On Wed, September 25, 2019 5:45 pm, Taka Kamiya via time-nuts wrote: > discussions take place both for GPSDO with Crystal Oscillators and > Rubidium modules. It appears there are two types of each. > 1) fixed frequency type (less jitter) > 2) frequency agile type (more jitter)
That is a bit oversimplified. GPSDO is "GPS disciplined oscillator," meaning that the output frequency of the oscillator is forced to follow the inverse of the time measurements derived from the Global Positioning System. Obviously if the oscillator has to follow another reference it cannot truly be fixed frequency on the output, but there are various different ways that the frequency can be varied. Depending on the important parameters for your needs the differences in mechanism may or may not be important. > I've read frequency agile Rb modules (ones you can change output > frequency) is one kind of Rb (sa.22c and fe5650, etc), and there is > another one that you cannot change frequency. (ie. T-bolt, PRS10, etc). You are mixing different types of equipment together, so it is a little difficult to know what you are asking. FE5650, PRS10 and SA.22C are rubidium frequency standards. A rubidium frequency standard is fundamentally an OCXO which is slaved in a frequency lock loop to a way to measure atomic transitions in rubidium vapor. There are various systematic errors which pull the frequency of that atomic transition measurement, so a rubidium standard will commonly have an adjustment mechanism that can adjust the output frequency far enough to compensate for those systematic errors, but you need a more accurate device for comparison for that adjustment to be worthwhile. A Thunderbolt is a complete GPS disciplined oscillator system which accepts GPS signals from an antenna, and outputs 10MHz which is slaved so that on average there are always 10 million output transitions per GPS-derived second estimation, and outputs a pulse once per second which occurs on the transition of the GPS-derived second estimation. When the GPS signal is not available for some reason (antenna failure, GPS system problems, etc.) the GPS disciplined oscillator has to run in undisciplined mode. Over that time the corrections to the oscillator frequency are not available, so only the intrinsic stability of the oscillator used sets the long term stability of the output frequency. In that particular case (no GPS available) a GPSDO using a rubidium controlled oscillator as part of the design will have more stable long term frequency than an oven quartz oscillator. While GPS signal is available to measure and control the output frequency there is little difference between an ovenized quartz and a rubidium controlled (which recall also has an ovenized quartz oscillator driving the output, but has a control loop using rubidium vapor transitions to correct any frequency drift of the quartz oscillator). > Words like phase noise and PLL are thrown out often in discussions. A very good place to start is searching for John Vig oscillator tutorial. John Vig has a lengthy presentation about all things relating to oscillators, if you can find the version which includes the notes along with the presentation slides you can spend a couple of days studying that > Is this because frequency agile type has the ultimate output from PLL > (subject to jitter) and fixed frequency type is from OCXO? Mixing different types of things into a single question again. The term PLL is phase locked loop, it is a particular type of control loop design which measures the edge transitions of a reference frequency input, the edge transitions of an output signal, and controls the output so that the edges match the reference. You can phase lock quartz, rubidium, pendulum. Maybe an hour glass, but the only transition I can think of is when you turn the hour glass over, so that would be an impractical PLL. > Even in main well known brands, I understand PRS10 > and sa.22C and fe5650 are fundamentally different. > I guess they are all "GPS disciplined" in some way No, SA.22C and FE5650 have no connection to GPS, they are not GPS disciplined in any way as stand alone devices, but can be used as part of a full GPS disciplined oscillator design. The SA.22C has a pulse-per-second (PPS) input, so it can be connected to GPS, but that gets into a lot of secondary questions relating to how that PPS is derived and whether it helps or hurts overall stability. -- Chris Caudle _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
