On Sun, Dec 3, 2017 at 8:41 AM, Forrest Christian (List Account) < li...@packetflux.com> wrote:
> One parameter in particular that I'm trying to figure out how to determine > is what the expected offset from UTC/GPS is, i.e. +- a certain number of > ns. I'd like to be able to look at the unit and determine how healthy it > is, and what level of uncertainty I should expect. I know the ADEV is > based on the deviation of the PPS signal, but it doesn't sound like it's > related to the deviation from the UTC second. I also see the PPS > parameter up at the top in Lady Heather, but this doesn't seem like it's > what I'm looking for. Can someone clarify this for me? > Lady Heather can't tell you the quality of what the GPSDO outputs. Without a better reference to compare against, the best you can do is infer from some "normal operation" parameters, that it your unit is likely operating within spec. First, some background about my unit: I'm currently running with a time constant of 1000 seconds, and a dampening of 1.2, with the DAC at around 2.1062 Volts. I have the satellite mask set for 10 degrees and 4 AMU; more restrictive values were found to be unusable because of the lack of a full sky view. I strongly recommend using much faster settings initially, like 100 seconds and 0.7 dampening, and letting it run like that until the DAC trend stays mostly horizontal over at minimum a week-long 360 minute/div view. It can take a while (days, months, or 2 years in my case). My Thunderbolt E has shown some behaviours that can be exhibited when there is poor satellite coverage: 1. There is no window for what is considered a valid offset, so the unit will chase wildly erroneous values. 2. Hold-over algorithm seems to fail with multiple short back-to-back outages, or constellation changes. This behaviour is probably related to #1. A minimum-valid timer would also have helped here. It typically takes my unit 3 days to recover from a poor-reception event with wildly erroneous offsets, which would typically last nearly 3 hours. Though, I've not had such an event since the unit stabilized recently (perhaps luck). What I, a total amateur, look for in Lady Heather: * Low DAC span, and predominantly horizontal trend over an 60 minute/div view, with some drift trend possibly showing over 360 minute/div or 1440 minute/div views. Expect some relationship to temperature change, with the DAC having to compensate for all the electronics located outside the crystal oven, which includes the DAC itself. * PPS rms around 3 ns (roughly 1 meter) and under 10 ns. I think mine is reporting just under 6 ns, last I looked. Don't know if this is good or bad, but it seems reasonable to me. The relative amount of cloud cover seems to affect this value slightly (lower during periods of clear sky). * Temperature can largely be ignored, until you see a trend that doesn't make sense. Make sure to properly terminate the outputs, and keep the loads driven constant, as there is a definite dependence on output loading! ~~ Andrew E. Mileski _______________________________________________ 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.