DIY RF Comb Gen Test
Following on from the initial test of the prototyped SI5351A clock driven RF comb generator (essentially adding a capacitor and diode), this last post captures the build and test of a final version of that device. The protoboard version worked pretty well, but might not survive use in the home lab (wires falling out of protoboard and all). With a few extra minutes, a smaller and more ruggedized version was constructed.
Parts at beginning of the build...
I started with a small strip of FR4 double sided copper circuit board and (carefully) sliced a few trace cut-outs into it to make the input and output circuit pads. A pair of SMA jacks and some copper tape top-bottom-shorts were planned for the board.
Before soldering the small signal diode, I was able to read the part number off of it: Fairchild FDH-600 (a fast switching diode). This is just the one I had on-hand and I'm sure that many other small-signal diode types will work similarly well in this application.
Ready to solder the connectors and copper tape (both sides).
It was about 40 Deg F in the garage today - leading to fast, not-perfect, but acceptable soldering of the top and bottom portions of the little board. All parts firmly mounted to the board.
With some IPA cleaning to remove the flux, this will turn out nicely. But how does it work? Setup the SI5351A clock board and set it to 10 MHz using SATSAGEN
Below is the test setup: UNO, EtherKit SI5351, DIY RF Comb Gen board, to TinySA-Ultra. The comb quality looks much better than the protoboard version!
The tighter build construction appears to have minimized parasitics seen in the protoboard test, leading to a nice consistent slope of the combs.
Following up, I extended the upper frequency limit up to 4 GHz on the TinySA. The combs continued their downward slope and pretty much disappeared at around 2 GHz. Not too bad for such an inexpensive and easily constructed setup!
Final remaining step is to clean the flux off of the solder joints. Calling it a wrap for the day...
Achievement Unlocked: DIY RF Comb Generator built and tested! It works even better than I had hoped...
All author photos captured with an iPhone 16e.