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

Wednesday, September 24, 2014

The Annual IEEE EMC Show in Dresden, Plus Elephant #4 ‘The Zip Code Lottery in Achieving Product Compliance'



Hurrah!! The 3D EM software has arrived and I have started to familiarize myself with it. I am going to enjoy this. Reading through the features, it is extremely powerful and looks perfectly suited to simulating the automotive test fixture. It always seems to take me about 40 hours at the keyboard to get up to speed with a new software package, so it will be a while. Meantime ....,

The Annual IEEE EMC Show – Why Dresden?

Last time we covered the vendors’ motives for attending a show, and made the claim that, despite the self-declared importance of the largest vendors, technical paper attendees rule when it comes to the survival of symposiums. Here is why: -

Shows come into existence when a small number of like-minded individuals (usually members of an engineering society) have a common interest in an emerging engineering discipline. Sooner or later it makes sense to meet up to share knowledge, often through the mechanism of technical presentations. This draws vendors like bees to a honey pot, and right from the start a ‘table-top’ sideshow becomes part of the event. As attendee numbers grow, so does the size of the sideshow. Exhibition hall fees soon make up a sizable part of the show revenue, which is why the largest of the EMC exhibitors see themselves as indispensable to a show’s survival. But when did you ever see even the largest vendor actually go out on a limb and pay for their own event on a scale equivalent to the annual IEEE EMC show? Why would anyone attend? It is like asking someone to subscribe to a technical journal with nothing but advertisements in it.

Vendors need to face facts, they are a sideshow, beholding to the symposium organizers; not the other way around.


The Presentation Attendee’s Tale


Given their importance, what draws a technical presentation attendee to a show?

Obviously, the papers/ workshops being presented. Now. unless they have a thousand dollars or two burning a hole in their pocket, these attendees are likely to be employer backed and likely be responsible for the EMC compliance of that company’s products. If newly appointed, an employer with an ounce of sense will want the new appointee ‘up to speed’ as quickly as possible. If the attendee is mid-level experienced, a worldly-wise employer will want that knowledge kept up to date. If highly experienced, the employer better keep that ‘hard to replace’ employee sweet should they wish to retain him/her.

Attendee Motivators

For the new guy, being responsible for product compatibility all of a sudden is disconcerting. Often due to an ad-hoc internal transfer or promotion, it is not uncommon for say a digital-electronics engineer to be saddled with the responsibility. The employer and new appointee both know they need to fill vast knowledge gaps quickly, either directly by way of attending ‘introductory’ technical presentations and workshops, or indirectly through making the acquaintance of experienced attendees at the show (so they can tap their brains later).

For the guy with mid-level EMC experience, keeping knowledge current is the chief motivator, and they will seek out and attend papers that update EMC personnel on standards changes, new technology, etc.

For the highly experienced guy, one pertinent topic can be enough to entice. They like to keep ahead of the curve with theoretical sessions, and during question time, can give vital input to leading-edge papers.

For both the mid and high level attendee, networking can be seen as important for job security

De-Motivators

Pressure at Work and No Travel Budget

Sadly, it is harder for staff to attend shows nowadays. Sometimes due to project pressures (everything seems to be project based these days), and sometimes due to increased pressure at work due to previous payroll cuts. It can be impossible for key staff to be ‘lost’ for a week, or even for a day. And in the rare situation where they can be spared, travel is often the first victim of budget cuts.

Regarding payroll cuts, I can state from first-hand observation that that the disappearance of support staff results in highly paid design engineers being reduced to busying themselves with jammed photocopiers, when they should be pressing ahead with the design of next year’s world leading product. Corporate madness prevails sometimes.


Types of Paper Delivered

Papers seem to range from the outlandish (university staff doing the ‘publish or die’ thing), to the same-old, same-old revamping of old topics. ‘Introduction to EMC’ type papers are always of use to a newbie, but to the more experienced engineers, I think perhaps the papers are less of a draw. Possibly as a result of the maturing of the industry.


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



Elephant in the Test Room #4 - The Zip Code Lottery


The room: Whether a manufacturer’s product passes the relevant emissions test can be a matter of where in the Country their premises are located

The culprit: The seemingly EMC industry wide acceptance that a 10dB variance in emissions measurements made on the self-same ‘golden’ emitter test-piece is OK

The consequence: This opens up the prospect of the unfair situation where one manufacturer’s design with comparatively superior emissions performance fails, while a second manufacturer’s design with inferior emissions performance (compared to the first) passes at another location

NOTE: Be sure to listen in on the EMC-LIVE webinar event  October 16 as this topic will be one of the roundtable discussions ( EMClive2014 ). This is sure to be fun because in truth I am simply a bystander with a pretty good understanding of the physics behind EMC tests, and the event is a chance for the experts on the panel to put me in my place.

 Continuing with the elephant at hand, in my view, the ‘root cause’ blame for its existence, and likely continuing existence, lies with the accepted design of open area test sites (OATS). The OATS test methodology involves the inclusion in the measurement of a deliberately created second field, routed via a deliberately created indirect path.

Here is the history of emissions testing as I see it. In the early days it made sense to build an open area test site (OATS) at a rural green-field site far from urban turmoil and its associated ambient RF noise. The trouble was it did not take long to realize that inter-site measurements varied enormously and there was no repeatability worthy of the name.

Figure 1 shows a key reason why these first failures of the method occurred. The figure depicts an unintentional path allowing a second field to be presented to the measurement antenna. The strength of this second field varied depending on the reflectivity of the substrate between the EUT and the antenna. Pavements and soil types at different sites would vary in reflectivity, and one site’s reflectivity could change depending on when it last rained.


Therein lay the problem, and it was decided not to attenuate or divert the second field, but rather to make it consistent in strength (site to site) by installing a fully reflective ground plane as shown in Figure 2. 


 This presented a problem in that the maximum combined field might not occur in direct vertical alignment with the EUT. So to rectify this, a further decision was made to add complexity by searching the vertical space at the measurement plane.

Once accepted as a method, chamber manufacturers wishing to claim equivalence had to replicate the second path (simple and cheap) and also had to increase the chamber volume to allow for the 4m height scan (simple, but far from cheap).

I wonder if the founding fathers of EMC realized the ramifications this method would have later on in the design and cost of semi-anechoic chambers.

Question - Could they have come up with another solution? Easy for me to say since I wasn’t there to face their constraints, but I wonder if they gave serious consideration to diverting the second field away from the antenna as shown in Figure 3.


 If you are going to search for a rural green-field site, why not choose one with a natural slope such as a hill? The hill slope might need to be graded to the best angle and access roads and unobtrusive scaffolding for the antenna mount would be an issue, but the impact on chamber designs would have been a lot kinder.

To be continued....

-Tom Mullineaux