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?
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 [email protected]