One should of course be aware that the number notation used is no longer
considered correct.
Also even after correcting for the non standard notation the last set of
numbers is incorrect:
eg
40.000 000 003 GHz is equivalent to an error of +7.5 parts in 1E11 not
7.5 parts in 1E-11.
Bruce
Tom Duckworth wrote:
Jim,
Attached is some info on how to measure/calibrate a time base or
oscillator which you might find useful per your e-mail.
Tom
Tom Duckworth
[email protected]
----- Original Message ----- From: "Jim Mandaville" <[email protected]>
To: "Discussion of precise time and frequency measurement"
<[email protected]>
Sent: Tuesday, January 05, 2010 11:17 AM
Subject: [time-nuts] Newbie questions
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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