On 6/10/23 11:04 AM, Roger Need via groups.io wrote:
Those interested in how the TDR was implemented in NanoVNA Saver will find the details in this groups.io post by Herb Walker.
/g/nanovna-users/message/9651
For more details on how the S11/IFFT method of TDR works this tutorial by Agilent is quite informative. It includes details on why "windowing" the data is necessary, how to determine the upper stop frequency required based on estimated cable length, why the lower start frequency should be close to DC and other considerations.
Roger
The Keysight Ap Note is a good reference.
For what it's worth, in the older NanoVNA-Saver version I have, there are some problems in the implementation, specifically because it does not deal with the "non zero" starting frequency. If you set 10 MHz to 60 MHz sweep, I don't know that you'd get the right results, because it just assumes that you're feeding the IFFT evenly spaced points from 0-{highest freq}.
It is possible that when you select TDR mode it changes how it builds the measurement segments so that the lowest frequency is equal to the spacing between frequencies, and then puts in a zero for the DC term.
I do know that the Keysight boxes (e.g. the fieldfox) do handle both lowpass and bandpass correctly. When you pay $20k, that's something you get. For other applications, with uneven spacing, I've written code to deal with it.
