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

[Lars Walenius]

Hello Charles and others

I have wondered what is meant by a proper digital filter below? Now I was reminded by the KS-24361 holdover graphs from Hal.

Is a proper digital filter something more than the LP-filter + PI-loop I use in the DIY Arduino GPSDO?
What is used in commercial GPSDO´s?

I am also quite sure that a home-built GPSDO will not "rival all comers," but you will learn a lot!

I also enclose an screen shoot from a two day run with my Arduino GPSDO in hold mode. This combination works well with about 1000-1500 secs of time constant and is an example that you need long time constants with good oscillators. It is also an example that you can learn a lot without expensive equipment. The TIC was linearized with a third order polynomial and data for linearization was gathered with the use of a PICDIV PD26 and processed in Excel. With the PD26 it is possible to have data with very accurate 200ns spacing together with the 1ns TIC.

BTW I saw that Nick Sayer now is using the ”Arduino” 1ns TIC in his open source GPSDO. His latest program also used a LP-filter+PI-loop what I could see.

Lars


Från: Charles Steinmetz<mailto:csteinmetz at yandex.com>
Skickat: den 5 april 2016 07:08
Till: Discussion of precise time and frequency measurement<mailto:time-nuts at febo.com>
Ämne: Re: [time-nuts] Advise on building a DIY GPSDO?

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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