[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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