Reducing EMI

The important thing to keep in mind when thinking about radiated emissions is that radiation comes from current loops.

In the crudest example, when a logic output changes state, the chip driving the output has to deliver a chunk of charge into the track carrying the signal to charge up its stray capacitance to ground along the trace, and at every input being driven by this signal along the track. For a 0-to-1 transition this means that current flows into the driving chip at the Vcc pin (or pins) and flows out of the ground plane and out of the Vdd pin (or pins) at each of the receiving chips. Ideally, the current flowing into the Vcc connection at the driver chip comes almost entirely out of the by-pass capacitor (or capacitors) at the driving chip (a ferrite bead can help here), so the current flowing out of the ground plane and the Vdd pins on the receiving chip eventually has to get back to the Vdd connection at the by-pass capacitor at the driving chip. If you look at the area included in these current loops, you have some idea of the size of the transmitting antenna that is radiating the consequent emission.

If the radiating loop is sitting on top of a continuous ground plane, the current circulating through the signal track and back through the ground plane will induce a very nearly equal and opposite curent in the ground plane, but the loop within the ground plane has a certain resistance, so the compensation can't be perfect.

Differential signal involve more or less equal currents moving in opposite directions, and if the tracks carrying the signals run close together, the radiating currents very nearly cancel, which helps a lot.

Differential signals running along twisted pairs radiate in opposite directions over every half twist in the two wires, which can further minimise the nett radiation.

In that sense twisted pair cable is better than flat cable.

Obviously, screened cable is better than unscreened cable, and cable which is only screened by wire braid - which only offers about 95% screening - isn't a good as cable screen by wire braid on top of of a wound layer of aluminised plastic (which offers 100% screening, but has a relatively high surface resistance).

Hope this helps.

-- Bill Sloman, Nijmegen

Thanks for the theory. I'll have to read it over a few more times to wrap my head around it.

AFAIK, all the signals are run over a continuous ground plane, and the USB signals are routed parallel to each other. I have followed USB design guidelines as far as I can, EXCEPT the EMI/ESD portion, which recommends a common mode choke. That's why I'm wondering if that would help.

Since this is a storage device, there's also an IDE/ATA component, which is translated to and from USB. As such, I'm wondering if the ATA portion also requires ferride beads for the power lines and common mode chokes for the signal lines. From my research only serial signals need these shokes(such as SATA/HDMI). I haven't found any for PATA, and I already have the necessary terminating resistors for PATA, so I guess that should also take care of EMI issues on top of SI issues...

On Jul 5, 12:00 am, galapogos wrote: [....]

Check the length of the wire or trace that connects the ground of the USB cable. The ideal is a very low impedance on this line.

Is the box metal? To radiate, you need either a current flowing in a loop or a voltage difference between two exposed surfaces. Connecting the housing to the ground of the USB via the shortest thickest path you can helps on the electrostatic issues.

If the box has other cables, such as a power line, filter the heck out of those connections. Ideally, those connections should be right next to the USB connection so that the path for grounds can be near zero length. You want to have the last item in the filter on them be the lossy inductor so that any RF energy trying to get out on that path gets eaten up.

A common mistake is to allow the return currents of your circuits to flow through the case or out one cable and back in another.

Also: It is better not to make the RF in the first place. Take a look at your design and see if you can source terminate clock signals, slow down edges, reduce how many line carry the fast signal and etc.

Th USB connector ground on my device is directly connected to the GND plane with a via, so that's the shortest length possible.

The box is partiallly metal(about 40-50% steel, 50-60% plastic). The entire box is not grounded nor are they connected to any part of the circuit, but I will keep this in mind. Unfortunately in my current design the platform that the PCB is mounted on is plastic, so it would be hard to connect the metallic part to the ground plane.

Sorry, I don't get what you mean by the last sentence. The box does have another FPC with a low current power line that is just supplied by Vcc. By filtering do you mean placing a ferrite bead in series to this? Also, this is a 2 PCB design that is connected by pin headers, with Vusb and gnd flowing between the 2 PCBs.

Since the case isn't connected, I don't think this is possible? As for the cable part, not sure if I'm doing that.

Just the one via?

Dave.

On Jul 5, 6:45 pm, galapogos wrote: [... Passing FCC limits ....]

Yes.

Imagine the cables laying like this:

----- PC =============! BOX !==================== Power supply -----

See how that looks like a dipole antenna. If you put something lossy at the power supply connection of the box, it will eat up some of the RF before it gets out to the cable.

How do the signals go with respect to the grounds. You want the grounds to wrap all around everything that can radiate. Using extra ground pins on the header helps.

Chances are you are to some degree. It is hard to avoid. At over

10MHz, the details of things like power supplied stop mattering. They are just big lumpy things that provide a path to the mains wires. The same is true of a PC. You have a loop fronm the box to the mains through the mains to the PC and back via the USB cable. You want losses somewhere in that loop.

Oh, you mean external cables...I thought you meant internally. The FPC cable is an internal cable and not to/from a power supply. The box is powered by the PCB via the USB cables, but there is also an external DC power supply jack. I have thought of filtering that with a ferrite bead. I will probably do that with the next PCB revision. However, as of now the EMI results of the box with or without the power supply connected isn't very different, and the spikes at the USB harmonic frequencies can't be caused by the external power supply can it? In any case they are still there when it is disconnected.

There are a few ground pins(8 of them) on the header, since this is a standard 44pin PATA header.

OK, that's assuming I'm using the external AC adapter to the mains right? Without it, there wouldn't be a loop since there's only a single point of contact to the mains through the PC via the USB cable? And assuming there's a loop due to the AC adapter, the way to induce losses in the loop is via ferrite beads?

another via from 1 of the 4 legs of the USB shield.

On Jul 5, 7:41 pm, galapogos wrote: [.... passing FCC ...]

Yes you want to put a ferrite over all of the power wiring of the power jack. This includes the "ground" connection of that connector. The reason those clamp on the cable things are so common is because people forget to do this inside the box.

Without the power cable plugged in, you have a much smaller dipole. If you can, you'd still want to get some losses into the picture.

RF is a little like light. If you block most paths with something refective, it will bounce around until it finds a way out.

The spikes at the USB harmonic frequencies do get into the power supply leads. Most of the current involved in the switching spikes circulates through the by-pass capacitors up against the chips on the board, but these do have appreciable impedances at 200MHz - take a look at the manufacturers data on their impedance as a function of frequency, and you can see voltage spikes (usually called "grass") on the Vcc lines anywhere on the printed circuit card. This generates apprecialbe high frequency current in all the power supply leads. Ferrite beads in series with the power supply leads can reduce - but not eliminate these currents.

Presumably the power lead in the USB cable is also radiating the same noise.

Star grounding doesn't work at RF frequencies. Connections that look like a short circuit at low frequencies look like inductors at RF.

-- Bill Sloman, Nijmegen

Gotcha, one in series on the Vcc, and one in series on the GND.

So I tried adding some ferrite beads...1 in series with the USB power, another in series with the DC jack power...couldn't find any pads to fix them on the grounds nor could I use any common mode chokes on the USB signals coz I don't have the footprints for them either. Turns out the ferrite beads did jack for EMI suppression. I'm still getting pretty much the same graphs, but I did manage to find a better cable with an integrated ferrite bead. Weird since the clamp ferrite beads did nothing for my other cables, even one brand that is listed on intel's USB design guide.

Anyway, with the new cable I'm able to reduce the EMI problem range to the 200-500MHz range, with another peak at 800MHz. The rest are within control...it seems that at least some of the EMI is radiating out from the unit itself rather than from the cable. I've got some EMI absorber mats that I just placed over the unit(didn't peel and stick em), but they seem to do absolutely nothing when measured with the near field probes that I have.

I also did another mini experiment. On my older design there's a metal plate that sorta covers almost the entire PCB area, but it isn't grounded. I tried to ground it by touching it with the USB shield(which is ground), but strangely, the EMI peaks went UP instead of down! Aren't grounded shieldeds supposed to make things better? Man this EMI suppression is turning into more of a black art than science... :(

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You should know that incomplete shielding can be as bad as no shielding at all. Check out "slot antennas"

A narrow slot or or slit in the shielding that is half a wavelength long acts as a dipole antenna. The wavelenth of you 800MHz component is 37.5cm in air (15 inches) so a 19cm (7.4 inch) slot would be a very efficient antenna.

Standard shielding completely encloses the printed circuit board. If you want to put a metal box on top of a multi-layer printed circuit board including a ground plane (which can work just as well), you need to couple the box to the ground plane every cm or so all around the periphery of the box. A sort of fringed skirt with lots of beryllium copper spring fingers is one way of doing this.

Howard S. Johnson and Martin Graham wrote a book "High Speed Digital Design: A Handbook of Black Magic" ISBN 0-13-395724-1 on the joys designing printed circuit boards for GHz logic. The index doesn't say anything about slot antennas, though they do have a decent sized section on cross-talk and do mention "ground slots".

-- Bill Sloman, Nijmegen

Never forget, lead length makes a hugh difference.

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller

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Ferrite beads aren't identical - Farnell have a page of or two of ferrite beads and chips and the impedances they present at 100MHz range from 60 ohms to about 1000 ohms.

And a ferrite bead just puts an series impedance into the connection. To reduce the high frequency current travelling around that loop, you also need a parallel impedance to local ground - a high frequency capacitor - to divert most of the high frequency current around a more compact loop.

One thing about shields tht you may need to know is that any extended slit in you shielding acts as a slot antenna.

A narrow slot that is half a wavelength long (even when quite narrow) functions as an efficient dipole antenna.

Even at 800MHz half a wavelenth is about 19cm or about 7.5 inches, but shorter slots can still be bad news.

Openable screening boxes usually come with a skirt of berylium copper fingers that make contact every few millimetres around the openable gap.

Speaking of black magic - Howard W Johnson and Martin Graham wrote a book "High Speed Digital Design: a handbook of Black Magic" ISBN

0-13-395724-1 which does talk about cross-talk and shielding, though it doesn't mention slot antennas.

-- Bill Sloman, Nijmegen