Spectrum Analyzer Aluminum Frame

Beautiful new frame for my SA

I've made fantastic progress on my spectrum analyzer build, and it's so much fun that I haven't had the will to pull myself away and post about it. The last post about the analyzer was about the ChipKit digital logic controller. That was going very well, so well, in fact, that I was able to finally diagnose an intermittent connection problem between modules. Intermittent problems are always the worst, especially when you don't trust other components in the system. The reason this is relevant to the discussion at hand is that the problem only manifests when one of the connectors has pressure one one side. I needed a reliable way the hold all the modules in fixed positions.

Since beginning this project, I've been inspired by the way that two people built their analyzers. Hans' is probably my favorite. I took the image below from his photo album in the Yahoo Spectrum Analyzer group.

Hans' analyzer "bottom view"

I love how clean and organized it looks. Much different than most of the others out there. His frame has holes that go all the way through the frame, and he has a back cover that screws on. His coax cabling is made of right-angle soldered-on connectors with what looks like RG-405 hard line.

Another inspirational build is Sant's. This image is also scraped from the Yahoo group.

Sant's build

This build is most probably the closest to mine. The pockets, or wells, for the component side of the boards don't go all the way through the substrate. Notice, in both designs, that there is a small lip around the perimeter of each well. This is there to hold the boards and to electrically connect to the ground vias on the perimeter. This design also uses right-angle connectors and hard line.

With these designs in mind, I sought out the things I would need. First, of course, was the aluminum itself. I had looked into McMaster-Carr (hopefully this link works), and a 1/2″ thick 12″ square costs about $40. Then, my brother suggested looking on eBay. I was able to find an equivalent sheet for about $30 after shipping.

Frame block layout

Once I had a cool hunk of 6061 alloy in my hands, I started designing the layout of the frame. I started with OmniGraffle (it's like Visio) because I could lay it out to scale, and the connections move in a natural way.

Once the layout was complete, I transcribed the design, complete with all the details into AutoCAD. By this time, about a month passed, and I was able to find someone willing to machine it for me as a favor. I also got a quote from another friend, which was about $250. This is a reasonable cost for something like this, in case you're looking to duplicate my results.

Close up of one of the wells

It took several weeks to get the parts back from the machinist, but the results are totally worth it! The larger hole was cut with a 1/8″ end mill, and the inner pocket was cut with a 1/4″ mill. With the majority of the modules, the inner radius is fine. There were a few exceptions, however.

Small relief for DDS capacitors

This photo shows some of the rework I had to do to accommodate a few capacitors right at the edge of the DDS module. It's very difficult to take a picture of a small notch in a shiny material, but hopefully you can see the cut into the side of this pocket. I made that mostly by making small, successive cuts using an exact-o knife. The PLO reliefs were a bit more aggressive (there is a power header right in the corner), so I had to use a Dremel cutter bit in my drill press.

Relief for the PLO module

Once important lesson learned in this process is that 1.2″ or 2.4″ set into a PCB specification is more of a suggestion rather than something that you can count on all that much. I had to sand almost every module to get it to fit. Once that was done, however, everything fit like a glove.

Making custom coax jumpers

The final piece in the puzzle is the coax. The perfect jumpers that both the other designs featured were definitely something that I wanted. It's possible to get these right-angle SMA connectors from China for about a dollar a piece, much less than the ~$5 that you'll spend at Digi-Key.

Connector end, ready for solder.

To make them, all you really need to do is measure the coax sections, strip the ends, and solder…

Soldered center conductor

Soldering around the shield of the coax is the hardest part, and it's not even that bad.

Final product!!

That's all there is to it! I'm really happy with how well things turned out.Certainly something to be proud of. Over the next few days, I'm going to try to keep posting about the other advancements. I have a bit of a backlog, so I should be able to keep them coming…

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