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

[Dimitri.p]

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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