NLTL Comb Generator (#2, v1)

I got the workbench pretty much all set up this past weekend. Work’s been slow so I’ve been able to unpack a lot of components and have been making good headway on building up the N2PK VNA. I need to make a Mini-Circuits order to finish that and some other things, so I decided to switch gears for a minute.

The first step in my DDS LO design is to create a 3.5GHz clock. Following the rabbit hole from Martein Bakker’s work on down to NIST’s timing group, I found the non-linear transmission line comb generator (multiplier, pulse generator) to compare favorably with other methods of frequency multiplication, especially with regards to phase noise. Thus, I set out to build one. This will all be detailed on the DDS LO page when I finish setting that up.

I laid out an 8-stage NLTL in Eagle with 0603 inductor pads and 0402 varactor diodes. I debated getting some boards professionally fabricated, but inspired by the fine folks who regularly do fine pitch SMT boards at home, I chose to test the limits of my laser printer -> laminator -> etchant setup.

Etching

Build and Test

I was at a sort of crossroads after etching this board. The plan is to have two multiplier stages, the first an x5 from the 100MHz OCXO output to 500MHz. The output of the first multiplier then gets amplified and sent to another NLTL to get an x7 output at 3.5GHz. Somewhere along the line I didn’t pay attention and got two different sizes for the inductors for each stage, so technically I should re-do the layout for the 0402 inductors I have for the first stage.

However, in the interest of testing my layout, concept, test equipment, etc., I went ahead and built up the NLTL for the 2nd stage, and tried it out with the 100MHz signal.

Conclusion

I’m very happy with the initial test. I may try and do the first stage board at the office with solder paste and the toaster oven since it’s all 0402.

Can’t wait to see what the pulse looks like after two NLTL stages! Would love to max out this scope. Then I’ll have to figure out a faster method. Sampling?