[time-nuts] GPSDO with all-digital phase/time measurement?

[Charles Steinmetz]

Bob wrote:

>You can achieve very good accuracy, but at the cost of waiting 
>thousands of seconds between "phase points"; i.e. where your 1PPS 
>coincides with the 10 millionth OCXO pulse.
>
>So, as your 1PPS pulse bobs back and forth, you will often encounter 
>an OCXO pulse up to 10ns early, or up to 10ns late.  So, might you 
>count 9,999,999 pulses from the OCXO immediately followed by 
>10,000,001 pulses.  Neither of those, by itself is a signal to 
>change the EFC voltage to your OCXO.  In fact, it is normal for your 
>count to alternate between the two for long periods, if you are very 
>very close to exactly 10MHz, just from the quantization error on the 
>1PPS.  It is also normal for 1/T to control the time between phase 
>crossings.  So you have to wait for two miscounts in a row in the 
>same direction to make a change.

I have been puzzled more than once by your comments about only 
changing the DAC count every several minutes or more.  I am not 
familiar with the circuit you are using, but in a digital PLL the 
errors (assessed every second) typically feed a digital filter that 
drives the DAC.  So, there is generally a very small correction every 
second according to the long running average of the individual 
errors, rather than a large correction after hundreds or thousands of 
seconds.  If you only adjust the DAC every two miscounts in one 
direction, you are guaranteed to get slipped cycles (which appeared 
to be one of the problems you were having when comparing 
oscillators).  This is a reasonable way to get an oscillator roughly 
on frequency if it is substantially off to start with, but it is not 
a good way to hold an oscillator within ppb of the desired frequency, 
and no way at all to hold it in phase lock with the reference.

If that is really the way the circuit you are using works, perhaps it 
would be better to implement a proper all-digital PLL with digital 
filter than trying to get better results out of the circuit you are 
using than it is capable of delivering.

Or, perhaps I'm not understanding what you are doing?

Best regards,

Charles