Jim,
We use a benchmark 1 ns per foot of coax (RG-59).
You could measure the delay by using a resistive splitter (50 ohms) and two
cables (say a 2 foot and a three foot, each terminated at the far end with a
50 ohm pass through terminator). Drive the splitter with your 10 MHz signal
and measure, at the far end, using an appropriate 2-channel scope or counter
with the necessary resolution, the difference in time delay between the two,
which will give you a pretty accurate delay per foot. Both cables should be
the same coax type.
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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