On 6/1/13 1:46 PM, Bob Camp wrote:
Hi
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 ….
Yes.. but still, a floor of sorts.. high at the low end, high at the
high end (for tau), and flattish in the middle.
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 …
Sure, but is ADEV *of the oscillator at constant temp* really relevant
when your application isn't at constant temp. I guess it is, because
your system ADEV can't be any better than the underlying oscillator, but
still, I'm not sure it's an entirely appropriate specification to be
calling out.
And, isn't "aging" (in the sense of slow long term drift) usually
excluded from the ADEV calculation (e.g. you fit a straight line to the
raw frequency data, and subtract that out)
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.
0.05 ppm in a TCXO over 100 degrees as a "assembly" or "component" level
device? That's quite impressive. I see that Vectron has the TX402 which
is 50 ppb -20 to +70 (for some frequencies, according to data sheet)
which is certainly in that ballpark.
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.
LEO doesn't require much in the way of radiation hardness. After all,
people live in LEO, so it can't be that bad. Single event effects
maybe. Or frequency change with dose/single events.
Seeing the recent RAD data from MSL only racking up <100 rad on the way
to Mars makes me wonder why we ask for 20kRad kinds of performances.
Bob
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