In all honesty I was not sure if the approach I was following to determine imperfections in a calibration kit made sense. Various wise people on this group have given me hints which, due to lack of knowledge on my side, I probably failed to understand.
So I decided to to some more investigations.
I used THREE different VNA's
VNA 1: NanoVNA (we all love)
VNA 2: Home build nanoVNA equivalent.
VNA 3: Home build 3 GHz VNA (uses same type of resistive bridge and measurement principles as nanoVNA but it does not use overtone mode)
and ONE calibration kit and ONE set of cables.
I measured on all VNA's the same calibration kit directly on the connector for calibration, the same 1 meter RG58 cable with OSL and the same 30cm semi rigid cable with OSL
All measurements where done till 900MHz. The calibration kit is assumed to be perfect. No compensation factors where used.
Any difference seen between these measurements should come from the difference in VNA's
After calibration all three VNA's show for the calibration kit a perfect calibration, no difference is seen.
In attached pictures you will see the calibrated plots of the measurements on the three different VNA's of the logmag S11 of the long cable (RG58) and the medium cable (semi rigid) with open and short at the end.
The only difference between the three pictures is the VNA. Cables and calibration kit are the same.
It is safe to conclude there are substantial differences between the three VNA's
First picture: VNA 1 (nanoVNA) seems at low frequencies to have a Z0 that best matches the impedance of the cables but shows a substantial deviation above 600MHz
Second picture: VNA 2 (home build VNA) has a bit worse Z0 match and seems to have a S11 open of both cables that goes off track above 800MHz
Third picture: VNA 3 (3GHzVNA) becomes very noisy above 550MHz (need to investigate why as this is new) but the oscillations of the logmag S11 show no sign of substantial deviation apart from gradual increase in amplitude and the cable loss in dB stays nicely linear with frequency
This comparison underpins my initial assumption that the nanoVNA needs a bridge error model to correct the differences (in particular above 300MHz) and until these threeVNA's (with their individual bridge error models) give the same measurement results I better not try to calculate calibration kit error parameters
Feedback is welcome
--
Erik, PD0EK
