[time-nuts] Determination of the placement of the first pps

Magnus Danielson magnus at rubidium.dyndns.org
Tue Jan 24 22:14:04 EST 2012

On 01/25/2012 02:41 AM, J. L. Trantham wrote:
> Darn.
> I was hoping for that feature.  I still think it should be there.

Indeed. Should be in there somewhere...

Didn't see these link hit the list:



Looking at the last one it says:

"NOTE: Although this unit is marked with both 10MHz and 1pps, research 
on the 'net seems to indicate that the '1pps' output has only a period 
of exactly 1 second when the frequency is set to 223 Hz (8.388608Mhz). 
According to those sources the '1pps' will have a period of 0.8388608 
seconds when the output frequency is set to 10MHz. This should be easy 
to verify. In any case I have no need for a 1pps output - I use a GPS 
module to get a 1pps signal which also has the advantage of being 
in-phase with real time seconds. "

Now... to speed-adjust the PPS phase, use the DDS and steer it 
intentionally of frequence with sufficient delta frequence for suitable 
time and you should home in pretty quickly.

Hunting some more:

"Without modification the units have just one output - 1pps (1 Hz). 
There is a simple modification to extract 8388.608 kHz, which is of 
course 223 Hz, and this frequency is used, through binary division, to 
generate the 1pps output. The 8388.608 kHz output is generated by a 
32-bit Direct Digital Synthesizer chip (AD9830). This output can be 
steered to any other frequency within the operating range, by 
interacting with the controlling microcontroller, with three provisos:

1. The unit has a peaked filter at the synthesizer output, and so the 
level at other frequencies varies wildly. This can be corrected with 
minor modifications.
2. When operating at any other frequency than 8388.608kHz, the 1pps 
output is of course incorrect.
3. The synthesizer operating frequency can be set to within ±5 mHz 
(milliHertz) of the requested frequency,
         - but ONLY if the calculations, on which the command sent to it 
is based, correctly use 32-bit maths. "

Which is a confirmation...



So, this would work for the 50,255+ MHz based FE 5680A. For 60 MHz 
variants it works a little different, but it has two MCUs sitting there, 
so some use for theme should there be.


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