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

[David C. Partridge]

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-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On Behalf Of Magnus Danielson
Sent: 03 April 2011 12:46
To: time-nuts at 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

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