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 <[email protected]> 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.) > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
