Dr Bruce Griffiths said the following on 04/08/2007 10:01 PM: >> By doing this, you can actually observe the measured value increase very >> slowly, one can even observe the sub 1ps values increase! Doing this, you >> can >> see about 3ps of added delay for every single turn of the SMA connector >> ground >> nut. >> >> Not sure many other instruments can do that. >> >> > There's no particular reason that they cannot if they have adequate > resolution and stability and a sufficient number of measurements are > averaged. > This should be possible even with an HP5370A/B albeit with a slower > response time.
I routinely measure coax cable lengths to 200 femtosecond resolution with my 5370B (though to be practical I normally round to at least 10 and sometimes 100 picoseconds, to take into account possible measurement errors like adapter lengths and cable tempco). I recently built 6 nominally 10 foot GPS antenna cables out of LMR-400. They all had N connectors on one end, but the opposite ends were two each of N, BNC, and TNC, which made measurement interesting because the needed tweenie configurations and lengths were different. After doing the best I could to normalize out the adapters, I measured the six cables and the spread from longest to shortest was 61 picoseconds (nominal length was 12.55 nanoseconds); I suspect that as much as 40 ps of that was due to uncalibrated tweenie lengths because the three pairs of cables with identical connectors matched within 15, 9, and 20 picoseconds. (And the coax was measured and cut by hand, so there was certainly some room for error there!) The technique was to feed a 1000 Hz pulse train (generated from a 5369A time synthesizer, but any good pulse generator would do) into the 5370B TIC by routing the pulses into a T connector connected to the counter START input; the other side of the T went to a four foot cable to provide some additional delay. The cable under test was connected to that cable, then to another four foot cable, and then to the stop input of the counter. I first measured the delay with just the two fixture cables and N adapters in place, which as 12.579 nanoseconds; I then subtracted the estimated length of the N barrel connector that substituted for the cable under test. Then I added the cable under test, remeasured, and subtracted out the fixture delay to get the cable length. I used a 100k sample average which yields (IIRC) 200 femtosecond resolution in the 5370B, and for each run checked the min, max, and standard deviation statistics to make sure nothing goofy was going on Standard deviation for a run like that on my 5370B is typically 30 to 40ps. John _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
