With any decent TDR you can SEE all the connections so you can measure the cable length (+/- a few mm) directly.
Regards, David Partridge -----Original Message----- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of Magnus Danielson Sent: 03 April 2011 12:46 To: time-nuts@febo.com Subject: Re: [time-nuts] T.I. experimenting - newbie question 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 _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.