[time-nuts] Rb vs.Crystal OCXO

EWKehren at aol.com EWKehren at aol.com
Sat Apr 26 21:16:08 EDT 2014

The thunderbolt is one of the best timing devices but not for frequency, if 
 you want high resolution. Over time it is ok but high resolution short 
gate  times and you see the frequency changes. They use the OCXO to correct for 
timing  error and if you have a Tracor 527E you can see it. Also how else 
do you think  they control the 1 pps.
Bert Kehren
In a message dated 4/26/2014 1:52:10 P.M. Eastern Daylight Time,  
csteinmetz at yandex.com writes:

Magnus  wrote:

>The PRS-10 have a nice little trick in it, it stores the  previous 
>OCXO steering value, so on power-up it sets the OCXO to  this

The PRS-10 has quite a number of nice tricks, in addition to  
particularly good engineering and high-quality construction of the  
basic physics package and support circuitry.  The OP (and others)  
should not expect the same level of performance from $30-$100 ebay  
Rubidiums (LPRO, FRS, FE-56xx, etc., etc.).

Very good to excellent  OCXOs are available readily for $5 to 
$50.  IMO, those should be the  standard of comparison for any 
aspiring time nut.  I'm not aware of  any economy Rubidium that has 
close-in phase noise or low-to-medium-tau  AVAR nearly as good as one 
of these very good OCXOs.  As mentioned by  others, some Ru may do 
better than a TCXO close in and at low tau.   But so what?  The TCXO 
should not be a time nut's standard of  comparison as far as a lab 
standard is concerned.

One quickly  concludes that a good GPSDO, which includes a good OCXO, 
is the optimal  solution for most time nuts.  The OCXO has excellent 
stability with  respect to close-in phase noise and low-to-medium-tau 
AVAR, and is  disciplined by the GPS for excellent stability at longer 
tau.   Probably the best turn-key solution is a Trimble Thunderbolt 
(although  prices have risen in the last few years, so they are not 
the bargain they  once were).  Other, less expensive Trimble units 
that are also  supported by the Lady Heather monitoring program are 
available on ebay,  and are probably the best bet today for 
bargain-hunters.  While I  applaud the recent efforts to build simple 
DIY GPSDOs using inexpensive  microcontrollers, from what I have seen 
so far most of them do not yet  have the programming sophistication, 
particularly in the PLL loop filter  and the houskeeping functions, to 
rival a good off-the-shelf GPSDO from a  quality manufacturer.

Final thought for specifying/designing/buying a  GPSDO for time nuts 
purposes:  Do not settle for a low-quality  crystal oscillator (and 
especially not a TCXO).  You will never  achieve best performance at 
tau < about 100 seconds that way.   Insist on a "10811-quality" OCXO 
(one of the many nice things about the  Thunderbolt is that it has a 
very good OCXO).

Best  regards,


time-nuts  mailing list -- time-nuts at febo.com
To unsubscribe, go to  
and follow the  instructions there.

More information about the time-nuts mailing list