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. Here's one for megawatt-range power gear. 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.
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.
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]
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).
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.
Also copper tape is wonderful but it will also cut the hell out of you so wear gloves.
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.
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)
First step is a doozy.
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.
"[...] 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."
This assumes single narrow and straight slots. Quite fancy radiation patterns, polarization and resonance points can be implemented with fractal shapes of slots.
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.
- 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?
SMPS I deal with now are much less challenging to certify, by comparison.