[time-nuts] Secondary phase noise standard & FE405

Bob Camp kb8tq at n1k.org
Sun Nov 13 14:00:55 EST 2016


Hi

The “sure fire” way to reduce the spurs is to go to a divisor that “fits” in to 
the DAC bit width. If you have a 12 bit dac, those points happen at Fclock / 2^12.
For Time Nut sort of stuff that’s pretty coarse tuning. 

Bob

> On Nov 13, 2016, at 11:03 AM, Tim Shoppa <tshoppa at gmail.com> wrote:
> 
> Some AD DDS app notes give examples of spurs and choosing nearby (but not
> exactly on freq) numbers that are much less bad for spurs - or at least
> that move the spurs outside the cleanup filters/loops. I don't know of a
> general example or even code that does this in a general way.
> 
> One paper I like on the subject is this:
> http://ttcla.org/vsreinhardt/DDS%20spur%20reduction%20techniques.pdf
> 
> P.S. I just like saying "Wheatley Jitter Injector". Not for those who are
> embarrassed easily... Almost as good as "Wankel Rotary Engine".
> 
> One of
> 
> On Sat, Nov 12, 2016 at 8:13 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
> 
>> 
>> kb8tq at n1k.org said:
>>> Yes, the FE-405 uses a DDS and a cleanup. Inside the cleanup loop the DDS
>>> spurs come  straight through. Since the FE-405 compensates for all sorts
>> of
>>> things, the DDS moves around a lot. Even a one bit change on a DDS will
>> move
>>> spurs around. With an ever changing  DDS, you have an ever changing
>> forest
>>> of “stuff” on the output.
>> 
>> Is there a web page or such telling me where the spurs will be on a DDS
>> for a
>> particular constant?
>> 
>> 
>> --
>> These are my opinions.  I hate spam.
>> 
>> 
>> 
>> 
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