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"
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.
If you need 0.1 ppm accuracy over -50 to +60C, you probably aren't going to get it with a TCXO.
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.
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