[time-nuts] oscillators

Bob Camp lists at rtty.us
Sat Sep 1 18:00:47 UTC 2012


Hi

I suspect that a good IT cut would probably do better than an SC in either application. In the deep space situation, a copper slug in a dewar sounds like a reasonable addition to the design.

Bob

On Sep 1, 2012, at 11:53 AM, Jim Lux <jimlux at earthlink.net> wrote:

> 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.)
> 
> 
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