[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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