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

[Bob Camp]

HI

Ok, let’s back up a bit. The first question is: why? 

There are a few basic answers ( and many not so basic ones):

1) Because you want to save money over the $40 GPSDO’s that are on eBay.

2) Because you want to learn how a GPSDO works.

3) Because you want to make a GPSDO that performs better in a specific application 

If it’s 1, I’d suggest that you will have a hard time doing a useful  one up design for less than 
you can buy a surplus unit for. I’d extend that to include the wide range of of units in the < $200
delivered price range. 

If it’s 2 or 3, you will need a pretty well equipped bench to make much headway. The cost and time
associated with those bits and pieces is not at all trivial. You both need good measurement 
capability *and* a lab standard to compare to. 

The next question is: what for?

Again a few basic answers: 

1) Anything that works at all is fine, it’s just an experiment. 

2) It’s going to be a lab standard that drives the following gear …..

3) It will drive my 100 GHz narrowband data radio project 

Focusing on 2 and 3:

Phase noise is going to be pretty important for a microwave system. Short term stability will be 
important for things like frequent counters. A design that does both *is* possible. It also is fairly
complicated.

Each of these decisions loop back and drive the bench gear you will need under the first question. 

Lots of branches and that’s only two questions.

Bob



> On Apr 3, 2016, at 6:04 PM, Nicolas Braud-Santoni <nicolas at braud-santoni.eu> wrote:
> 
> Hi,
> 
> I've been slowly becoming a fellow timenut over the last few years,
>  though said nuttery had yet to go beyond adding some wiring to
>  get the PPS signal out of my GPS and into my NTPd.
> 
> 
> 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:
> 
> - Does it indeed make sense to build a GPSDO using an “ordinary”
>  high-quality oscillator? (as opposed to using a Ru standard)
> 
>  It seems that decomissioned rubidium standards are large, rather
>  expensive (hundreds of €), consume lots of power and have
>  uncertain lifetime.
> 
> - Are there recommendations people can make for not-too-expensive
>  VCOs to use in a GPSDO?
> 
> - 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.
> 
> 
> - 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)
> 
> - While trying to design this on my own is fun and educational, are
>  there existing designs for DIY GPSDOs that I should look at?
> 
>  I saw Jim Harman's message from last month[0], but this only
>  included the schematic, without the Arduino code that controls the
>  PLL.
> 
>  [0] https://www.febo.com/pipermail/time-nuts/2016-February/096134.html
> 
> - Should I prefer using an IC implementation of the PLL (this seems
>  simpler) or should I consider having my own implementation?
> 
>  Option 2 is what Jim has gone with[0], and in principle that could
>  let me learn correction coefficients for ageing and temperature,
>  but this seem like a big overreach for a first attempt.
> 
> 
> For reference, my use-case (beyond simply building it) is two-fold:
> - I want an accurate ref. clock for my local NTP setup.
> - I need a frequency reference for QRSS (low-power RF transmissions),
>  and getting a 8MHz reference out of the GPSDO would help a lot  :)
> 
> 
> Best regards,
> 
>  Nicolas
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