[eagle] Re: Jim's Phase Noise Number!!
wb4gcs at amsat.org
Mon Jun 11 19:34:57 PDT 2007
Some thoughts. . ..
Juan Rivera wrote:
> I spent some time this evening trying to dig into the CAN-Do noise.
> I was able to reduce it by 3 or 4 dB by temporarily shielding the
> module with aluminum tape. Before getting too excited we need to
> answer the question - is this noise going to be a problem?
I think it IS going to be a problem. As Bob has said, we really need to
> I think as a general suggestion we should move the RF to the far end
> of the enclosure. We have the space. I think it would be best to
> just extend the traces and not get involved trying to decouple the
> leads with ferrite beads and a cable - or perhaps build in some
> filtering into the PCB and implement with SMD components. Extend the
> input and output traces so we can stick with the three PCB mount SMA
> connectors. It might be nice to know what steps have been taken in
> the past to reduce CAN-Do noise. Stephen?
> I have another idea that would involve a milled enclosure with
> multiple compartments. I can see a modular structure where the
> sections are assembled into a chassis and pulled together with
> threaded rods. The individual sections would be kept in alignment
> with pins. That would take care of all the RFI and mechanical flexing
> issues. If you're willing to consider this idea I'll follow up with
> details. Unless the existing chassis is made rigid and plumb it is a
> bit concern of mine.
I AM willilng to consider this. Milled enclosures have been raised for
discussion elsewhere for other reasons.
> Moving to microphonics... I did a quick check of available
> information on the Internet and found this: "Barium titanate, which is
> the base ceramic material for most dielectric systems, will exhibit
> microphonic effects..." That article went on to suggest tantalum as a
> non-microphonic choice. Since it should be pretty quiet out there in
> orbit microphonics shouldn't be a problem. I did find out where the
> bulk of is coming from however - C27 and C28. You'll find them on
> schematic page 4 between U4 and U5. Almost every capacitor on the
> board appears to exhibit some microphonics but those two seem to be
> the worst.
IT is good news that you found the most notorious offenders. While I
might agree that once stably on orbit microphonics won't be an issue, I
worry that if we don't get this under control, microphonics will bite us
at a very dynamic time when we MUST be able to talkl to the bird. I'd
also worry about thermally induced creaks and groans. What do I mean?
When I stand next to my barn (sheet metal slats) or my neighbors during
a sunny winter day, the creaks and groans due to sun induced temperature
changes are LOUD. I expect what we see on orbit to be worse by several
orders of magnitude...
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
Thanks & 73,
wb4gcs at amsat.org
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