On Sun, Mar 21, 2021 at 10:13 AM, <reuterr@...> wrote:
Roger Need has made a comparion of two different RF Demo kit PCB's.
See: /g/nanovna-users/message/20702
and the attached photo: RD-Demo-Kit_PCB_comparison.png
Thank you for pointing out the other thread. I haven't seen it yet. I agree that the board on the left has a better layout - all three standards are in the same position.
I have the variant of the board shown on the right. Here "short" and "open" are directly at the u.fl connector while "load" is on the end of approx. 2mm of a microstrip line. This adds some additional phase shift to the load (around 30 deg. at 3 GHz), but I don't think this matters much for calibration. If there is approx. 0 reflection off load, a phase shifted 0 is still 0.
The offset of the "load" might matter if I could better characterize the load resistor (e.g. describe it with inductance + resistance, like in the Agilent white paper I cite in my blog post). However so far I haven't found a method to do that that would work within the limitations of my equipment.
Also, I should stress that my 0.6 pF estimate of the stray/fringing capacitance for "open" depends only on the measurements of "short" and "open". These two seem to have pretty well matched lengths on my board (although I can't say how the PCB looks like underneath the u.fl connector)
I might order the other kind of board as well. It would be interesting to see how it compares. However I would still expect the other board to have some stray capacitance on the "open". Even professional calkits have that - the difference is that they usually come with the capacitance value specified (the C0 parameter)
Best regards
Toma?
