[time-nuts] DDS in GPSDO design?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon May 28 08:33:11 UTC 2012


The frequency word is the same width as the accumulator, however the 
output of the accumulator is truncated.
The phase completion and amplitude adjustments that the Taylor series 
correction method employs dont use the initail accumulator value 
although this has some effect (see heroic DDS parallelism thesis link in 
previous post).

Bruce

Mike Feher wrote:
> Bruce -
>
> Is the FPGA the same width as the accumulator? Still it seems to me that the
> initial conditions of the accumulator at the time of a frequency chance
> would matter as well. At least it used to. I agree, in as pipeline
> situation, If the FPGA is no more than a look up, there would be no delay.
> However, if it actually had to do some number crunching. It would have to be
> pretty quick to fit into a pipeline time slot. - 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 1:44 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] DDS in GPSDO design?
>
> 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.
>>>>
>>>> _______________________________________________
>>>> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to
>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>>> and follow the instructions there.
>>>>
>>>>
>>>>
>>>>          
>>> _______________________________________________
>>> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to
>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>> and follow the instructions there.
>>>
>>>
>>> _______________________________________________
>>> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to
>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>> and follow the instructions there.
>>>
>>>
>>>
>>>        
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to
>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
>>
>>
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to
>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
>>
>>
>>      
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>
>    




More information about the time-nuts mailing list