[time-nuts] Interesting paper: Don't GPSD' your Rb...

[Magnus Danielson]

Hi Poul-Henning,

On 05/05/2012 08:35 PM, Poul-Henning Kamp wrote:
> In message<CDAA55B4E94149A6972740E6FD0C8C53 at Warcon28Gz>, "WarrenS" writes:
>> Magnus wrote:
>
>> This is not a paper about "Don't  GPSD your RB", as the nut subject line
>> suggest.
>
> Actually, that is their point, or maybe more specifically:  Don't GPSDO
> it the way we do with OCXOs.
>
> At least as I read the paper.

Well, I just don't read the paper like that.

> It chimes pretty well with my own experiments and measurements.
>
> The short term noise and offsets of GPS-PPS signals in most cases
> is quite detrimal to disciplining a Rb.
>
> Case in point:  Oncore + PRS10.
>
> If you do not apply the negative sawtooth, there is no guarantee
> that the PPS signals average will be coincident with the epoch the
> receiver tries to mark.
>
> Tom has some plots of the "hanging bridges" you will experience
> and they can have durations so long that it starts to leak through
> the PLL steering the Rb, thereby ruining the result.
>
> This is the reason why you need to configure a PRS10 with a
> timeconstant a fair bit shorter than theory predicts if you
> just hook it up to a GPS-PPS signal.
>
> Interestingly enough, applying the PRS10's 1/256th filter makes
> worst case behaviour worse, because the filter makes the PRS10
> "latch onto" even shorter hanging bridges.
>
> If you cannot apply the negative sawtooth, you will get better
> results by disciplining almost any random quartz xtal, ovenized
> or not to the GPS, divide it down to PPS and then discipline
> the PRS10 to that.
>
> I belive that is the same thing the paper advocates, although
> they communicate it very badly.

I think you read to much into it.

The OCXO discipling will grab the hanging bridge much quicker than a 
rubidium loop would. There is no really good way to papper over the 
hanging bridge, and if so you would have to use the stability of a 
source to achieve it. Hanging bridge could be detected and phase be 
extended, it's really not that hard to achieve, TimeLab does it.

I read this article quite different. It illustrates a slightly different 
way to lock a rubidium to GPS. It's not particularly fancy, but it does 
the trick and it has pretty good performance for many purposes. It's not 
bad, it's not wrong, it's just not very fancy. A PICTIC II would be 
cheaper than their Agilent 53132A TIC.

Cheers,
Magnus