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

[Charles Steinmetz]

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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