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@dlharmon what are your thoughts on direct coax-to-PCB solder connections? Any experience, good or bad? How high of a frequency is it feasible to push this sort of launch to?
I've got several applications in mind, one only needs like 2 GHz of bandwidth while the other would ideally run from DC to Ku band.
The general goal is high density multi signal probing applications using .047" micro coax and jamming them in as tight as possible. Two lanes for initial testing using a GSSG or GSGSG tip geometry. Separate 50 ohm paths usable for either a diff pair or two independent single ended signals.
Right now I'm using SMPM which is pricey (especially for duals) and also still a little bigger than I'd like.
@azonenberg @dlharmon @g4dbn
Friend has simulated some stuff relating to that and it seems to simulate up to 20-30GHz with 0.086 semirigid at least.
The thing apparently is to get the launch really good.
So cutting a slot to the PCB for the coax and soldering it to the grounds was apparently critical for good return loss.
Our discussions and his simulations were mostly from the point of feeding LO to homebrew harmonic mixers.
Because I wanted to push 18GHz or preferably 38GHz to a 76GHz rig.
@ftg @dlharmon @g4dbn So slot in the PCB with the center conductor flush with the surface?
I was hoping I could get decent results with the shield soldered to a top side ground layer then the center bent over. KF047 is only 1.2mm OD with the jacket removed, so there's not a whole lot of height variation to deal with vs .086.
@azonenberg @dlharmon @g4dbn
I'll link this thread to him and ask for input :D
But bending the center over gives an impedance bump and that limits the performance.
The main reason for cutting the slot was get rid of that center conductor to PCB trace bend and impedance bump.
@ftg @dlharmon @g4dbn Yeah, I get the concept. It's more a question of how bad things would be without it on really-small cable (where you might not even need the bend, just use more solder than usual).
The center conductor is 290 μm, dielectric 920, shield 1190.
So the difference in radius between the dielectric and center conductor is only 315 μm.
I guess the other question is how cheaply I could get my PCB fab to do controlled-depth milling...
2/x
- A direct, unmatched connection to a microstrip with some kind of coplanar-like top & bottom ground plane shape and plentiful amount of vias might do 15 - 20GHz.
- If soldered just directly on top of the PCB ground plane, then 5- 10GHz.
- The transition works better on a thicker PCB, because the microstrip being soldered to is wider.