[time-nuts] GPSDO using 100Hz

[WarrenS]

I am having trouble keeping my responses from starting new threads, 
If anyone could shed some light on what I'm doing wrong it would be a 
great help to me. It will be interesting to see what thread this post ends up in.

(Now back to the original subject; I've cleaned up things and added my responses in the text below)


Re: [time-nuts] GPSDO using 100Hz
from Tom Van Baak
Mon, 24 Nov 2008 17:17:34 -0800


by WarrenS
>> Besides asking if anyone is using the 100Hz output, 
>> I would like to know why it is that the generally available 
>> GPSDO don't use the 100Hz. I have found that this can 
>> give about 1 ns of certainly with a simple PLL and analog 
>> RC filter, whereas the 1 PPS  has more like 100 ns of uncorrected 
>> uncertainty in it, and for the most part the 1PPS needs a processor 
>> to use it instead of simple Phase Lock Loops that I am able to 
>> be used on the 100 PPS

by /tvb
>Where do you get your "1 ns" and "100 ns" figures? 
> Have you actually measured these values? Also, 
> over what time average (tau) are you assuming 
> this level of resolution?


by WarrenS
The 1ns and 100ns are measured peak values of the 
nominal phase Jitter over a few seconds time as 
displayed on a scope and averaged from a time interval meter.
The averaged time has to be long enough to let the GPS Phase 
noise cycle thru a few of the Oncore's sawtooth cycles, 
and short enough so as not to include the added received GSP 
signal errors.
The GPS signal errors that I see are around 10 ns over a 10 plus 
seconds period, with short excursions of an additional 30 ns 
every few minutes generally lasting no longer more than a few seconds. 
These Phase noise values I see are pretty much independent of time, 
as long as the viewed average time is not too long or too short. 

There is also another nominal 50 ns phase error that comes along maybe 
every hour on the 1 PPS and once or twice a day on the 100 PPS both 
lasting from 1 to 10 minutes or so. This happens when the various 
different frequencies involved sync up for a while.


by WarrenS
>> On the  LeapSecond.com site
>> http://www.leapsecond.com/pages/gpsdo/
>> It would seem that much of the noise what they have 
>> plotted is the results of  NOT setting the GPSDO  tracking 
>> TC slow enough.  The GPS is adding noise, to the low noise 
>> oscillator.
>> This is why the Allan Deviation increases by more than 3 db in 
>> the tracking  mode at mid averaging times below a 1
>> 1000 seconds, (15 minutes)
>> For a low noise Rubidium Oscillator like the SRS PRS10 

>> Bruce early said, "where the ADEV for various GPSDO remains 
>> below 1E-11 over the Tau range of [0.1s, 100,000s]. 
>> I said: That Tau has nothing to do with the tracking time 
>> constant that the GPSDO has been is set to, which is usually 
>> recommended to be set for a TC of several hours for optimal results.
     ( for low noise Rubidium Oscillators)

from /tvb
> For ideal results start with the TC = the tau where the ADEV
> of the GPS engine & phase detector crosses the ADEV of the
> OCXO. However there are other practical considerations.

>At short averaging intervals when there is a phase difference
>it is easy for the GPSDO to assume the OCXO is stable and
>the GPS engine has noise. So you average more samples.

>At long averaging intervals when there is a phase difference
>is it easy for the GPSDO to assume the GPS engine is more
>correct and the OCXO has drifted. So you steer using EFC.

>At the tau of the ideal TC, the GPSDO sees an average phase
>difference but can't totally blame either the OCXO or GPS for
>the error. By definition at this tau, half the noise is due to each
>subsystem. Hence you nearly always see an ADEV hump that
>goes above where you'd really like it. Best case sqrt(2).

>The ADEV hump itself is not indication of a mistuned TC; it is
>an indication that the GPSDO is working correctly.

>You can move the hump left and right and distort its shape by
>changing the TC.

>In practice I suspect most commercial GPSDO have a TC that
>appears too low for your liking. I can discuss why if you wish.
>/tvb

from WarrenS
Thanks, Very Good information to know and remember, and It 
gives me a much better understanding of what is happing.
I see that in many of the graphs shown on the Leap Second plots,  
the tracking hump is more like 4 to one instead of 1.5 to one.
Is this because the tracking TC is too fast for best noise?
I would love to hear more about it, Please 'Discuss why'.

Also if you would, I'd like to have a better understand of
what seems like an over obsessions with Low Noise GPSDO.
I do understand the need (or at least the desire) 
to have low noise oscillators when using them directly for high 
frequency and/or short time scale data taking applications, 
and the need for a good stable oscillator where it must maintain 
frequency if the GPS signal is lost, 
BUT it sounds like this is not what a large percentage of the 
second hand GPS trackers discussed here are used for. 
It does not seem like it would matter what the noise of the internal 
OSC is, if the unit is only being used for averaging things over longer 
periods such as looking at phase drift over time when doing freq checks. 
Any data taken at a period of say 1000 second to several days is 
pretty much limited by the noise of the GPS and not the Oscillator, 
so why is there the desire for the low noise oscillators in these cases?

Warren
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