One often sees experts online insisting that s11 reflection impedance measurements of common mode chokes is inaccurate, and the ONLY way is s21 series measurement.

This one is built upside down so the connections are visible.

In this experiment using the SDR-KITS VNWA testboard (above), a series of measurements are made of an inductor, and the test setup.

The nanoVNA-H (modified) was calibrated using the test board and its associated OSL components. The test board is used without any additional attenuators, it is directly connected to the nanoVNA-H using 200mm RG174 fly leads.

## s11 reflection measurement

Above is the s11 reflection measurement. It is quick, and most tools directly display R and X vs frequency (though some of the new age tools display X as pF or µH equivalents). This is nanoVNA MOD v3 (a derivative of nanovnasharp).

Ok, so we have 4856-j852Ω.

## s21 series measurement

A s21 series measurement was performed and the s21 magnitude and angle retrieved from a saved s2p file. At 19MHz, the data was:

19000000 0.980218409 -0.200792857 0.021893894 25.165374550 1.0 0.0 1.0 0.0

s21 magnitude and angle is the fourth and fifth column.

So, that is not impedance, it is not direct reading in most analysis tools. Never mind, let’s calculate it with Calculate impedance from S11, S21 using S11 reflection, S21 series & S21 shunt .

Ok, so we have 4034-j1942Ω. Hmmm, that is quite a bit different to the s11 reflection measurement.

Let’s perform a s21 series measurement of the 50Ω cal part.

Well that is not what we expect.

Let’s perform a s11 reflection measurement of the 50Ω cal part.

Well that is just fine.

## Why is there such a disparity between the s21 series measurement and s11 reflection measurement of the 50Ω cal part?

One of the contributing factors is that the formulas for conversion of the measured s21 value to impedance depend on an assumption that both the source and the load have Thevenin equivalent impedances of 50+j0Ω

The plot above shows that the input impedance of the extended Port 2 (CH1 in nanoVNA speak) has quite a departure from 50+j0Ω.

This problem is solvable, you just insert an instrument quality 20dB attenuator on the rx side of the test board, and calibrate in that configuration. That brings its own problems in that it has effectively raised the noise floor… so the very reason for preferring s21 series impedance measurement is diminished. Depending on the VNA, you may get away with just 10dB of attenuation… that is better but still degraded noise performance.

Trying attenuators is left as an experiment for the reader.

## Conclusions

There is great benefit in direct reading of R and X, benefit that should not be overlooked.

Uncertainty in Zin of Port 2 rolls up into uncertainty of s21 series impedance measurement.

An attenuator can be used to better control Port 2 Zin, but at the expense of noise performance.

Common mode chokes with high impedance are very sensitive to stray capacitance and distributed inductance of connections, and the measurer must consider the design of a test fixture appropriate to the deployment.

In the case of common mode chokes with high impedance one is less interested in the peak impedance values, or even impedance at some specific frequency (since it may be very sensitive to stray capacitance), but rather the range of frequencies where common mode impedance exceeds some criteria.

## References

- Agilent. Feb 2009. Impedance Measurement 5989-9887EN.
- Agilent. Jul 2001. Advanced impedance measurement capability of the RF I-V method compared to the network analysis method 5988-0728EN.