Hi The CPLD may have enough stuff going on to nuke the output without any help from the rest ou the board.
Bob On Feb 9, 2012, at 6:34 PM, Magnus Danielson <[email protected]> wrote: > On 02/10/2012 12:22 AM, Javier Herrero wrote: >> El 10/02/2012 00:13, Javier Herrero escribió: >>> El 09/02/2012 22:28, Magnus Danielson escribió: >>>> >>>> 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. >>> Yes, you're right... I was thinking on the alternating rate (that in >>> fact I measured, at 416.6666..Hz, but the other number came first ;) >>> ), and in the fact that the FRS, that uses 127Hz as alternating rate, >>> has notorious spurs at 127 and 254Hz (al at a lot of their harmonics), >>> so I was expecting someting similar for the FE5680A at 416.6666Hz and >>> harmonics, but seems not to be there (or the spur forest makes not >>> easy to see these trees :) ) >>> >> I answer myself. Perhaps they are there quite notoriously, since in the >> spectra plots that I took when I got mine, now it is clear why there are >> two peaks at around -70/-75dBc at somewher that seems very near >> +/-416.7Hz: http://www.nebulosa.org/images/FE5680A/FE5680A4.jpg Probably >> in the phase noise plot they are masked by all the phase noise floor and >> other spurii that are not so apparent in a quick measurement with the >> spectrum analyzer > > All being as expected then. > > I think we can focus on tracing the DDS spurs as injected through another > path. Could be lack of isolation of the 60 MHz crystal oscillator from the > rest of the design, or it creeps onto the 10 MHz in the CPLD, or just plains > sneaks in on the output buffer, possibly via the power lines. > > Cheers, > Magnus > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
