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

Joseph Gray jgray at zianet.com
Tue Apr 5 05:11:15 UTC 2011


Hal, replies inline.

On Mon, Apr 4, 2011 at 10:21 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
>
>> Here are the results I got using your suggestion. The numbers may be a bit
>> different than last night as I'm not sure I'm using the same cable.
>
>> A+, B+ = 18.9 ns
>> A-, B- = 19.4 ns
>> Obviously there is some difference in delay between the A and B channels.
>> Otherwise, the two numbers would have been identical - correct?
> ...
>> If I take the average of the two readings, I get 19.05 ns, which is more
>> precise than the readings I'm taking. I'm rounding to the nearest 0.1 ns on
>> the readings.
>
> There are several possibilities for asymmetry.
>  The input signal could have other than a 50/50 high/low ratio.
>  The trigger levels could be set at other than the half-way point.

See my reply to John about trigger levels.

> Do you have a scope?

Yes, I will dig out the scope and check. The last time I checked the
Spectracom 8140 (long ago), it was ok. I'll check it again.

> There are probably second order problems with different rise/fall times.
>
>
>> A+, B- = 5014.6 ns
>> A-, B+ = 5023.5 ns
>> If I take the average of these two readings and subtract out the 5 us for
>> 1/2 period of the 100 KHz square wave, if get 19.15 ns. This agrees
>> very closely with the above average.
>
> I read that as everything mostly works.

That was my assumption, but it's nice to have someone more
knowledgeable confirm it.

> The question is how well does it work and/or what can you do to get
> more/better data?
>
> You can get another set of numbers by swapping the A and B inputs so the
> signal goes from B to A rather than A to B.
>
> Some of the errors might be more obvious if you deliberately offset the
> trigger levels.
>
> NB:  That's for all 4 modes:
>  A+ to B+ will be slightly less than a whole cycle.
>  A+ to B- will be slightly less than a 1/2 cycle.

I'll try swapping inputs and see what happens. I'm not sure if I'll
get to this tonight. If not, tomorrow night.

Joe Gray
W5JG




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