[time-nuts] T.I. experimenting - newbie question

Magnus Danielson magnus at rubidium.dyndns.org
Sun Apr 3 11:45:36 UTC 2011


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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