Actually it is (n*Fs +/-Fo), however, they do go down in amplitude as a SinX/X. Theoretically, they would also be there after the Sin ROM, and the only one really of concern in the real world is the Fs - Fo, hence the LPF. When I used to design DDSs in the early 70's, we typically tried to use an Fs that was about 2.5 times the max desired Fo, to make filtering easier. We also used discrete parts then :). Now, with the higher speed devices available a real high Fs can be selected to meet a desired Fmax. Of course the limit is always being pushed, and, the devices are never fast enough. Regards - 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 Graham / KE9H Sent: Monday, June 20, 2011 11:12 AM To: time-nuts@febo.com Subject: Re: [time-nuts] DDS'ery Luis: No, not the same. The most significant bit out of the accumulator has the alias information on it (Fs +/- Fo), so it still needs to be run through the low pass filter to clean off the alias signals. The alias signals manifest themselves as jitter, so no amount of just clipping will remove them. If your application is not sensitive to the alias frequencies, then OK to drive out of the DDS directly. If you are driving something like a mixer in a wide band radio, then you still need to use the low pass filters. They don't call them anti-alias filters for no reason. --- Graham / KE9H == On 6/20/2011 9:46 AM, Luis Cupido wrote: > Folks, a quick one... > > A DDS, that is an accumulator with a DAC followed by a low pass filter > and comparator (zero crossing) to produce a square wave to drive a PLL > or a MIXER or else (at logic levels). > > Isn't it the very same thing as just using the most significant bit of > the accumulator. > > Or am I missing something here ? > > Comments appreciated. > thanks. > > 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.