Magnus Danielson said the following on 12/23/2006 07:21 PM: >> Magnus Danielson said the following on 12/23/2006 05:22 PM: >> >>> Such as temperature and humidity. One of the Cesiums seemed to have a rather >>> large frequency offset. Did you drift-compensate your ADEV measures or not? >> No, I ran the ADEV in Stable32 without removing drift. My understanding >> is that linear drift shouldn't affect the calculation. I was also a >> little concerned whether drift removal would work very well given that >> both Cesiums had periods where the offset changed for quite a long >> period (CS1 near the end of the data, CS2 at the beginning) and I wasn't >> sure what impact that might have. > > I know next to nothing about Stable32, so I don't know what that black box > actually is doing. > > From a theory standpoint, it is trivial to show (I've done the exercise to > convince myself, but it is already covered in literature) that linear > frequency > drift does affect the result. Time/phase offset and frequency offset cancels. > > From a practical standpoint, linear frequency drift may or may not be small > enought not to interfere with the measure of the noise power. To be sure, > measure the drift rate.
I think I've been sloppy in my terminology above and in other messages -- I meant to say, as Magnus does, that linear offset doesn't affect the stability measurement. Drift such as aging certainly would. In the data I've been working with, there is a component of linear drift in the CS2 results; if you chop off the first 1250 records where the offset was essentially 0 and look at the rest of the data, the offset is about +7x10e-13 and a quadratic fit shows a drift of 6.4x10e-15/day. What's interesting about CS2 (a fairly old 5061A upgraded with the later model 10811A OCXO and a replacement FTS tube) is that it ran for over a year prior to this experiment with very, very low offset; there were certainly wobbles but the long-term offset was in the low 14s. That shows at the beginning of the current plot, but about 20 days into the run something changed (and I don't know what) that caused the offset to increase. Similarly CS1 (a later model 5061A also with late OCXO and replacement FTS tube) ran at about -2.5x10e-13 for the first 70 days of the run, and for quite a long time prior to that. But starting at the 70 day point, it went into a series of slow oscillations with a mean offset of about zero; recent data shows that it may be returning to the historic offset. Looking at the first 70 days with the offset removed, there's very little discernable linear drift; instead most of the instability appears to be various cycles ranging from daily to a couple of weeks. And, for anyone who's interested in playing with the data, I've uploaded it to http://www.febo.com/time-freq/uploads/. There are three files in the zip, which is about 327k. Each consists of MJD and phase data with a base tau of 600 seconds. John John _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
