Following on from Common mode choke measurement – length matters…
Lots of people have reported experiments to show gross failure of s11 reflection measurement of high impedances such as those encountered measuring common mode chokes.
Above is a chart of a “10k resistor with leads” from (G4AKE 2020), the curve of interest is the s11 curve which he describes as unsuitable. He did not publish enough information to critique his measurement… so I will conduct a similar experiment.
My experiment
Above is a pic of my experimental setup. The resistor on Port 1 is a 10k 1% metal film resistor. The NanoVNA has been SOL calibrated at the Port 1 jack.
Above is a screenshot of the measurement. Quite similar result to that shown in (G4AKE 2020). (Note R, X, s11 phase at the marker.)
Note the s11 phase plot, we see the phase of s11 falling at a uniform rate from 1 to 101MHz.
What should we expect?
s11 for a 10k+j0 DUT should be 0.990050+j0 or 0.990050∠0.000° independent of frequency.
But we see this linearly decreasing phase. It is a big hint, transmission lines do this sort of thing.
So what if we attempt an approximate correction using e-delay.
e-delay of 54.5ps flattens the phase response, and the R and X values are closer to ideal, not perfect, but much closer than the uncompensated plot earlier.
A SimNEC simulation
Above is a SimNEC simulation of my experiment.
Conclusions
- An experiment to duplicate G4AKE’s measurement achieves similar response.
- Drilling down on the detail of the experiment response hints that the resistor pigtails contribute transmission line effect that are the main cause of the poor response.
- An approximate compensation of the transmission line effects gives an impedance measurement that is much better than G4AKE’s recommended s21 series through measurement.
- G4AKE condemns s11 impedance measurement as unsuitable, but there is good reason his fixture was the main reason for poor results… length matters.
- Read widely, question everything.
References
Mar 2020. G4AKE. Measuring high and low impedance at RF.