Hello Martyn, >Rubidium's oscillators usually stay within 1E-10 accuracy, about five times >better than any OXCO unit I've measured.
yes, your data shows this. This is actually also reflected in your ADEV measurements, your data shows that at 100s and 500s the Rb has about 10x better ADEV performance than the OCXO, and looking at your 3600s plot the OCXO wanders around in about that time frame, so there is some correlation here. So the ADEV numbers are not all that unrelated to frequency accuracy :) Please see Toms' ADEV measurements for four GPSDO's that he posted a couple of days ago, the ADEV numbers for those units are a bit, to significantly better than your ADEV measurement numbers, I think the reason is that the units he tested very likely all had DOCXO's. >From your stability measurements it looks like the OCXO was a single oven unit. A double oven unit's stability spec versus temp is usually 10x to 100x better than a single oven, so should perform much better at a marginal cost increase. Frequency errors in GPSDO's are primarily caused by two sources: 1) The accuracy of the GPS. Any GPS errors especially from 200s and longer will of course affect the frequency accuracy since the OCXO follows the GPS tightly at averaging times longer than 1000s (typically). Rb's are usually locked with much larger averaging times, so the OCXO has to be very good in the interim. 2) The stability of the OCXO. The more inherently stable the OCXO, the less error correction control the loop has to do, the better the frequency accuracy (and ADEV), and the further-out the time constant for the GPS locking can be pushed. Many OCXO vendors claim their DOCXO units to be Rb replacements. I think with the aid of a good GPS receiver, this can actually be accomplished for most applications, or at least GPSDO's can get close to the same performance. But it does require very good OCXO's and GPS performance, and you are right Rb's are on average more accurate due to this requirement. GPSDO's still have some advantages: * a good Rb with a built-in GPS is still more (to a lot more) expensive than a very good DOCXO-based GPSDO, but the accuracy differences may actually be small * the Rb requires much higher power consumption * The Rb generates more RF noise (due to all of the RF related frequencies and switching regulators etc inside the unit) * an Rb has a lamp-design-related limited lifetime, and their MTBF is lower than typical GPSDO's * some commercial GPSDO's can operate up to +85C which most commercial Rb's cannot do (SRS PRS-10 is limited to 65C on it's baseplate for example, meaning the ambient has to be much <<65C) * GPSDO's are typically smaller and lighter than GPS-disciplined RB's I think the choice is still not that clearly differentiated. bye, Said **************The year's hottest artists on the red carpet at the Grammy Awards. Go to AOL Music. (http://music.aol.com/grammys?NCID=aolcmp00300000002565) _______________________________________________ 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.