Jim, More technical stuff @ http://www.sotiriadis.info/
Pete Rawson On May 2, 2010, at 10:27 AM, jimlux wrote: > Hal Murray wrote: >> [email protected] said: >>> If there is no electronic tuning available one can use a DDS based >>> synthesiser to produce a corrected output frequency. However close in spurs >>> will be problematic unless one use a couple of simple mix and divide stages >>> or resorts to a Diophantine synthesiser using phase noise truncation spur >>> free output frequencies from the DDS chip(s). >> I think I understand the classic spurs from a DDS. >> I wasn't familiar with Diophantine techniques. Google found this >> http://www.hindawi.com/journals/ijno/2008/416958.html >> which is readable at my level. >> But I don't think I understand the big picture. The example numbers they >> give involve mixing 500 Hz with 10 MHz. Assuming I want the sum, how do I >> get rid of the difference? It's going to be a good strong signal, as strong >> as the one I want. I think anything that leaks through the filter into the >> next mixer is likely to make mirror sidebands that are right where we don't >> want them. >> Why is that going to be easier to get rid of than traditional spurs? >>> Alternatively if one implements the DDS in an FPGA its possible to >>> virtually eliminate such spurs using a modified algorithm. However this >>> requires an external DAC to produce the required output. >> Got a URL? What's magic about a FPGA? Why don't traditional DDS chips use >> that modified algorithm? > > > Commercial DDSs are sold in large quantities for generalized applications, so > they tend not to use exotic techniques for spur reduction over small ranges. > You can also burn gates in exchange for performance, a decision that would be > tough to make for a manufacturer concerned about power dissipation, etc. > > It's easy, for instance, in a FPGA, to implement several different length > cosine lookup tables, so that all the frequencies you want to generate > exactly match the table length. You can also do things like error filtering, > various spur cancellation techniques, etc. > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
