Re: Reducing EMI

On Jul 6, 4:41 am, galapogos <gois...@xxxxxxxxx> wrote:
On Jul 6, 10:12 am, MooseFET <kensm...@xxxxxxxxx> wrote:



On Jul 5, 6:45 pm, galapogos <gois...@xxxxxxxxx> wrote:
[... Passing FCC limits ....]

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.

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?

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.

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?

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.

In any case they are still there when it is disconnected.

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

Also, this is a 2 PCB design that is connected by pin headers, with
Vusb and gnd flowing between the 2 PCBs.

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.

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

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.

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.

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.

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,

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

.

Relevant Pages

  • Re: Reducing EMI
    ... those connections should be right next ... Imagine the cables laying like this: ... powered by the PCB via the USB cables, but there is also an external ... You have a loop fronm the box to the mains ...
    (sci.electronics.design)
  • Re: Reducing EMI
    ... those connections should be right next ... powered by the PCB via the USB cables, but there is also an external ... losses in the loop is via ferrite beads, ...
    (sci.electronics.design)
  • Re: Reducing EMI
    ... those connections should be right ... powered by the PCB via the USB cables, but there is also an external ... losses in the loop is via ferrite beads, ...
    (sci.electronics.design)
  • Re: Reducing EMI
    ... those connections should be right next ... powered by the PCB via the USB cables, but there is also an external ... losses in the loop is via ferrite beads, ...
    (sci.electronics.design)
  • Re: Media Center TV Has no Sound
    ... There are no samples in my media center under My Tv/recorded. ... happened, but it shows the video but once again, no sound. ... I feel confident the connections are NOT the problem. ... TV connections cables, and what they are. ...
    (microsoft.public.windows.mediacenter)