[time-nuts] Ublox neo-7M GPS

SAIDJACK at aol.com SAIDJACK at aol.com
Tue Aug 19 16:02:57 EDT 2014


Hal,
 
I guess that depends on your definition of "disciplined".
 
The products that I am familiar with don't consider adjusting phase length  
of an asynchronously running oscillator on a cycle-to-cycle basis thousands 
 of times per second to try to fit 10 million of them (or whatever your 
desired  frequency is) "disciplining". Best case you could call it  
phase/frequency hopping to try to achieve some sort of frequency average in  my 
opinion. 
 
However if you used a DDS to adjust the frequency of an asynchronous clock  
digitally and control that frequency by digital adjustment that would be 
true  "disciplining" of your frequency source. So analog versus digital has 
nothing to  do with it.
 
If your DAC had only a few bits you still would have many orders of  
magnitude less phase errors than the NCO approach; you can do the simple  math:
 
Let's say your VCXO had only 4 bits and a +/-20Hz frequency adjustment  
range. Pretty nasty considering any low-ball GPSDO these days has at least 21  
bits EFC resolution.
 
Now changing one LSB on our 4 bit DAC would thus result in a  massive 
frequency change of +/-2.5Hz. This would result in a phase drift of  2.5E-07 or 
250ns drift over ONE ENTIRE SECOND.
 
That means 250ns divided by 10 Million (!!) cycles or a cycle to cycle  
change of only 25 femtoseconds when the DAC changes state. Theoretically that  
cycle length change would only happen ONCE if the system was a digital DDS 
type  system.
 
How does a single 25 femtoseconds cycle length change on our  hypothetical 
4 bit EFC DAC compare to a 10ns cycle to cycle change that  happens 
thousands of times or more per second on typical NCO's?
 
My point is we are talking performance differences of 5 or 6 orders of  
magnitude between a GPSDO (digital or analog) and an NCO. We are not comparing  
apples to apples. These are not even apples to oranges in my opinion.
 
bye,
Said
 
 
In a message dated 8/19/2014 12:32:02 Pacific Daylight Time,  
hmurray at megapathdsl.net writes:


SAIDJACK at aol.com said:
> its not a GPSDO though, not even  a simple one :)
>   It does not discipline an oscillator. It  generates the output by
> mathematically calculating how many phases it  has to add/drop in a 
second,
> then   digitally  adds/drops/extends/retards the phase of the output 
clock to
> achieve an  average of number of desired clock cycles.

Is there something about the  term GPSDO that says I have to do the "D" in 
the 
analog domain rather than  the digital domain?

I agree that current technology doesn't give  results that are useful for 
many 
applications that currently use  GPSDOs.  What if the clock ran at a GHz?  
10 
GHz?  Sure, it  would have spurs, but would it be useful for some 
applications?

Is a  GPSDO still a GPSDO if the D/A driving the VCXO only has a few bits?  
 
How many bits does it need to be a real GPSDO?

Is a battery powered  wall clock listening to WWVB at 2 AM a WWVDO?  It's 
got 
a pretty good  ADEV if you go out far  enough.



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