On 6/14/23 2:17 PM, Jim Lux wrote:
On 6/14/23 12:20 PM, DiSlord wrote:
I hope you disable internal NanoVNA calibration and use only NanoVNA Saver calibration
Linear Interpolation (used in internal calibration if points count or frequency range not equal) can give errors
This is a good point..
In addition, you might use a sim program to simulate an ideal line with a small discontinuity, and see what kind of wiggles you get.
Think of this - you're doing an S21 measurement - a small reflection at either end will constructively or destructively interfere with the main line signal.
On a group delay plot like this, you're looking at fractions of a degree in phase -
A reflection (or spurious signal) that is 90 degrees out of phase, and down 40dB would give you approx atan(.01) = .6 degrees.
just to illustrate how a small error can show up in group delay, I simulated what you'd get with a 2 meter cable, and adding a "triple transit" echo (i.e. a reflection at the Port 2 end, then another reflection at the Port 1 end). So what winds up inside the Port 2 (Ch1) receiver is the sum of the direct signal and an attenuated delayed signal.
60 dB (essentially a -30dB mismatch at each end) gives you about .03 ns of ripple on 9 ns. 40dB is tenths of a ns, etc.
I ran it for 3 cases, 20dB, 40dB, and 60dB down.
