[time-nuts] Two types of GPDSO / Rubidium

[Didier Juges]

I have a number of revisions of the John Vig tutorial in my Manual pages.
The most recent was sent to me directly by John but I am not sure if it has
the notes.
www.ko4bb.com
Go to Manuals and search for Vig

Didier KO4BB

On Thu, Sep 26, 2019 at 7:03 AM Chris Caudle <chris at chriscaudle.org> wrote:

> 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
>
>
>
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