[time-nuts] DDS in GPSDO design?

[Bruce Griffiths]

Since the Taylor series correction is done entirely within the FPGA 
there should be no settling time issues other than the associated 
pipeline delay.
It makes use of the untruncated data available (before phase truncation) 
within the FPGA.
Of course this doesn't correct for DAC nonlinearity, however this only 
contributes harmonics and doesnt generate other spurs.

Bruce


Mike Feher wrote:
> Thanks - I'll look those over. Just seems the initial accumulator setting
> always needs to be known, or preset to a pre-defined number for anything to
> help. On top of that to apply a series, even with a limited length, just
> would add to settling time or introduce other issues. Regards - Mike
>
> Mike B. Feher, EOZ Inc.
> 89 Arnold Blvd.
> Howell, NJ, 07731
> 732-886-5960 office
> 908-902-3831 cell
>
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Bruce Griffiths
> Sent: Monday, May 28, 2012 12:36 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] DDS in GPSDO design?
>
> These Xilinx datasheets indicate the performance of some techniques:
>
> http://www.xilinx.com/support/documentation/ip_documentation/ds794_dds_compi
> ler.pdf
>
> http://www.xilinx.com/support/documentation/ip_documentation/dds.pdf
>
> The Taylor series (amplitude and phase) correction technique is the most
> effective.
>
> Bruce
>
> Mike Feher wrote:
>    
>> With a finite number of bits for phase and amplitude, regardless of
>> total number of bits. How do you eliminate spurs by design? Thanks -
>> Mike
>>
>> Mike B. Feher, EOZ Inc.
>> 89 Arnold Blvd.
>> Howell, NJ, 07731
>> 732-886-5960 office
>> 908-902-3831 cell
>>
>> -----Original Message-----
>> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]
>> On Behalf Of Bruce Griffiths
>> Sent: Monday, May 28, 2012 12:10 AM
>> To: Discussion of precise time and frequency measurement
>> Subject: Re: [time-nuts] DDS in GPSDO design?
>>
>> Michael Tharp wrote:
>>
>>      
>>> On 05/27/2012 06:23 PM, Bruce Griffiths wrote:
>>>
>>>        
>>>> The principal problem with conventional DDS implementations is phase
>>>> truncation spurs which can occur close to the desired carrier.
>>>> Virtually all commercial DDS chips produce such phase truncation spurs.
>>>>
>>>> It is possible to eliminate such spurs if one implements a custom
>>>> DDS using an FPGA and an external DAC.
>>>> In this case the performance is limited by the DAC.
>>>>
>>>>          
>>> How specifically does the FPGA resolve the problem? I have a FPGA
>>> already for the phase comparator, it's just a "simple" matter of
>>> figuring out how big of a DAC to get and what data to feed it...
>>>
>>>
>>>        
>> By implementing a design thats phase truncation spur free like:
>>
>> /Ultra low Phase noise DDS/ , Fred Harris, Chris Dick, Richard Jekel
>> in Proceedings of SDR06 Technical conference and Product exposition.
>> Both amplitude and phase errors arise due to phase truncation and its
>> essential to correct both.
>>
>> rather than the simplistic technique used in AD (and other) DDS chips
>> where the phase and amplitude errors due to truncation remain uncorrected.
>>
>>      
>>>> Another approach is to use a cascaded mix and divide technique
>>>> <http://www.karlquist.com/FCS95.pdf>   to restrict the effective
>>>> tuning range of the DDS.
>>>> The amplitude of DDS generated spurs is thereby significantly reduced.
>>>>
>>>>          
>>> Great paper, this looks like it could be interesting as a standalone
>>> filter. It's just a little over my head (self-taught digital guy) but
>>> since I don't need to hit a home run on the first try, I'll keep it
>>> in mind.
>>>
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>>>        
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>
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