L-L? I remembert numbers in the range of 10 to 100pF were
maximum practical values. With 10pF being the preferred
value, but this is from memory. Then the BALANCED L-G were
around 20 to 200 pF each. Else, you get very "sloped"
edges on the wave shape and then you get a lot of "rattle"
in the decision point upon reception. Remember, in digital
signals faster edges reduce noise (jitter) in the data.
Wow! 4700pF to kill 35MHz, now that's a low impedance
source. You're going to have to increase that impedance a
bit.
No access to the PCB? Need a large common mode choke.
THEN a small capacitance, even parasitic, will stomp on
that emission.
- Robert -
On Tue, 28 Feb 2006 06:21:33 -0800
"Price, Ed" <[email protected]> wrote:
> I have a twisted-pair Ethernet receive and a twisted-pair
> Ethernet transmit
> set of 4 wires penetrating what would otherwise be a
> shielded box. Internal
> box noise, as low as 35 MHz, is getting out along this
> path.
>
> I put a 4700 pF feedthrough capacitor on each of the four
> lines. That worked
> pretty well, so well that the Ethernet shut down. Does
> anyone have a
> guideline about how much capacitance, L-L & L-G, that is
> typically OK?
>
> Thanks in advance!
>
> Ed Price
> [email protected] <mailto:[email protected]>
> WB6WSN
> NARTE Certified EMC Engineer & Technician
> Electromagnetic Compatibility Lab
> Cubic Defense Applications
> San Diego, CA USA
> 858-505-2780 (Voice)
> 858-505-1583 (Fax)
> Military & Avionics EMC Is Our Specialty
>
>
>
> -
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