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[time-freq] What Use Clock-Block?

K&J Aggett kj.aggett at optusnet.com.au
Wed Apr 25 00:02:21 UTC 2007


Having driven some country miles recently I had plenty of time to 
consider the Clock-Block and, I must confess, have some queries.  Is it 
an essential bit of kit or simply a 'must have'?  Let me explain.

The average system would probably comprise a GPS receiver, a GPS 
controlled oscillator producing, amongst other frequencies, a "standard" 
1PPS, Clock-Block driven by the "standard" PPS and a computer and 
operating system.  Clock-Block drives the kernel clock and to set the 
actual system time in hh:mm, NTP is used to derive it from the GPS 
receiver, although I concede that this could be set from the hardware 
clock.  To make the system time accessible to other than locally running 
programs, the computer would be designated as an NTP time server on the 

Now, it seems to me that there are several ways of arriving here.  One 
way is to set up the local clock as being externally disciplined and 
introduce some complications I have not yet thought through, while a 
simpler way seems to be simply set the stratum of the local clock to, 
say, 1 or 2.  Whichever way one proceeds, my understanding of NTP is 
that it 'trains' the system clock according to some well honed 
algorithms to obtain the 'best result', whatever that means.

With the 'right' GPS controlled oscillator, I can see that the system 
can ride out periods of GPS signal loss, but that is not a function 
provided by Clock-Block.  A time server with PPS implementation driven 
from GPS controlled "standard" PPS would do the same, so my question is:

"Does anyone have any idea of the in-practice 'best result' of a system 
with Clock-Block compared to that of one without?"

Having a GPS source with PPS on it, one could succumb to the temptation 
to use both Clock-Block and GPS derived PPS (or even the 'standard' 
PPS).  Big or small mistake?

Or have I got the whole thing completely wrong?


Keith Aggett

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