In the old days, HI, we used to use 6 dB/bit for SFDR for the DAC as a rule of thumb. In practice, it needed to be somewhat better. So, even with 6 dB/bit it would require a minimum of 25 bits. Good thing back then, in the early 70's I was working on ASW stuff at acoustic frequencies and some of this was doable. However, we did not require a great SFDR. More recently in the late 80's and 90's, I was concerned in getting DDSs in the 500 to 1 GHz range. Lots of R&D money and lots of promises, but, no cigar for a real usable product. 73 - 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-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of Ulrich Bangert Sent: Tuesday, June 21, 2011 5:03 AM To: 'Discussion of precise time and frequency measurement' Subject: Re: [time-nuts] DDS'ery 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-boun...@febo.com > [mailto:time-nuts-boun...@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-boun...@febo.com [mailto:time-nuts- > boun...@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. > > >> > > >> _______________________________________________ > > >> time-nuts mailing list -- time-nuts@febo.com > > >> To unsubscribe, go to > > >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > >> and follow the instructions there. > > >> > > >> > > > > > > > _______________________________________________ > > time-nuts mailing list -- time-nuts@febo.com > > To unsubscribe, go to https://www.febo.com/cgi- > > bin/mailman/listinfo/time-nuts and follow the instructions there. > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.