[time-nuts] Building a GPSDO & trouble using Jupiter-T

Ray Xu rayxu123 at gmail.com
Sun Jan 29 22:20:40 EST 2012

Hi Chris

Thanks for your helpful input.

What do you mean by "average"?  Do you mean that the GPS and PLL must be
kept on for "20 minutes to hours", or did you mean that the PLL loop filter
must have a time constant of 20 minutes to several hours?  To me, the
latter seems really unpractical for analog filters...Yet I have seen many
of them built using analog filters.  Especially JAmes Miller's
http://www.jrmiller.demon.co.uk/projects/ministd/frqstd.htm and his FAQ
says that the time to wait is perhaps 15 minutes or so to be usable.  The
previous GPSDO that James built has its schematic; the filter he used
doesn't look like they're anywhere close to a time constant of 20 minutes.

I may consider the Rb standard, but I'm more inclined on using GPS since I
actually get to build some stuff on my own :-)

Thanks again
Ray Xu

On Sun, Jan 29, 2012 at 6:52 PM, Chris Albertson
<albertson.chris at gmail.com>wrote:

> On Sat, Jan 28, 2012 at 10:50 PM, Ray Xu <rayxu123 at gmail.com> wrote:
> > Also, what is the advantage of using a OCXO instead of a VCXO in terms of
> > short-term accuracy?  If the PLL time constant is only a few seconds,
> then
> > a crystal shouldn't deviate in frequency by too much within a few
> seconds,
> > assuming I'm using a crystal bought from a well-known manufacturer...or
> > could it? I am inclined towards using oscillators that do not require any
> > significant warm up time...
> GPS is only a good reference if you average it over a long time
> period.  (1000 to 10,000 seconds) There is more short term jitter in
> the GPS then in a decent crystal oscillator.   So a very short time
> constant does you no good.   Why use an OCXO?  Because of the required
> long time constant.  You need to average GPS for such a length of time
> (20 minutes to hours) that the ambient temperature will change during
> the averaging time.  Of course you could take care that the
> temperature does not change but that is what an oven does.    You can
> buy a pretty good OCXO for $20 or $25
> How long?   That depends on the required accuracy.  You want 1Hz at
> 10GHz.  That is 1E-10.  Not super hard but no way will you have that
> 10 minutes after you apply power.
> If you need a portable "standard" look at those $40 rubidium nuts that
> are on eBay.    It the 1E-10 level, after you calibrate it, if would
> stay on-frequency for days and not require much warm up.
> Chris Albertson
> Redondo Beach, California
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73, Ray Xu

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