diyer,
You make a good point about the harmonic content of the square wave. They do extend out to much higher frequencies than 50 kHz. I am not sure how these harmonics would impact the 50 kHz result in a case where both inputs are terminated in 50 ohm loads. And specifically, how they would contribute to an answer at 50 kHz of 0.0 + j0.0.
I do see zero S21 value at frequencies above 50 kHz. I see intermittent zero values for S21 up to about 10 MHz but not usually above that. An example is shown the attachment "S21Isolation-singlescan.png." I am not sure why. I have a nanoVNA and a nanoVNA-F and the -F model shows this behavior more frequently. The nanoVNA-F model seems to have smaller S21 values in the isolation measurement below 50 MHz than the nanoVNA that I have as can be seen in the S21Isolation files attached. These were created by averaging 20 scans. When averaging 20 scans, they don't show 0.0 + j0.0 or 0.0 dB for S21 at any frequency.
I attribute most of this issue with zero S21 values to the likelihood that the S21 signal with both inputs terminated in 50-ohm loads at low frequencies is not really noise from the input but some sort of very low-level, repeatable internal leakage signal. I am wondering if the S21 isolation calibration process causes the S21 calibrated answer to be identical to zero for repeatable internal leakage which is 100 dB or more below the reference signal.
None of these actual indications in the device are a concern for me in the actual measurement. I would just like nanoVNA-Saver to deal with the 0.0 + j0.0 value received from the VNA in a little more appropriate way than to indicate that S21 is 0.0 dB in this case. Apparently in the nanoVNA device itself the 0.0 + j0.0 answer is dealt with properly because I never see an S21 indication of 0.0 dB in the logmag plot when terminating both inputs with 50-ohm loads.
--
Bryan, WA5VAH
