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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Jan 29 23:08:29 EST 2012

Don't confuse the PLL loop "time constant " with the time constant of 
the analog or digital filter.
They are not the same.


Ray Xu wrote:
> 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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