On 10/19/22 3:26 PM, Leif M wrote:
"You can calibrate for any resistance"
Sure. But if those things can be made would it improve accuracy, when measuring low and high impedances.
Sure - and people do that - sometimes as part of a research project or dissertation.
Ultimately, what you do is look at what you need to measure, assess the uncertainty with your "50 ohm" equipment and go from there. Usually, it's a matter of SNR and accuracy of measurement of small changes. Looking at a 5 ohm circuit, the reflection coefficient is huge (0.8=45/55) and if you want 10% accuracy, you need to look at telling the difference between about 0.02 out of 0.8 (2%).
Z mag gamma
4.5 .834
5 .818
5.5 .801
The NanoVNA can easily make measurements to 2% or even 1%.
OTOH, if you need 1% accuracy, you're looking at needing a measurement to 0.2%.
The NanoVNA can still do that, but you'd better have really good cal standards, etc.
It's also going to be painful making accurate S21 measurements of lossy components, because you've got 15dB loss through the first reflection and the same at the output, so it's like having a 30dB pad in series with your UUT.
If you want to get really wild, look into "dembedding" - it's of vital importance in measuring circuits on a semiconductor die and they're rarely 50 ohms.
In addition, waveguide has a frequency dependent impedance that is also propagation mode dependent. And it calibrates out.
I've used near field antenna ranges which use a VNA as their core, with a open waveguide as the probe, and it's nothing even remotely close to 50 ohms. But it *can* be precisely calculated, and verified.
There's also VNAs that don't use the coherent transmitter/receiver like the NanoVNA. Look up 6-port analyzers - which use only power detectors (which can be made very accurate).
