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, [email protected] writes:
[email protected] 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. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
