Yury Kuchura EU1KY
Interesting idea! I will try to model this approach in Python + numpy. But maybe not for this particular project:
This device is not a lab grade VNA. First of all it is just a good antenna analyzer used to finetune very unstable system which actual antenna is, considering that in real life it is exposed to extremely varying outside weather conditions which affect its parameters. I believe the provided precision is quite enough for such a system. Also, I don't think LO phase noise makes any significant influence while tuning real antennas, especially on short waves where the incoming signals are sometimes very strong comparing to this noise.
Let me clarify what is the math is under the hood.
Single precision floating point RFFT is used for phase and magnitude calculations. 512 samples used, with 48 kHz sampling rate, IF is 10031 Hz (bin 107), then Blackman windowing. I've found no improvements in precision when I used more samples, but the sampling and calculation time grows significantly.
Then the voltage magnitude ratio between channels is calculated as a root of the sum of powers in five bins where the most of the signal power is concentrated. Even if the IF is not exactly in the center of bin, this approach makes scalloping loss negligible. But even this is an overhead I think because I use the ratio of magtitudes for further calculations, it remains the same when both coherent signals are offset from the center. The phase difference is taken only from the central bins, modelling showed that it remains constant when frequency offsets up and down within the bin. With known magnitude ratio and phase difference, it is then easy to calculate the raw impedance of the DUT connected to the bridge. It is then easy to convert it to complex reflection coefficient for chosen Z0, and to apply OSL calibration.