2009/8/6 Magnus Danielson <[email protected]>: > Ulrich Bangert wrote: ... >> Well, stability over time is what exacly is displayed in a >> tau-sigma-diagram >> of an oscillator. Since only a few words before he is saying that he is >> NOT >> intersted into Allan Deviation plots, then he is perhaps interested into >> something else? > > Yes. Sigma-Tau plots of the Allan Deviation fame (with friends) addresses > the instability of the noise part of things. For crystal oscillators and > other non-atomic oscillators "linear" factors in frequency drift is not best > specified, described or measured using that method, which was invented > purely to be able to handle the phase noise side of things, not the slow > frequency drift.
For these sorts of measurements on drifting oscillators would it not be prudent to use the Hadamard Deviation? Cheers, Steve > As for frequency drift, it has been shown that using a model of > > f(t) = A*ln(B*t+1) > > or for some cases > > f(t) = A*ln(B*t+1) + C*ln(D*t+1) > > best models the frequency drift properties. Notice that the drift rate is > not constant but rather > > d(t) = AB / (B*t + 1) > > This is not very well handled by the Allan Deviation calculations, so it > needs to be estimated and removed from the data before hitting the Allan > Deviation core. > > Estimating A and B is fairly trivial if assuming t = 0 for the first drift > sample and then let t be tau for the next drift sample. > > Once stable values for A and B is established, the drift properties can be > scetched out into the future. > > As for the drift chaning direction, this comes from the case when A and C > has different signs. All this is covered in literature. > > For a complete picture of frequency stability, "linear" or mechanical > changes, environmental changes and noise values all needs to be combined. > Just looking at the Allan Deviation plot is as foolish as just looking at > the frequency drift. The experienced designer may however know for which > tau-range either of them is expected to dominate, and thus cheat a bit in > the analysis. > > Cheers, > Magnus > > _______________________________________________ > 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. > -- Steve Rooke - ZL3TUV & G8KVD A man with one clock knows what time it is; A man with two clocks is never quite sure. _______________________________________________ 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.
