Hi Bob,
On 02/09/2012 07:03 PM, Bob Camp wrote:
Hi
There are at least two possible sources of the spurs in the FE. One is the
DDS, the other is cross talk in the CPLD divider. In one sense it really
does not matter which it is from, they are indeed there on the output. In
another sense it does. If they are from the CPLD, you *could* hack into the
unit and buffer up the VCXO at 60 MHz to use it as the output. Not as cute
for a standard for your counter, but fine if you want to head off to
microwaves.
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 low frequency DDS variations will go through however.
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.
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.
Cheers,
Magnus
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