[time-nuts] Advise on building a DIY GPSDO?

Attila Kinali attila at kinali.ch
Sun Apr 3 18:44:29 EDT 2016


Salut Nicolas,

On Mon, 4 Apr 2016 00:04:29 +0200
Nicolas Braud-Santoni <nicolas at braud-santoni.eu> wrote:

> Lately, I have been looking into designing & building a home-brew
>   GPSDO (and my copy of TAoE 3rd ed. came in quite handy), but quite
>   a few questions came up:

I have put together a short list of GPSDO designs and their 
advantages/disadvantages a few weeks ago, you might want to have
a look at this:
https://attila.kinali.ch/blog/2016/02/07/gps-disciplined-oscillator

> - Does it indeed make sense to build a GPSDO using an “ordinary”
>   high-quality oscillator? (as opposed to using a Ru standard)

It largely depends on what you want/need.
An Rb reference is more stable and you can integrate over several hours
or even days instead of minutes to hours.
 
>   It seems that decomissioned rubidium standards are large, rather
>   expensive (hundreds of €), consume lots of power and have
>   uncertain lifetime.

That's the big disadvantage of the Rb approach.


> - Are there recommendations people can make for not-too-expensive
>   VCOs to use in a GPSDO?

the Connor-Winfield DOT-050 seems to be quite decent, if you look at
Nick Sawyers measurements. Other than that, a 10811 is pretty decent
and quite stable, but also costs usually >100USD on ebay.

People have used Morion's MV-89, Isotemp 131, etc...
 
> - Are there GPS modules that people here can recommend?
> 
>   I have been looking at the uBlox NEO-7 and the GNS TC6000GN-P1
>   GPS modules.  Both retail around 40€, and promise <100ns PPS jitter.
>   I would probably prefer the NEO-7, because uBlox makes more precise
>   PPS jitter claims for GPS, with 30ns RMS and 60ns for 99 percentile.

Get a LEA-6T, NEO/LEA-M8T or LEA-M8F.
You really do want the "fixed position" feature.
Unless you want to go "low cost"


> - Some GPS chips offer higher-frequency pulse signals: are those
>   generated with an on-chip PLL?  If not, does it make sense to feed
>   this to the PLL, instead of the 1Hz pulse, to get higher loop gain?
>   (On the other hand, the loop gain must not be too high, as the PLL
>   is meant to get rid of the phase noise in the reference signal)

Depends on the chip.
Usually there is no "PLL" per se on the chip/module but rather they use
the on board TCXO/XO with an counter and use pulse skipping to get to
the right frequency and phase. Note, the LEA's have two modi, one where
they have "controlled" frequency and one where they have "controlled" phase.


> - While trying to design this on my own is fun and educational, are
>   there existing designs for DIY GPSDOs that I should look at?

See above blog post.

> - Should I prefer using an IC implementation of the PLL (this seems
>   simpler) or should I consider having my own implementation?

Depends. What loop bandwidth do you expect? Is it seconds? That can be
done with a PLL chip. Or is it minutes/hours/days? Then you have to
do it digitally.

> For reference, my use-case (beyond simply building it) is two-fold:
> - I want an accurate ref. clock for my local NTP setup.

For this you don't even need a GPSDO. Just use a normal GPS receiver
(which can be bought for less than 30USD on ebay), place it somewhere
with good skyview and you will get something in the order of 30-60ns
jitter, which is way beyond what your PC can measure anyways.

> - I need a frequency reference for QRSS (low-power RF transmissions),
>   and getting a 8MHz reference out of the GPSDO would help a lot  :)

How good has this reference to be? How much frequency deviation does it
tolerate? How much phase and frequency drift does it tolerate?

It might be, that a good OCXO is already stable enough for QRSS.
Or just an cheap OCXO locked to an FE-5680 with a low (<10Hz)
loop bandwidth.

			Attila Kinali


-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson


More information about the time-nuts mailing list