With vector network analyzers, the commercial offerings seem to come in two flavors: relatively inexpensive but limited capabilities, and full-featured but scary expensive. There doesn’t seem to be much middle ground, especially if you want something that performs well in the microwave bands.
Unless, of course, you build your own vector network analyzer (VNA). That’s what [Henrik Forsten] did, and we’ve got to say we’re even more impressed by the results than we were with his earlier effort. That version was not without its problems, and fixing them was very much on the list of goals for this build. Keeping the build affordable was also key, which resulted in some design compromises while still meeting [Henrik]’s measurement requirements.
The Bill of Materials includes dual-channel broadband RF mixer chips, high-speed 12-bit ADCs, and a fast FPGA to handle the torrent of data and run the digital signal processing functions. The custom six-layer PCB is on the large side and includes large cutouts for the directional couplers, which use short lengths of stripped coaxial cable lined with ferrite rings. To properly isolate signals between stages, [Henrik] sandwiched the PCB between a two-piece aluminum enclosure. Wisely, he printed a prototype enclosure and lined it with aluminum foil to test for fit and function before committing to milling the final version. He did note some leakage around the SMA connectors, but a few RF gaskets made from scraps of foil and solder braid did the trick.
This is a pretty slick build, especially considering he managed to keep the price tag at a very reasonable $300. It’s more expensive than the popular NanoVNA or its clones, but it seems like quite a bargain considering its capabilities.
Damn that’s impressive, I have no need of much more than a NanoVNA for my purposes, but I want one of Henrik’s
High frequency PCBs always look so damn good vs. the more typical ones. I recognize that they aren’t a corner you want to find yourself in, on economic grounds; but gorgeous.
Oh My….
This looks really good.
I have a NanoVNA-F I bought a few years ago. I worked as an RF tech and for just playing around it is a good starter VNA. For the less acquainted with RF test gear, things like real VNAs and Spectrum analyzers can be blown up easily… A really dedicated RF hobbyist will build their own thermister power sensor.
There is a nice rabbit hole for you; Building RF power sensors.
Yeah, soon as I saw the board and the SMAs used I was like “that’s gonna leak.” Been there, done that (as in, had a leaky cavity due to edge-mount SMAs, not built a 15 GHz VNA!). Did the same PCB sandwich trick on that, too.
If you extend the case a little outward, extend walls upwards to let the PCB fit inside it (so now essentially it’s fully enclosed) with a bit of slop on the holes (just add a bit of slot to them), they make bulkhead SMAs which have panel gaps (so there’s a flush face front/back) so you get a solid seal straight around (e.g. CONSMA006.062). Bit more fiddly to design with but they work really well and they barely cost any more.
The other option I’ve seen that works is just don’t bother with the edgemounts at all and use a square flange SMA screwed in that extends to a land on the PCB, with ground connection through the enclosure. Downside to that is that removing it requires desoldering the connector, whereas the other ones just slot in. The setups we use need absurd levels of isolation so it’s been a lot of experimentation.
If you read the article, you will see that the designer did consider bulkhead connectors.
That’s why I mentioned the ones with panel gaps (the CONSMA006.062). The square flange ones I mentioned because you can get very extreme isolation with them, but they do have the drawbacks mentioned. Tends to get used on more professional stuff though.
The ones with panel gaps give you a front/back flange so the top/bottom sandwich between it, and there’s no straight-through gap (and if you want you can just stick gasket material around it too and you’ve got an extremely solid seal.
You can kinda-sorta do it with the long-barrel bulkheads (you trap the square flange) but that’s awkward.
This gives me hope of someday having a 4 port VNA.