[time-nuts] Re: GPSDO - GPS1300-10-1000 by RFX Ltd. UK

[Bob kb8tq]

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 <john at scawbydesign.co.uk> 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
> 
> 
> 
> 
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