[time-nuts] DDS in GPSDO design?

[Mike Feher]

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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