Said, The CNS-II outputs a raw hardware sawtooth corrected 1PPS and this is by design; it's not a mistake. There is or may be a GPSDO version of the CNS-II clock but I don't need it. What I (and some of the VLBI sites) use the CNS for is to monitor the long-term performance of masers and cesium clocks.
[ Rick can jump in quickly to correct me if needed ] In this application you don't need a 10 MHz GPSDO; you don't even need a "clean" 1PPS derived from a disciplined OCXO. Over hours or days, the raw 1PPS from the receiver has all the information, has all the stability, that you need and it is not "polluted" by any disciplining algorithm or jitter and wander of the OCXO. Does this make sense or shall I explain a bit more? I think one can make a case that for *long-term timing purposes* anything resembling a GPSDO will perform slightly worse than the bare GPS engine upon which it is based. It has to. Consider: On the ADEV plot, to the left of the crossover point, the GPSDO performance is essentially that of the OCXO minus a bit for system noise minus a bit due to the "drag" of the GPS engine at short tau. It follows then that to the right of the crossover, the GPSDO performance is essentially that of the GPS engine minus a bit for system noise minus a bit due to the "drag" of the OCXO at long tau. So if you don't need a CW RF output frequency, and only need long-term timing, and are post processing all the TIC data anyway, then the OCXO -- or anything else that's part of a disciplining system -- has no value added. Might as well use the best raw 1PPS you can get; which is what that version of the CNS-II does. /tvb ----- Original Message ----- From: [EMAIL PROTECTED] To: [email protected] ; [EMAIL PROTECTED] Sent: Thursday, December 14, 2006 19:21 Subject: Re: [time-nuts] Best GPS 1PPS Accuracy In a message dated 12/14/2006 16:46:10 Pacific Standard /tvb writes: The beauty of a GPSDO is that you're only changing the DAC every few minutes so there's plenty of time to look carefully at the quality of your 1 Hz samples before you commit the DAC update. It would be a more difficult problem if you needed to update the DAC in realtime. /tvb Hi Tom, one thing I noted in the PTTI paper on the CNS Clock II block diagram that was recently published here was that the CNS clock II is outputting a raw but sawtooth corrected version of the 1PPS. ftp://ftp.cnssys.com/pub/PTTI/PTTI_2006.pdf This 1PPS will suffer from the jitter not recognized by TRAIM that we are talking about. I wonder why they did not use the OCXO output DIV 10E6 to generate a clean 1PPS, since this is so much more stable as you mentioned? Or is there a mistake in the block diagram? bye, Said _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
