Hi To find the intercept point, you need very long ADEV runs on both the OCXO and Rb. Both should be in the "been running for a lot of days / very stable environment" mode (you do control the local environment). Then you need the same thing on your GPS receiver, except it should be more towards the worst case situation (you don't control the weather / sat constellation / what ever). Lots of data before you can begin doing loops.
Often you find that the GPS has some interesting bumps at long tau's. Twelve hours / 24 hours / 48 hours are good places to look at. Your Rb *may* be better than your GPS at much longer tau's than you might first think. The same could be true of your OCXO. Bob -----Original Message----- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of John Ackermann N8UR Sent: Thursday, January 12, 2012 11:02 AM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] FE-5680A Mechanical Question I think you have to define "better," though. A GPSDRbO will have better holdover performance (e.g., stability when the GPS signal goes away) than one using an OCXO, but the OCXO is quite likely to have better short term stability than the Rb. If holdover isn't an issue, you need to find the "Allan Intercept Point" (term stolen from David Mills and NTP) to see where the GPS and oscillator ADEV curves cross over. That will tell you how long a loop time constant you want, and will let you look at the combined ADEV for each type of oscillator. You may find that a decent OCXO gives better performance overall than an Rb. John ---- On 1/12/2012 10:37 AM, ewkeh...@aol.com wrote: > with the good long term performance of Rb's and the presently available > 5680A for $ 40 a Shera Type controller will make available to any one on the > list a reference better than a Tbolt for less money than a Tbolt. Repeating > my self I would also add a "clean up" OCXO in order to get the very best. > Bert Kehren > > > In a message dated 1/12/2012 8:34:54 A.M. Eastern Standard Time, > jlt...@att.net writes: > > David, > > I have been following discussions on the list about a GPSDRbO for a year or > so. Some interesting challenges and probably best implemented in a > 'stable' > environment rather than portable operation but as best I can tell, it would > require a very stable and good antenna location, stable and clean power, > and > I was thinking of using something like an LPRO-101 with an Ext C Field > input, a Shera type controller, and a GPS timing receiver, though, I > suspect > there are likely to be some 'multi-tasking' receivers out there that will > look at several sources including GPS, GLOSNASS, etc. > > While the project might be fun, for portable work, it is likely far easier > and probably just as good (almost), to use a Tbolt. > > In any event, something to think about for the future. > > Joe > > -----Original Message----- > From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On > Behalf Of David > Sent: Wednesday, January 11, 2012 11:12 PM > To: Discussion of precise time and frequency measurement > Subject: Re: [time-nuts] FE-5680A Mechanical Question > > > How would a GPSDRbO work? Phase lock the DDS output to the GPS? Phase lock > a VCXO to the GPS and then phase lock to the RbO on loss of GPS lock? > > On Wed, 11 Jan 2012 22:55:58 -0600, "J. L. Trantham"<jlt...@att.net> > wrote: > >> Bill, Brian, Bill, and Peter, >> >> Thanks for the info. All I need now is a 'project' to incorporate the >> unit into. >> >> In the back of my mind, I have the thought of a 'box' that will be >> battery powered or 110 VAC powered (perhaps with an internal SLA >> battery) and include a GPSDO and Rb unit (possibly a GPSDRbO) for the >> purpose of a 'reference' for portable operation in the microwave >> regions. > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
