[time-nuts] DDS in GPSDO design?

Ulrich Bangert df6jb at ulrich-bangert.de
Mon May 28 12:12:56 UTC 2012


Gentlemen,

in general i can second Bruce's comments on the topic. But since I
constructed a DDS based GPSDO of my own and constructed two DDS based offset
generators of my own I would like to add some comments that arise mainly
from experience:

1) The absolute size of the phase and amplitude truncation errors are a
direct function of the accumulator width and the dac bit width. Two designs
that are different in these parameters my behave VERY different.

2) A AD9850 32 bit accumulator / 10 bit dac design is definitely NOT suited
for serious time applications.

3) A AD9852 48 bit accumulator / 12 bit dac design is - whith some
compromises - suited for serious time applications.

4) If one wants to do better a fpga solution is needed. Note that the
current XILINX and ALTERA DDS ip cores offer no higher accumulator
resolution than 48 bit, so you won't be able to increase the frequency
resolution with them against the AD9850. Nevertheless they feature a number
of interpolation and dithering tricks to decrease the phase and amplitude
truncation errors. Note that you can take advantage out of these tricks then
and only then when the DDS is allowed to produce an output word with
sufficient bit width. As long as the NCO is completely INSIDE the fpga (as
in John Miles's Timepod project) wide output words are not a problem.
However if you are thinking of producing a real world signal keep in mind
what bit width your external dac has to avoid an overshot in complexity in
the fpga compared to what the dac can deliver. I would judge that a 16 bit
output width in conjunction with a 16 bit dac as the AD5762 or the LTC1668
is currently the best that can be done. The AD offers the higher update rate
but the LTC is easier to connect to the DDS ip cores due to its parallel
data input design. Design a low tempco reconstruction filter for the dac and
please don't ask me how to do because I don't know.

5) Although Rick is a member of the group I am always very astonished that
his synthesizer publications specially 

http://www.karlquist.com/FCS95.pdf 

are not well known among the group members. I assume the design described
here makes an excellent tuning circuit for use in an GPSDO albeit the
repeated dividing and mixing increases the complexity. I am not aware which
impact the repeated filtering has on the overall tempco. My guess is that
the Symmetricom AOG 110 must be similar in design.

Best regards
Ulrich Bangert 

> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Bruce Griffiths
> Gesendet: Montag, 28. Mai 2012 10:41
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] DDS in GPSDO design?
> 
> 
> Poul-Henning Kamp wrote:
> > In message<4FC2A5E9.7050304 at partiallystapled.com>, Michael Tharp 
> > writes:
> >
> >    
> >> Does anyone have any comments or experience with DDS-based 
> frequency 
> >> references?
> >>      
> > I belive that is basically how the optional "output 
> generator" works 
> > for hydrogen masers.
> >
> >    
> The Symmetricom AOG 110 has a very small fractional frequency 
> adjustment 
> range of 5E-8 albeit with a resolution of 1E-19.
> Thus it is relatively simple to ensure that the effect of any 
> resisual 
> DDS phase truncation and other spurs is very small.
> 
> Bruce
> 
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