Hi There are multiple categories of GPSDO:
1) eBay surplus items pulled out of one or another piece of gear. They are commercially made for whatever application that gear needed. Typically they have a 10 MHz and 1 pps output. The normal target market is telecom so the phase noise may not be great. 2) Home brew designs out of various places, also sold on eBay. Some of these are just a package put around a module from category 1. Others are “from scratch" basement lab designs that may ( or may not ) have issues. 3) A kit built design done in the basement. There aren’t a lot of these out there anymore (as full up kits). They do pop up from time to time. Folks don’t seem to be as excited about breaking out a soldering iron as they used to be. 4) One off DIY from scratch designs done down in this or that basement. More than anything else, the comments about “tooling up” relate to this sort of design. 5) Brand new off the shelf modules from this or that OEM. These are the parts that will be going into new designs. In a few years they will become category 1 items. As time goes on and more folks get into this business, the range of parts expands. Making any blanket statement about “everybody” will get harder and harder. It also can easily get into a bias issue. ( = I did this for a living for many decades ). For a lot of years a whole bunch of us have been pretty happy with various items out of category 1. They come and go on eBay with a new batch of gear being scrapped out and this or that module flooding the market at a cheap price. Wait a few years and they become scarce and expensive. The Trimble TBolt is one great example. The HP Z3801 is another. In general all of this or that model work pretty much the same. They do break and need repair eventually. Category 2 covers an enormous range of things. The BG7TBL based designs have been out for a while. There have been reports of design bugs over the years. Is the one you get from this or that seller bug free? Who knows. A box around a category 1 item is nice, it also is something you probably could do yourself …. (though maybe not as cheaply) Category three becomes a bit vague. A lot depends on the exact kit and how well the various parts you dig up work together. Since things like the master oscillator (TCXO or OCXO) play a big part, it can get “interesting”. Most of the conversation so far has been about GPSDO’s when locked. That is the mode we typically all use them in. Holdover is to be avoided. You come up with an antenna location that lets you keep things running on GPS all the time. That’s fine for a stationary bench application. It might not work at all in other use cases. Indeed the telecom Rb’s are an alternative when you start to look at a wider range of use cases. Holdover in the commercial world is normally a “so many microseconds in so many days” ( or nanoseconds in hours …) sort of spec. The target application cares more about time than frequency. That makes holdover pretty easy to measure. One thing we benefit from is that hitting a holdover spec forces a pretty good level of frequency control when locked. That and the typical locked timing error (which also gets into holdover) for most systems forces most of the category 1 units to be pretty good. Fun !! Bob > On Mar 1, 2022, at 3:54 AM, John Moran, Scawby Design > <[email protected]> wrote: > > David - thanks for the reply, but these seem designed for SDR and I wanted > 1pps. Nice and cheap though. > > Paul - thanks too; it seems that you are saying that the performance of all > GPSDOs are the same, but that wasn't the impression I had got from listening > to discussion here. Fine when the thing is locked (except for sawtooth stuff > and these hanging bridge things, and digital vs analogue (British) control > loops, etc.) but what about hold-up performance when it's not. > > So, are they all the same ... or not? If they are, why not buy the cheapest > Chinese device available rather than making your own? > > John > > > > > _______________________________________________ > time-nuts mailing list -- [email protected] -- To unsubscribe send an > email to [email protected] > To unsubscribe, go to and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there.
