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

EWKehren at aol.com EWKehren at aol.com
Mon Jan 30 12:16:49 EST 2012


With a Rb we are talking loop time in excess of 20 minutes. Any thing other 
 than digital is out of reach. To day I should get my second Rb and I will 
put it  on aging test. The present one is better than 2 E-12 per month! I am 
still  hoping some one else will do an independent test. Key is temperature 
control.  Higher temperature may speed up aging.
Bert Kehren
 
 
In a message dated 1/30/2012 11:39:57 A.M. Eastern Standard Time,  
albertson.chris at gmail.com writes:

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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