Hi
On Jun 1, 2013, at 4:24 PM, Jim Lux <jim...@earthlink.net> wrote: > On 6/1/13 10:35 AM, Bob Camp wrote: >> >> Both suffer from people talking about levels (-120 dbc or 1x10^-11) without >> mentioning the offset or tau. Since both are highly dependent on the offset >> or tau that's not a good thing. My observation is that ADEV is much more >> likely to be mentioned without an associated tau than phase noise without an >> offset . I've also observed that when the error is mentioned you are likely >> to get a "oops I'll fix that" on phase noise. On ADEV people often simply >> don't get the fact that tau matters even after it's pointed out. >> > > For ADEV, a lot of oscillators have a sort of "floor" where the ADEV is > relatively constant, say from tau in the range10-1000 seconds, and then it > rises up (from thermal effects and such), so the shorthand is that the number > quoted is that "floor value" You see a lot of different ADEV plots. Some would suggest flat from 0.1 seconds out. The real world is rarely that simple …. > > >> Looking at what the systems using OCXO's are actually doing, about half the >> time ADEV is probably the better / more important measure than phase noise. >> The system is more sensitive to the OCXO wandering around over 100 or 1000 >> seconds than it is on the level of a sideband offset how ever many Hz off >> carrier. Once you get past ADEV, you rarely see an OCXO specified for any of >> the other related specifications. That's a shame, since some of them are >> better measures of certain things than ADEV. Again, I blame the fact that >> people just don't understand / trust the measurements. >> > > Certainly for "OC" applications this might be true. Although, a sort of > trend is that the TCXO resonator has to have a lower Q, so the temperature > compensating components can "pull" it to the right frequency over > temperature, so the phase noise of a TCXO isn't as good as that of an OCXO, > which can have a higher Q. > > A lot of times, though, an OCXO is chosen because a TCXO doesn't have > frequency stability needed over environmental changes. I don't think ADEV is > really the right measure when you're looking at aging or temperature effects. Well, I've certainly seen TCXO's spec'd and 100% tested for ADEV in the 50,000 pc / year quantities … > > If you need 0.1 ppm accuracy over -50 to +60C, you probably aren't going to > get it with a TCXO. Again, a "that depends" sort of thing. There are several outfits that will sell you a 0.01 ppm TCXO over a 100 degree span. -50 is not normally paired up with +60C, so there isn't a lot out there for that exact range. Doing 0.05 is not unreasonable over that range. > > For example, the Space Network using TDRSS on S-band (2.2 GHz) requires you > know the actual frequency to within 700Hz. That's 0.3 ppm and tough to get in > a TCXO over space qual temp range. Temp range isn't the issue as much as the range plus the radiation hardness required. Bob > > > _______________________________________________ > 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.
