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
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