RF shielding  - Page 1
Forum section: electronics / projects

author: hpmaxim
author_rank: Regular Contributor
timestamp: September 27, 2021, 07:51:13 pm
content: I have a design that has sensitive analog circuitry and it will be operating near an aviation band (118-136 MHz) transceiver. Operating the transmitter can affect the circuitry. Someone told me he had solved a similar situation by installing 1nF caps between each pin on the DB25 connector (which feeds power, ground, RS232, switches, etc) and ground.This seems like a good enough solution, and I plan to do the same thing, except... I will have the following things exiting the box: 1-wire, I2C, SPI, USB 2.0. I have two common mode chokes (DLW21HN900SQ2L) and (ECMF02-2AMX6) on the USB, although I was also thinking of using ferrite beads in place of the DLW21HN900SQ2L. The question is, is it safe to put 1nF shunts on the USB, I2C, and SPI (pretty confident the 1-Wire should be safe). Hopefully this would also decrease the chances of the USB affecting the receiver (it does have a habit of doing that if you see my other post).

author: David Hess
author_rank: Super Contributor
timestamp: September 28, 2021, 12:00:56 am
content: I have done the same thing with RS-232, but RS-232 relies on slow edge rates compared to I2C, SPI, and especially USB. I2C and SPI can be slowed down enough to allow easy RFI suppression at bulkheads but USB cannot. They make special RFI suppression blocks for USB, or a more complex CLC network can be used.In an extreme case, maybe the external interfaces would be galvanically isolated from the rest of the circuit to aid RF immunity.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 28, 2021, 05:43:50 am
content: Is this relevant to the other thread (stuff in conduit)?1-wire, I2C and usually SPI should always be entirely within the enclosure.They are not general purpose wired methods.As long as the enclosure is closed and metal, you only need to consider what exits it.I2C can be extended with differential interfaces, and SPI can be extended with logic drivers, isolators or via RS-422 or better. Still, neither is very good, as they are edge-sensitive (CLK), so also noise-sensitive, and lack any kind of correction or compensation for line noise (error detection, CRC etc.), at least intrinsically so.CAN is probably the best bus for compact networks, of modest bandwidth, with graceful tolerance of noise. General purpose devices, other than industrial peripherals, tend not to use it however (e.g. lots of MCUs do come with the onboard hardware protocol, but you still have to add a bus interface chip).Low-speed interfaces, exiting the enclosure, can be filtered as suggested, yes. High speed must be shielded where it exits (more exactly: the enclosure ground should be contiguous with the cable ground, so that it's all still the same enclosure, electromagnetically speaking, just topologically stretching it), so that the signal can be preserved within its shield.Notice that BNC cables attaching to a bulkhead connector, do exactly this. The cable shield is crimped or bolted inside the cable connector housing, which fits snugly inside the bulkhead connector using some spring tabs, and the bulkhead connector is sealed to the enclosure with screws, and preferably an EMI gasket too. The pin inside the enclosure, is an accurate representation of what's going on at the far end of the cable -- at least at the connector anyway, if maybe not further inside the enclosure (as the circuit might interfere with itself too!), and minus any problems with, like, leaky cables and stuff, or at low frequencies there's ground loop too.You can do exactly the same thing with any number of wires, as long as all the same assumptions are made, and if you don't mind that the wires might couple to each other, as in ribbon or multiconductor cable. Which isn't always easy, as you might have for example, a terminal block with a ground connection provided, but this means the length of wires between cable and block have to be unshielded, and yes just a few inches of unshielded wire can make all the difference sometimes. (Example: a shielded industrial bus, maybe it's 0-10V, or 4-20mA, or serial like RS-485 or CAN.)So for the USB case, the power and data pairs are screened separately inside the cable (or at least, they damn well ought to be), so they can do their own things inside their screens, inside the braid shield. And thus you can get good data through the inside, without worrying about noise outside. As long as the shield is tied solidly to circuit ground, anyway.Tim

author: hpmaxim
author_rank: Regular Contributor
timestamp: September 28, 2021, 07:01:50 am
content: It's related but not the same. It is the same environment, but the other is a USB aggressing on a RF transceiver (in RX mode). This is the RF transceiver (in TX mode) aggressing on a different device (which I'm designing).The 1-Wire is inherently external, but again can be filtered. The SPI will likely be internal (within an aluminum box), but it will have a cable which could act as a receiver-- in reality, I wasn't planning to use SPI, but I have the capability, so I figured I'd ask. I am using I2C. The I2C will either be entirely internal with a cable, or it may be external, or it may be both (I'd probably try to avoid the hybrid, because the internal stuff is the sensitive stuff, and the external is less so). The USB is for a GPS receiver, I'm not sure CAN bus GPS receivers are readily available (or temperature sensors).In terms of shielding, I presume the shield should be grounded to the radio chassis (or similarly the same can be said of the USB), but you only want it connected on one side, and I'm not sure how you'd guarantee that in most cases.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 28, 2021, 08:36:28 am
content: This is starting to sound like a slow-motion train wreck...I hope you don't expect to rely on any of these avionics working, at any given point in time or space, or perhaps several other things around them. As long as you have your map, flight computer, radio and transponder handy at all times, fine, but then, what's the point of adding anything? If you can't count on it, it might as well not exist, right?I'm not sure offhand what the FAA's rule is for personal GA craft, or if any electronic standards apply. I'm assuming rather flexible, it's your own vehicle after all. At least to the extent that it doesn't plummet out of the sky. (Airlines of course are especially cautious about those sorts of things, but they have reason to be. And different standards apply to commercial craft. Though I don't know offhand if any relevant rules apply here? That might be more of a manufacturer's mandate -- i.e. "we don't guarantee operation with any unapproved electronic devices onboard", etc.)Industrial and instrument buses are well known and used, for good reason. They must be highly predictable, and reliable, in harsh environments. USB is not. SPI is not. I2C is definitely not. These are interfaces designed for use entirely within a single circuit board, or in the case of USB, a shielded environment; and that at commercial EM levels. They can be extended over cables, but the effort quickly rises from modest to heroic as the environment gets rougher and the demands get stricter.Maybe I'm fishing for excuses here, imagining the worst for something that's actually just some dumb passive logger or something. I don't know. I haven't heard plans either way, and you'll forgive me for preferring to air on the side of caution until I hear otherwise.Shields must be (AC) grounded to whatever circuits, AT BOTH ENDS, otherwise they are as good as invisible. Sometimes there are limitations on DC or low-frequency AC (including mains (ground loop), and also audio) where one or more ends must be allowed to float, whether in a modest common-mode range, or by complete isolation; this is okay, it can be done both ways at the same time. The key is, for a receiver for example, to make a proper differential measurement of the transmission line's output, then to bypass or terminate that input with respect to circuit ground, to also limit common mode error at high frequency.Where a transmission line is any cable -- a collection of one or more wires with respect to one or more reference or "ground" wires -- with consistent cross-section, thus having a nominal impedance, velocity factor, coupling between wires if multiple, etc.Where a circuit is grounded, galvanically (at DC/LF), is irrelevant: that is only a safety issue, typically for where fault currents go in event of failed supply connections. Galvanic grounding can be achieved independently of, or incidental to, the ordinary wiring of power, ground and signal to modules in a system.Tim

author: David Hess
author_rank: Super Contributor
timestamp: September 28, 2021, 12:45:08 pm
content: Quote from: T3sl4co1l on September 28, 2021, 08:36:28 amI have seen all kinds of weird behavior with PS/2 (basically I2C) keyboard ports in EMI hostile industrial environments. Some keyboards could not be used without modifications. I do not know if Apple had the same problem with their Apple Desktop Bus.One example of a serial peripheral interface (not SPI) which handled hostile EMI well was MIDI which uses optical isolation at both ends.

author: hpmaxim
author_rank: Regular Contributor
timestamp: September 28, 2021, 02:45:42 pm
content: T3sl4co1l,I definitely appreciate the feedback, and yes, you're starting to sound a bit like a drama queen. First, what I'm doing is legal. Second, it's not as dramatic as you are making it out to be. An instrument which is not critical for safety of flight can be temporarily adversely affected while talking on the radio which is done sporadically. The other problem (which in my mind is more serious) is that the squelch function on the radio doesn't work, which is... annoying. But, now on to the technical issues.At present, there is no SPI or I2C that is exiting the aluminum case with the electronics on it, it is entirely on a single board, although there is a 10-pin ribbon cable (perhaps 10 cm long) that runs between two boards in the case which does contain I2C. I would like to be able to use an external temperature or temperature/humidity sensor, of which the best options appear to be I2C based. Presently, I'm using a 1-Wire based DS18B20 for temperature sensing, which is obviously external.There is USB going to the box, but not to the board itself. I am redesigning the board and will include a USB port. The intention is to hook up that board to a USB GPS dongle. The problem is, frankly, USB is ubiquitous. Pretty much all Single Board Computers and devices are communicating over USB so it is fairly desirable to communicate via USB -- because everything else does.Here is my confusion, someone previously (on the other post) stated that you must not allow current to travel through the shield only the internal ground. If the shield is grounded on one end only, there will not be a current path, but if it's grounded on both ends (whether AC coupled or not) why would current prefer the inner ground wire to the shield. The shield is almost certainly lower resistance and lower inductance and will take the majority of the current. Presumably whatever current goes through a data pin, must return through the ground, and hence you are almost guaranteeing current flowing through the shield. The only potential exception is a differential swing output (like USB) where current presumably makes a circuit between D+ and D-.If you can point me to a good tutorial on this topic, that might be very helpful.