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: [email protected] [mailto:[email protected]] 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: [email protected] [mailto:[email protected]]
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: [email protected] [mailto:[email protected]]
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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