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
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