
Electromagnetic compatibility, come and see - sohkamyung
https://helentronica.com/2017/07/23/emc-come-and-see/
======
Animats
In metal shielding cases, slots, not holes, are the big problem. Slots are
long openings and leak on wavelengths up to their length. Small round holes
leak well up in the gigahertz range, where high-power noise is less likely.

A high-powered switching power supply, like this guy is building, is the worst
case. It's inherently a high-power RF emitter. Switching power supplies are
spike generators, so they emit lots of high harmonics. Of course this guy has
an EMI problem.

I've run into this with a device I'm building that's powered from a USB port
and has a switching power supply. The switcher runs around 300KHz, and I can
see noise spikes down to 25ns, which is 40MHz. Keeping it from injecting that
back into the USB power end is tough. If the device is powered from a
computer, that's likely to cause trouble. USB ports usually have current limit
detection, and it's instantaneous current that matters. Exceed 500mA for a
microsecond and the port shuts down until power cycled. That's good; port
shutdown occurs before the computer crashes due to noise on the power bus.
Right now, I'm waiting for some 6.8μH ferrite beads to arrive. 1μH wasn't
enough.

The gold standard for EMI testing is to test outdoors in a big, open field far
from any RF emitters. Even for some IoT devices, an outdoor test range is used
for the final check.[1] Here's one for megawatt-range power gear.[2] For
outdoor testing, the unit under test sits on a wooden or Fiberglas turntable,
all alone, several hundred feet from anything else. Now you can make far-field
measurements. RF anechoic chambers are used for convenience, so you don't have
to go out in the boonies for test runs.

[1]
[http://www.echelon.com/assets/blt2781ab44cb62fe72/PC_BOARD.P...](http://www.echelon.com/assets/blt2781ab44cb62fe72/PC_BOARD.PDF)
[2] [http://incompliancemag.com/article/lessons-learned-from-
the-...](http://incompliancemag.com/article/lessons-learned-from-the-design-
and-construction-of-an-open-area-test-site-oats-and-sound-measurement-
building/)

~~~
dom0
It is actually impressive how wide-band disturbances are. A hard-switches
MOSFET can easily give you disturbances with bandwidths greater than 1 GHz.
Closing a switch on a mains circuit can create high-energy wide-bandwidth
disturbances in the power line, easily coupling everywhere.

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pjc50
Ah yes, EMC. If you've ever opened something up and seen lots of metal fingers
round the edge of the case, or those flat metal cans on the PCB, that's
because it wouldn't pass EMC otherwise. The ferrite rings are another common
trick and you can sometimes see them on power or USB cables.

Multilayer PCBs also help a lot; it's not mentioned in this article but
putting the 100MHz clock on an interior layer with ground planes on the
exposed layers would be a good idea.

You do have to design EMC defence in from the start, rather like defensive
programming. Sometimes people take the other extreme and cover v1 of the board
in extra capacitors, ferrites, TVS diodes etc, then see how many they can
_remove_ while still passing EMC. This may explain unpopulated pads you see in
consumer equipment.

One of the reasons cheap Chinese Alibaba gear is so cheap is ignoring these
requirements. I have a car USB charger that has so much conducted EMI that you
can't listen to the radio with it plugged in.

~~~
dfox
In many high priced low-volume systems you will find another approach: go to
the extreme with EMI supression, pass testing and just leave it as is in the
final product. Perfect examples are 90's DEC and IBM workstations which are
full of completely redundant ferrite beads (I've even seen snap on ferrite
bead on cable for power LED).

Obvious problem with that approach is cost, somewhat non-obvious one is that
there can be cases when unnecessary EMI supression/shielding/whatnot can be
actually detrimental. Part of EMI susceptibility issues on one customer's
system were resolved by replacing 20cm run of shielded twisted pair with
unshielded one, shielding formed parasitic capacitor with the overall chassis
of the product which coupled noise from chassis ground into analog ground.

[Edit: also the fact that your product passes EMC testing says nothing about
the EMC issues it will have in field]

~~~
brians
Well, yeah: the cable shielding should have been bonded to the chassis at both
ends.

~~~
leggomylibro
I thought you should only ground shielded cables on one end to avoid forming a
ground loop.

~~~
dfox
As said in other comments in depends on the nature of the ground. In this case
it should have been bonded to chassis ground and not to the ground plane of
analog board, which wasn't feasible due to mechanical constraints.

In general cable shielding should be connected to shielding ground of devices
on both ends of the cable as long as the devices have purpose-designed
shielding enclosure, which they often don't and in that case it should be
connected at the end with lower impedance ground. Another problem are cables
where the shielding is also used as return path for normal operation (RF
coaxial cables, consumer electronics...) and in that case it obviously needs
to be connected at both ends.

My rule of thumb for this is that shielding should be connected only on the
lower-impedance ground end except when it is not obvious which side that is or
the shield is also return path. Neat solution to the "non-obvious side
problem" is used for IEEE1355/SpaceWire and related interfaces: outer shield
of S/STP cable is connected at both ends while the per-pair shields are
connected at transmitting side for given pair (used cable has isolation
between shields).

~~~
leggomylibro
Very interesting, thanks!

It's funny, part of what drew me to electronics and circuitry as a hobby was
how predictable it is, compared to how sloppy chemistry and biology are. But
when you get into high frequencies and EMI and wireless signals...woof.

It's really amazing how much work goes into making that sort of thing work
like, at all. I have enough trouble getting a simple bit-banged serial or
parallel interface to work without having to worry about occult ley lines.

------
IndrekR
Running an engineering house and dealing with emission/immunity issues all the
time. Also have a small precompliance capability in house. This is a very good
summary to the topic! I really like the pointers to the pitfails like "notice
the vertical ground plane next to the LISN".

One thing to remember: if you see external EMC measures (shielding fingers,
extra ferrites, copper tape, line filters) on the final product you can be
sure the development has been too agile and regulatory compliance an
aferthought. Happens way too often.

One thing with precompliance in the office environment to keep in mind. The
"magic environmental noise substraction" on EMI receivers does not work that
well if noise is stronger than the signal. Just pure statistics at play.

------
aidenn0
No mention of using a spread spectrum clock source. Nearly all modern clock
generator chips have such an option. There is debate as to whether or not it
actually improves EMI, but it definitely helps you beat the regulations.

Also, as a non-hardware person, a ferrite choke on the cable is the very first
thing I think about for EMI just because it's the single most visible EMI
reducing device in anyone's daily life (e.g. that annoying round lump on the
end of your VGA cable)

------
QAPereo
Unbelievable amount of depth, well written, and I learned a lot. The bottom
line is important too: _If the EMC is not taken into consideration from the
beginning of the design, it will be a devil’s job to make it work in the end.
Chances of being able to fix EMI when your product is finished will drop
closely to zero, while the costs of doing so will skyrocket._

First step is a doozy.

~~~
jeroen94704
Yes, you do need to take it into account, but even so, there are no guarantees
you _are_ going to pass the first time around. EMC is as much dark art as it
is engineering, and some things you simply cannot predict up front. We always
tell our customers we do the best we can up front, but to expect EMC issues
nonetheless.

------
jeroen94704
It may be worth adding that EMC compatibility is only one step in getting your
product ready for market introduction. Depending on the category of the
product (consumer, medical, automotive, industrial, high-power, wireless etc
etc) there may be many more tests you need to pass and many more standards you
need to adhere to before you are allowed to sell your product.

------
IndrekR
Henry Ott's /Electromagnetic Compatibility Engineering/ [1] is a really good
book about the topic. Also mentioned in the article.

[1]
[http://www.hottconsultants.com/EMCE_book_files/emce_book.htm...](http://www.hottconsultants.com/EMCE_book_files/emce_book.html)

------
madengr
Great article! Going to forward it to my manager (non EE) who I keep denying
requests for MILSTD-461 testing.

The switching supplies ICs (with integrated FETs; like from Linear Tech, TI,
etc) are noisy where you might not think. Extremely fast rise times on the FET
generates white noise through 2 GHz, even if it's only running at a few 100
kHz. Putting wrong bypassing/filtering parts on can only make it worse as they
will act like an antenna. It may not even show up in a EMC test but enough to
degrade RF receiver sensitivity a few dB.

------
code_duck
Lots of interesting info in here! I have a DSL modem that smells like burning
plastic at all times, so I put it outside in a metal box. I had wondered about
the extent to which it diminishes the signal in the house. This passage
helpfully explains:

"[...] any opening, or slot, on the shield reduces its effectiveness. The slot
behaves as antenna, with the same radiation pattern as a wire of the same
length."

~~~
IndrekR
More like wire of the half length (monopole) with good reference. Slot is
pretty much equal to a dipole and resonates best at double the wavelenth of
its longest axis.

This assumes single narrow and straight slots. Quite fancy radiation patterns,
polarization and resonance points can be implemented with fractal shapes of
slots.

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grandalf
It's amazing how many products cut tiny corners which end up resulting in a
lot more EMI than would otherwise be necessary.

One other phenomenon I've learned about recently is that some switching power
supplies can create all sorts of weird harmonics when in the presence of
strong RF fields. Ideally you can test your device to eliminate such behavior
even if its own generated emissions are to spec already.

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darklajid
I'm a bit confused. Can someone point out my error?

\- device gets tested, fails

\- temporary fixes and "hacks" on top of the product were improvised

\- device passes

How is the CE conformance of the real devices endured? The device under
testing was a one off with lots of tape, right? Do you need to come back at
some point with your final, straight off the manufacturer's line device? If
not, what is certified?

~~~
dfox
CE isn't certification but declaration by the manufacturer that the device
complies with relevant regulations. Testing by independent laboratory only
serves as source of paperwork that limits manufacturers liability should
somebody want to contest that declaration.

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phasetransition
In the not so distant past I was working getting rf pumped light emitter
engines through a variety of EMC tests. All of this rings very familiar.

SMPS I deal with now are much less challenging to certify, by comparison.

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dsjoerg
"gravy silence" :0 fun to read, thanks!

