Planning for EMI testing - where to begin? - Page 1
Forum section: electronics / beginners

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: January 29, 2021, 09:07:59 pm
content: Hi,I'm beginning to develop a commercial version of a scientific instrument that I have built. A key part of the design is a pair of acousto-optic modulators (https://en.wikipedia.org/wiki/Acousto-optic_modulator) that are driven by approx. +32dBm 40MHz continuous fixed amplified sinusoidal signals (1.7W/50 ohm). There will also a variety of digital circuits including 40MHz DDS, FPGA, SBC etc. In addition, there is a helium-neon laser with a traditional HV supply.Attached image shows an early prototype to give you an idea of size and context.I'm looking for pointers to where to start reading about best practices for reducing EMI, detecting it myself etc. My attempts to find out what to do so far have left me overwhelmedThanks.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 30, 2021, 12:02:54 am
content: Well, shoving it all inside a metal box is a good start. As long as that is contiguous, it is an effective shield, keeping inside noise in, and outside noise out. That leaves the penetrations, cables going through the enclosure -- most look to be shielded (coax BNC), which as long as those are well tied to the enclosure, will maintain that shielding. In effect, the cable's shield becomes a topological extension of the enclosure, and when it connects to whatever's at either end of that path, those things in turn also become part of that extension. If the complete path is closed in this way, then there will be effective isolation between the inside and outside volumes defined by that shield.Difficulties arise when the coax cables/connectors aren't perfect, and when shielding cannot be maintained (e.g. one-off control signals on a terminal/wiring block or something).- Connectors aren't always perfect, they can get dirty or corroded, etc. Any gap can let in some signal, slightly coupling the inside and outside. That's tolerable within some bound, whatever your noise floor and emissions limits are. All things that can be tested and measured; well, causing a connector to misbehave, maybe isn't the easiest thing to test; but given a way, yeah.Cables too, aren't ideal. RG-58 for example is notoriously leaky, due to its loose braid construction. Tighter weave, and foil shield, is preferable. Sometimes you can find shielding vs. frequency plots for cables, just select based on that and your need.The challenge often comes in, when multiple ground points form loops. Or projections (e.g. a cable coming out from the enclosure, but the thing it connects to, is not itself grounded) can form stubs or resonators -- to much the same end, but without the obvious DC path for those loop currents to flow. (So, study up on transmission lines, stubs and resonators to spot those sorts of situations better.)The challenge arises, because loops and resonances allow higher currents to flow, given an unlucky source of stimulus coupling into that path. Higher currents running along the outside of the shield, couple that much more easily through a leaky shield.Loops can be broken by minding grounding (don't make DC ground loops in the first place?), adding ferrite beads or common mode chokes to add series impedance at AC, or full circuit isolation (assuming good enough (low capacitance) isolation is possible -- logic and opto-isolators are usually good examples; DC-DC power supplies, YMMV).Loops can also be minimized, by keeping wiring tight to the chassis. Use tie-downs, cable clamps, ties, etc. This doesn't eliminate resonances -- a cable over a ground plane is a variant of microstrip transmission line -- but it reduces loop coupling, say between other cables (when routed separately, not all in one harness), or generally to fields within the enclosure. This also reduces the impedance of the loop, which does mean more current flow for a given induced voltage -- but it also means a given series impedance (ferrite bead, etc.) is more effective.You can also get grounding cable clips: strip back the outer jacket and insert into the clip. Bolt the clip to the enclosure. Now the cable's loop area is, say, halved; or, it's grounded to a penetration through an interior bulkhead, say, without having to add more connectors. Economical!- You'll probably have to consider some tradeoff between DC power or signal routing, and shielded/coax signal routing. The shields obviously(?) take priority, so, all the modules are already grounded through them. So you might not need to wire DC power grounds to them. But this does mean DC flows through the shield, which will create a small offset (ground loop) voltage, and DC is not shielded by cabling, that shield has the same DC resistance with respect to interior or exterior current flow. Maybe DC isn't an issue in your application, but maybe fluctuations in that DC (AC, at low frequencies) is?Unfortunately the only good solution to this, is isolated DC supply to all modules where this is a problem/concern. That way, DC current supplied to the module, is balanced by current returned through its power-ground pin. No shield current. (In principle, you could, like, sense supply current, and draw a parasite current, from ground to ground (really, it would be to a slightly negative rail, since your sink needs some voltage to work), equal to the supply current, thus balancing the shield current -- but what a pain, an isolator module is probably easier. Alas!)And yeah, you still need shielding for connections with high-speed signals. Say you have a module with a coax output,- If you have a bunch of enclosed (shielded/metallic) modules, already grounded through shields like this -- you don't really need to ground them to the enclosure, by themselves. But it is probably still a good idea: this simplifies the network. Instead of one coax cable, coupling to say two other things, and so on between every module in the system -- you'll have effectively single loops, and much less coupling between them all. Instead of an extended, branching network, it's a set of loosely coupled, short loops.It's probably not so much that this has direct, beneficial impact on EMI response -- you're making smaller loops, it could be that they resonate at new, unlucky frequencies. Or because the impedances are lower, the currents are higher, and now maybe the cable or connector shielding isn't good enough! So it's not a slam dunk. It's more that, because the loops/branches are less coupled, it's much,mucheasier to diagnose and treat. Maybe new resonances pop up, but now it's simply whack-a-mole to find the loop(s) that need a ferrite bead, and bam, down it goes.This, by the way, extends completely to PCB layout. Keep local loop currents small (e.g., switching supply inverter and bypass; logic chip supply pins and bypass; etc.). Where currents leave the local area (high speed signal traces), provide solid ground around them, avoid crossing paths, avoid parallel coupling, and stitch grounds around them as needed (especially for 2-layer designs; less important in multilayer where a contiguous ground plane is usually possible). High speed traces need some sort of termination; sometimes this is provided by the trans/ceiver pins, sometimes external resistors are needed. Power supply traces too can resonate, by themselves (as stubs between bypass caps), or between bypass caps (PDN (power distribution network) analysis).Making a simpler network is easier to diagnose and treat.Whereas, without well-damped bypasses, or without adequate ground plane support, who knows what traces are coupling into what -- a random-wired layout with no plane, may bepossible-- but who knows if it's really done right, and good luck trying to fix it if it fails. Worse, it may fail randomly under certain data patterns, say. Spooky! (Fortunately, most of these designs went out of vogue in the early 80s -- coinciding with the move away from slow TTL and metal-gate CMOS, towards modern LVCMOS with sharp, noisy edges and fast clock speeds!)- As for loose wiring, filtering them with respect to the enclosure, where they penetrate it (or anywhere along a shielded shell -- again, certain topological extensions of the enclosure are fine, too), is best practice. This is fine for slow control signals, not much bandwidth is required in the first place. When it is required, consider using coax or differential pairs. The coax in turn can be shielded as above, or diff pairs can be common-mode filtered (using a variety of networks to further improve CM filtering, give or take DM insertion loss (damping / termination resistors), maybe including highpass filtering where tolerable (CT chokes, isolation transformers).So, garden variety industrial signals, just fine -- 0-5 or 10V, 4-20mA, that sort of thing, just slow DC levels, easy. Or switch closures at whatever, 5, 12, 24V, etc. -- just mind the rising/falling edges or arcing if using mechanical contacts, they can get crazy! For digital communications, RS-232 (single ended) is fine when it's slow (e.g. 9600 baud), again, easily filtered. RS-422/485 is preferred when it's fast (differential), or anything already widely available like Ethernet (and with good reason: it's literally transformer coupled!).As for literature, I don't have much to offer; my own insights have covered this subject very well, so unfortunately I am not well read (I know, humblebrag..). Henry Ott does come to mind. Not sure if that'll be... a bit advanced, not introductory enough maybe? Others may have better suggestions.Tim

author: TimNJ
author_rank: Super Contributor
timestamp: January 30, 2021, 01:07:17 am
content: Regarding literature, I unlike other (much smarter) Tim above, need some supplemental reading material to help me make sense of some EMC phenomena. I opened this thread last week:https://www.eevblog.com/forum/projects/emc-book-with-emphasis-on-electromagneticsphysics-fundamentals/I haven’t gotten a chance to read through some of the tables of contents of suggested books, but you may wish to check some of then out.

author: msat
author_rank: Regular Contributor
timestamp: January 30, 2021, 01:22:57 am
content: First off, congrats on the project so far! I do love me some scientific instruments!Are you looking to have this certified by some governing body, or are you just interested in reducing potential local interferences in a lab setting? Is the final product going to be substantially different from your prototype? As T3sl4co1l mentions, you'll want plenty of shielding, and other than some effects it might have on transmission line impedances (to my understanding), I'm not sure if you can have too much of it, so unless you're trying to cut costs as much as possible, when in doubt just shield it. I don't actually have much to add because I know little on the subject, but I'd be surprised if there wasn't a lot of resources online regarding EMI shielding. I'd also take a peek at teardown images of various professional devices that might have some of the kinds of functionality yours does just to get a feel for what might be involved in accomplishing what you're seeking.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: January 30, 2021, 11:28:07 am
content: Quote from: T3sl4co1l on January 30, 2021, 12:02:54 amThank youI must print your reply and keep it as a reference pointAs I suspected, there's a lot of commonsense stuff (once it's been pointed out) and some pretty advanced things, too.My suspicion is that the modulators will be a big emission source. More by luck than judgement, I chose ones that work at 40MHz which ties in nicely with the FCC band used for RC vehicles.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: January 30, 2021, 11:29:00 am
content: Quote from: TimNJ on January 30, 2021, 01:07:17 amThanks, Tim - I'll take a look at your recent thread.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: January 30, 2021, 11:33:40 am
content: Quote from: msat on January 30, 2021, 01:22:57 amI plan to sell it. I've already had a couple of companies ask if something is available. As with most prototypes, it's not ready for someone else to fiddle with - too many quirks - and it's made with whatever I had available to do the job.