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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