundle, and had measured to the center of the cable – which added 1 cm to the overall height.
He said the next thing he saw was a document stating the cables needed to be on standoffs 5 cm high based on the past tests. And he did not correct it.
Or this could just be a story he told me.

Fin:
FYI, some history . I knew Ken would have the lowdown. He has a pretty good museum of old equipment and books on EMI. And a pic of some of our old generators!!
Fin,
The answer involves some heavy math.
The first part of the heavy math is if one picture is worth a thousand words, how much is two pictures worth?
The first pic shows a radio room in a WW II-era bomber. On the left is the radio, on the right the unshielded antenna lead from the radio is visible held above aircraft structure via porcelain standoffs, to minimize capacitive loading of the high impedance signal.
The second pic shows the standoff dimension, in inches.
The rest of the heavy math is the conversion to MKS, which is left as an exercise for the reader...
- Ken Javor


Patrick:
Oh, of course. And I should have known Ken would know.
And I kinda figured John Day was handing me a line. Kinda.
Thanks! This is great stuff.

To be continued........

For more information on the roundtable, visit EMC Live's website here.

Tuesday, October 7, 2014

Continuation of the Annual IEEE EMC Show in Dresden, Plus Updates on Other Topics



Welcome back. Before we get started - two updates:-

3D EM Software Analysis of the Automotive Emissions Test Fixture

The tutorials in the handbooks are completed (there are quite a few because there is an entire suite of analysis tools). Also done are the ones that are downloaded with the software. With all tutorials, a lot of time is devoted to constructing the models to be analyzed. This makes sense since an accurate 3D EM analysis of a real-life component depends entirely on an accurate 3D model.

Attention has now moved to the more comprehensive tutorials on the software house’s website (you have to sign in with the license details).

It can be hard not to try running before you can walk. One of the booklet tutorials is on the analysis of a monopole. It crossed my mind that if you lay the monopole on its side, placed a conducting sheet below it, and terminated the far end in 120 ohms, then ‘hey presto’, you have the basis of the automotive emissions test arrangement. Probably better to be more familiar with the many tools available before going down that path. For instance, maybe creating the test set-up in the cable analysis tool, and then transferring it into the microwave tool is the best way to go.

By the way, I added a new task to the EM analysis list: – Establishing the change of attenuation when an RF wave strikes a pyramidal absorber at an acute angle.

It is all to do with a proposal that could improve the performance of 3m semi-anechoic chambers. An article titled ‘Applying Stealth Technology to EMC Test Chamber Designs’ will appear in the next Interference Technology publication (2014 EMC Europe Guide).

The pictures show the first efforts at defining and placing primitives and then using the knowledge gained to create a pyramidal absorber shape.

To be continued.......

UPDATE ON EMC-LIVE ROUNDTABLE EVENT

Theme: Elephants in the Test Room
Date: October 16
Time: 1:30 pm EST
Venue: Your computer

A prize-winning academic has joined the panel, and one of the expert panellists at the round table actually has access to a study that collected and compared test-house emission measurements on a ‘golden’ test piece.

I, armed with a jaundiced eye, am at odds with the entire premise of the study and see the ‘problem’ being addressed as systemic, even before operator competence is factored in. I think due to its longevity, and hence people growing up with it, many in the industry are in awe of the OATS set up. To my mind they worship a false idol.

And that’s just one of the topics - be sure to mark your calendar, be sure to register beforehand (http://emclive2014.com/elephants-test-room/), and when the big the day arrives, be sure to set the alarm on your cell phone. And then listen in on the fireworks!

The Annual IEEE EMC Show – Why Dresden?

So far we have covered the motives of those at the show to attend presentations and those at the show to sell (vendors).

We concluded that vendors are a sideshow, and that the survival of the symposium is down to the audience count, that is the number of registrants there to learn from papers and workshops.

Obviously the number of registrants that sign up to learn is directly proportional to the relevance and quality of the papers / workshops on offer.

So what entices a person to go to the trouble of composing and then delivering a paper / workshop?

The Presenters Tale

We start with papers. We will focus on workshops next time.

Presenters of Papers

For the purposes of this discussion, academic papers will not be a factor. In my view the main reason academic staff present papers is down to the ‘publish or die’ syndrome, and as such, content is rarely a compelling reason for prospective registrants to attend. There are exceptions, but they are few.

That pretty much leaves industry professionals as the technical-paper presentation candidates. So if we accept the main body of presenters is made up of industry professionals, what do they get out of it?

Presenter Motives

These include but are not limited to: -

·         The travel and subsistence costs are covered by their employer

·         To raise or maintain their profile in the industry

·         Straightforward self promotion (consultants)

·         Have stumbled on something big in their work (hands-on professionals)

·         Have a gripe about a technical issue, and it is gnawing at them

·         They are selling something

Most of these are self explanatory so I will expand on only a few.

Vendor staffs often get to witness life at ‘the coal face’, so on occasion they have something useful to contribute. Their flight and hotel are already paid for, so expense is not a barrier. As much as they would like to deliver marketing dressed as technical discourse, this won’t get through the paper acceptance filter. The paper has to have relevance and a good measure of technical content, so the only reward the company backing the endeavour can expect is their company name in the presenter’s title.

Papers are a great way to raise the profile of a newcomer to the industry, and one of the quickest ways to get to know key players in the industry.

One man companies such as consultancies know papers are an excellent way to promote their services

If something is seriously new in the industry and an innovative ‘mousetrap’, with the addition of a bit of gratuitous math, these have been known to get through the symposium filter process.


Presenter De-motivators

These include but are not limited to: -

·         Travel and subsistence costs not covered by a sponsor

·         Pressure at work

·         Employer has not budgeted for staff excursions

·         No experience at writing papers

·         Not good at public speaking (jitters)

·         Cannot guarantee will be available in 8 months time (too far ahead, fluid situations at work and at home)

Let me pose some questions: -

1.     If you were on the symposium board, what recommendations on raising the relevance and quality of papers would you suggest?
2.    Do any of your suggestions tie in with the strategy behind the European venue?

To be continued......

The 3D EM tutorials are digging into to the time available for blogging, so I will pick up Elephant #4 ‘The Zip Code Lottery in Achieving Product Compliance’ next time.

-Tom Mullineaux