[eagle] Re: Jim's Phase Noise Number!!
greencl at mindspring.com
Mon Jun 11 21:51:41 PDT 2007
> There are issues with Tantalums. From my perspective (and I could be
> very WRONG!!!!) Lyle, and Chuck Green are AMSAT's corporate knowledge
> on space-suitable components. We should make no changes without
> consulting them, and possibly Jan King as well. Lou McFadin may have
> some useful insight, also.
Don't look for consensus on this topic. Like most subjects, you can
find "experts" to support any position you want to take.
We've used a lot of Tantalums with good success. My choice is to use
Tantalum rather than Electrolytic whenever possible. But I lay out PCBs
and populate them, hence my bias. I don't normally have to make the
The position taken by Karl Meinzer is that you should never use
Tantalums. The argument I have heard to support this is that the
failure mode of a Tantalum is a short. Of course, depending on the
circuit, if the capacitor fails, it might not make much difference what
the failure mode is. But for many circuits, if the capacitor fails, the
circuit will probably keep working if the capacitor does not short. I
have read that this is no longer a problem for modern parts (newer than
about 10 years) because it is no longer the failure mode. I do have one
recent experience with this when I put a part in backward which resulted
in the predictable explosion. The failure mode... shorted.
Note that the filter capacitors on the CAN-Do! are not Tantalum (except
one). This is because of Karl's rule. He did allow the one when we
severally de-rated the voltage rating.
Electrolytic capacitors have their own problems. The biggest is their
tendency to dry out, especially in a vacuum. We address this by sealing
the open end of the can. Yes, this creates the potential for an even
bigger bang, but we haven't seen this yet. Epoxy has been used. I also
use the conformal coating for this purpose as appropriate. You are not
likely to use conformal coating on an RF circuit because of the bad
things it's dialectic constant will likely do to your carefully tuned
My advice is to choose the technology that best suits your needs. They
both work and we have a lot of success with both of them when we take
precautions as described above.
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