With the coordic (yeah, sometimes cordic), you need to build it a few more bits
wider than the DAC. Then it closely matches the lookup table. One of the best
references for the coordic I found was a PhD dissertation at Stanford. The
author's last name was Ahmed IIRC.
It is a very versatile algorithm. You can vary the magnitude of the initial
vector to AM the signal. PM is done by directly. The coordic can also multiply,
divide, and compute inverse trig functions. It was used (and might still be) in
scientific calculators.
We did a FSK demod with the coordic. Basically unwrap the phase of the signal
and least mean square fit the phase to a straight line, where the slope of the
line relates to the frequency.
-----Original Message-----
From: Javier Herrero <jherr...@hvsistemas.es>
Sender: time-nuts-boun...@febo.com
Date: Mon, 20 Jun 2011 21:46:11
To: <time-nuts@febo.com>
Reply-To: Discussion of precise time and frequency measurement
<time-nuts@febo.com>
Subject: Re: [time-nuts] DDS'ery
Inside an FPGA, CORDIC can be implemented performing quite fast. An
open-source implementation of CORDIC algorithm and a DDS using look-up
table or CORDIC can be found in OpenCores, gh_vhdl_lib.
Jim, thank you very much for all the excellent references, and
particularly the last one about the CORDIC variation. The Altera
simulations of its NCO showns significant more spurii for CORDIC
(traditional one) than the other alternatives for same phase accumulator
and magnitude precisions, and angular same angular resolution, but it is
by far the most effective in terms of FPGA usage.
Probably I will use the CORDIC approach in the implementation of my
digital down-converter, but the OpenCores core instead of the Altera IP,
if it is clean enough (the signal I need to downconvert is 8-bit
quantized and with poor S/N ratio, so some -40dB spurious will not have
significance).
Regards,
Javier
El 20/06/2011 21:17, li...@lazygranch.com escribió:
> Just a FYI, you don't have to use sine lookup tables. You can generate sine
> and cosine on the fly with a coordic. Perhaps not easy at RF speed, but very
> common in audio DSP.
>
> -----Original Message-----
> From: Jim Lux<jim...@earthlink.net>
> Sender: time-nuts-boun...@febo.com
> Date: Mon, 20 Jun 2011 12:04:24
> To:<time-nuts@febo.com>
> Reply-To: Discussion of precise time and frequency measurement
> <time-nuts@febo.com>
> Subject: Re: [time-nuts] DDS'ery
>
> On 6/20/11 9:46 AM, Luis Cupido wrote:
>> Gracias, Javier.
>>
>> As you read in my previous email I'm basically
>> worried about close-in spurs (those that
>> will pass through the PLL loop filter).
>>
>> will digest that 4th section... tks.
>>
>> ...
>>
>> Since I'm inside an FPGA... I'm eager to get
>> spurs down without leaving the digital world...
>> Anyone knows any literature covering that ?
>>
> Tons..
>
> Jouko Vankka wrote whole books about it.
>
> Direct Digital Synthesizers and transmitters for software radio
> Direct Digital Synthesizers: Theory, Design and Applications
>
>
> You might want to look at various Error Feedback/Error Filtering schemes
> which allow you to use a smaller cosine table and/or smaller DAC and
> have better spur performance.
>
> Vankka, J, "A direct digital synthesizer with a tunable error feedback
> structure", IEEE Trans on Comm, V45, #4, pp416-420, 1997
> Vankka's EF technique works quite well at suppressing spurs close to the
> carrier (at the expense of pushing them farther out).
>
>
> Reinhardt, V, "Spur Reduction Techniques in Direct Digital
> Synthesizers", Proc Intl Freq Control Symp, 1993
>
> Flanagan, M., Zimmerman, G., "Spur-reduced digital sinusoid synthesis"
> IEEE Trans on Comm, V43, #7, pp2254-2262, 1995
> (this one is about using dither to spread the spurs out)
>
> O'Leary, P., Maloberti, F., "A direct-digital synthesizer with improved
> spectral performance", IEEE Trans on Comm, V39, #7, 1991
>
>
> You might also look at some of the spur cancellation things, such as the
> one implemented in some of AD's DDS parts.. Basically, it's a second NCO
> that generates a coherent signal that is subtracted/added to the primary
> signal to "notch" out the spur.
>
>
>
>
>
>
>
>
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>
--
------------------------------------------------------------------------
Javier Herrero EMAIL: jherr...@hvsistemas.com
Chief Technology Officer
HV Sistemas S.L. PHONE: +34 949 336 806
Los Charcones, 17 FAX: +34 949 336 792
19170 El Casar - Guadalajara - Spain WEB: http://www.hvsistemas.com
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