Togetherness is Another Way of Saying Well Bonded

Electromagnetic compatibility (EMC) is often described in terms of RF coupling phenomena. Usually the big four: Radiated and Conducted Emission and Radiated and Conducted Susceptibility/Immunity. These four elements cover all of the EMC issues in a very broad sense. Occasionally lightning and ESD will be divorced from susceptibility and described as if these two items were something other than just a high level transient susceptibility condition. That’s very understandable if you’ve witnessed a nearby lightning strike or been zapped on a cold winters day by a seemingly harmless doorknob. Sometimes in the clandestine world of communications security even the low level emissions are carefully examined for their content. It’s all a matter of interest ... the reality is that EMC simply means that electronic widgets can live together in peace and in harmony with one another and with the electromagnetic environment, and we get to define peace, harmony, and environment. It’s all a part of togetherness. 

As I mentioned in the last post on MPG systems, as wavelengths (λ) approach the dimensions of boxes, cables, and grounding connections parasitic capacitive coupling tends to interconnect circuits having changing voltage potentials to other circuits and to ground references whether that coupling is desired or not.  There are two primary ways of handling parasitic capacitance depending on whether a circuit to circuit connection is permitted. If the circuits must be kept isolated then shielding is used. The shielding can be a simple grounded fence between the cir-cuits or the circuits (one, both, or all) can be completely enclosed. For a peek at how this is done with a small system take a look inside a cell phone. Enclosure can be accomplished using PCB layers, small postage stamp sized boxes, or entire multistory buildings. Shielding will be covered later in the EMC-Zone.

The parasitic capacitance can be shorted if the circuits can be interconnected or connected directly to the ground reference. The intent is to keep the circuits at the same voltage potential. Unfortunately any conductor has inductance which results in voltages being developed from any changing currents through the inductance:  V = L di/dt. In addition, the combination of the parasitic capacitance with an inductance may create either a parallel or series resonant circuit (only Murphy knows for sure) with the first resonance occurring at a frequency (F) = 1/ (2 π √(L C)).  At this frequency the inductive reactance cancels the capacitive reactance leaving only the resistance of the conductor. Above and below the resonant frequency either the capacitance or the inductance will dominate. Consequently as the frequency increases, long individual wires cannot be used for grounding and bonding because their higher inductance (L) lowers the frequency where the grounding/bonding system fails.. 

The figure shows the migration from a wire to direct bonding as the frequency increases.  The symbol L is used to honor Heinrich Lenz, even though Joseph Henry actually discovered both self and mutual inductance. However, L is also used by most people (especially the mechanical types) as the symbol for length (L). This turns out to be really appropriate because inductance (L) is primarily related to conductor length (L)!

Here’s a memory hook just to illustrate the point (Click to enlarge). Imagine that the figure shows a solid-flat bonding conductor with length (L), width (W), and thickness (t) where  L >> W >> t. If the thickness (t) is increased by 10x the inductance is decreased approximately 10%.  If the width (W) is increased by 10x the inductance is decreased approximately 30%.  If the Length (L) is decreased by 10x (to 1/10th the original length) the inductance is decreased approximately 93%.  That’s ninety-three percent!  So keep conductors as short as possible, make them as wide as possible, and don’t worry too much about their thickness.

Before there is true togetherness, there are some other factors to be considered such as material types, surface finishes, and methods of connection. Stay tuned to EMC-Zone.com for the next piece on togetherness.

- Ron Brewer

And Now, a Shameless Plug….

The Joint Spectrum Center (part of DISA for those of you who support the US DoD) sponsors and populates the premier electromagnetic environmental effects and spectrum supportability web site on the World Wide Web.


We in the business know that compliance with Spectrum Supportability (SS) requirements and control of electromagnetic environmental effects (E3) can be critical performance parameters for platforms, systems and electronic equipment acquired by the DoD. If these technical areas are not given the proper level of attention during the acquisition process, you can be sure that fielded systems will suffer from some level of operational degradation up to, and including, catastrophic failures or unacceptable operating restrictions. As part of their charter, the Joint Spectrum Center manages the DoD E3 Program, which includes a mandate to provide outreach and training services to the DoD acquisition community so that they understand E3 and Spectrum requirements and how to implement them in their specific programs. Thus was borne the Spectrum and E3 requirements Community of Practice (previously Special Interest Area) as part of the Defense Acquisition University Acquisition Community Connection (that’s a mouthful). The site is available at acc.dau.mil/e3 or acc.dau.mil/spectrum (no www!). Let me give you a quick tour….

The figure above is from the front page of the site and the icons provide and entry into the various sections of the website. Basically it starts with the Spectrum and Electromagnetics 101 icon at the 11 o’clock position and goes around counter clockwise, providing information on basic technical concepts, policy requirements and various DoD process and procedure requirements to ensure that E3 control and spectrum supportability is fully considered in the design, development, testing and fielding of military spectrum dependent systems.

Beyond these areas of technical and procedural guidance, there are various areas that provide links and information to relevant organizations, technical articles, and website, both commercial and military, that provide users with all the contacts and additional information they need to ensure EMC in their systems.

The site has a feature where you can become a member…don’t worry, it doesn’t cost anything and takes only a minute to join. Benefits of community membership include the ability to:

• Ask and/or answer questions;
• Connect with members/experts with a shared specific interest;
• Receive community-wide emails;
• Access restricted community knowledge.

Future plans include the development of a compendium page of EM-related blogs so that you could find all your favorite blogs (like this one!) in one spot! The Joint Spectrum Center is very interested in constructive feedback from the EMC and Spectrum communities on the site content (primarily) and construction. While the site layout and functionality is dictated by the DAU, we do have some control over the look. So dive in, join up and let the JSC know what you think!

-Brian Farmer

The Ground Loop

Several readers have made comments regarding multipoint grounding (MPG) systems.  The concern is that this arrangement creates multiple ground loops.  As a result, we need to address that.  As some of you know, I make a lot of presentations and teach a lot of EMC seminars.  A long, long time ago, I was discussing ground loops in one of my training classes when a student interrupted me and told me that I obviously didn’t know what a ground loop was, so I let him tell me.

As it turned out, he was a member of a Civil Air Patrol (CAP) squadron flying an antique Piper J-3.  This is a little wooden and metal framed tail-dragger covered with canvas, which looks a lot like a large version of a model airplane that we—with the help of our kids— might build at the kitchen table.  That said, in the world of aviation, a ground loop is an on-the-ground spin-out of the airplane, where the pilot is suddenly looking back at where he just came from.  This usually only happens with conventional landing gear (not tricycle), where the center of gravity is behind the wings. I suspect that this kind of ground loop is very scary . . . especially if going fast.

We all had a good laugh at the airplane definition of ground loop, but ever since then I have tried to use the more technical term: common impedance coupling. However, that term is hard to visualize.  Regardless, the MPG creates a lot of ground loops and they are just as scary to the EMC engineer as they are to the airplane pilot. 

Figure 1 illustrates the issue of loop creation within a MPG system.  This figure shows five cases of grounding connections, i.e. none (0), 1, 2, 3 and 4.

None (0) and 1 (which is the single point ground) do not create a loop.  Two ground connections (one loop) might be all right if there are no ground currents and no radiated electric or magnetic fields.  Yeah right!  With three or more intentional or unintentional ground connections, there are progressively more loops than ground connections, and the loop complexity gets worse and worse.  This is a graph theory problem, with the number of loops (L) related to the number of grounding (G) connections as follows: 

If G = n, then L = n (n-1) / 2.

So, four interconnected cabinets might have 6 loops, but a PCB with 20 devices might have 190 loops.  A similar situation occurs when the number of interconnecting cables is considered, but that is another problem for another day.

Unfortunately, as frequencies increase to where wavelengths approach the dimensions of boxes, cables and grounding connections, single point ground systems fail and capacitive coupling creates MPGs whether they are wanted or not.  Since many of the high frequency ground connections (if we want to call them that) are the result of capacitive coupling, they are not visually identifiable.  We can’t see where they are or determine what loops were formed.  Consequently, the grounding design for high speed systems must be adapted to work at these higher frequencies.  Individual grounding wires cannot be used because their resistance and inductance is too high.  Instead, ground planes are used.  This may be a dedicated PCB layer, the chassis or a grid underlying a complete installation.  It all depends on the frequencies of the device. 

Although there are many loops created by MPGs, that doesn’t mean that they should be avoided.  MPG ground systems have connections that are shorter and more direct. In addition, they are easier to build, their loop areas are smaller and they can operate to higher frequencies before standing waves become a problem.  Nevertheless, like anything else (even SPGs), MPGs can’t be ignored, especially the large-scale grounding systems.  There needs to be routine maintenance to counter the effects of corrosion, shock and vibration, and mechanical damage to prevent the introduction of unwanted high impedances.

- Ron Brewer

 

Do You Feel Lucky?

Do You Feel Lucky?  Well do ya…..PUNK?

Classic line from a classic movie, Dirty Harry, who was also in the Enforcer. Frankly I forget which one came first. So who is our enforcer of electromagnetic environmental effects (E3) requirements, specifically for US DoD program? 

The requirement to field electromagnetically compatible systems is mandated in DoD Directive 3222.2, “DoD Electromagnetic Environmental Effects (E3) Program.”  It’s pretty straightforward about the need for E3 control and defines a wide variety of roles and responsibilities for the implementation of E3 control, but at this point it’s a bit out of date.  The current version is dated 8 September 2004, so its going on 10 years old. That can be a lifetime in technical discipline like E3 as many things can and obviously have changed over the last decade in the E3 business. Priorities change, definitions change, organizations change (including people), technology changes causing the electromagnetic environment to change (that’s a biggie!), all contributing to the need for policy to change or at least to be updated. This version of DoDD 3222.3 has seen changes to MIL-STD-461, MIL-STD-464, the creation of MIL-HDBK-240 (HERO Test Methodology), DoD Instruction 4650.01 (DoD Spectrum Use), the disbanding of the Office of the Secretary of Defense Networks and Information Integration (office responsible for E3 and spectrum directives) and a wide variety of other changes.

There is some good news – there is an updated version of the directive waiting for signature.  DoDD 3222.3 will become DoD Instruction 3222.03, which is a significant change.  Its actually an improvement as an Instruction is allowed to include more information (its physically allowed to be longer, more pages).  This will allow the E3 community to ensure that more guidance is provided on HOW to implement the E3 program, not just the fact that an E3 program is required.  Better guidance allows program offices to plan more effectively, with a better understanding of what is expected of them.  The final coordination draft of DoDI 3222.03 contains significantly more information on specific actions that must be taken by acquisition programs to effectively implement E3 controls with the acquisition process. In particular, there is an entire enclosure that details the procedures that should be followed during the acquisition process to achieve electromagnetic compatibility.  Just as importantly it requires that the DoD provide awareness and training to the acquisition and operational communities, so there is a better understanding of the importance of E3 and related technical disciplines and the processes associated with them.

The bad news is that the draft has been stuck for quite some time in the final signature cycle, hostage to various recent reorganizations and never quite getting high enough on “the list” to actually get promulgated. But there are good people pushing for it and we’re all confident that it’ll be out soon. Then hopefully fielding electromagnetically compatible systems won’t just be a matter of luck ….

- Brian Farmer

Can an Ordinary Human Stop RF Noise?

When my son was working on his EE degree he found an article titled: Stop That Noise in the IEEE Potentials, October - November 1995. Being a dutiful son, he brought it home to his EMC-dad and I read it. It is a great little article giving an overview of the cause and effect of EMC problems. The articles primary thrust is to introduce EMC to college students and encourage them to become EMC engineers. It does a superb job with that, which is really good; because the field needs some newbie's to replace the retirees

The article contains a list of valuable attributes for becoming a successful EMC engineer.  It says, to solve electromagnetic compatibility problems, a person needs:

•        Common sense,

•        The right education,

•        Some experience,

•        A logical diagnostic procedure,

•        Good observation skills,

•        A sense of humor,

•        Persistence, and an

•        Occasional bit of luck.

Doesn’t that sound like the attributes of all successful people?

The article goes on to say that the right education for EMC means a good understanding of:

•        Basic circuit theory,

•        Electromagnetic theory,

•        System concepts,

•        Electrical modeling,

•        Electronic circuits,

•        Transmission lines, and a

•        Little antenna theory.

The article was written by the gang from U of Missouri, Rolla: Van Doren, Hubing, Drewniak, Fei Sha, and Hockanson. I don’t know all these guys, but the ones I know certainly live up to the published list of requirements and they are all very willing to share their experiences and expertise.

I might add to their list of attributes, a comment I made to my boss when he asked me how I knew that the solution I proposed would work. I told him all that was needed to develop a good EMC solution was to think like an electron, i.e. what would I do if I were one. 

EMC is one of those fields (if you pardon the pun) that subtends all disciplines so to the above education list could also be added thermodynamics/heat transfer, shock and vibration, and electrochemistry. The reasoning behind adding these three design areas is that they affect the EMC behavior of a system. 

Heat affects the operation of semiconductor materials -- some more than others. Adding shielding to solve an RF problem creates thermal barriers which then require some method of getting the heat out. If the heat removal is convective and depends on the movement of ambient air then ventilation openings are required. Vent openings are not RF tight so we still have a problem. 

Vibration changes the separation distance between components, PCB’s and subassemblies at the vibration frequency – or in the case of shock at their natural mechanical resonance frequency. The movement changes the capacitive and inductive coupling resulting in vibration induced modulation.  In the vacuum tube days this was called microphonics.    

Corrosion is an electrochemical effect in which the more reactive material is consumed creating non-conductive and/or nonlinear junctions where conductors ought to be. If mixing occurs, a non-linear junction can generate frequencies that aren’t part of the systems operation.  Non-conductive junctions destroy the bonding between shield panels. Given enough time, corrosion can eat through the panels. Corrosion is a major EMC problem, especially for systems located in harsh environments or requiring long term reliability. 

All of these things create potential EMC problems which should be considered during the systems/equipment design. These design efforts can be worked into the schedule during the off times when the EMC engineer is not trying to outrun speeding bullets, stopping powerful locomotives, and leaping over tall buildings. But, they absolutely have to be done before PDR, i.e. the preliminary design review.

- Ron Brewer

Where are the Resumes?

Sometimes it’s a bit of a struggle to come up with something relevant to write. I try to write about topical subjects in EMC or spectrum supportability, but sometimes things are just slow. Right now we’re trying to hire a RF Spectrum engineers for some Navy work and it's just hard to find quality resumes. I started wondering why, and I’ve concluded it’s a combination of high employment in this particular niche business and a lack of training in that particular area.

In the DoD spectrum management (and EMC, by extension) worlds, demand for work is far exceeding supply of capable people to do it. This can be directly attributed to the ongoing spectrum reallocation and repurposing (I LOVE that term!) from a variety of spectral auctions in past years to the current 500MHz Presidential Initiative. When these actions affect government spectrum users (with the DoD being the largest user by far), it spawns an endless supply of analysis work to determine the possible impact to DoD operations. I’m certainly not knocking this work; it needs to be done and done well because billions of dollars are at stake. It also tends to be high paying work done by senior engineers that have certain expertise in spectrum engineering. In addition, mandates in the form of DoD Directives and Instructions related to spectrum use and EMC help to ensure that those technical requirement areas are included when procuring spectrum dependent equipment and other electronic systems. Suffice it to say, there’s no lack of work, both civil service and contractor, in the military communications-electronics and platform integration world. There isn’t always enough funding to do the work properly, but that’s another story!

The rosy employment scenario is one reason it’s hard to fill positions. The other big reason, at least from what I’m seeing right now in resumes, is that a lot of what the military needs regarding spectrum engineering work, just isn’t taught anywhere. Sure, the actual uniformed military trains its spectrum managers, and they do it well, but these guys aren’t the RF engineers needed for all the technical analysis, testing, etc. The current crop of DoD contract spectrum engineers seem to be very senior, started with EEs, maybe working in the non-DoD manufacturing sector and transitioned at some point to this nebulous engineering support world. From there on, there is an awful lot of on-the-job training that turns into experience that is only available in that one person. I could easily name several very senior EM/Spectrum engineers that have spent the majority of careers working on a single program until they are literally irreplaceable. Until they retire, that is! So its nigh on impossible to find a junior or even mid-level spectrum engineer for DoD work with any significant experience in DoD spectrum certification processes. Most of the experienced ones have a lot of telecomm companies on their resumes doing cell phone and wireless internet work. Close, but no cigar!  That leaves a pretty big learning curve for the client to have to consider for most candidates we’ve come up with.

- Brian Farmer