[time-nuts] 4046 experiment for gpsdo

[Bob Camp]

Hi

Why not do a FLL based on the counter and let the TDC run at 5 MHz (with 5X
the resolution)?

It’s reasonable to believe that if you run the FLL for a while you will get things 
quite close. That should allow you to run the TDC at 10 MHz.

Bob

> On Sep 26, 2015, at 7:22 AM, Jim Harman <j99harman at gmail.com> wrote:
> 
> 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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