I paid $45 on ebay for a telecom surplus TruePosition GPSDO, implemented on a card that plugs into a rack-resident mother board, which I dont have. But the instructions to use it are on the web
and also here, (a long and informative thread about them).
The TruePosition is built far better than anything I could ever build myself. Its likely as accurate as one of the Trimble or Symmetricom units that cost more than twice as much, even used.
It's gradually emerged that it has a lot of configurability, thanks to a number of people's contributions. I wish we had the manufacturers specifications but what we do have is still quite good.
This post is way too long and pieces are redundant.
Basically, your needs are totally dependent on what you are doing, but the availability of this specific unit makes that choice a pretty easy one for most users here who are seeking a lab quality 10 MHz time and frequency standard.
Why not buy a standalone timing GPS?
For some uses, that makes sense too. For example, for an NTP server, I think a timing GPS is a better choice. A plain GPS which gives you 25-40 ns accuracy is much more accurate than you need.
All of the better GPSs end up giving the exact same results to me, all the time, with NTP once I figured out how to configure the offset correctly.
(Anybody who has more info, please share it here!)
Any good GPS thats successful in a mobile or UAV application likely has a good TCXO, already (or they wouldn't work in the wide range of environmental conditions they encounter, or at best they would work unreliably)
Timing applications in telecom and especially space to earth applications like GPS (where there is a very low signal to noise ratio are much more demanding than other radio applications.
An OCXO smooths out the frequency vs. time. Otherwise the GPSDO's frequency will vary in a sawtooth manner between two extremes, within a range of typically 50 nanoseconds or so. The best ones might do better, maybe half that.
Experts all will tell us that the jitter on any GPSs (and all GPSs have this problem) is too high for some (RF) applications (for the GPS-resident oscillator to act as a frequency standard in the most demanding applications) all by itself without an oscillator with a long time constant- a means of smoothing this signal out.. A quartz or rubidium oscillator - 'free running' will emit a pure sine wave that changes in frequency very slowly, but it may be off a bit unless its aligned with a GPS both periodically (calibrated to cancel out the crystals natural aging) and also on an ongoing basis by the GPS's atomic clock.
One feature one should look for in a timing GPS is "Timing Receiver Autonomous Integrity Monitoring" (abbreviated "TRAIM")
Once the GPS is installed in its permanent home and you know its not going to move using TRAIM you can train it so that after that it can maintain its lock for timing purposes with only one satellite.
The commercial GPSDOs that originated in cell phone equipment, although used, are likely the cheapest high quality signal source youre likely to find. Some of the GPSDOs on ebay, taobao, etc. are comparable in quality or better. But I really doubt if most of the homemade ones - with the possible exception of the very best designs - perhaps ones that were in essence crowd sourced, improved by a community of high knowledge users.
- without that I doubt if most attempts are going to be as good as a telecom surplus unit thats not at the end of its useful life due to aging or broken in some way.
Personally I would make sure that I got one with an oven if I planned on serious radio use in the future. There probably is a lot of variation between ebay sourced GPSDOs with some of the best being made by a Chinese ham I think his call is bg7tbl. See this thread here.
https://www.eevblog.com/forum/testgear/bg7tbl-gpsdo-master-reference/But, here is the problem with his unit, if somebody wants to have the ability to tweak the settings, you really can't.
Some of the functionality on most GPSs is accessed by sending commands to the unit via a serial port (well, more often than not on newer units it is a 3.3 or 5 volt- serial port, make sure you know which - lower voltage UART)
As far as I know some of the homemade units don't have any way of sending commands to the units to do much in the way of configuration. Some may have a button to initiate training the unit for a new location, and thats it. Some don't even have that.
OTOH, some of the GPS-only units are very flexible and configurable to emit square waves over a wide range of frequencies, basically you get a DDS. Considering that some Ublox 6T units that have that dual output capability, two configurable square wave outputs can be found for as little as $25, thats an amazing deal.
IF you can live with the jitter. Or get both.