[time-nuts] Newbie questions
Tom Duckworth
tomduck at comcast.net
Wed Jan 6 02:22:41 UTC 2010
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
tomduck at comcast.net
----- Original Message -----
From: "Jim Mandaville" <zygo at dakotacom.net>
To: "Discussion of precise time and frequency measurement"
<time-nuts at febo.com>
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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