[time-nuts] oscillators

[Jim Lux]

On 9/1/12 8:32 AM, Bob Camp wrote:
> Hi
>
> Observing a curve and being able to compensate it are often two different things. Hysteresis is one very obvious example. Another is simple sensor lag. A some what less obvious one is that the temperature performance is also influenced by the rate of change in temperature.
>
> Here's another thing to consider:
>
> If your crystal is running 3 ppm / C, and you are after 3.0 x 10^-11 stability at one second - You will need to either have a rate of change at ~ 1x10^-5 C/sec (0.6 mC / min) or you will need to compensate for some pretty small changes. That of course makes a bunch of assumptions ….
>
>

In this application,  the requirement for frequency accuracy has to do 
with initial acquisition.. that is, you want the signal (or receiver 
tuning) to be within some few hundred Hz of where it's expected to be 
(because the receiver is narrow band).


The ground station typically has a Doppler predict based on orbit 
knowledge, that predict has some uncertainty.  Added to the radio 
frequency uncertainty.  (SNUG - Space Network User Guide, has more info)

Once you've acquired, the receiver and ground station will track (i.e. 
the ground station puts in the estimated Doppler, so all you're really 
tracking is the variation in the local oscillator).  (for a LEO 
satellite at 2.3 GHz, the 7km/s orbital velocity already puts tens of 
kHz variation on it)

(and this completely neglects that a modern radio could use something 
like an FFT for acquisition)

Temperature changes are pretty slow.. I'm seeing 5-10 degree cyclical 
variation over 90-100 minutes.  Actually, the bigger change is during 
the warm up transient, going from off and cold to on and warm over 10 
minutes or so.

In other applications, where you're not going in and out of the sun 
every revolution (i.e. deep space, rather than LEO) and you were 
interested in Allan deviation type measurements for gravity science 
(where we're looking for 1E-13 over 100 sec sort of performance), what 
we'd probably do is warm up early.. Turn it on, compensate based on the 
measured temperature, and then hold the compensation fixed during the 
measurement, letting the ground worry about the apparent frequency 
change due to Doppler.  We'd have a high quality narrow band signal, 
just at an unknown (but reasonably stable) frequency.  What the science 
team is usually interested in is small relative changes in phase & 
amplitude(occultations) or in small changes in frequency (Doppler, for 
gravity science).

(we regularly measure velocity to cm/sec precision for outer planet 
orbiters like Cassini, Juno, etc.)