Another interesting aspect of this is: the interaction between the transformer's nonlinear response vs load and freq, and the workings of the Open-Short-Load math.

In a nutshell, the OSL math works by assigning, at each measured frequency point, real & imag values that define the short and open conditions, plus one "known" impedance value. All measured impedances are then mapped or scaled linearly to those values.

For this to work, the transformer has to have a transfer response that is linear with load and frequency within the ranges you're wanting to use it. This is a tall order into low Z loads.

I decided to evaluate some xfmrs over my desired range of use: from 0 to 10 Ohms, and 10Hz to 1MHz. Other than xfmr saturation, there were no linearity problems at low freqs, so I evaluated them at four freqs (10kHz, 100kHz, 500kHz, and 1MHz) and seven resistances from 1 Ohm to 10 Ohms. (With a 1 Ohm current sense R, this corresponds to a measurement range from 0 to 9 Ohms). I converted the S21 to real/imag values and plotted them on a linear scale. This will give immediate visual indication of what is the best OSL "Load" resistor to use with that xfmr, and over what impedance and freq ranges the xfmr can be used with OSL compensation to give accurate results.

Since Tim's 3:1 xfmr with paralleled secondary gave the best HF performance, it was first. The plots shows S21 at the 4 freqs and 7 impedances for each, with a line from the 1 Ohm point to one of the upper-impedance points to highlight the linearity of the data points.

The first plot shows the 3:1 with linearity to 10 Ohms. Up to 100kHz, it is excellent. So used with a 9 Ohm Load R, this xfmr would deliver accurate results from 0 to 9 Ohms up to 100kHz.

But at 500kHz and 1MHz the OSL math would not correct the measurement accurately. The measured points are highly nonlinear. The 2nd plot shows that its linear range at high freqs is only to 2 Ohms. So this xfmr would give good results up to 1MHz with a 1 Ohm Load R measuring 0 to 1 Ohms, with increasing error at higher Z's.

This is a much narrower operating range than expected or hoped. It wipes out any advantage gained by the unit's better HF response.