[time-nuts] Newbie questions

Jim Mandaville zygo at dakotacom.net
Tue Jan 5 19:17:57 UTC 2010


I am new to the list (although lurking now a while) and also new to 
the more precise species of  frequency and time measurement.  I have 
recently powered up an LPro and a Thunderbolt, both of which appear 
to be working by the book.  Connecting the TBolt to my scope external 
sync and the LPro as an "unknown" I see the pattern moving one 
division (cm) to the left in 295 seconds with a 0.05 us-per-division 
setting on the scope (the fastest setting available).  This, if my 
newly-learned calculations are correct, indicates a difference of 1.7 
X10-10 (0.0017 Hz).  This appears to be confirmed by my HP 5335A 
counter, which shows the LPRO 1or 2 thousandths of a Hz low, using 
the TBolt as an external time source. An HP manual I have indicates 
that a low unknown pattern should be moving to the right, not the 
left, on the scope, so this sort of puzzles me.

I have a few questions that I'm hoping some of you more experienced 
hands can help with:

1.  Can someone tell me the meaning and significance of the "Timing 
Outputs" numbers in the lower left corner of the TBolt monitor 
window?  (Mine right now is showing plus 3.75 ns and plus 0.01 ppb). 
The TBolt manual does not describe these, although on one page it 
lists them as "estimates of UTC/GPS offsets."  Do these numbers show 
the difference between my receiver outputs and the time being kept by 
my present satellites?  Or is it the difference between my receiver 
outputs and master gps time (somewhere)?  Neither of these?  The use 
of two decimal places on nanoseconds implies great accuracy.  Is this 
obtained in practice?  My ppb on 10 MHz usually lies between plus 0.1 
and minus 0.1, often hanging around 0.01 or 0.02.  I have not so far 
put in any compensation for cable delay.

If the TBolt "knows" what these differences are, why doesn't it just 
factor them into its outputs?  Or does it?

2.  What is a reasonable expectation of TBolt accuracy (at any given 
time that I use it for measurement) for the 10 MHz relative to NIS? 
How accurate would it be, say, 90 percent of the time? (Looking for 
just an experienced guesstimate here).

3.  What format do I use to put in pps nanoseconds compensation for 
cable delay (I use about 19 feet of RG-58U).  I understand this 
should be a negative number.

4.  Does anyone know a way to force the 5335A counter to display 
another decimal place in frequency measurements?  I am getting to 
0.001 Hz by using the "mean of 100 counts" function on the counter, 
but I think the counter has at least one more digit available which I 
would like to use when accuracy justifies it (e.g. when using the 
TBolt as an external time source).

Any comments and suggestions appreciated
Jim, KF7A




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