[time-nuts] FE-5680A FAQ update: question about frequency synthesizer architecture

[Javier Herrero]

El 28/01/2012 00:17, claudio.girardi at virgilio.it escribió:
> Hello John,
> thanks for maintaining the 5680A FAQ;
> regarding its
> frequency resolution, have you checked the DDS FSELECT pin (and maybe
> also PSEL0/1) to see if they are doing some kind of "dithering" of the
> DDS frequency ?
I've just checked it... yes, it does. In the unit I've at hand, a quick 
check shows a square wave at FSELECT, rate 416.6666666...Hz, duty cycle 
50% (exact as per a fast check, will try to measure with higher detail 
later).

I've not reprogrammed the frequency adjustment on this unit, so I 
suspect that the adjustment is done by a combination of varying the duty 
cycle at FSELECT and reprogramming the DDS. Next step: to snif the SPI 
buts to the AD9832 :)

Regards,

Javier