[time-nuts] GFS-6A - was: Advise on building a DIY GPSDO?

Bob Stewart bob at evoria.net
Tue Apr 5 11:53:57 EDT 2016


Hi Charles,

I've changed the subject line, so I hope this isn't perceived as a thread-jack.

It's always dangerous to post anything substantive to this list, but I think it's about time.  Here's a timelab file of my GPSDO, which I've been working on for quite some time with help from Dan K and others.  It is just about ready to release.  My unit is a frequency standard, not a time standard.  IOW, I do not have a disciplined PPS output.  What I do have is 10Mhz, 5MHz, and 1MHz outputs.  

The file is here, which is on my personal domain.  It is compressed using gzip.

http://www.evoria.net/AE6RV/GFS-6A/225.80.15.32.tim.gz

Bob

--------------------------------------------
On Mon, 4/4/16, Charles Steinmetz <csteinmetz at yandex.com> wrote:

 Subject: Re: [time-nuts] Advise on building a DIY GPSDO?
 To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
 Date: Monday, April 4, 2016, 8:56 PM
 
 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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