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

[Bob Camp]

Hi

This comes back (very much) to: 

What is your objective? 

If the full performance delivered by a surplus part is your objective, then the Miller is not what you 
are after. Since it sells for about 2X the current crop of surplus parts … that is a bit of an issue as
well. There are a *lot* of different aspects of these devices beyond what any one plot can show. That
is why the testing part of it is an issue. It is not a deal breaker. It is not something to stop you dead. It’s
something to plan and budget for.

Bob


> On Apr 5, 2016, at 2:16 AM, Dimitri.p <dimitri at dotp.com> wrote:
> 
> Miller GPSDO vs HP Z3801A,  Jackson Labs Fury GPSDO,  vs Trimble Thunderbolt  compared Feb 2008
> 
> http://leapsecond.com/pages/gpsdo
> 
> It seems the Miller GPSDO, for an "unlikely" design, it can hold its own pretty good.
> 
> 
> 
> At 06:56 PM 4/4/2016, you wrote:
>> Don wrote:
>> 
>>> 5.  Design a voltage tracking / filter to match the OCXO
>>> control requirements.
>>> 6.  And, ...ta-dah;
>>> 7.  You have a 10MHz, bench frequency standard that will
>>> rival all others
>> 
>> Not very likely.  The whole point of a GPSDO is for the frequency to be controlled by the more stable source (OCXO or GPS) at all integration times (tau).  But the OCXO will typically be more stable than the GPS for tau less than several hundred seconds (see graph below -- black line is GPS, brown line is a typical OCXO).  So, the PLL needs to have a time constant of hundreds of seconds.  Such a PLL filter cannot practicably be designed in the analog domain, so one needs to design a digital filter with appropriate time constant and damping.  Because of the very long time constant, it is almost necessary for the filter to have more than one, switchable time constants to avoid extremely long lock times.
>> 
>> Very few home builders are capable of designing a proper digital filter suitable for this application (the counter-based loops of most published DIY GPSDO designs are not proper digital filters).
>> 
>> So, no -- it is very unlikely that a home-built GPSDO will "rival all comers," whether the builder designs his or her own circuit or uses one of the many published circuits.
>> 
>> Best regards,
>> 
>> Charles
>> 
>> 
>> Graph below.  Note that a properly designed GPSDO would show stability that follows the OCXO (brown line) at low tau, and the GPS (black line) above the point where they intersect -- here, about 350 seconds.  Note that a loop filter with proper damping will NOT exhibit a "hump" near the crossover (many GPSDOs do exhibit a pronounced hump, betraying that their loop filters are not properly designed).
>> 
>> 
>> 
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