In a message dated 12/16/2006 12:31:31 Pacific Standard Time, [EMAIL PROTECTED] writes:
In message <[EMAIL PROTECTED]>, Hal Murray writes: > >> You still want to produce some form of quality measure for the ADEV >> shape in order to form some form of control loop. > >Suppose I build I good GPSDO. How do I determine how good it is? (or even >if it is any good) Since you pretty much know that the ADEV of the GPS receiver is a straight line, you can get quite a long way by doing an ADEV calculation over your measured errors. Hi there, one poor man's way to check longer term performance without the help of ADEV is to show your GPSDO's output (the 10MHz, or preferrably the stabilized 1PPS output) against the GPS receivers raw 1PPS output on a scope that can do color grading with infinite trace persistance. Color grading is nice to have, but not even needed for this test. All you need is a scope with infinite persistance. If you trigger on the the raw 1PPS rising pulse, the 10MHz trace should move back and forth within the 1PPS typical sawtooth range (+-13 to +-20ns for an M12+) and never wander outside of this range on your scope display if the OCXO is perfectly locked. The left and right short-term wander (sawtooth) is really well defined, and you can see over time periods of 10 minutes to hours how your OCXO output wanders beyond this sawtooth. If it doesn't, you've got a really stable unit :) If your OCXO is locked really tightly, the displayed range will fit neatly into a 100ns pulse of the 10MHz OCXO output, and you won't get aliasing. If it wanders more than +-50ns, the measurement is inconclusive due to aliasing (the screen is just a white bar in this case) Say now that the width of the wander is never more than +-35ns over 12 hour periods, this means your unit is probably locked to the GPS receiver with better than 70ns/(12*3600s) = 1.6E-012 to the GPS receiver for measurement periods of 12 hours. This figure actually includes the GPS sawtooth error not likely to be present on your OCXO output! That's pretty good already if you can achieve such a small wander window for such a long time! Of course you can wait exactly until the wander reaches the +-50ns window, then do the math for that particular time-period. If you have a raw GPS 1PPS and a stable OCXO 1PPS you are not limited by aliasing, and can measure much less stable OCXO's. This method won't tell you much about shorter term stability unfortunately. bye, Said _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
