[time-nuts] FE 5680A "new version" - Filtering the 10 MHz

Javier Herrero jherrero at hvsistemas.es
Wed Nov 16 04:18:52 EST 2011


El 16/11/2011 07:22, Sylvain Munaut escribió:
> But a DDS would require a DAC somewhere. Here, I see can see the trace 
> from the PLD pin, to the LC filter to the RF connector ... and I 
> probed the PLD output, it's a square wave. 
A good DDS does, but (as was discussed here some time ago) you can use a 
1-bit DAC (i.e. only the MSB). The output frequency would be correct, 
but quite jittery. Have you performed any jitter measurement at the PLD 
output? In any case, it seems to me a small PLD to have implemented a 
NCO, but of course that depends on the NCO implementation and perhaps 
could be there... but it is an speculation.

In any case, if there is only a simple LC filter from the PLD to the RF 
connector, I would not be too surprised for artifacts in the output of 
the PLD. The ones and zeros have usually a lot of noise coupled from the 
remaining logic.

I hope I can play also soon, when I got mine :)



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