[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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