[eagle] Re: Receiver Spec vs. ATP, a few Suggestions and a Question or Two
juan-rivera at sbcglobal.net
Sat Jul 28 08:35:58 PDT 2007
I know this is an unpleasant subject and I apologize, but I don’t think the existing enclosure is going to satisfy the real-world requirements for SMT PCB flex. I’m basing this on several things. The first is a paper written by AVX (see below), one of the largest surface mount capacitor manufacturers in the world. They recommend a maximum PCB flex specification based on bend radius to prevent component cracking. It works out to 0.0084” in any 1-inch segment. They also caution against placing any components near mounting screw holes as they are areas of especially high stress. As you know, this board has ceramic chip capacitors (the most brittle components on the board) extremely close to the heat sink mounting hardware. This imposes even more need for absolute rigidity and flatness across the entire mounting surface. Given the current expectation to cold soak this Receiver down to -60C it places almost impossible demands on the PCB, the components, and the enclosure. Coefficient of thermal expansion mismatches between these items is aggravated by this huge ∆T the receiver is expected to survive. I have yet to find anyone who has experience with a payload that was expected to survive repeated excursions down to -60C.
Next is my experience with this prototype enclosure. Yesterday I disassembled the PCB and heat sinks and took a few measurements. Here’s what I found:
Heat Sink No.
As you can see, the tolerances on the heat sink thickness are pretty bad. I’m not able to measure the standoffs because of the material extending out the opposite side of the base plate. I checked the parts list for this chassis and those standoffs are not listed so I don’t know what the manufacturer’s tolerance is on height.
I also found that seven out of the ten PEM standoffs are loose and can be rotated with your fingers. This raises a concern over grounding. But the main problem is that the enclosure is flexible and warped. I think there are several reasons for this. The sheet metal base might have been slightly bowed to start with and then the large amount of work that is done to it (34 PEM standoffs and captive nuts) might have aggravated the warp. And of course milling holes in the base to gain access to the connectors might have contributed to this. I also notice that the base becomes more warped after the front connector panel is attached. I inspected this closely and found that some of the PEM captive nuts that are applied from the bottom are not completely flat on the top surface. When the connector plate is drawn down to the base plate the base plate is flexed over those protruding PEM nuts causing more warp. This changes the dynamics of the enclosure. Attaching the lid and finally mounting the enclosure to the satellite frame will all cause the PCB mounting surface to shift, tilt, and bend in unpredictable ways.
And lastly, we may need to split the enclosure into two compartments with the CAN-Do module and other switching power supply components in the front and analog circuitry in the back. This may be necessary to resolve radiated EMI issues.
So there are my concerns and findings. To me they all point to the need for a milled enclosure. Such an enclosure would have a flat plate on top as a cover. A milled enclosure would allow the power supply regulators to be mounted on the inside surface of one of the walls, greatly improving and simplifying heat dissipation. Most importantly, all of the heat sink and PCB mounting surfaces would be part of the same block of aluminum as the enclosure itself and would be very rigid and flat.
Here’s a link to the AVX paper I mentioned --> http://www.avxcorp.com/docs/techinfo/smzero.pdf (see page 4)
Juan – WA6HTP
From: Dick Jansson-rr [mailto:rjansson at cfl.rr.com]
Sent: Saturday, July 28, 2007 5:26 AM
To: juan-rivera at sbcglobal.net; 'John B. Stephensen'
Cc: 'David Smith'; 'Dave Black (Work)'; 'Dave Black (Home)'; eagle at amsat.org; 'Samsonoff at Mac. Com'; 'Juan.Rivera (Work)'
Subject: RE: [eagle] Receiver Spec vs. ATP, a few Suggestions and a Question or Two
>From a practical aspect, the hanging of electronics on the outside of an E05 20 module is rather implacable as there is no place to put them. The thin metal cover is not a place as the heat would not be spread out for dissipation, and the cover would not be “divorceable” from the rest of the module. You would not want to place them on the front connector bracket as you are already scrambling for space for connectors. Those are the only areas that are open to the outside world.
We already have solutions for the mounting of these electronics on the inside of the module, where they belong, so I do not see where you otherwise plan to place these power conditioning electronics?
Dick Jansson, KD1K
kd1k at amsat.org
kd1k at arrl.net
From: eagle-bounces at amsat.org [mailto:eagle-bounces at amsat.org] On Behalf Of Juan Rivera
Sent: Saturday, 28 July, 2007 05.23
To: 'John B. Stephensen'
Cc: David Smith; Dave Black (Work); Dave Black (Home); eagle at amsat.org; Samsonoff at Mac. Com; Juan.Rivera (Work)
Subject: [eagle] Receiver Spec vs. ATP, a few Suggestions and a Question or Two
I took a few minutes to look over your new specs and compare them against the Acceptance Test Procedure. I've got a number of tests in the ATP for which there are no specs:
* Image rejection
* Internally generated spurs
* Local oscillator leakage
* Input and Output VSWR
Items that need to be addressed that aren’t in either document:
* EMI conducted susceptibility
* EMI radiated susceptibility
* Short and long-term frequency stability
I also take issue with the -60C minimum power-down temperature. I think this is unrealistic just from a CTE mismatch perspective. The reliability of anything subjected to that wide a temperature spread is going to suffer. A way must be found to raise that temperature.
I have a few thoughts... The CAN-Do switching step-down converter is only supplying 11 milliamps. If we take a slight efficiency hit we could just go to a simple linear regulator and completely eliminate the radiated and conducted EMI emission problem from CAN-do. That eases the EMI filtering and shielding requirements for every single payload. That seems like a good trade-off to me.
Rather than worry about trying to conduct heat through a PCB why not just go to externally mounted regulators for the CAN-Do and the Receiver right on the case itself. We’re not that pressed for space. That eases the heat sink and the associated thermal gap filler issues. EMI will still be an issue, but only for external sources instead of one that is inside the enclosure itself. That strikes me as a huge bonus. Why not design a single-sided PCB with the regulators hanging over the edge and the whole assembly, PCB and regulators, mounted directly to the enclosure. Stick it in with the CAN-Do module in a separate cell. Filter all the signal and power through the common bulkhead.
I’m also curious as to why you are specifying two different types of SMA connectors.
Juan – WA6HTP
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