[time-nuts] FE5680 GPS Disciplined Controller Update

[Bob Camp]

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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