Can placement of frame ground to ground connections cause a resonance?

Yes.

Much is possible with microwave frequencies. You could also introduce absorbers, caps of say 1nF with 50 Ohms in series. While it isolates DC, it poses 50 Ohms at frequencies above say 100MHz.

A standard is also to use metal cases for the pcb.

Rene

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There should be no current, AC or DC, between digital ground and frame ground. To get the digital noise out of the frame ground, look at loops. Where does the power supply current flow, and where can you short the AC component to tighten the loop?

Good Luck! Rich

Yes it can. You don't even really need the system to resonate for it to radiate. All you have to do is cause a large enough AC current to flow in the case.

So you've created something like a 12" long folded dipole with the drive point on top. Do you have metal blocking the 16" edges of this shape?

I'd suggest that you try isolating one 12 inch side of the PCB. If the radiation decreases greatly, this is the way it is getting out.

The ideal situation is to have a PCB in completel metal box with all connections to the outside world in one place and the PCB grounded right at that place. This prevents currents from flowing in the case.

[...]

You never said what the frequency is.

Remember that the length of a wavelength in free space is not equal to its length in some funny structure with FR4 and sheet metal. The wave length in the product will be shorter than the wavelength in air.

There is another reason to add a middle mounting point. If this thing has to withstand vibration, you want another mounting point not exactly in the middle.

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kensmith@rahul.net   forging knowledge

Ken,

Thanks for your insights, a few follow ups below.

Understood about the 'large' enough currents. My basic question though is that given the X,Y locations of each of the tie points do these points define resonant frequencies that would cause the board to radiate more at and around those frequencies than at others?

Not sure what you mean by 'metal blocking'. The metal comes up the two

16" sides with five mounting screws on each side. Across the top and bottom of the board (i.e. the 12" direction) there is no metal.

Given the description above I think I already am isolated, or missing on what you're suggesting.

Correct, speed of light divided by sqrt(Er) for the FR4 material. As an example, the (X,Y) coordinates of two of the 32 points are (2.035",

15.515") and (8.675", 0.245") which results in a straight line distance of 16.65" or 42.3 cm. Ballparking Er as 4.1 then the speed would be 3E10/sqrt(4.1) = 1.48 E10 m/s. Given that, I compute the wavelength that corresponds to 42.3 cm as being 350 MHz. Since the tie points are the nodes, it would seem to be that a 350/2 = 175 MHz standing wave resonance could possibly exist.

Anyway, that was how I did the calculations, and when I sort the list of 512 such frequencies that resulted from the placement of the 32 tie points, 170.8 MHz is the lowest frequency and the list extended quite a ways up. While I didn't have any frequencies that exactly matched the

4th harmonic of the oscillator I'm presuming that this isn't a terribly discriminating filter and that each of those 512 frequencies really moosh out into a range that probably overlaps with the next one somewhat.

My oscillator was 44.2 MHz, the 4th harmonic being 176.8 MHz, the two nearest possibly resonant frequencies from my calculations are 175.15 and 177.20 MHz. Given that there are approximations and assumptions in the calculations I wouldn't expect to find a point that exactly matches. I just found it to be an odd coincidence (and therefore maybe not actually a coincidence) that the 3rd harmonic at 132.6 did not cause problems (it's way below 170 MHz) and yet the 4th harmonic for some reason did. So if this is not just a coincidence, then what is the mechanism in play here? Why am I getting AC current at these frequencies in my 'case' (i.e. the frame ground layers on the PCB that tie to the physical metal frame)?

Actually the board is rather thick (.125") for mechanical stiffness, it is in a somewhat vibrating system (i.e. a stationary machine that has motors for moving stuff around). This board has 11 connectors of various species all oriented along one of the 16" sides, that need to be removable for servicing.

The 'middle point' connection that I was talking about also would be a connection to the top and bottom PCB layers not to the physical metal frame since the physical metal frame only runs along the two sides. These two PCB layers spread over the PCB surface and make contact with the mounting holes and then to the metal frame as well as pressing against the metal along the two sides.

The metal box is not all that ideal, it's an assembly problem, potential airflow problem and generally speaking is more of a means to contain emissions that have not been properly controlled in the first place. In any case, the top/bottom frame ground PCB layers that tie to the physical metal frame where it is available are an attempt to form such a Faraday cage. It's not quite the perfect cage since there are obviously cut outs for where the parts exist but since the parts are small so are those cutouts. All in all though not that much unlike the commercial cages that one can purchase for such applications.

KJ

Rene,

I'm kinda missing how increasing the impedance by adding a fixed series resistance would help. Seems like that would make the problem worse.

Generally speaking, the 'standard metal case' contains what was not controlled properly in the first place. My top and bottom PCB layers being copper should make for a reasonably good Faraday cage for containment.

I'm trying to understand the mechanism that must exist that is allowing the higher frequencies AC to possibly be flowing in these layers and radiated out. At least that's my hypothesis since clamping the ferrite cores around the I/O cables did the trick and most (but not all) of these connectors were shielded and tied to frame ground where they connected to the board. Sniffing around on the board itself didn't light up anything to indicate that there were any of the obvious things going on (i.e. signals crossing plane cuts, gross impedance mismatch, etc). I'm surmising that I'm dumping high frequency AC on to 'frame ground' and trying to understand why in this case and does it have to with the relatively large area and the physical distance between 'frame ground to digital ground' tie points.

KJ

[....]

Yes, the placement of the connections can effect the radiation. It increases it at frequencies where the RF signal can find a resonant circuit with one part exposed to the outside. Consider this end view:

=================================== Sheet metal ! ! > So you've created something like a 12" long folded dipole with the drive

So it is like by ASCII art above and if you look at it edge on, you can see the hole between the PCB and the chassis without looking through metal. RF works a lot like light. It can "shine" out this opening. If you plug the hole with something conductive (eg: sticky copper tape), you may find the RF doesn't get out as much.

Doesn't the PCB electrically connect the mounting screws on the two sides, together? I thought it did. I suggest you have it not do this.

That isn't the only mode. Consider the gap between the PCB and the sheet metal. That will have its own speed of light. If will form yet another bunch of resonances.

[...]

Also remember that the calculation was based on estimates and simplifications. You have components on the PCB and the actual bit of FR4 vs the ideal one and the inductance of the mounting holes and etc all trying to make nonsense of any calculations. As such, I'd take any number that comes out as close as a suspect and try to defeat that resonance to see if it is the real trouble maker.

[...]

If it was me, I'd just assume that Maxwell hates me.

[....]

So there is no sheet metal near the surface of the PCB then?

Trained ants :>

Once you make some RF, it is "emitted". If you can't prevent its creation you have to stop it somehow. The first line of defence in the layout of the PCB. You'd like it to trap the RF on the conductors it is supposed to flow. Adding things that convert the RF to heat can also be helpful. "Source termination" on fast signals lowers the amount of RF put onto the trace and thus the amount that can get away.

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kensmith@rahul.net   forging knowledge

It is a coincidence, the 4th harmonic has to 80dB down on even the signal line. How and where are you measuring this so-called radiation?