[time-nuts] DDS'ery
Ulrich Bangert
df6jb at ulrich-bangert.de
Tue Jun 21 09:03:22 UTC 2011
John,
as usual I second your opinion and I did have already on my mind to suggest
XILINX's DDS compiler to the group too.
However your statement
> to provide SFDR up to 150 dB (and I'd notice it if I were
> getting much less than that in practice.)
has pushed me up! When I tell the compiler to generate me a 150 dB SFDR DDS
then it produces an block with 28 (!) bits output witdh for the DAC. So, I
am asking myself what wonder-chips you may be using as DAC for your DDS that
features a dynamic range high enough to really measure a 150 dB SFDR?
Best regards
Ulrich Bangert, DF6JB
> -----Ursprüngliche Nachricht-----
> Von: time-nuts-bounces at febo.com
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von John Miles
> Gesendet: Dienstag, 21. Juni 2011 00:52
> An: 'Discussion of precise time and frequency measurement'
> Betreff: Re: [time-nuts] DDS'ery
>
>
> I'm not familiar with Altera's DDS options, but I will say
> that Xilinx's DDS compiler is superb. It can be configured
> to provide SFDR up to 150 dB (and I'd notice it if I were
> getting much less than that in practice.)
>
> As Javier hinted, the reason you can't use the MSB directly
> is that its transition point is not necessarily stationary
> between cycles of the frequency you're trying to synthesize.
> It will flop around all over the place. You need at least a
> few more bits in most applications -- remember that in an
> n-bit word, the magnitude represented by the n-1 LSBs is
> almost as much as the bit-n MSB.
>
> When DDS technology was first becoming popular in the 1980s,
> Qualcomm was one of the main vendors, and they required
> external DACs. High-speed DACs were pricy and used a lot of
> power, so I imagine that a great many people tried feeding
> the MSB directly to the filter, as I did. It could be
> feasible at some selected frequencies or at very high
> clock/output ratios, but in the general case the output
> signal is just comically awful.
>
> You would need a truly massive filter to provide the needed
> flywheel effect to make up for those missing bits. And it
> would need to be a BPF, not just an LPF, because not all of
> the artifacts associated with output quantization are above
> the desired carrier frequency. Sometimes the MSB's toggle
> period is going to be shorter than it should be, and
> sometimes it's going to be longer.
>
> -- john, KE5FX
>
>
> > -----Original Message-----
> > From: time-nuts-bounces at febo.com [mailto:time-nuts-
> bounces at febo.com]
> > On Behalf Of Luis Cupido
> > Sent: Monday, June 20, 2011 9:46 AM
> > To: Discussion of precise time and frequency measurement
> > Subject: Re: [time-nuts] DDS'ery
> >
> > 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 ?
> >
> > Thanks.
> >
> > Luis cupido.
> > ct1dmk.
> >
> >
> >
> >
> >
> > On 6/20/2011 4:52 PM, Javier Herrero wrote:
> > > To reduce the spurii due to quantization distortion. Here is an
> > > explanation, in Section 4
> > >
> > > http://www.analog.com/static/imported-
> > files/tutorials/450968421DDS_Tutorial_rev12-2-99.pdf
> > >
> > >
> > > Regards,
> > >
> > > Javier
> > >
> > > El 20/06/2011 17:39, Luis Cupido escribió:
> > >> Well, if we really need to filter it out
> > >> we better filter the MSB and square it
> > >> again...
> > >>
> > >> Why having a DAC for ???
> > >>
> > >> Right ?
> > >>
> > >> Luis Cupido.
> > >> ct1dmk.
> > >>
> > >> _______________________________________________
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> >
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