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

[Nicolas Braud-Santoni]

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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