Karel Kulhavy wrote:
On Thu, Mar 02, 2006 at 03:05:35PM -0800, joeft wrote:
If I understand your description correctly, you need to raise the
impedance of the 2.5cm wire at 200 MHz to keep it from conducting
interference to the outside of the box. Is this wire used for power?
If so, get a ferrite core (toroid) and put it over the wire; pass the
wire through it a few times if you can.
Also, 100 nF caps may be beyond self resonance at 200 MHz depending on
their construction. Check the manufacturer's data sheet. It may help
to put a smaller cap (100 pF) in parallel with the 10 & 100 nF if 200
MHz is your problem.
But together with the other caps they form a complex RLC network.
What's the typical ESR of these caps so I can simulate it and look what
happens there?
CL<
Again, you'll need to check the manufacturer's data to find specs
appropriate for the caps you are using. For small (0603) SM packages,
ESR can be just a few milli-ohms. Also be aware that the ESL and
parasitic inductances and capacitances resulting from your layout must
be accounted for. You can simulate this, but you will have to make the
board parasitics part of your simulation. Getting accurate values for
these can be a challenge. For your situation you might consider placing
a 100 pF cap as close as possible to the part which you believe is
causing the interference. In addition place a .01 uF cap very close to
the first cap. Then maybe add a 0.1 uF cap as close as possible to the
first two. Connect power to this circuit via a ferrite bead. Use the
smallest body size for the caps you can and don't get them in any higher
voltage than you need for reliability (high voltage means larger body
size which means more inductance in the caps). Keep all traces short
and make sure the caps connect solidly to ground.
When you exit the enclosure, use feedthrough devices such as were
discussed by others earlier in this thread.
Joe T
