OwO 2020/06/19 20:49
Yes, this is true, but normally with the ordinary SOLT calibration it's
not possible to calibrate out the port 2 match error in a T/R VNA. Only
port 1 is fully calibrated out, and S21 is a simple response calibration
that normalizes the gain and phase to 0dB/0 degrees for the thru
measurement. However there is a technique that allows you to calibrate
out the imperfect port 2 match if you measure the DUT both ways. This
isn't yet implemented in NanoVNA-QT, but the scikit-rf python library
has it:
https://scikit-rf.readthedocs.io/en/latest/api/calibration/generated/skrf.calibration.calibration.TwoPortOnePath.html
Why is it not possible to calibrate out port 2 match error? When
measuring the thru standard, we did discover the port 2 reflection
coefficient. However, it's not possible to calculate how this will
affect the DUT's response. We know how much signal is incident on port
2, since it has a receiver. We know how much signal is reflected from
port 2, because we know its reflection coefficient. However, we don't
know the S12 of the DUT, so we don't know how much of this signal goes
back to port 1. By measuring the DUT both ways, we learn of its S12 and
S21, which allows you to formulate a system of equations to solve for
the true S parameters of the DUT without the influence of port 2 match
error. This still depends on measurements being stable after unplugging
and reversing the DUT, so you need connectors with good repeatability
and phase stable cables.
On 19/06/2020 20:23, iz1fks wrote:
> Sorry, but I haven't understood. If the full calibration is performed at the port1 and the path is calibrated using through it should be possible to measure both S11 and S12. Reversing the EUT connection is possibile to measure the other 2 S parameters, isnt'it?
>
> regards
>
> Phil
>
>
>
