macroPFA (Benchtop Phase Frequency Analyzer based on the nanoVNA-H4 / tinyPFA)

This is essentially a docking station for the nanoVNA-H4. When coupled with the tinyPFA firmware (which I have no association with), it becomes a pretty powerful (and cheap) Phase Frequency Analyzer. If you aren't familiar, PFAs allow you to characterize the phase noise and other characteristics of clocks, measuring how “good” the signal from your favorite crystal oscillator is. I use mine to check OXCOs against an old Rubidium atomic clock.

The tinyPFA firmware can be used alone or in conjunction with TimeLab take measurements.

Instructions

Printing

The version shown above was printed on a Prusa XL (5 toolheads) in PETG (although PLA would likely work fine). If you don't have the ability to print in multiple colors, there's a blank version as well. You could also try printing the faceplate alone without the colored bits, leaving gaps in their place.

There are two versions of each faceplate style: macroPFA-faceplate-blank/graphics-original and macroPFA-faceplate-blank/graphics-clocked. The second one is due to an issue I found while assembling, that the front SMA connector screwed onto the attenuator for Channel B interferes with the BNC connector for Channel A. The interference was slight, and I was able to get around it by loosening the BNC bulkhead for Channel A while installing the cables. In the second version, the attenuators are clocked a few degrees off to prevent this interference. While I think this will work, I haven't tested it so I am still providing the old version. If I end up printing a new one, or someone in the comments tries it out, I'll remove the original file.

Print the faceplate face-down with supports on the build plate only.

Print the box with the USB cutout touching the build plate, with supports only in the large rectangular cutout.

The cap is optional but recommended.

Parts & Assembly

You'll need:

• The 3D printed parts
• 1x nanoVNA-H4 with tinyPFA firmware
• 4x M3 heat-set inserts (McMaster 94180A331)
• 4x M3 6mm socket head cap screws
• 2x Right-angle SMA-terminated coax lines (make your own or buy from Amazon)
• 2x Right-angle SMA/BNC-terminated coax lines (make your own or buy from Amazon)
• 2x BNC bulkhead connectors (Amazon)
• 1x Right-angle USB-C cable (Amazon)
• 1x USB-C bulkhead connector (Amazon)
• 2x JFW 50DR-109 attenuators. Other attenuators will work fine as long as they attenuate sufficiently and cover the frequency range you need. You may need to modify the design to make others fit. I got mine from Ebay.
• Epoxy
• 4x Adhesive rubber feet (optional)

Of course, you should only proceed with assembly if you are comfortable with the processes involved. The author assumes no liability for any issues that arise during assembly or use!

After printing the parts, do a test fit to make sure everything lines up the way you'd like. Next, carefully install the heat set inserts. Once they have cooled, mix a small amount epoxy and add a small dab to the notch where the USB-C cable sits below the nanoVNA. I also carefully scored the back of the USB-C right-angle connector to promote adhesion. Be careful not to get any epoxy in an area where it could touch the nanoVNA, and don't add so much such that it will ooze into the nanoVNA pocket. Slide the USB connector into into the notch and onto the small dab of epoxy. Next, slide the nanoVNA into its pocket, being careful not to get any glue on it. Gently move the cable as-needed to plug the USB-C connector into the nanoVNA, and leave the epoxy to set. Once it has set, gently remove the nanoVNA and mix up some more epoxy. Add it to the USB-C connector notch, potting the cable in place. Again, be careful not to add so much such that the nanoVNA won't fit. Allow for the epoxy to set thoroughly.

Once the glue has set, install the attenuators. Set both knobs on each attenuator to zero. Loosen the set screws and remove the knobs, then slide each attenuator into it's place and install the screws. Re-install each knob, aligning the zero vertically with respect to the face plate. 

Now, install the BNC bulkhead connectors and the USB-C bulkhead connector. The BNC connectors are installed from the front and secured from the back. The area around the nut is a bit tight but you should be able to at least get it finger tight, or use pliers if you need to.

Now, wire it up. Install SMA-SMA cables between the two ports of the nanoVNA (top port on the VNA to Port A, bottom port to Port B). I don't believe it matters which port on the attenuator you use. Install the SMA-BNC cables between the attenuators and their respective BNC bulkheads. Plug the USB-C cable into the bulkhead.

Finally, slide the box over the faceplate lip and secure with screws. If you printed the cap, you can install it now.

Use

To use the analyzer, remove the cap and switch the nanoVNA on. You can use it as a standalone tool, or plug the USB-C cable into your computer and take readings with TimeLab. You can remove the nanoVNA by sliding it upwards to clear the USB-C connector, pushing it backwards to clear the lip, and slowly pulling it out. Once removed, the SMA connectors can be removed. You may want to mark them to indicate which one corresponds to which output. To re-install the nanoVNA, reverse the process.

Modifications

If you'd like to change the design, you can copy my OnShape project (although it's pretty sloppy).