[time-nuts] FE5680 GPS Disciplined Controller Update

EWKehren at aol.com EWKehren at aol.com
Tue Jun 24 20:15:31 EDT 2014


Lets be clear the 1 E-13 is a totally different project and does not relate 
 to the FE 5680 A.  Yes most likely we will use the same universal  
controller but with different code, same board. GPS crosses the 1 E-13 line at  
100000 seconds little more than a day. I whish it was just temperature as I  
mentioned we are able to hold the back plate of a FRK at 0.01 C. But that is 
the  easy part. 
Bert Kehren
 
 
In a message dated 6/24/2014 8:06:52 P.M. Eastern Daylight Time,  
kb8tq at n1k.org writes:

Hi

If:

1)  You are after better than 1.0 x  10^-13 accuracy
2) You are getting 1 to 9x10^-9 at one second ADEV out of  your GPS 
3) You have a telecom Rb with 1 to 5x10^-10 temp coef over a 70C  delta
4) Your Rb self heats 20 to 30C in still air

Here’s some  math:

You will need at least 10,000 seconds to get a single frequency  estimate 
and likely 100,000 seconds.

You are likely to go through temp  cycles in a room at a 30 to 90 minute 
rate. (1800 to 3600 sec).

Your  room temp swings are *way* outside your likely loop. The Rb will have 
to deal  with them by it’s self.

——

Your Rb is “sort of” compensated in  the temcom units. It’s more like a 
TCXO than an OCXO. 1x10^-10 over 50C would  give you 2x10^-12 per degree C. 
That may be better or worse than the sample  you have. The “worse” really 
comes in  when you have one that’s a  parabola or third order temp curve. 

At least around here a room swing  of 2 to 4 C is pretty normal with the 
heat or air-conditioning turned on. That  gets you into the 4 to 8x10^-12 
swing range. In a typical garage you are at  10C and 2x10^-11. 

If you want that to be *below* your 1x10^-13 goal,  you have to knock it 
down by about 100X. 

——

Is the goal  rational? Well this is Time Nuts …. It is roughly the sort of 
goal Bert has  said they are after. 

Most of the lightweight Rb’s have a major ADEV  hump when the temperature 
compensation cuts in. Without *good* temperature  stabilization, you will not 
disable this correction. There’s no real way to  know what the ADEV is 
without this hunting until you do it. Because of the  self heating (and 
gradients), it’s not a real easy thing to do. 

If you  are using an ensemble of parts to get the ADEV, temperature likely 
will  correlate between them. You will not get the “group vote” to suppress 
it the  way you some other sources of ADEV.

——


Yes, I’m sure Bert  can cross the T’s and dot the i’s better than I can, 
but that’s a pretty good  outline of the problem.

Bob




On Jun 24, 2014, at 4:34  PM, Tom Van Baak <tvb at LeapSecond.com> wrote:

>> However it  is disappointing that no one has stepped up to tackle the  
>>  temperature problem. how many have looked at the temperature attachment 
 and  
>> clicked on the N5TNL link. Let me make it clear that  yes the GPSDO will 
work but 
>> there will be one or two orders of  magnitude degradation without active 
>> fan  temperature  control unless the internal temperature compensation 
is  
>>  disabled.
> 
> Can you clarify the "two orders of magnitude"  claim? That's hard for me 
to believe, I think, without seeing the ADEV plots  or actual lab report.
> 
> I mean, even a cheap XO or TCXO or OCXO  can be disciplined against GPS 
and achieve superb results. Temperature (or  rather, temperature rate of 
change) has little effect short-term. Temperature  also has little to zero 
effect long-term. So it's only in the, what, tau 100  to 1000 or maybe 10000 
second range that temperature even matters. As long as  the LO is locked to GPS; 
I assume you're not talking about holdover.
>  
> Obviously you'd want a slightly shorter loop time constant for a  
non-temperature-controlled Rb than a fancy temperature-controlled Rb. But does  
this really make a one or two *orders of magnitude* difference?
>  
> /tvb
> 
> 
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