[time-nuts] Metastability in a 100 MHz TIC

Tom Van Baak tvb at LeapSecond.com
Fri Jul 20 20:07:06 EDT 2007


Ah, I see now. Sorry. You are adding 120 TIC values together,
dividing by 120 to get one mean TI value.

You are also, independently, adding 120 sawtooth corrections
together and dividing by 120 to get one mean correction value.

Then, every two minutes, you add these two numbers and call
it your OCXO-GPS phase and derive your DAC adjustment
from that. I understand now. Mathematically your 120-batch
scheme is identical to sawtooth correcting every 1pps live
every second and averaging for 2 minutes.


----- Original Message ----- 
From: "Richard H McCorkle" <mccorkle at ptialaska.net>

The GPS 1ns sawtooth corrections are accumulated in a 16F688 during the
120 second sample period with care that they match the same 120 1-second
10ns resolution phase samples collected. The accumulated sawtooth
correction is read at the end of the sample period before the sawtooth
correction for the next sample is sent by the GPS, scaled to match the
accumulated phase count resolution and added to the phase count before
the value is sent to the filter. This simplifies the design and has the
same effect as adding a scaled sawtooth correction matching the counter
resolution from each sample once per second. I do indeed make sure both
numbers cover the same samples and have the same LSB resolution before
adding so the results are valid.


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