Hi A lot depends on how good your OCXO is.
1) With a typical OCXO, the 1 second tau should be around 2x10^-11 against a typical Rb. You are limited by the Rb. 2) With a good reference and OCXO the 1 second tau should be about 10X better than that (2x10^-12). 3) With some OCXO’s you can do 5 to 10X better still. Keep in mind that for 1 you will need a ~10ps counter. For the other cases, forget about a counter. Typically things like DMTD’s and TimePod’s get used in these cases. Chances are, you are looking more at your counter and Rb than at your GPSDO. Bob On Sep 2, 2014, at 9:31 PM, Bob Stewart <b...@evoria.net> wrote: > Hi Bob, > > Thanks for the review. I thought I knew a good deal of that, and promptly > forgot it in the heat of battle. > > Here's a plot of about 8,000 samples vs my trusty FE-5680A. The blue is the > OCXO output vs the Rb as measured on my 5335A w/10811 clock. The red is the > OCXO vs corrected 1PPS as measured by the TIC in the GPSDO. As this is my > first one of these, I don't know what to make of it. The two lines > intersect, which I assume is the point where the OCXO stability gives way to > the GPSDO stability. But, is ~1E-9 a good starting point for this old > 34310-T, or is it an indication that there is some underlying problem that I > haven't addressed? Perhaps the ADC in the PIC? > > > http://evoria.net/AE6RV/TIC/ADEV4.png > > Bob > > > > ________________________________ > From: Bob Camp <kb...@n1k.org> > To: Bob Stewart <b...@evoria.net> > Cc: Discussion of precise time and frequency measurement <time-nuts@febo.com> > Sent: Tuesday, September 2, 2014 5:39 PM > Subject: Re: [time-nuts] OCXO Phase Noise Measurement in Primitive Conditions > > > Hi > > There are a lot of GPSDO ADEV plots out there. The T-Bolt has a lot of plots > run on it, there are others. > > Ideally the plot should start out looking like an OCXO: > > 1) Left hand side rising as you go left. Rate of rise 1/Tau. > 2) Left hand side intersecting a flat region somewhere in the 0.1 to 1 second > range. > 3) Flat region running out to a few hundred or a few thousand seconds > 4) Rising region past the flat area more or less straight line as you go > further to the right. > > That’s what you start with before you have any GPS involved at all. The flat > region could be at 1.0 x 10^-11, it could be 10X, or 100X lower than that > depending on the OCXO you have. The flat region could run out to 1,000 > seconds at 1.0x10^-13 if you get really lucky. One way to measure all this is > to take two or three identical OCXO’s and compare them with a device capable > of < 1.0 x 10^-13 accuracy at one second tau. > > —— > > Now you have your GPS. Most modern GPS plots are pretty simple. You have an > error (say 2 ns) at one second. That gives you an ADEV of 2.0 x10^-9 at a tau > of 1 second. At 10 seconds the same error gives you an ADEV of 2.0 x10^-10. > At 100 seconds the same error gives you an ADEV of 2.0 x10^-11. It just keeps > on going down and down as tau gets longer. At some point it hits a floor and > then runs flat from there on. Depending on how you do your corrections the > floor is somewhere below 1.0x10^-13. > > —— > > Lay those two plots down on top of each other. > > An ideal GPSDO would have the best of both and only the best of both devices. > You would have the close in ADEV and flat area from the OCXO. You would then > hop on the falling plot of the GPS and ride it down to who knows where (let’s > say 1.0x10^-14). It would magically have no peaking or odd behavior (it’s > ideal). The closer you come to that ideal, the better your GPSDO is doing. > > —— > > In the real world we need to measure things like GPSDO’s. They can not be > measured against themselves. They need to be compared against something else. > Sorry, but there are no shortcuts. If you want to show how one is working, > you need a comparison standard. It can be a second GPSDO. It could be a > Hydrogen Maser. It might be a Cesium standard. What ever you use to do the > comparison must be up to the job. A counter with a 1 ns (or 100 ps) > resolution isn’t going to show you 1 second tau data on a good OCXO. > > Can you estimate what’s going on - sure. The gotcha is that you will never > know if your estimates are correct without a comparison. If you have made a > few hundred of these things, all done the same way, you probably can guess > pretty well. If you have a new design, first time up and running, your guess > may not be very good. > > —— > > Yes, that’s the short / abridged version. The full version goes on and on for > a few hundred pages. There are lots of special cases and a multitude of ways > to deal with each of them. There are also a number of end goals you can > target with a design. The optimum outcome will be different for each of the. > > Lots of Fun. > > Bob > > > > On Sep 2, 2014, at 12:02 PM, Bob Stewart <b...@evoria.net> wrote: > >> Hi Bob, >> >> I take it that the 3 segment ADEV pretty much defines a GPSDO. So, is my >> plot an indication that I'm riding the OCXO too hard to get the results I >> think I should be getting? What I think I should be getting is a flat phase >> line. But is a GPS receiver, even after applying quantization error >> corrections, accurate enough to give that? I see three things at play here: >> hardware, software, and my expectations. I'm trying to figure out which one >> (or several) of these needs to be addressed. I suppose I could hook the Rb >> back up and see the effect on the ADEV of the Rb vs OCXO as I loosen the >> gain values. >> >> Bob >> >> From: Bob Camp <kb...@n1k.org> >> To: Bob Stewart <b...@evoria.net>; Discussion of precise time and frequency >> measurement <time-nuts@febo.com> >> Sent: Tuesday, September 2, 2014 6:38 AM >> Subject: Re: [time-nuts] OCXO Phase Noise Measurement in Primitive Conditions >> >> Hi >> >> Your ADEV is (mostly) a plot of 1-2 ns resolution out over your sample >> range. It’s not a “typical” three segment ADEV. It’s more likely that any >> departure from a straight line is random rather than systematic. Yes I’ve >> spent a lot of time looking at random plots that I was *sure* had >> significant information in them….. >> >> Bob >> >> >> On Sep 1, 2014, at 9:31 PM, Bob Stewart <b...@evoria.net> wrote: >> >>> Hi Poul, >>> >>> I've made a few hardware changes and some software changes, and I was >>> wondering if you (and/or others) would take a look at this plot. The green >>> is phase over 70,000+ seconds with ~100ps resolution, while the blue is the >>> ADEV. (The sharp departures in phase are from large temperature changes >>> and not quite perfect temperature compensation software.) I don't see your >>> horizontal line down at the bottom, but there are some worrying horizontal >>> lines between about 3tau and 150tau. Should I take that to mean that I >>> have some noise/oscillations from about 1/3Hz to 1/150Hz? Those up at >>> about 150tau I think I can see by eye on another plot of different data. >>> I've got a lot of data collected from this run (still running) including >>> pTerm, iTerm, dTerm, and tTerm (temperature) updates to the DAC. I hope to >>> try to tease something out of that later, though the pTerm and iTerm are >>> mostly +1,0,-1 stuff. FWIW, in real terms, the dGain is at about 4.2. The > phase >>> starts to get ugly if I bring it much below that. iGain is .01 and pGain >>> is .05. There are misc damping and limiting factors applied, as well. >>> >>> >>> Just about ready to do a second board with updates from what I've learned, >>> so any help is appreciated. The plan is to release the source code at some >>> (not too) future date and make boards available, if there's interest. But >>> I've still got a lot to do before that. >>> >>> >>> http://evoria.net/AE6RV/TIC/ADEV3.png >>> >>> >>> Bob >>> >>> >>> >>> >>> ________________________________ >>> From: Poul-Henning Kamp <p...@phk.freebsd.dk> >>> To: Bob Stewart <b...@evoria.net>; Discussion of precise time and frequency >>> measurement <time-nuts@febo.com> >>> Sent: Thursday, August 28, 2014 1:04 AM >>> Subject: Re: [time-nuts] OCXO Phase Noise Measurement in Primitive >>> Conditions >>> >>> >>> -------- >>> In message <1409175707.2080.yahoomail...@web142706.mail.bf1.yahoo.com>, Bob >>> Ste >>> wart writes: >>> >>> This looks like a step in the right direction. >>> >>> The "correct" allan plot will have a clearly visible horizontal >>> segment somewhere in the 100-10000 second range (depending on OCXO quality). >>> >>> This is where the GPS long-term stability "takes over" from the OCXO's >>> better short-term stability. >>> _______________________________________________ >>> 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.