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

Chris Albertson albertson.chris at gmail.com
Mon Jan 30 11:39:11 EST 2012


On Sun, Jan 29, 2012 at 7:20 PM, Ray Xu <rayxu123 at gmail.com> 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...
here was talk here a while back abut if you even could build a GPSDO
using a simple analog PLL.   The problem it turns out is the long time
constant and of leaking or temperature sensitive caps will be a
problem.   Most people end up useng a micro controller, a PIC or the
like.   And then the filter is in the uP memory.    I think you need
hundreds or even 1000 seconds in the loop.

Look at the Allen deviation of your GPS.  When does it reach the 1E-11
figure that you want?  Most GPSes are not that good in the short term.
 You can answer the question of "How long" if you have a plot of your
GPS.

Don't worry, if you use the Rb then you get to build about the same
thing.  How else would you calibrate it?  You ned to compare the phase
of the Rb to GPS and send rs232 commands to adjust it.  Pretty much
the same thing.


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
> KF5LJO
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-- 

Chris Albertson
Redondo Beach, California




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