Some time back there was another thread on this subject that was very helpful. The content was:
4a.
TDR setup for NanoVNA #tdr
From: John Gord
Date: Thu, 09 Apr 2020 01:07:40 CDT
The following setup for the NanaVNA-H, -H4, or -V2 will give a display much like that on a conventional TDR such as the Tektronix 1502:
Set stimulus for a wide sweep, say 50kHz - 900MHz or more.
Do an SOLT calibration at the desired measurement point (if not already done)
Set TRACE 0 for Real format
Turn TRACE 1 off
Set TRACE 2 for Smith,
Turn TRACE 3 off
Turn TRANSFORM ON
Select Transform LOW PASS STEP
The resulting display should be similar to that on a step-type TDR like the Tek 1502. With 900MHz max, the range is about 43ns, with 3GHz it is about 8ns.
Impedance along any connected cable can be read by moving the marker to the desired time (distance) and looking at the Smith chart marker values.
Save via the CAL menu when you have it all set up.
An open cable will show as an initial middle value depending on the cable impedance followed by a step up. A shorted cable will have a step down. A properly terminated cable will just be a steady middle value.
I used this to check my lab cables and found several that I had assumed were 50 ohms to be 75 ohms instead.
--John Gord
I found that I could customize the time span on the screen by changing the Stimulus STOP frequency according to a formula. If you have an idea of the length of your cable, choose a frequency span that is slightly longer than the propagation time of your cable.
Time selection on vna screen
1/f = t(max)/39
Tmax=39*1/f
f=39*(1/tmax) or f=39/tmax
So if you have about 50 feet of cable with a v of 0.66, the prop time of the cable is t=L/Cv =50/(984E6*.66) or 77 nS. (C is the speed of light in feet/seconds 983.6x10^6 f/s)
You would want a maximum time on your screen of 80-100 nS.
for 100 nS max, the STOP frequency would be 39(1/100e-9) (Or 39/100E-9) or 390 MHz
This will give you a displayed pulse representing the entire length of the cable. Any discontinuities along the line will show up as variations along the pulse.
At the end of the pulse, the trace will indicate the nature of the termination. An open end will step up, a shorted end will drop to zero, a matched load will continue at the same level without ending.
