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Re: gEDA-user: Edge ringing filtering

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.

Joe T


Karel Kulhavy wrote:

From my Twister a 200MHz ringing 10mVpp leaks out, with the exact 
frequency of the clock signal. The gates inside are HC. The thing is in
a shielded enclosure. There is a hole 5mm in the enclosure through which
1mm thick copper wire goes. In the place where it leaves there are 3 1uF
and 1 100nF ceramic caps. Soldered with 2.5mm leads as close as
possible. The place at the hole is connected with 2.5cm thin wire (from
UTP) to the board. The board has 100nF and 10nF blocking. Farther into
the board there is airwire coil 500nH to filter the power line. However
it's not enough - there is still those 10mV noise going out and
interfering with some people's TV.

I found some time ago that the mechanism how it gets out is that
it catches on the 2.5cm wire like on an antenna and goes through the
blocking caps on leaving the shielding enclosure because their impedance
is too high to kill it.

Do you think it will be solved when I put 47nH SMD 1210 size polymer
core coil just before the leave-the-box caps? Or will it catch on the
47nH like on an antenna again? I would like to get it down at least to
1mVpp. I calculated the 47nH to have 100x attenuation with 300mOhm
internal resistance of the capacitor.

CL<