[time-nuts] SE880 GPSDO

[Attila Kinali]

On Mon, 25 Apr 2016 21:41:57 +0200
Ilia Platone <info at iliaplatone.com> wrote:

> I'm trying to build a GPSDO with a FVXO-HC53BR-10 (Fox Electronics) VCXO 

Please note that the HC53 family is what is commonly called "programmable
crystal oscillator". Which means it is a combination of a crystal oscillator
with a VCO and a PLL. The crystal runs at an arbitrary frequency and is
used to stabilize the VCO. The use of a standard crystal makes these devices
very cheap, but their phase noise performance is rather bad.

Quite interestingly, Fox has a quite extensive specification for
the jitter performance and  splits it into random (aka noise) jitter
and deterministic (aka spurs) jitter. Curiously, there are no spurs
visible in the phase noise plot, which I find rather odd.

You might want to spend a couple of more bucks and get something
like the DOT050 from Connor Winfield that is a true crystal oscillator.

> and a Telit Jupiter 
> <http://www.digikey.com/Suppliers/it/Fox-Electronics.page?lang=en>SE880<http://www.digikey.com/Suppliers/it/Fox-Electronics.page?lang=en>, 
> this serves for a datalogger an I want to achieve a highly reliable clock.
> 
> This GPS receiver has two clock inputs: a 16MHz, and a 32KHz XT input 
> for the RTC functions.
> My problem is this: The crystal must be driven by a voltage depending to 
> the phase difference between the GPS 1PPS output and the uC 1s cycles, 

It does not need to. The GPS module does not care about whether its
oscillator is exactly on frequency or not and what the phase of it is.
You can of course phase lock it to the PPS and to make it stable, but
you dont have to.

> but must I drive (feedback) the RTC of the GPS also from a divider of 
> the main clock, or the 16MHz TCXO input, or both (or none...)?

The RTC is only there to give the GPS module a rough estimate of the
time so it can use the stored ephimeris data to speed up the search
for satellites. Ie there is no need to phase lock or stabilize the 32kHz
in any way.


> Suggestions of any other component, for driving both the VCXO, and 
> possibly for a RTK implementation, are welcome.

For RTK you need satellite phase data. Most of the GPS modules do not
provide this information. I could not figure out whether the SE880 does
or does not provide it. There is quite a bit of lack of documentation
for that module, especially the communcation protocol (no, "NMEA standard"
does not mean anything). 

Unless you already bought a SE880 already, i would recommend to
go with one of the u-blox modules (preferably the timing modules)
or get a LTE-Lite from Jackson Labs.


			Attila Kinali

-- 
Reading can seriously damage your ignorance.
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