[time-nuts] Dithering vs. locking all the clocks to the OCXO?

Robert Lutwak Lutwak at Alum.mit.edu
Tue Jul 4 10:34:01 EDT 2006


Are these permanent installations or portable?  If portable, how quickly do 
they need to lock up to within sub-nanoseconds?

In a permanent (or semi-permanent) installation, it's hard to beat 
GPS-steered cesium, with a loop-tau of DAYS to eliminate all the GPS jitter, 
ionosphere effects, etc.

-RL

------------------------------------------------------------
Robert Lutwak, Senior Scientist
Symmetricom - Technology Realization Center
34 Tozer Rd.
Beverly, MA 01915
(978) 232-1461   Voice           RLutwak at Symmetricom.com   (Business)
(978) 927-4099   FAX             Lutwak at Alum.MIT.edu  (Personal)
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----- Original Message ----- 
From: "Stephan Sandenbergh" <stephan at rrsg.ee.uct.ac.za>
To: <time-nuts at febo.com>
Sent: Tuesday, July 04, 2006 8:49 AM
Subject: [time-nuts] Dithering vs. locking all the clocks to the OCXO?


> Hi All,
>
> Earlier, I explained that my application require very good relative
> stability between various GPSDOs.
>
> A rough estimate of my requirements is:
>
> -Baselines of 100s of meters to 10s of kilometres.
>
> -Sub-nanosecond relative stability (this I forgot to mention earlier -
> thanks to TvB for reminding me).
> -Time scales of maybe 100s of seconds to 10s of minutes.
>
> -The lower limit on my stability requirement is maybe the 200ps of jitter
> that the FPGA will add to the processed data.
>
> My question is this:
>
> At this stage I'm not sure what all the various causes are for the error 
> in
> the 1PPS output of a GPS receiver. (I sure I will find the answer to this 
> in
> all that references TvB and Magnus gave me).
>
> However, a quick guess would be the delay caused by atmospheric effects (I
> don't think thermal noise would play a big role since the antenna is 
> looking
> straight up) Also, there will be errors higher up in the food chain, such 
> as
> changes in satellite orbits etc. I guess these errors are fairly 
> systematic.
> Lower down in the food chain, I presume the M12+T adds further errors to 
> the
> signal (viz. the antenna, LNA, TCXO jitter, etc). I presume these errors
> would be on faster time scales, smaller and much more stochastic in 
> nature.
>
> If the resolution of my phase comparator is about 100ps, and keeping in 
> mind
> that I want relative stability, wouldn't it make sense to lock the M12+T's
> on-board TCXO to the OCXO (probably not straight forward to do)? I realise
> that one would lose the advantage of any dithering effect which would
> quickly average any zero mean effect. I guess this will depend on the 
> nature
> of the errors introduced due to clock jitter: Is it Gaussian and zero 
> mean?
> I guess one will have to investigate what happens at down-conversion to IF
> etc. And, that ultimately it will depend on the size and nature of noise
> caused by the TCXO.
>
> If one could closely follow the drifts in atmospheric effects (which would
> be the same for short baselines) one will have very good relative 
> stability.
>
> Regards,
>
> Stephan
>
>
>
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