[eagle] Re: Jim's Phase Noise Number!!

[Jim Sanford]

Juan:
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 
minimize.

> 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?
>
Good question.

>  
>
> 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 
insight, also.

Thanks & 73,
Jim
wb4gcs at amsat.org

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