Well, I thought I might actually have the project done by now. After all, it is all buttoned back up and fully functional. ...Almost.
After spending many days designing, tearing apart, reconfiguring, thinking about, and generally messing this project royally up, I finally put the thing back together and turned it on. That was when I realized all of the mistakes I had made.
First off, a recap. I had intended to make a number of changes to the original Junk Box SDR, so much so that I almost considered calling it the Junk Box SDR 2.0. However, that sounded extremely lame, considering that the main electronics (the SDR and upconverter) were unchanged. The changes that were actually made were: better shielding by filling in some of the holes, completely isolating the internal power from USB by adding a 12v to 5v DC-DC converter (a simple 7805 with plenty of smoothing caps) and drawing power from the Astron RS-35M that I use to power the rest of my ham radio gear, isolating internal components from each other, adding switchable filters and an attenuator, adding a bi-color LED to indicate which mode the SDR was in (upconverter active or bypassed), and generally cleaning things up and doing a nicer job of things.
Here are my more obvious mistakes: The filter circuit I designed never stood a chance of working due to an oversight so obvious I am too embarrassed to share it with you. The photo I posted is probably sharp enough for you to figure it out. Total facepalm moment. Speaking of facepalm moments, I had a similar feeling when I realized why the bi-color LED showed up as orange instead of either red or green. Yup, I'm feeling pretty stupid right about now.
The one thing I'm not blaming myself for is the discovery that the SDR and upconverter are actually generating much of the noise I was hoping to be able to get rid of with this project. That was an unexpected blow, but one I would not have been able to discover without going through all of this trouble.
Here it is, warts and all (minus the top cover):
Now that I've made a complete fool of myself in this somewhat public forum, I can start to work on coming up with fixes for the various problems I have created. However, I am feeling a bit burnt out today, so the Junk Box SDR will go up on the shelf in front of me until I feel like cracking into it again. I hope those following this project will be patient with me. I'm also open to suggestions, provided they can be achieved with little to no monetary expenditure.