On 04/03/2011 12:40 PM, Hal Murray wrote:
If I understand you correctly, then I did not have things setup
symetrically. Now I have a tee on each of the A and B inputs. The unused
port on the B tee has a 50 Ohm terminator. Is this what you mean?
Symmetry makes it easier to see what is going on. If you use a Tee on the B
input, then the timing from the center of the Tee on the A input to the
internal logic should match the same setup on the B port. That lets you
focus on what is between the two Tees.
Trouble is that you have added the electrical length of a tee inbetween
the center-to-center distance. You still need to calibrate that out of
the equation. Still not much, but it's a bias.
Consider that you have three cables (tc1, tc2, tc3), a barrel (tb) and
input offset (toff).
You measure them
t1 = toff + tc1
t2 = toff + tc2
t3 = toff + tc3
t4 = toff + tc1 + tb + tc2
t5 = toff + tc1 + tb + tc3
t6 = toff + tc2 + tb + tc3
6 measurements for 5 unknowns should work out well. Remove one cable and
you only make 3 measurements for 4 unknowns.
With the above setup, using the shortest cable I have gives me 1.5 ns.
Adding the longer cable to the shorter, using a F-F BNC coupler gives me 21
ns, for a difference of 19.5 ns.
Adding the barrel complicates things. What is the delay through the barrel?
(My guess would be about an inch at 50% of c, so that's ballpark of 0.2 ns.)
If you swap the short cable for a longer one, then the only difference is the
change in length.
Only helps for diffrence in delay, not actual delay.
Cheers,
Magnus
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