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EMC Design Reviews: Some Lessons from Our Design Review Career (to date!)

1508_F3_coverOne of our favorite EMC sayings is “an ounce of prevention is worth a pound of shielding.” Like vaccinations for children, an EMC design review can prevent serious problems later, such as a failed EMC test. Or worse, a failed product in the field.

When we began full time EMC consulting in 1987, most of our clients were already in EMC trouble. Within a year or two, however, we started doing EMC design reviews on new projects for many of those same clients. Most realized it was far less painful (and costly) to design for EMC in the first place, rather than to simply test for EMC at the end of a project.

Since that time, we have done hundreds of design reviews for a wide range of clients with very positive results. In our experience, a little effort during design goes a long way towards EMC success. We’ve discussed this several times in our newsletter (Kimmel Gerke Bullets). Here are some general thoughts and comments (both old and new) on EMC design reviews.

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So, just what is an EMC design review?

First, an EMC design review is NOT a full blown review. Rather, it focuses on specific EMC issues. It does not address other issues like reliability, thermal, power, weight, etc. These are better left for more formal reviews.

Depending on the product, and EMC design review can address circuit boards issues, mechanical issues, or systems-level issues. At the same time, it addresses requirements (regulations and/or threats), constraints (costs, volumes, etc.) and design strategies.

We prefer an interactive approach to EMC design reviews. Rather than dictate directions, we like to get the design team (both electrical and mechanical) actively involved. Together, we identify and assess the risks, and discuss the design options and tradeoffs. We understand EMC issues, while our clients understand their products and constraints. Together, we can come up with practical EMC solutions.

What is the best time for an EMC design review?

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For most projects, an ideal time for an EMC design review is during the initial electrical and mechanical design phases. For, circuit boards, a good time is when the board layout is complete, and the first artwork is ready. At this stage, the design is usually solid enough to make recommendations, but fluid enough to make changes.

In some cases, you may want to consider EMC issues in the early concept stage. This is particularly helpful when dealing with packaging issues, such as cabling and shielding. This can extend to circuit boards when considering connector placement, or bus or I/O design.

Most EMC reviews can be done in a day or less. For simple systems or a single circuit board, even a few hours may be adequate. But don’t do it all by yourself. Grab a colleague and go over the issues together. Often the act of explaining the design to someone else uncovers unexpected issues. Of course, your colleague may have unexpected suggestions too.

Four general tasks

Before jumping into the design assessments, there are four preliminary tasks that must be done, as follows:

1. Identify and assess the EMC threats—Typical threats include radio frequency (RF) energy from nearby transmitters, electrostatic discharge (ESD) from humans or other sources, power disturbances, and conducted/radiated emissions (which may adversely affect other electronics).

These are often specified as test requirements, but you may need to modify them based on the actual anticipated environment. For example, at one review for a medical device, we asked if it would be used in ambulances, both land and air. When the answer was yes, the original office/home requirements were deemed inadequate.

Incidentally, that manufacturer ultimately developed two product lines. The ambulance product was hardened to higher levels than the home/office products and sold at a premium. What could have resulted in some sticky EMC issues yielded additional profits while preemptively solving unexpected customer problems (an example of good engineering!).

2. Identify the key circuits or assemblies that affect or are affected by these threats—Digital circuits (particularly resets and control circuits) are very vulnerable to spikes and transients, and analog circuits are very vulnerable to RF. Digital clocks (and other highly repetitive sources) are rich sources of radiated emissions. Power circuits are vulnerable to power disturbances, and can also contribute to conducted emissions.

3. Identify other design constraints that may affect EMC design decisions—These include costs, volumes, weight, space, and the cost of noncompliance (CONC). Incidentally, in very cost sensitive situations, we often advocate designing in place holders (such as pads for capacitors) that can be populated later as needed with EMC components. Don’t overlook the latter – as engineers, we always need a fallback plan.

4. Identify the appropriate EMC design features—This is where the design fun begins. The circuit board is an ideal place to start. After all, all EMC problems ultimately begin and end at a circuit. Of course, if you don’t design the boards, you will work at the system level. That may include mechanical issues (shielding) along with cables, connectors, power, and grounding. Many (but not all) defense projects fall into the latter category.

Here are some comments on both levels. At the board level (inside the box) we like to work from the inside-out. We look at both the circuit schematic and the board construction, along with the board interfaces (power/signal/grounding.) At the systems level (outside the box) we like to work from the outside-in. We look at the cables/connectors, enclosure construction, and systems interfaces (once again, power/signal/grounding.)

Printed Circuit Board Reviews

The majority of our past EMC design reviews were at the board level, so we will start there. We divide board reviews into two parts: a schematic (circuit) review, and a construction (artwork) review.

Here are ten key points to check on your circuit boards. Much of this is from a short tutorial we gave several years ago. Feel free to use it as a design review check list.

1. Clock circuits—These are the primary sources of high frequency radiated emissions. Also, check any clock-like circuits that are highly repetitive. Some memory control and bus control signals fall into this category.

Design recommendations include:

  • High frequency decoupling at Vcc (series ferrites provide even more protection)
  • Series resistors in clock outputs (10-33 Ohms typical)
  • Crystals or resonators located adjacent to the oscillator

2. Reset/interrupt/control circuits—Resets are very vulnerable to ESD, EFT, and transients. Interrupts and control (read/write) are also vulnerable. External reset lines to mechanical switches are extremely vulnerable.

Design recommendations include:

  • High frequency decoupling of Reset Vcc, reference, and output with trace lengths over two inches of trace length. Consider series ferrites for additional protection.
  • Similar fixes as needed for vulnerable interrupt/control circuits

3. Analog circuits—Very vulnerable to RF energy. In addition, parasitic oscillations may cause unwanted radiated emissions.

Design recommendations include:

  • High frequency decoupling of Vcc
  • High frequency filtering of both circuit inputs and outputs ( 1000 pF typical)
  • Similar protection at all analog sensors

4. Voltage regulators—Like analog circuits, these are also vulnerable to RF. Due to increasing component bandwidth, parasitic oscillations are now common in VHF/UHF frequency ranges.

Design recommendations include:

  • High frequency decoupling of Vcc
  • High frequency decoupling directly at input and output pins to chip neutral pin (1000 pF typical). Highly recommended to prevent those pesky parasitic oscillations.

5. RF transmitters and receivers—These circuits bring a whole new set of potential EMC problems. Onboard receivers can be jammed or desensed by digital harmonics (GPS receivers are extremely vulnerable). On board transmitters can jam nearby analog circuits. Multiple radios may result in intermodulation and cross modulation problems.

Design recommendations include:

  • Protect receiver inputs (may need special design).
  • Internal shielding of RF modules
  • Clock management (avoid harmonics on receiver inputs).
  • Check antenna locations and cable routing.
  • DSP or other software techniques may also be necessary.

6. Board stackup—Good board construction critical for good EMI control. Fortunately, most of these fixes are free.

Design recommendations include:

  • Keep every signal layer next to an adjacent plane.
  • Keep respective power/ground planes adjacent.
  • Maintain a symmetrical stackup.
  • Consider outer buried planes.

7. Split planes—Traces crossing cuts and mismatched planes can seriously negate even the best EMI controls on the board. We’ve seen 10x improvements (20 dB) after fixing these problems. So it is in your best interest to prevent them in the first place.

Design recommendations include:

  • Check high speed traces with “over and back” routing across cuts in adjacent planes.
  • Note that low speed traces across cuts can also cause problems if high frequency energy sneaks onto these traces.
  • Always align the power/ground planes as mirror images.

8. Floor planning and routing—Random placing of components, and random trace routing can result in EMC problems. Given the opportunity, autorouters often route to maximize EMI (a variation on Murphy’s Law).

Design recommendations include:

  • Segregate components according to frequency. Group digital, audio, power, and RF circuits together, rather than sprinkling them all over the board. Separate the traces too.
  • Pay attention to routing of critical traces (clocks, resets, control lines).
  • Avoid placing critical circuits near I/O ports.
  • Consider manually routing critical traces for better EMC control.

9. Protect the periphery—Since power and I/O connect to the outside world, they need special attention. This begins at the board level, and may also be applied at the systems level.

Design recommendations include:

  • Filters and transient protection as needed. As a minimum, place 0.01 uF capacitors across all power inputs.
  • Pay attention to ESD protection on external I/O lines.

10. Grounding—Another issue that needs to be addressed at both the circuit board and systems levels. This could be the subject of a whole other article, or even a book. But there are a number of things to check at the board level.

Design recommendations include:

  • Consider separate ground paths for digital, analog, and power.
  • Multi-point ground connections are preferred for high speed digital (and RF) circuits.
  • Single point ground connections are preferred for low level/low frequency analog circuits.
  • Hybrid grounds (caps and inductors) can be used for mixed technologies.
  • Note that additional grounding constraints may apply in harsh environments.
  • NEVER violate safety grounding to solve an EMC problem.

Systems Level Reviews

At this level, we often work from the outside-in, focusing on mechanical construction, interfaces (both power and signal), and system grounding. Much of this assumes shielded enclosures. For unshielded equipment, the EMC design goals must be met at the board level.

1. Mechanical—At this level we are interested in the EMC shielding performance. As such, we look at the materials, mechanical joints (seams/covers/ventilation) and discontinuities (penetrations and openings).

Design recommendations include:

  • Check the material. Thin conductive coatings (including foils, paints, and plating) are effective for high frequencies, but often inadequate for power frequency magnetic fields. In the latter, permeable material (steel or mu-metals) may be needed.
  • Check the discontinuities. Any seam over two inches is problematic for ESD or RF above 300 MHz. At 1 GHz, even 1/2 inch can be pretty leaky. You may need to fill the seams with conductive gaskets. See the next paragraphs for penetrations due to cables.

2. Interfaces—At this level, we examine both the signal and power and their connections. This includes internal cable placement and routing.

Design recommendations include:

  • For signal interfaces, use bulkhead connectors for shielded cables. Filtered connectors are even better. No holes. Passing cables through a hole in the enclosure can completely destroy EMC shielding at high frequencies. We’ve seen it happen too many times.
  • For power interfaces, bulkhead filters are preferred at the point of penetration. If internal modular filters are used, they must be placed as close to the penetration as possible.
  • External cables—If possible, examine the mating cables. Connectors are a key area of concern. Full circumferential bonding of the cable to the connectors is preferred. If it leaks out the external cables or connectors, all your efforts at the box level are for naught.

3. System grounding—Most EMC grounding issues are addressed inside the box at the circuit level. The main concern here is not violating system grounding guidelines, particularly for safety grounding.

Is all this effort worth it? Absolutely!

In one case, we had a design team “dancing in the lab” when they passed their EMC tests on the first try. Never having tasted that kind of success, they became firm believers in EMC design reviews. They also beat their competition to market by a month, which pleased their management to no end. After all, engineering is also about economics.

In another case, we supported a defense client that had already implemented a formal EMC design review process, with numerous “EMC check points” throughout the design. We reviewed several dozen boards as part of a multi-year contract. Their chief EMC engineer revealed they rarely failed their EMC tests – and when they did, they were easy to fix. Each check point review typically lasted a couple of hours.

After an EMC review, we usually document the findings in a memo. You can and should do this too, often in an hour or two. But keep it simple. Grab a buddy—two sets of eyes are better than one. Or you can always call your favorite EMC consultant for help. Either way, the payback is there. Remember, an ounce of prevention… INC

author_gerke-darylDaryl Gerke, PE, is the surviving partner of Kimmel Gerke Associates, Ltd. Sadly, Bill Kimmel, his good friend and business partner of almost 40 years, passed away in April after a short battle with pancreatic cancer. This article is dedicated to Bill.

Kimmel Gerke Associates has been addressing EMC issues on a full time basis since 1987. Bill and Daryl solved or prevented hundreds of EMC problems across a wide range of industries—computers, industrial controls, medical devices, vehicular electronics, defense systems, telecomm, facilities, and more. They also trained over 10,000 designers through their public and in-house EMC courses.

You can reach Daryl at dgerke@emiguru.com or at www.emiguru.com. Curious about consulting? Visit Daryl’s personal blog at www.jumptoconsulting.com where you can also learn more of the Kimmel Gerke story.

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