You missed the part about the 10MHz being divided by 10 million
to produce a 1PPS signal that is compared to the second 1PPS signal...
-Chuck Harris
Will wrote:
Hi,
I'm new and trying to get to grips with things.
If I understand correctly, please forgive if I have it wrong, This
locks a 10MHz signal to a 1Hz (1pps) signal. What makes it lock to 10
000 000Hz instead of 999 999Hz or 10 000 001Hz? Just the hope that the
10MHz is exactly that?
Cheers,
Will
On 26/09/15 08:32, Jim Harman wrote:
To further demonstrate the Diode - R- C- approach, here (hopefully) is a
screenshot of the raw DAC output vs time on my Arduino Micro (32u4) based
system. For this test the oscillator is free running with an error of about
1 usec per 460 sec or 2.17x10^-9. The horizontal scale is 125 sec/div (1000
sec total) and the vertical is 1024 DAC counts (0-2.56 V) which
corresponds to 1 usec of offset between the oscillator and the reference.
You can see that there is some curvature because the capacitor is being
charged through a resistor and not a true current source, but as I
mentioned earlier this does not affect the system's ability to lock the
oscillator to the pps reference. When locked with a time constant of 1000
sec, the phase detector output is almost always less than +/- 100 counts
from the setpoint of 500.
The noise is due mostly to jitter in my PPS reference, which is generated
by an Adafruit GPS module. Presumably it would be less if I had a real
timing receiver.
.
If the inserted image does not come through, I will re-send as an
attachment.
--
--Jim Harman
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