[time-nuts] FE-5680A Mechanical Question

[Bob Camp]

Hi

To find the intercept point, you need very long ADEV runs on both the OCXO
and Rb. Both should be in the "been running for a lot of days / very stable
environment" mode (you do control the local environment). Then you need the
same thing on your GPS receiver, except it should be more towards the worst
case situation (you don't control the weather / sat constellation / what
ever). Lots of data before you can begin doing loops.

Often you find that the GPS has some interesting bumps at long tau's. Twelve
hours / 24 hours / 48 hours are good places to look at. Your Rb *may* be
better than your GPS at much longer tau's than you might first think. The
same could be true of your OCXO. 

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of John Ackermann N8UR
Sent: Thursday, January 12, 2012 11:02 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] FE-5680A Mechanical Question

I think you have to define "better," though.  A GPSDRbO will have better 
holdover performance (e.g., stability when the GPS signal goes away) 
than one using an OCXO, but the OCXO is quite likely to have better 
short term stability than the Rb.

If holdover isn't an issue, you need to find the "Allan Intercept Point" 
(term stolen from David Mills and NTP) to see where the GPS and 
oscillator ADEV curves cross over.  That will tell you how long a loop 
time constant you want, and will let you look at the combined ADEV for 
each type of oscillator.  You may find that a decent OCXO gives better 
performance overall than an Rb.

John
----


On 1/12/2012 10:37 AM, EWKehren at aol.com wrote:
> with the good long term performance of Rb's and the presently available
> 5680A for $ 40 a Shera Type controller will make available to any one on
the
> list a reference better than a Tbolt for less money than a Tbolt.
Repeating
> my  self I would also add a "clean up" OCXO in order to get the very best.
> Bert Kehren
>
>
> In a message dated 1/12/2012 8:34:54 A.M. Eastern Standard Time,
> jltran at att.net writes:
>
> David,
>
> I have been following discussions on the list about a  GPSDRbO for a year
or
> so.  Some interesting challenges and probably  best implemented in a
> 'stable'
> environment rather than portable operation  but as best I can tell, it
would
> require a very stable and good antenna  location, stable and clean power,
> and
> I was thinking of using something  like an LPRO-101 with an Ext C Field
> input, a Shera type controller, and a  GPS timing receiver, though, I
> suspect
> there are likely to be some  'multi-tasking' receivers out there that will
> look at several sources  including GPS, GLOSNASS, etc.
>
> While the project might be fun, for  portable work, it is likely far
easier
> and probably just as good (almost),  to use a Tbolt.
>
> In any event, something to think about for the  future.
>
> Joe
>
> -----Original Message-----
> From:  time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of  David
> Sent: Wednesday, January 11, 2012 11:12 PM
> To: Discussion of  precise time and frequency measurement
> Subject: Re: [time-nuts] FE-5680A  Mechanical Question
>
>
> How would a GPSDRbO work?  Phase lock the  DDS output to the GPS? Phase
lock
> a VCXO to the GPS and then phase lock to  the RbO on loss of GPS lock?
>
> On Wed, 11 Jan 2012 22:55:58 -0600, "J. L.  Trantham"<jltran at att.net>
> wrote:
>
>> Bill, Brian, Bill, and  Peter,
>>
>> Thanks for the info.  All I need now is a 'project'  to incorporate the
>> unit into.
>>
>> In the back of my mind, I  have the thought of a 'box' that will be
>> battery powered or 110 VAC  powered (perhaps with an internal SLA
>> battery) and include a GPSDO and  Rb unit (possibly a GPSDRbO) for the
>> purpose of a 'reference' for  portable operation in the microwave
>> regions.
>
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