[time-nuts] 4046 experiment for gpsdo

Jim Harman j99harman at gmail.com
Sat Sep 26 11:22:07 UTC 2015


Good question Will.

First, it divides the 10 MHz down to 1 MHz, so the oscillator would have to
be off by 10 Hz for it to lock onto the wrong cycle.

Second, the full implementation also feeds 5 MHz from the oscillator into
one of the processor's counters and checks the count every second. It
performs several checks on this to detect if the frequency is way off,
missed PPS, etc.

On Sat, Sep 26, 2015, 12:15 AM Will <ZL1TAO at gmx.com> 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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