<azonenberg>
My fixture is resonating or something
<azonenberg>
this is s11 of the open circuit on the fixture with no probe connected
<monochroma>
how did you find it's not the probe?
<azonenberg>
i took the probe off the fixture and vna'd the fixture :p
<azonenberg>
and it didnt go away
<monochroma>
ohhhhhhh
<monochroma>
i thought you had already characterized the fixture?
<azonenberg>
i was trying a different one that was easier to solder to for this purpose
<azonenberg>
Mistake
<azonenberg>
this fixture is also less lossy
<azonenberg>
so it should have delivered better results
<azonenberg>
ok now i am even more confused
<azonenberg>
I'm seeing a giant spike with the vna cable floating and nothing hooked up
* azonenberg
recalibrates vna
<azonenberg>
Mystery solved
<monochroma>
vna cal issue? :<
<azonenberg>
Yes and no
<azonenberg>
The connector saver between the SMA-N cable and the VNA front panel had loosened over time
<azonenberg>
i torqued the cable onto the saver but didn't re-torque the saver because its a different size that my torque wrench won't fit
<monochroma>
ahhhh
<azonenberg>
The saver is, i guess, about a quarter wavelength long at ~3 GHz...
<monochroma>
heh
<azonenberg>
So my data is indeed invalid, but the fixture was not to blame
<monochroma>
does the saver have a flat for a torque wrench? in the WISP industy i'm used to the N connectors just being knurled without any flats for hand tightening only >_>
<azonenberg>
These have nuts for tightening them. but it's the wrong size for my wrench
<azonenberg>
i have a normal wrench that fits them which i used to fix it
<azonenberg>
but it's not properly torqued which i'm not thrilled about
<azonenberg>
i should measure the nut and get a proper torque wrench just for the saver
<azonenberg>
you know, this calls my older S21 measurements into question as well
<azonenberg>
i recall a bump around that freq...
<azonenberg>
ooook so now that THAT problem is found, gonna take a break before repeating the whole night's measurements :p
<azonenberg>
i found something that looks good from fairview microwave but i think i will have to special order it
<azonenberg>
they have different combinations of preset torque and head size
<azonenberg>
the heads seem to be changeable
<azonenberg>
but they dont ship a 18mm one with 8.85 lbf-in
<miek>
would that have shown up by doing a sweep on the open fixture with no probe soldered on? if so, that'd be an easy check to add to the test plan
<azonenberg>
Yes
<azonenberg>
I'll add that
<azonenberg>
anyway, so all the data i sent you before is now suspect
<azonenberg>
I found a few other small procedural issues that i've since corrected too
<azonenberg>
trying to be as consistent as possible
<azonenberg>
they're diverging a bunch past 3 GHz but still like +/- 0.1 dB consistency
<miek>
very nice
<azonenberg>
This isn't even half of the total batch i have to do
<azonenberg>
but i stopped to crunch the numbers and see how wide the pread was
<azonenberg>
it's looking quite nice
<azonenberg>
ok, got a bunch more done. Looking at S11 phase I see a +/- 8 degree spread at 6 GHz
<azonenberg>
The probe is 925 ps delay and 160.3 mm in length
<azonenberg>
So our velocity factor is about 0.578 (which sounds slow, but we'll use it for now)
<azonenberg>
Wavelength of 6 GHz in free space is 49.97 mm, scaling that by the velocity factor gives a wavelength on the probe of 28.88 mm
<azonenberg>
8 degrees is 0.022 of a cycle
<azonenberg>
Which means our difference in S11 path length is +/- 640 μm
<azonenberg>
Sounds plausible for a combination of resistor misalignment (due to soldering drift) and slight variations in placement of the probe on the fixture
<azonenberg>
Pretty cool that the VNA measurement has the resolution to see sub-millimeter variation like this
<azonenberg>
intuitively, i wouldn't have expected that with only 6 GHz BW