Except the 1nS/ft figure is a good approximation to C (speed of light in vacuo). In RG58, you would expect to see 0.66C or about that.
So 1nS for 8" of cable is a good ROT. Or put the other way round, about 1.5nS per foot. Dave -----Original Message----- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of Tom Duckworth Sent: 06 January 2010 02:23 To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Newbie questions 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 tomd...@comcast.net ----- Original Message ----- From: "Jim Mandaville" <z...@dakotacom.net> To: "Discussion of precise time and frequency measurement" <time-nuts@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 > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.