[time-nuts] DDS module

[Hal Murray]

lists at rtty.us said:
> A number of them are sigma delta PLL's and have some really nasty jitter and
> spur issues. Anything that shows up listed as "user programmable" - watch
> out. 

But "user programmable" won't find all the nasty ones.

I haven't been looking recently.  There used to be some that were field 
programable.  The idea was that the distributor would do it so you didn't 
have to wait for the factory to grind the crystal to order if you wanted an 
oddball frequency.  They didn't mention the PLL in the data sheet.  You had 
to read between the lines.

I assume the distributor stocked a handful of basic frequencies and there was 
a PC that told them which one to use and did the programming if you told it 
what frequency you wanted.

It looks like a fun math problem.  How many frequencies do you need to stock 
in order to cover a given frequency range with a given PLL chip?  The result 
doesn't have to be a direct hit.  You can include some fudge as part of the 
overall accuracy spec.  (If you claim 100 PPM, but cut the crystal within 50 
PPM, then you have 50 PPM left for the PLL.)



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