Sorry I didn't reply again - I was on lunch break and had to go back in to work (thankfully I can still go to work!)
As John noted, you can set the Velocity Factor: it is at the end of the transform menu.
Another important note for TDR use, which builds on what John was saying: when doing very high frequency measurements, there is ambiguity introduced if the reflection delay is longer than the frequency allows.
So to do TDR on short lengths of cable, with high resolution, use a high stop frequency.
But to do TDR on long lengths of cable, you need to use a lower stop frequency.
For TDR, you should always leave the start frequency at 50KHz (the lowest, which is essentially 0 compared to the stop frequency) - this makes the FFT math work right. And you should set it to "Low Pass Impulse" for length measurements, and "Low Pass Step" for reading impedance changes along the cable (also shows length, but less distinctly).
Tables don't work too well here, but see if this makes sense, all at a velocity factor of 1.0 (100 on the H4 display):
StopFreq:1500MHz gives tmax=26ns --> maxlen=3.9m or 12.8ft
StopFreq:1000MHz gives tmax=39ns --> maxlen=5.85m or 19ft
StopFreq:500MHz gives tmax=78ns --> maxlen=11.7m or 38.4ft
StopFreq:200MHz gives tmax=195ns --> maxlen=29.2m or 95.8ft
StopFreq:100MHz gives tmax=390ns --> maxlen=58.5m or 192ft
Those figures are a bit misleading, because with a lower velocity factor, the lengths are shorter by that factor.
So when you want to measure a cable that is ~100ft long, you need to use a StopFreq of about 100MHz to get a true reading without the ambiguity factors - otherwise the 'pulse' doesn't have time to make it down the cable and back to be measured.
This also means that the measurement accuracy is smaller for longer lengths (since each measurement point is larger in time/length, corresponding to the frequency bin being smaller). I haven't yet tried to figure out the accuracy numbers, but I know it is within a millimeter at 1500MHz/26ns, and I suspect it is only within several inches at 100MHz/390ns. And as for John's idea of using the Electrical Delay: I don't know what happens if it is set longer than the max lengths above: it will shift the window, but if used beyond the range of the 'tmax' value, it still can't do away with the ambiguity that results from multiple waves reflecting (when set to have a tmax shorter than the cable length requires).
I hope some of that makes sense; don't know how well I described it.
Stan
