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

[Charles Steinmetz]

Lars wrote:

>Could you please explain a little more about this:
>      *     *     *
>What is the systematic ripple?
>Is the comparison frequency normally 1Hz (1PPS)?
>What is the algorithms used?
>If I sample the difference between the PPS and the divided 10MHz 
>output every second and have a third order loop driving a DAC will I 
>get systematic errors?

Ummmmm -- please do not take this the wrong way, but you seem to be 
asking for short-cuts and cookbook solutions to very complex design 
issues in advanced PLL design, apparently without having a good 
understanding of the basics of PLL design (as indicated by your 
questions: what is the comparison frequency, what is systematic 
ripple, etc.).

That is just not the way it works.  System designs are organic -- 
everything affects everything else.  One needs a comprehensive 
understanding of the whole subject to deal with all of the 
consequences of each contemplated design choice.  There are just too 
many degrees of freedom in a design at that level of complexity and 
performance to make short-cuts and cookbook solutions possible.

Most anyone can throw together a first-order PLL using cookbook 
solutions and make it basically work (i.e., the loop doesn't 
oscillate and it converges to lock).  But this doesn't even begin to 
make a decent GPSDO design.  Not only must the loop converge and not 
oscillate, the loop tuning must match the OCXO being used, it must 
hold errors down to PPT levels and below, and it must address errors 
at the second, third, and higher levels of analysis.  There is just 
no easy, or formulaic way to incorporate results of the required, 
interdependent analyses without actually performing those 
analyses.  There are no short-cuts.

Here is a post from October on the same subject:

      <https://www.febo.com/pipermail/time-nuts/2015-October/094472.html>

And here are links to the best PLL design texts I'm aware of:

      Gardner:  <http://www.amazon.com/dp/0471430633>
      Best:  <http://www.amazon.com/dp/0071493751>
      Egan 1:  <http://www.amazon.com/dp/0470118008>
      Egan 2:  <http://www.amazon.com/dp/0470915668>
      Stephens:  <http://www.amazon.com/dp/0792376021>
      Wolaver:  <http://www.amazon.com/dp/0136627439/>

I suggest that you get all of these books (or, at minimum, three of 
them, after reviewing the tables of content and some relevant pages 
using Google Books or "look inside" on Amazon) and study them -- 
really study them.  (I'm betting that if you study any three of them, 
you will end up getting the other three and probably other books as 
well.)  In time, you will be able to answer the questions you are 
asking, and to pose the next several hundred questions you will need 
to answer to design a well-optimized GPSDO.

Best regards,

Charles