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.
you might also look for some "feedthrough caps". These are emi filters
that can be inserted in a hole in a shield can which give very low
inductance caps to ground and some include a ferrite on the main line
which makes it look resistive at higher frequencies (resistive due to
the high core losses).
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.
You know... I don't completely buy this argument anymore. What do I
care if my bypass cap is beyond self resonance? What I care about is if
the impedance to ground that it presents is very low. So pretend I have
2 caps that are exactly modeled by a series RLC network and I keep R and
L constant. 4x the C gives 1/2 the self resonant freq, but except for
right near the resonance of the 1X cap, the 4x cap has a lower
impedance. At low frequencies, it has 1/4 the impedance and at higher
frequencies it is the same. So my take on the "put smaller caps in
parallel" old wisdom is that you have to make sure you're really getting
a benefit. The last I looked at some low voltage 0603 (or were they
0402) ceramics, the data seemed to suggest that bigger was always better.
I think the place where you can get a benefit is when your bigger caps
put you into a different package or technology that caused significant
difference in R and L or the required board parasitics to handle the big
package.
Just my 2 cents.
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.
Are your caps leaded? The HF behaviour can be less than stellar.
You might look at using some ferrite based EMI filters on the power line
instead of an inductor. Get one with good rejection at the freq. you
care about.
You might also consider adding some resistance and local bypassing on
the supplies for your HC gates.
I feel your pain, this sort of problem can be one of the most frustrating.
-Dna
