[time-nuts] Low-Cost Rubidium Performance

[Magnus Danielson]

Hi Javier,

On 02/09/2012 10:04 PM, Javier Herrero wrote:
> Hello,
>
> El 09/02/2012 21:06, Magnus Danielson escribió:
>>
>> I was just thinking about that. For these more modern 5680A, just the
>> Q of the resonance and the loop filter / loop bandwidth would not
>> allow for so much high frequency side-bands of the DDS to pass through.
> The loop filter at least should have good rejection to the 1400Hz
> frequency presumed to be used for the detection... although in the phase
> noise plots I've not seen a very significant spur at that frequency.

Consider that it is de-modulated and then low-pass filtered.

Also, it is the alternating rate and not 1400 Hz difference in DDS 
setting which is the key parameter here. The 1400 Hz gives a hint of the 
Q-value however, which seems to be lower on these than on any of my 
larger rubidiums, but it is maybe to be expected.

>>
>> The low frequency DDS variations will go through however.
> Yes :)
>>
>> I'd expect that the DDS noise creeps onto the 10 MHz signal one way or
>> another, such as the CPLD or other location where separation is poor.
>>
>> It would be a bit fun to hunt around and see where the noise creeps in.
>>
> Quite a bit. Would be helpful to see how bad is the 60MHz signal
> available inside :) All those 1Hz spaced spurs (and multiples...) could
> be originated by the CPLD.

Hmm, picking up the 60 MHz is quite easy! I just don't have the matching 
connector at home. Maybe I can borrow one from work.

>> It would also be fun to see what a mixer based PI-loop OCXO cleanup
>> (using say a spare 10811) would do. Using a pre-filter and mixer (to
>> avoid severe intermodulations) while still getting a decenting
>> filtering effect.
> Also... perhaps using the 60MHz instead if it is cleaner... I've a spare
> 10544 and a 10811 that would love to use for cleaning up the output (but
> lack of time for now to play around as much as I would like :) )

You could build a synchronous re-generative divider to divide 60 MHz 
into 10 MHz (and 50 MHz). It's not that much components for a low-noise 
divider. A re-generative divider could also produce 20 MHz and 40 MHz, 
where 20 MHz is useful for among other things Z12T.

Cheers,
Magnus