On 11/1/17 10:59 AM, Attila Kinali wrote:
Hi Jim,
On Wed, 1 Nov 2017 06:17:31 -0700
jimlux <jim...@earthlink.net> wrote:
That's why I wish they'd sell OCXOs, cheap, without the oven. Or maybe
look for regular XO (no TC). Those might have a more "pure" (read lower
order) freq vs temp characteristic.
It feels like I have asked this before, but I cannot remember and
cannot find the mail....So: What do you mean by "OCXO without oven?"
I often have requirements for "good phase noise" but no particular
requirement for "good temperature stability" or Allan Deviation for tau
> 200 seconds - we get frequency knowledge from other sources (e.g.
just like a GPSDO, or it can be inferred from the measurement)
I often have a system where the environment is pretty benign - a typical
on-orbit temperature variation might be a degree or two over 90 minutes
(or longer), if that - we don't know what the temperature will be (in
advance), but once it's up there, it's pretty stable. Maybe the temp
changes a bit due to relative orientation to the sun and similar
effects, so there's a annual variation. My current spacecraft is
expected to change maybe 5-6 degrees over the year.
For instance, in a software defined radio, the input oscillator is often
fed into some sort of NCO or DDS for tuning - knowledge of the frequency
is what you really want, because the ultimate requirement is on the
frequency accuracy at the input or output of the radio - the regulators
care not a whit what kind of oscillator is inside (well, they DO ask on
the license application, but it's really not relevant)
THere are also systems like Doppler radars - they need good pulse to
pulse stability, and low phase noise to avoid reciprocal mixing
degradation of the noise floor - but they care not what the exact
frequency is.
In general, OCXOs have crystals with high Q -> low phase noise,
especially compared to a TCXO, which *can't* have high Q, or the
temperature compensation circuit can't do it's work.
So, it would be nice to have a *cheap* lowish power packaged part that
has the Q of an OCXO, but without the power consumption of the oven
(typically measured in watts).
yeah, I'd be operating it *way* far from the optimum turnover temp, so
the tempco might be huge (in oscillator terms), but I don't really care
- in fact, that might give me a way to measure the temperature of the
system.
I have gotten quotes for OCXOs with the oven disabled- but that's
making it a custom part and as Bob has pointed out, the moment you
deviate from the "catalog part", the cost (and often more importantly,
the delivery time) goes up.
I think the ideal, of course, is to get an oscillator high performance
crystal cut with the turnover temp around 20-30C - useless for a OCXO,
but might be real useful for GPS/temp compensated oscillator where the
oscillator drives a DDS (or or provides an output giving the estimated
frequency).
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