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I'm designing a carrier board for an INS module. The carrier board is small (64 mm x 92 mm). Due to providing isolated GPIOs, I had to use 4 isolated power supplies on this small area, one for the main power and the others to power the digital isolators I have on the board.

A six-layer stack up (high speed SIG/GND/low speed SIG/PWR/GND/high speed SIG) is used for PCB layout.

There are Ethernet 100BASE-T, CAN BUS, and 1MPS RS232 lines on the board. The routing room was so small that I couldn't efficiently design the return path, so I decided to use a seamless internal ground plane on the entire board and tied it to the main signal ground.

I have provided a diagram of the system for clarification:

Carrier board diagram

The whole board will be in an aluminum enclosure and the signal ground will be attached to it through some metal stand-offs.

I have already built this carrier board and it is working as expected, but I need to get EMC certification on this.

Will this PCB layout create any EMI or SI issues?

I appreciate your thoughts and opinions on the most efficient way of routing such PCBs.


The standard is EN 50121-3-2. Galvanic isolation is required for the GPIOs, and all used DC/DC converters are certified. Here is the layout.

The cable is shielded, but the connectors are not; at the moment, the cable shield is floated. Although the metal shell of the connector is attached to the enclosure.

An anodized aluminum enclosure was picked for some mechanical reasons.

This device is installed on the roof where no other devices are mounted or used, and it is considered a non-rail device.

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  • \$\begingroup\$ Hi and welcome! If you could fill in several questions, we should be able to answer this-- How much isolation is required? What kind of noise might this module be exposed to (i.e. how much isolation at what frequencies)? What standard(s) are you certifying to? What does the layout look like? \$\endgroup\$ Commented Jan 3, 2023 at 2:08
  • \$\begingroup\$ I suspect anything in an aluminium enclosure is going to do better on EMI tests than a lot of consumer products :) \$\endgroup\$ Commented Jan 3, 2023 at 3:34
  • \$\begingroup\$ It looks well done to me. Make sure that your isolated sections are still RF-bonded to their sourroundings, so they don't have uncontrolled common-mode voltages. caps across the converter isolation, and I/O caps to the chassis. Serially filtering the IOs doesn't hurt if the protocol allows it. Return currents only in neighboring planes. But answer @TimWilliams questions for better advice. \$\endgroup\$
    – tobalt
    Commented Jan 3, 2023 at 5:00
  • \$\begingroup\$ Is the unshielded cable part of the product/DUT? If so, then during EMC testing you should be able to reason and say that GND1, GND2 and GND3 should be grounded down externally, outside the scope of the DUT. \$\endgroup\$
    – Lundin
    Commented Jan 3, 2023 at 10:11
  • \$\begingroup\$ @TimWilliams, no significant noise in the environment the device is used, however EMC and passing EMI tests is the goal here. The standard is EN 50121-3-2. Galvanic isolation is required for the GPIOs, and all used DC/DC converters are certified. Here is the layout \$\endgroup\$
    – M.A.T
    Commented Jan 3, 2023 at 21:41

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Keep in mind, EN 50121-3-2 specifies immunity requirements, including 2kV EFT, 10V conducted (RMS, 0.15-80 MHz, 80% AM 1kHz), and up to 20V/m radiated (RMS, 80-1000MHz, 80% AM 1kHz). Even if your box isn't located in a normally troublesome location, it may still be subject to noise from occasional or proximate sources, like overhead wires (catenary arcing is powerfully noisy), or antennas in infrastructure (railside or station) or others. And who knows where the cables are routed, and by what.

The conducted emissions limits are very lax (99/93 dBµV (QP) for 150-500kHz / 0.5-30MHz respectively, so you're not likely to have problems there, following basic (appnote level) advice at least.

Radiated emissions is somewhat stricter, at 40/47 dBµV/m (QP, 10m) for 30-230 / 230-1000MHz respectively. This is comparable to commercial levels (e.g. CISPR 22 class A), so requires at least as much care as that kind of equipment.

I might be most worried about the IMU connector overlapping the area with the isolated signals. Whether the ground plane solves that, or if the CM capacitance is low or high enough to deal with it, I don't know. The fact that there's several inches of overlap (a resonant length for the upper test frequencies), and that radiated immunity could carry significant voltage (~10s of V) along these paths, is a matter of concern. Perhaps ferrite beads, on as many signals as you can afford (including power and ground, but not shield; and CMCs on differential pairs -- I see CAN there already), might be prudent.

The metal enclosure does give the option that, if the cable shields can be well bonded to it (doesn't have to be galvanically, but should involve very little -- preferably zero -- lead length on the shield connection, and preferably a wide connection made with multiple capacitors in parallel), most of the external noise can be excluded, and radiation needn't manifest as radiation as such, but will only be carried inside the box on those connections (hence the value in solidly [RF-]grounded shields), and can be filtered directly at the connectors, keeping noise away from the bulk of the circuit. In other words, using the enclosure as a ground plane, and all external connections act as ports with respect to it.

Remember that shielding is only as good as the weakest link in the chain. If the cable shields will not be grounded at both ends, then any incident energy upon the cable will find its way inside the shield and thus onto the signals within; it's almost as bad as nothing at all (unshielded).

Likewise, any gaps in the enclosure allow RF energy through; gaps of just an inch or two can transmit low ~GHz, as well as significant amounts (10s to 100s of V) of ESD when struck in the middle of the gap.

Aluminum can be difficult or inconsistent here, as painted and anodized enclosures are common, which will only make contact around the screw holes, if at all. Bare mating surfaces are preferred, or even better, nickel plating (not environmentally friendly, though), and EMI spring stock or gaskets filling the gap is best. There may be compromises including IP6x sealing (EMI-shielding IP65+ rated gasket material is available, remarkably enough).

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  • \$\begingroup\$ Thanks for your thoughts and advice on this. Regarding the IMU connector, I am worried too, but I cannot do much about it due to limited room for routing the high-speed signals, although I have tried to separate the vertically and placed two ground planes in between(L2&L5). \$\endgroup\$
    – M.A.T
    Commented Jan 6, 2023 at 0:51
  • \$\begingroup\$ What high speed signals? It sounds like Ethernet is the highest, which is only low 100s MHz. Rotating the IMU is probably fine. \$\endgroup\$ Commented Jan 6, 2023 at 0:58
  • \$\begingroup\$ Yes, it is. I cannot rotate the IMU, because of mechanical reason. This is the only possible way to put it on the PCB unfortunately. \$\endgroup\$
    – M.A.T
    Commented Jan 6, 2023 at 2:27
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    \$\begingroup\$ No, that's fine. \$\endgroup\$ Commented Jan 13, 2023 at 10:37
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    \$\begingroup\$ Doubtful you would find differences with the EMI stuff, maybe if you have a lot of fields near a surface, or at frequencies high enough to resonate in cavity modes. On-board solutions should be more than adequate here. As for RF bonding, imagine a bulkhead coax connector, but instead of bolting it to the bulkhead, a high-capacitance gasket were used. More practically, we might mount a coax/shielded connector on a PCB, and join shield to circuit GND with multiple capacitors in parallel. \$\endgroup\$ Commented Jan 25, 2023 at 0:26

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