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

Magnus Danielson cfmd at bredband.net
Tue Jul 4 09:29:38 EDT 2006


From: "Stephan Sandenbergh" <stephan at rrsg.ee.uct.ac.za>
Subject: [time-nuts] Dithering vs. locking all the clocks to the OCXO?
Date: Tue, 4 Jul 2006 14:49:19 +0200
Message-ID: <002101c69f68$44983a00$401c9e89 at Stephan>

> 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.

Actually no, not so much as you might expect. Your receivers are *close* to
each other in GPS terms, you will certainly experience what is referred to as
GPS common view. The receivers will experience almost identical shifts due to
atmospheric effects since they are so close to each other. You will most
definitly have the same satelites overhead, except where local foilage
prohibits the view of a certain satelite.

> 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. 

You should be looking at making sure you have a fairly unobstructed view of the
sky and minimal of ground-reflections. A good antenna could reduce dependence
on reflections. BG made some similar comments privately as we met during the
weekend, and I agree.

> 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)?

Infact, BG and I is going to engange in such an experiement, we both had been
playing around with that thought. The details vary depending on which
particular receiver one has, both chipset and software plays a factor.
I do infact have a suitable GPS receiver board here in my hand (thanks to BG!).

> I realise that one would lose the advantage of any dithering effect which
> would quickly average any zero mean effect.

Dithering effect is what you use if you can't do better. It is one of several
methods. If you have better single-shot resolution you will directly lower the
measurement noise-floor and averaging will just work with you.

> I guess this will depend on the nature of the errors introduced due to clock
> jitter: Is it Gaussian and zero mean?

Yes and no. Yes, there is a large white and Gaussian noise in there, no, it is
not all Gaussian and as you go down in frequency (i.e. up in time - tau) you
will experience non-Gaussian noises too. However, most of that should come from
the GPS system and not the local clocks, really depends on the clocks and
PLL bandwidth, a topic which have been discussed lately.

> 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.

Indeed. A good TCXO is needed anyway, low noise is crutial since high noise
will effectively lower the selectivity of the receiver.

> 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.

Indeed.

Cheers,
Magnus



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