On 7/14/21 12:25 AM, Uwe Lange wrote:
On Tue, Jul 13, 2021 at 03:27 PM, David Eckhardt wrote:
So what if the "bull's eye" on the Smith Chart is not appropriately placed
in the center and appears to the right of center halfway between the center
and where the 100-ohm circle intersects the real axis? I know that is
75-ohms. So, what is the problem??
There are two issues:
First the one from OP: he built at 50/75ohm pad and calibrated the NanoVNA with
a 75ohm load. The pad transforms every impedance on its 75ohms side into the
50ohms world of the NanoVNA and the reported impedances are then off which
confuses the OP.
A resistive pad does NOT transform from a reference impedance of 75 ohms to a reference impedance of 50 ohms.? A transformer would (over some frequency range).
All a resistive pad (of whatever loss) does is provide a 50 ohm impedance on port A when port B is terminated in 75 ohms, and provide a 75 ohm impedance, when port A is terminated in 50 ohms. It does not, for instance, provide a 100 ohm impedance when Port B is terminated in 150 ohms.
There are several solutions to this: The probably easiest is to not use the pad
in the first place and calibrate with a 50ohm load. This would also avoid the
5.6dB loss of dynamic range incurred by the pad.
Right, and then you can just change the way the smith chart displays and VSWR, etc. are computed so they turn the 50 ohm measurement into some other reference impedance.
Another option is to keep the pad and calibrate with a 75ohm load but *inform*
the NanoVNA that the cal load was 75ohm. The NanoVNA then has a chance to
display correct impedance values. My patch to NanoVNAsaver achieves this.
Sure, you could do this.? But why not just calibrate with a 75 ohm load, leaving out the pad (which in this case would reduce dynamic range).
And then, you'd need to change some other code to make sure that displays and the calibration use the 75 ohm reference (and calibration) impedance.
