[time-nuts] GPS-disciplining an ordinary VCXO?

Said Jackson saidjack at aol.com
Sun Sep 28 01:50:11 EDT 2014


In the analog domain you can probably do a PLL with a 1Hz loop BW. Using a PLL chip like ADF 4002 or similar. This means all the nasty noise from the NEO will taint your PN up to 20Hz or more, very significantly close-in. If you don't care about noise (jitter) below 100Hz then this is fine. If you do as it will dominate your ADC jitter then you can't use an analog PLL.

With that VCXO you want to have a 5s to 10s or more loop time constant (0.1Hz BW) which typically can only be done in the digital domain.. This allows you to use the excellent 1Hz to 100Hz PN of that VCXO without tainting it by the noisy NEO.

An even better setup would be to lock a very low noise 5 or 10MHz ocxo to the GPS with >100s time constant, then use the analog PLL with wider bandwidth (say 30Hz) to reduce the VCXO PN close-in even further by using the ocxo to supress the vcxo PN.

Welcome to our world, if you look at the archives there are 10++ years of discussions about exactly doing this...


Sent from my iPad

On Sep 27, 2014, at 21:01, "Mark A. Haun" <haunma at keteu.org> wrote:

> In my quest to learn Verilog and get my hands dirty with
> software-defined radio, I'm currently designing a direct-sampling
> shortwave receiver.  This uses an 80-MSPS ADC, which requires a
> low-phase-noise oscillator, e.g. Crystek CVHD-950 or Abracon ABLNO.  It
> would be nice to have some provision for locking this oscillator to an
> external reference, hence my question:
> All of the amateur GPSDO designs I've seen are disciplining an OCXO.  I
> understand this is easier because the excellent short-term accuracy of
> the OCXO means the feedback can run slower, so even a 1 PPS signal can
> be used.
> I am wondering what sort of performance could be achieved by
> disciplining my VCXO directly with a good GPS module.  I have a NEO-7N
> (Ublox) with configurable timepulse up to 10 MHz.  Someone mentioned
> that this is derived from 48 MHz, so jitter is reduced if you pick an
> integer divisor.  That is fine, but I don't have a feel for what other
> irregularities may be present in the timepulse output, and how they
> would affect the performance.  I also don't know how to go about
> designing a PLL loop filter.  I understand the goal is to marry the
> long-term GPS stability with the short-term VCXO stability but all I
> have is a phase-noise plot for the VCXO.  How do you know where to
> split the difference?
> It is not essential to the larger project, but what I am ideally going
> for is 1 ppb frequency match between two ends of a radio link, and 1 ppb
> stability over data symbol times.  That is, carrier stability of ~ 1/10
> cycle at 10 MHz over one-second symbols.  (Channel coherence imposes
> this limit.)  I know the experts here can tell me whether this is
> impossible, totally doable, or somewhere in between!
> Thanks,
> Mark
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