Month: June 2024

An experiment with NanoVNA and series through impedance measurement concluded with:

The NanoVNA-H4 v4.3 and Dislord firmware v1.2.30 using 5 term calibration does not appear to give good accuracy on series through measurement of impedance.

The method

The focus is on the corrected s21 measurement as that is what drives the conversion to series through impedance.

The test fixture was designed to be very close to ideal at 1MHz, the test Zx is a 1% SM 200Ω resistor, taken as being 200+j0Ω.

The method of error correction based on a one path calibration does not correct all errors. In particular it does not correct load mismatch error, but an ‘enhanced response’ calibration of s21 (done properly) should correct source mismatch errors. Of course there remain other errors that cannot be corrected but in a good implementation and effective application, they should be relatively insignificant.

Above is an extract of the published schematic of the NanoVNA-H4 v4.3, the circuitry around Port 1 and Port 2 jacks is of particular interest. Continue reading An experiment with NanoVNA and series through impedance measurement… more

s21 series through impedance measurement

HP and their successors describe this technique in many publications, the following is from 5988-0728EN.pdf.

Note in the second formula the use of 2Zo and 100 synonymously. The formula depends on the Port 1 source being a Thevenin source with source impedance Zo and the Port 2 input impedance Zo (which they take to be 50+50Ω).

Derivation of the expression for the unknown impedance in an s21 series through measurement shows that the critical value is actually Zsource+Zload or Zs+Zl. If Zs+Zl ≠ 100 then that formula is not correct, the measurements will have error. Continue reading An experiment with NanoVNA and series through impedance measurement